It is accepted that during the Late Heavy Bombardment (LHB—4.2–3.8 Ga) in the Solar system, as preserved on the Moon, the terrestrial upper mantle-crust system was dominated by interactions between internal mantle processes and extraterrestrial impacts, and that following this period, the impact flux decreased by two orders of magnitude, from 4–9×10−13 km−2 year−1 to 3.8–6.3×10−15 km−2 year−1 (for asteroids Dc>=18 km), a rate consistent with a cratering rate of 5.9±3.5×10−15 km−2 year−1 estimated for near-Earth asteroids (NEA) and comets. Geology being a geocentric science, the assumption is generally made that from about 3.8 Ga the impact factor can be neglected in the context of models of crustal evolution, including the emergence of early continental nuclei and plate tectonics. This paradigm is questioned in this paper. From the observed minimum number of 6 continental impact structures with Dc>=100 km [Vredefort (300 km); Sudbury (250 km); Chicxulub (170 km); Woodleigh (120 km); Manicouagan (100 km); Popigai (100 km)], assuming an Earth surface occupied by time-integrated >=80% ocean crust since 3.8 Ga, the lower limit of post-LHB impacts is deduced at >=30 craters with Dc>=100 km and >=10 craters with Dc>=250 km. From the lunar crater counts and the present-day asteroid flux the impact incidence was likely to have been higher by an order of magnitude, with a possible decline in the impact frequency of the largest bodies Dp>=20 km. Evidence for maria-scale impact basins in the Archaean emerges from 3.24 Ga-old impact vapor condensation-fallout layers in the Barberton greenstone belt, Transvaal, pointing to multiple oceanic impact basin of Dc>=400 km. This impact cluster falls within error from the c. 3.18 Ga impact peak documented by lunar spherules—suggesting a mid-Archaean impact cataclysm in the Earth–Moon system. Models of crustal evolution need to account for the inevitable magmatic and tectonic consequences of these events, particularly on impacted geothermally active oceanic crust and lithosphere. A combination of the internal heat engine and the impact factors is capable of accounting for the early sialic nuclei, for the spatial and temporal localization of major faulting and rifting events, and for ensuing plate tectonic patterns.
We present new modal Q measurements of the 0S0 and 0S2 modes, and first modal frequencies and Q measurements of 2S1 and 0S3 modes.The high quality of the GGP (Global Geodynamics Project) superconducting gravimeters (SGs) contributes to the clear observation of seismic normal modes at frequencies lower than 1 mHz and offers a good opportunity for studying the behaviour of these modes.The interest of scientists in the gravest normal modes is due to the fact that they do contribute to a better knowledge of the density profile in the Earth, helping to constrain Earth's models.These modes have been clearly identified after some large recent events recorded with superconducting gravimeters, particularly well-suited for low-frequency investigations. The Ms = 8.1 (Mw = 8.4) Peruvian earthquake of June 2001 and the Ms = 9.0 (Mw = 9.3) Sumatra-Andaman earthquake of December 2004 provide us with individual spectra which exhibit a clear splitting of the spheroidal modes 0S2, 0S3 and 2S1, and a clear identification of each of the individual singlets, with a resolution never obtained from broad-band seismometers records.The Q quality factors have been determined from the apparent decrease of the amplitude of each singlet with time, according to a well-suited technique [Roult, G., Clévédé, E., 2000. New refinements in attenuation measurements from free-oscillation and surface-wave observations. Phys. Earth Planet. Inter. 121, 1–37]. The results are compared to the theoretical frequencies and Q quality factors computed for the PREM and 1066A models, taking into account both rotation and ellipticity effects of the Earth. The two observed datasets (frequencies and Q quality factors) exhibit a splitting on the observed values different from the predicted one. That seems to point out that some parameters as density or attenuation values used in the theoretical models do not explain the observations.A new dataset of frequencies and Q quality factors of the whole set of singlets of the gravest spheroidal modes is thus under construction. That dataset includes the five individual singlets of the 0S2 mode clearly identified on the SG records, the three singlets of the 2S1 mode recently observed for the first time by [Rosat, S., Hinderer, J., Rivera, L., 2003b. First observation of 2S1 and study of the splitting of the football mode 0S2 after the June 2001 Peru earthquake of magnitude 8.4. Geophys. Res. Lett. 30, 21211, doi:10-1029/2003L018304], the 0S0 radial mode, and the seven individual singlets of the 0S3 mode.
A 100 m laser strainmeter system was installed in a deep tunnel about 1000 m below the ground surface in Kamioka, Gifu, Japan in 2003. The system consists of three types of independent interferometers: (1) an EW linear strainmeter of the Michelson type with unequal arms, (2) an NS–EW differential strainmeter of the Michelson type with equal arms and (3) an NS absolute strainmeter of the Fabry–Perot type. These are configured in L-shaped vacuum pipes, each of which has a length of 100 m. (1) and (2) are highly sensitive (order of 10−13 strain) and have wide dynamical range (10−13 to 10−6). Based on data obtained from these strainmeters (1) and (2) during the period of about 4 months in 2003, we analyzed tidal strains by employing the tidal analysis program BAYTAP-G. Observed tidal strain amplitudes of eight major constituents were compared with theoretically expected ones that were obtained from the GOTIC2 program. As a result, it was revealed that the “observed” amplitudes are about 10–20% smaller than the “expected” amplitudes. In order to explain these discrepancies, we examined the topographic effect using three-dimensional finite element model (3D FEM), and succeeded in reducing the discrepancies within several percent. This result shows that our laser strainmeter system in Kamioka has sufficient reliability to detect small strain changes of the order of 10−10 in the tidal frequency band.
During the devastating earthquake of 13 May 1995, in the Kozani-Grevena area (Western Macedonia, Greece), many surface ruptures formed in the epicentral area. Most of these fractures were due to faulting, but some were secondary ground ruptures and landslides.Geological field work in the area has shown that the Aliakmon river neotectonic fault consists of several (three or more) fault strands: the Servia, the Rymnio and the Paleochori-Sarakina strands. Using geological criteria, all of these fault strands were judged to be active faults affecting recent (Holocene) deposits and scree. The main new surface fractures caused by the earthquake, and particularly those clearly of tectonic origin, follow systematically the traces of the last two neotectonic fault strands, forming a new fracture line. This tectonic line, trending ENE-WSW (N 70 °), coincides with the focal mechanism solution and the satelite image major lineament. Both the geological and instrumental seismological data suggest that the seismogenic fault is a segment of the Aliakmon river neotectonic fault zone situated among the villages of Rymnio, Paleochori, Sarakina, Kentro and Nisi. The total length of the reactivated fault segment is about 30km long overall and is separated from the non-activated Servia fault segment by a geometrical seismic segment barrier near the village of Goules.The seismic fault is a normal fault trending ENE-WSW and dipping to NNW, with high angle at the surface and low angle at depth. The majority of the epicentres of the seismic sequence were distributed on the hangingwall of this reactivated fault segment.Additionaly a series of subparallel antithetic surface fractures, mainly striking E-W or ENE-WSW and dipping to the South, following previous neotectonic strike-slip faults, were reactivated during the earthquake with the geometry of normal faults antithetic to the main seismic fault. The most important of these are the Chromio-Varis-Myrsina fracture line (length 15km), along the Vourinos corridor dextral strike-slip structure and the Felli fracture line (length 6 km) along the Felli sinistral strike-slip fault.An interpretation of the geometry and kinematics of the reactivated faults is shown in the proposed geological model with simplified cross sections.
Thirty-five years ago the introduction of the plate tectonics paradigm led to a new understanding of orogeny. Subsequently, the development of advanced instruments for remote collection of information and for analysis of elemental and isotopic composition of materials, and the increases in computing power have enabled an unprecedented number of high-precision data about the Earth to be collected, analyzed, modelled and displayed. Within this revolution in global tectonics, the metamorphic petrologist has developed methods to unravel the depth, thermal, temporal and deformational history of orogens using detailed observations at map, hand sample and thin-section scales in combination with elemental and isotope data, and using inverse and forward modelling. Two exciting new directions in metamorphic petrology in relation to geodynamics concern the kinship between earthquakes and metamorphic reactions in subduction zones, and the petrology of the Earth’s mantle. Evidence of the changes in pressure (P) and temperature (T) in the Earth’s crust and upper mantle during the break up, movement, and collision of pieces of the continental lithosphere is sporadically recorded by the mineralogy and microstructures preserved in rocks exhumed to the surface. Better calibration of phase equilibria, the use of internally-consistent thermodynamic data sets and the development of techniques to retrieve close-to-peak P–T conditions from metamorphic rocks have yielded more precise P–T data that enhance our ability to characterize the path followed by individual rocks in P–T space. An improved ability to date segments of the P–T path, and to separate the length of time associated with the prograde (increasing T) evolution from the age of close-to-peak P–T conditions has enabled better understanding of the rates and processes involved in lithosphere thickening. At the same time, better constraints on the retrograde thermal history have contributed to our knowledge of the several tectonic processes that may operate during exhumation, although these are less well understood. The expanding database of key information, combined with predictions from modelling, has allowed the identification of characteristic P–T–t evolutions expected for rocks that have undergone distinct tectono-metamorphic histories. However, relating structural events recorded by rocks to specific points along the P–T evolution remains problematic, particularly regarding complex overprinting patterns of inclusion trails in porphyroblasts. These advances have improved our understanding of the tectonic evolution of orogens. At the extreme of conditions for crustal metamorphism are the recently discovered ultra-high pressure (UHP) and ultra-high temperature (UHT) facies of metamorphism. Both are problematic given our limited knowledge of processes at these conditions, particularly the return of UHP rocks from peak-P conditions and the mechanism for extreme heat in the crust in UHT metamorphism. The extreme depth inferred for metamorphism in some UHP terranes raises the issue of whether theoretically plausible tectonic overpressures can be dynamically maintained to affect metamorphic reactions. If the pressure gradient recorded by UHP rocks is greater than lithostatic, the UHP metamorphism may have occurred at depths shallower than currently believed. These studies have provided a reliable first-order framework for the comparison of rocks of ancient suture zones where the plate tectonics situation is less certain. However, orogens are spatially and temporally extended nonlinear systems with feedback relations. Such complex systems generate apparently simple behavior by self-organization, and the influence of unique histories must be respected.
A new medium-wavelength gravity field model has been calculated from 110 days of GRACE tracking data, called EIGEN-GRACE02S. The solution has been derived solely from satellite orbit perturbations and is independent from oceanic and continental surface gravity data. This model that resolves the geoid with an accuracy of better than 1 mm at a resolution of 1000 km half-wavelength is about one order of magnitude more accurate than recent CHAMP-derived global gravity models and over two orders of magnitude more accurate than the latest pre-CHAMP satellite-only gravity models. This progress in accuracy together with an increase in resolution are the result of the dedicated instrumentation of the twin GRACE satellites. The instrumentation allows for continuous GPS–GRACE high–low satellite-to-satellite tracking, on-board measurement of non-gravitational accelerations, precise attitude determination, and—being the most important component—the observation of the intersatellite distance and its rate of change.
A study of the historical tsunami that occurred on 30 July 1627 in Gargano, Apulia, southern Italy, has been conducted by performing numerical simulations based on integrating shallow water equations via a finite-element (FE) technique. The tsunami was generated by an I = XI earthquake which produced severe damage in the Gargano promontory. Macroseismic observations are not sufficient to determine, unambiguously, the epicentre and the generative fault position. In this study we have assumed a dip-slip focal mechanism which is known to be the most effective tsunami generation. Since the source location is uncertain, different simulations have been carried out assuming that the fault is placed on land (two cases) and offshore (two cases). The Adriatic basin facing the northern coast of Gargano has been covered by a triangle-based mesh, with triangle sizes adapted to the variable bathymetry, which proved to be advantageous in describing the irregular coastlines. The available historical observations concerning the tsunami are scarce and chiefly qualitative. Yet, comparing data with the results of the numerical simulations gives some important hints on the position of the genetic fault. Of all the sources studied, the one matching the observations better is the inshore fault causing the uplift of the sea block facing the Lésina lake. Accordingly, at the present stage of research, this fault can be assumed as that responsible for the 1627 earthquake and tsunami.
We analysed the 1688 Sannio earthquake, one of the most destructive events that occurred along the axis of the Southern Apennines (Calore River valley, Italy). It was characterised by a very large damage area and by several ground effects. Nonetheless, there is still a lack of specific and multidisciplinary geological studies focused on the active tectonics of the area where this earthquake occurred. Therefore, this work is aimed at integrating subsurface and surface data to provide a new reconstruction of the present day structural setting and active tectonics of the region struck by the 1688 Sannio earthquake.We interpreted deep well logs and reflection seismic lines, and integrated them with the results of original geomorphic and mesostructural analyses and with new radiometric dating (40Ar/39Ar) of Pleistocene pyroclastic layers. The latest Pleistocene brittle deformation observed in the Calore valley suggests a NE dipping main fault related to a NW-SE oriented active extensional system (Calore River fault system: CRFS). This extensional system is tentatively interpreted as the seismogenic fault of the 1688 Sannio earthquake. The reconstruction of the deep structural setting of the study area, especially in correspondence with the CRFS, allowed the buried Apulia units to be identified. The active extensional fault system develops within a set of thrusts that strongly uplift the Apulia platform succession and possibly the underlying Paleozoic basement. At surface, instead, the extensional fault system projects within the most external parts of the Apennine unit, in proximity to its leading edge. A comparison with other extensional seismogenic sources of the Southern Apennines suggests that the occurrence of the described features could represent a key for the location of the major seismicity of the region and could provide an interpretative model for the identification of areas of possible seismic gap in Southern Italy.
The 1755 Lisbon tsunami was felt all over the North Atlantic, being one of the first major events of this kind relatively well documented by historical sources. However, in spite of the extensive research work on the historical reports by a considerable number of authors, the epicentre location of this event is still uncertain and its focal mechanism is still not well understood, implying a great uncertainty in the tsunami generating mechanism. The generally assumed epicentre, inferred from isoseismal maps, is located slightly north of the Gorringe Bank (SW Iberia) and the rupture mechanism has been assumed in the past to be similar to the well studied 1969.02.28 event.While all previous studies have used a seismic-based approach, this paper uses all that is known about the tsunami parameters at the coast — presented in a companion paper — to define the location and geometry of the tsunami source. For that purpose some backward ray-tracing techniques were developed and their results were used to define the initial fields in a number of shallow water simulations of the water height at the coastal locations where the most reliable historical data are available. The source parameters also took into account the estimated seismic energy released.The results obtained here suggest that the 1755 tsunami probably originated on the continental shelf, implying an epicentre area located between the Gorringe Bank and the Iberian coast, in a geodynamic context quite different from the one implied in the 1969.02.28 event. The amplitude of the initial movement in the source region, required by the shallow water simulations to account for the reported magnitudes, suggests an elongated but shallow rupture area, extending along the shelf. It is suggested that this location of the rupture would have significant implications in the geology of the region.
The 1857 Basilicata earthquake (Imax=XI° MCS; Me=6.9) is one of the most destructive events that occurred in peninsular Italy; shaking effects and ground breaks were recorded over a large area extending from the Vallo di Diano (Campania) to the Val d'Agri (Basilicata) for a length of about 60 km and with a width of more than 10 km. Within this seismogenic belt, only another strong earthquake, with maximum intensities in the range of X° MCS (Me=6.4), occurred in 1561. In the epicentral area of the 1857 earthquake, two regional fault systems (i.e. the Val d'Agri and the Vallo di Diano fault systems) offset the main features of the southern Apennines fold and thrust belt; both systems show evidence of activity during Pleistocene times. The Vallo di Diano Fault System (DIFS) includes mostly NW–SE and WNW–ESE trending faults displaying long-term displacements of a few hundred meters; slip data from the latter faults record a kinematic transition from almost pure normal motion to dextral/oblique motion, whereas the NW–SE oriented faults are mostly dominated by normal/transtensional (sinistral) motion. The Val d'Agri Fault System (VAFS) is characterized by fault zones of different size; it is a kinematically coherent system including roughly N120° trending left-lateral strike-slip faults and N090°–N100° trending left-lateral transtensional faults. Inversion of fault slip data indicates that the stress field conditions responsible for the genesis and evolution of the two fault systems are quite different, with σ1 being: (1) sub-horizontal and WSW–ENE trending, in the case of the VAFS, and (2) sub-vertical, in the case of the DIFS. However, the two fault systems are characterized by a roughly N–S oriented extension, and by R-values indicating that σ1≌σ2>>σ3. This suggests the possibility that, in these areas, permutations between the principal maximum and intermediate axes of the stress ellipsoid may have frequently occurred during the faulting process. In this paper, we present new data for both the VAFS and DIFS and discuss the inferred modes of interaction between the two fault systems; this, in turn, suggests possible implications for seismic hazard analyses (SHA) in this sector of the southern Apennines.
Many earthquakes have been recorded from the coastal margin of the Indian peninsular shield during the last 200 years. Largely made up of Precambrian assemblages with variable cover of Jurassic to Quaternary sedimentary rocks and Cretaceous-Eocene volcanics, the peninsular shield was long held to be aseismic. Recent measurements, however, show that this continental fragment is being pushed northeastward by the Carlsberg and Central Indian ridges; and the Indo-Myanmar subduction zone is exerting vigorous slab pull towards the east. Repeated cycles of sea level change during the Quaternary have also induced continuing hydro-isostatic adjustment due to variable melt water loading in the Bay of Bengal and the Arabian sea. All these forces produce space-time fluctuations of strain around many small to large faults, which occur in the upper crust of the shield. Some of the faults have been intermittently active (during the past 100 kyr); others were active earlier. Although the Shillong plateau and the associated hill ranges of northeastern India and Myanmar are subject to the maximum seismic hazard, the peninsular coast is also vulnerable to intermittent seismicity. We present illustrative evidence of some active faults, which are recognisable (a) on coastal land by displaced Pleistocene weathered cover, hot springs, leakages of native mercury and allochthonous geochemical anomalies of base metals and (b) offshore below the inner shelf by horst-shaped uplifted segments and intra-formational slump folds on and below the top shallow seismic (3.5 kHz) reflector. On the other hand, there are long stretches of the east coast at Vishakhapatnam and Manappad Point, which do not show active faults. Step-like marine terraces, which occur up to+6 m above the low tide level (LTL) preserve records of relative sea level fluctuations during the Holocene and the Last Interglacial. In such sectors, absence of tectonic disturbance during the last 100 ka is also corroborated by lateral continuity of shallow seismic reflectors below the inner shelf over many kilometers. Since authentic historical (200–1000 years B.P.) records of seismicity along the Peninsular coast are virtually unavailable, the likely recurrence interval between earthquakes in each sector cannot be gauged. We, therefore, propose a scale of seismic risk, based on geometry of the mappable faults and available seismic records of the last two centuries. These could be used in combination to rank the densely populated coastal tracts sector-wise.
We combined field mapping and structural analysis of Landsat imagery in order to identify active faults in the broader area of the Simitli graben and to the east towards the cities of Razlog and Bansko, in southwest Bulgaria. We mapped five large active fault segments with normal-slip kinematics and down-to-north displacement and three smaller, antithetic faults near Razlog. Our work suggests that: (a) present-day deformation in SW Bulgaria is extensional and is accommodated by seismic slip along E–W, NE–SW and WNW–ESE normal faults; (b) inversion of fault slip data shows a σ3 axis oriented 336–356°; (c) the Krupnik fault comprises one earthquake segment with a general NE–SW strike and dip to the N–NW; its length is about 20 km so its earthquake potential is of the order of Mw = 6.7 ± 0.3; (d) as the 4 April, 1904 earthquake comprised two events, a static stress triggering hypothesis may apply, which is also compatible with the fault segmentation, geomorphology and the macroseismic reports. Source faults for the first event (10:02 a.m.) may have been either the 12 km long Gradevo fault or the 11 km long Elovitsa fault. We estimate a moment magnitude of 6.3 for this event. The first event triggered the second one (10:28 a.m.) on the Krupnik fault.
The crustal movements associated with the 1923 Kanto, Japan, earthquake of magnitude 7.9 are deduced from the results of network adjustments with the pre-seimic and post-seismic geodetic data including up to the third order triangulation. The average spacing of the third triangulation is 4 km of higher density as compared with the first order triangulation and the second order triangulation, so we can expect to find much more detailed behavior of the released crustal strain. The main rupture of the 1923 Kanto earthquake that had occurred in the Sagami Bay has been estimated from the crustal deformations deduced from repetition of the first order triangulation and the second order triangulation, while the detailed deformations associated with the secondary faulting are deduced for the first case from the repeated third order triangulation. We find evidence for the secondary faultings in the south Kanto district, Japan, mainly from the pattern of the released strain with the order of 10−4 and some of them are associated with the surface breakages. We carefully investigate whether these secondary faultings had originated with fracturing at the depth or not. We cannot find any evidence for deep fracturing, though we find remarkable high earth strain release along some secondary faultings. This is mainly due to the fact that the area of high released strain is very limited and narrow along the direction perpendicular to the faulting.
The 1953, Ms = 5.7 Corinth (Central Greece) earthquake was associated with a hitherto unknown, at least 3 km long, NW-trending seismic surface normal fault with its SW block downthrown by about 8 cm. This fault abutted to the less than 2000 years old solfatara, in the low enthalpy Sousaki geothermal field, at the NW end of the Aegean volcanic arc. This result confirms and refines previous hypotheses for a structural control of the Sousaki geothermal field by crossing E-W and NW-SE trending faults, and reveals that the circulation of geothermal fluids and gases is controlled by the fault that broke in 1953. The most prominent area for geothermal exploration can therefore be identified with a narrow zone along the edge of the hanging wall block of the Sousaki seismic fault; this zone coincides with the area of maximum subsurface temperatures.
The Makarska earthquakes of 1962 represent one of the most important seismic sequences in Croatia in the last 50 years. The mainshock (ML=6.1) occurred in the area between the mainland and the islands of Hvar and Brač. Fault plane solutions for the three largest events indicate reverse faulting on a steeply dipping, WNW–ESE striking fault, which also agrees with the position of the relocated hypocentres. The dip-slip component of the motion on the fault was large enough to cause maximum vertical sea-bottom displacement of about 15 cm, which in turn could have generated a small tsunami. The horizontal dimensions of the source area are estimated by empirical formulas to be of the order of 10 km. As the bottom depth in the area ranges between 10 and 100 m, a linear shallow-water model may be used to simulate the tsunami. The model equations have been solved using a finite-difference numerical scheme (resolution 500 m/10 s). The modelled tsunami reveals some interesting features: short waves are detained in the source area, probably due to scattering caused by the irregularities of the coastline; long waves propagate outwards, through the Brač, Hvar and Neretva Channels; and the tsunami is somewhat amplified near the coasts. The simulated tsunami is compared to the corresponding tide-gauge record from Split. The observed arrival time and amplitudes agree well with those of the synthetic tsunami, if it is assumed that initial disturbance was due to the subsidence of the foot wall of the seismogenetic fault — in agreement with tectonic motions in the area.
Public statements about volcanic activity at Mount St. Helens include factual statements, forecasts, and predictions. A factual statement describes current conditions but does not anticipate future events. A forecast is a comparatively imprecise statement of the time, place, and nature of expected activity. A prediction is a comparatively precise statement of the time, place, and ideally, the nature and size of impending activity. A prediction usually covers a shorter time period than a forecast and is generally based dominantly on interpretations and measurements of ongoing processes and secondarily on a projection of past history. The three types of statements grade from one to another, and distinctions are sometimes arbitrary.Forecasts and predictions at Mount St. Helens became increasingly precise from 1975 to 1982. Stratigraphic studies led to a long-range forecast in 1975 of renewed eruptive activity at Mount St. Helens, possibly before the end of the century. On the basis of seismic, geodetic and geologic data, general forecasts for a landslide and eruption were issued in April 1980, before the catastrophic blast and landslide on 18 May 1980. All extrusions except two from June 1980 to the end of 1984 were predicted on the basis of integrated geophysical, geochemical, and geologic monitoring. The two extrusions that were not predicted were preceded by explosions that removed a substantial part of the dome, reducing confining pressure and essentially short-circuiting the normal precursors.
A three-dimensional creep model of the driving mechanisms of the 1976 Tangshan earthquake is presented, with consideration of both the body force and the regional tectonic force. The upper crust is assumed to be a visco-elastic body, the strain rate for the viscous component is assumed to be linearly proportional to the deviatoric stress, but the lower crust and the upper mantle respond to the stress as a non-linear Newtonian fluid obeying the power law of creep. This model is solved numerically using the principle of additively of elastic, plastic and viscous strains, as well as the initial strain method with a tangential modulus for the upper crust and with a secant modulus for both the lower crust and the upper mantle to represent the energy dissipation of creep flow.Computed results show that: 1) to match roughly the pre-seismic ground surface leveling and the co-seismic dip- and strike-slip, uplifting beneath the doming zone of the crust-mantle boundary plays a more important role than a horizontal driving mechanism; 2) the lower-crust fault first relaxes the strain induced by the uplift, resulting in stress accumulation in the upper crust, expecially at the front of the tip of the Tangshan fault lying above the crustal low-velocity zone; 3) significant co-seismic and post-seismic ground-surface displacements along the newly faulted belt in Tangshan result from a combination of elastic strain rebound and change in gravitational potential.
In order to detect thermal anomalies prior to an eruption with remotely sensed thermal infrared data, fifteen images collected by the Advanced Very High Resolution Radiometer (AVHRR) on board NOAA7 satellite were acquired over a period of three months before the eruption of Mt. Etna which took place on 28th March 1983. This data set was processed in order to get a surface temperature as close as possible to reality and to suppress the spatial variations not directly linked with internal activity (i.e. altitude effects, climatic environment, sea proximity, etc...). Subsequently two anomalies were made conspicuous; the first one is related to the permanent activity of the summit craters and the second one, visible during the month prior to the eruption seems to be highly correlated to the intrusion.A possible interpretative model assuming natural convective heat transfer is then proposed and discussed.
The NW Bohemia/Vogtland region situated at the western part of the Bohemian Massif is characteristic in a frequent reoccurrence of earthquake and micro-earthquake swarms. We present a comprehensive, integrated pattern of the space and time distribution of seismic energy release in the principal NK (Nový Kostel) focal zone for the period 1991–2001 and for the intensive 1985/1986 swarm. More than 3000 earthquakes, recorded by the WEBNET, the KRASLICE net and by temporary stations VAC, TIS and OLV operating during the 1985/1986 swarm, were located or re-located using the master event technique. Swarm-like sequences were identified and discriminated from solitary events by detecting local minima of the inter-event time using a standard short-time/long-time average (STA/LTA) detection algorithm. Most of the seismic energy in the NK zone was released during the two intensive 1985/1986 and 2000 swarms and in the course of the weaker January 1997 swarm. Further 27 swarm-like sequences (micro-swarms) and many solitary micro-earthquakes (background activity) were identified in the NK zone for the period 1991–2001 by the inter-event time analysis. Relative location revealed a pronounced planar character of the NK focal zone. Most of the events, including those of the intensive 1985/1986 and 2000 swarms, were located at the main focal plane (MFP) striking 169° N and dipping 80° westward at depths between 6 and 11 km. A singularity was the January 1997 swarm together with a micro-swarm that were both located across the MFP. The position and geometry of the MFP match quite well the Nový Kostel-Počátky-Zwota tectonic line. The space distribution patterns of larger events and of micro-swarms at the MFP differ: larger events predominantly grouped in planar clusters while the micro-swarms lined up along two parallel seismogenic lines. The temporal behaviour was examined from two aspects: (a) migration and (b) recurrence of the seismic activity. It was found that (a) the seismic activity in the time span 1991–2001 migrated in an area of about 12×4 km and (b) several segments of the MFP were liable to reactivation. The activity before, during and after the 2000 swarm took place in different parts of the MFP.
The western part of the Bohemian Massif is characterized by repeated earthquake swarm occurrence, juvenile carbon dioxide waters, mineral springs, mofettes, Quaternary volcanics etc. To study the geodynamic activity of the region, repeated geodetic measurements of recent crustal movements, repeated gravity measurements and continuous recording of groundwater level have supplemented the seismological investigations since the fall of 1993.The December 4–5, 1994 weak earthquake swarm, the strongest seismic sequence between 1993–96, remarkably coincided with the absolute minimum of the groundwater level and a maximum of gravity. Repeated gravimetric measurements confirmed the existence of temporal variations of gravity, characterized by a certain correlation with the local tectonic setting.The results of GPS did not prove any displacement >5 mm–a in horizontal component. Precise levelling indicated small negative changes in elevation at several points of measurements.
In this study we rigorously combine 18 old campaign GPS data sets from Greece covering the period 1994–2000. Although the majority of these old datasets have been analyzed and reported previously, it has not been possible to combine them into a single velocity field and apply strain analysis. Here a uniform, final coordinate solution is given by reprocessing 43 global, long-running International GNSS Service (IGS) sites together with 280 local sites. The 221 daily SINEX files are then combined in a least squares approach and the geodetic horizontal velocity field in ITRF2000 and Europe-fixed reference frame is derived. Two methods are used to compute the geodetic strain rates: (i) discrete estimates within contiguous polygons, and (ii) a continuous curvature surface fitted to the velocity field. The seismic hazard potential can be determined by comparing the geodetic and seismic strain rates. The published 300 year earthquake catalogue best describes the major active tectonic features at the scale of geodetic strain determination. The geodetic strain appears larger than the seismic strain for the majority of the region, suggesting that accumulated strain has not yet been released by earthquakes. The geodetic field is consistent with the detailed constraints implied by the observed orientations of faulting as these are given in the 300-year catalogue. We have shown that with the GPS dataset used in this work and following this processing scheme reasonable results can be obtained comparable with more recent studies, CGPS data and by recent earthquake activity.
The seismicity in the Vogtland/NW-Bohemia region is mainly characterized by the occurrence of earthquake swarms. A key to a better understanding of the reasons of earthquake swarms can be provided by focal mechanism investigations. Here we present focal mechanisms for 12 of the strongest events (ML⩾3.0) for the new swarm of 2000. With more than 10,000 events and magnitudes up to 3.7 the new swarm is the most prominent one since the big swarm in 1985/1986. The focal mechanisms of the swarm 2000 show different styles of faulting, namely strike-slip, normal and reverse faulting. There are indications for systematic temporal variations in the dislocation type. A comparison with the mechanisms of the preceding swarms of 1985/1986, 1994 and 1997 which all took place at the same location shows similarities in the faulting types and orientations of the nodal planes for the swarms of 1985/1986, 1994 and 2000. However, the focal mechanisms of 1997 do not fit into the scheme of the others. The focal mechanisms have also been used to determine the regional stress field. It turned out that the stress field in the Vogtland/NW-Bohemia region does not substantially differ from the known stress field in West and Central Europe. It is a strike slip regime with a SE–NW directed σ1-axis and a NE–SW directed σ3-axis.
Accurate locations of aftershocks, fault plane solutions for the main shock and for a large number of aftershocks as well as geographical distribution of macroseismic intensities led to a reliable estimation of the fault parameters and to a better understanding of the rupture process for the 1995 Kozani-Grevena destructive earthquake. The corresponding fault has a length L = 30 km, a width w = 10 km, strikes in an ENE direction (N65 °E), dips to NNW and the mean displacement on the fault during the generation of this earthquake is about 50 cm. Aseismic preshock slip in an aseismic area of the central part of the fault induced high tectonic stress in the rest of the fault. This stress reached the foreshock barrier's strength near the shallow western boundary of the aseismic area, where the two largest foreshocks occurred, and then the ultimate mainshock barrier's strength near the deep eastern boundary of the aseismic area where the mainshock occurred. From the focus of the mainshock, which was located in the deepest and northernmost part of the fault, the rupture propagated both up-dip and bilaterally and terminated vertically at a depth of about 4 km (blind fault) and horizontally at the eastern and western end of the aftershock zone. The vertical termination of the rupture not at the surface but at some depth is attributed to properties of the uppermost part of the crust or to slipping on minor shallow faults, while the horizontal east and west termination to the intersection of this fault with other faults which strike in a NE direction (geometrical barriers).
Occurrences of anomalous electro-magnetic phenomena at varied frequency ranges, covering ELF to VHF, have been reported in relation to the 17 January 1995 Kobe earthquake (M7.2), by several independent research groups. Prominent pre-seismic peaks, which could have been emitted from the focal area, were observed on 9-10 January in ELF, VLF, LF and HF ranges. Whether these changes were truly related to the earthquake is not certain, because atmospheric (thunderbolt discharge) activities also peaked on 9-10 January. The nomalous changes were markedly enhanced toward the catastrophe in agreement with many reports on unusual radio/TV noise. Anomalous transmission of man-made electromagnetic waves in VLF and VHF ranges was also detected from a few days before the earthquake, indicating the possibility that the ionosphere above the focal zone was disturbed at the final stage of the earthquake preparation process.
From January 1995 to December 1997, about 74 earthquakes were located in the Pannonian basin and digitally recorded by a recently established network of seismological stations in Hungary. On reviewing the notable events, about 12 earthquakes were reported as felt with maximum intensity varying between 4 and 6 MSK. The dynamic source parameters of these earthquakes have been derived from P-wave displacement spectra. The displacement source spectra obtained are characterised by relatively small values of corner frequency (f0) ranging between 2.5 and 10 Hz. The seismic moments change from 1.48×1020 to 1.3×1023 dyne cm, stress drops from 0.25 to 76.75 bar, fault length from 0.42 to 1.7 km and relative displacement from 0.05 to 15.35 cm. The estimated source parameters suggest a good agreement with the scaling law for small earthquakes. The small values of stress drops in the studied earthquakes can be attributed to the low strength of crustal materials in the Pannonian basin. However, the values of stress drops are not different for earthquake with thrust or normal faulting focal mechanism solutions. It can be speculated that an increase of the seismic activity in the Pannonian basin can be predicted in the long run because extensional development ceased and structural inversion is in progress. Seismic hazard assessment is a delicate job due to the inadequate knowledge of the seismo-active faults, particularly in the interior part of the Pannonian basin.
To determine the present day kinematics of the major tectonic elements in Bulgaria, part of the northern extent of the Aegean extensional system, we established and surveyed a regional GPS network in 1996, 1997 and 1998. Based on our estimates of the velocities of 17 stations surveyed at least twice, we identify two areas of significant deformation. Between the central Danubian Plain and a 200-km-wide area adjacent to Black Sea, we find 3±1 mm/year of shortening, consistent with compressional focal mechanisms for the Gorna Oriahovitsa fault zone and a NW-trending flexure between higher topography in northeast Bulgaria and the area to the west. South of the Danubian Plain, there is a broad zone of N-S to NNE-SSW extension encompassing the E-W trending Sub-Balkan graben system and Thracian basin of central Bulgaria and with a total differential motion of 3–5 mm/year. The zone of extension appears to continue into the Sofia graben and through a topographically low part of the Stara Planina mountains. This zone of extension marks the northern boundary of the Aegean extensional region, but its western continuation remains unclear. Velocities of 3–4 mm/year to the south and 3 mm/year to the ESE in south central Bulgaria express a clear transition to the Aegean extensional region farther south.
We use interferometric synthetic aperture radar (InSAR) observations to investigate the coseismic deformation and slip distribution of the 1997 Mw7.5 Manyi earthquake, a left-lateral strike-slip earthquake occurred on the west portion of the Kunlun fault in the northern Tibet, China. The fault trace is constrained by the combination of interferometric coherence image and azimuth offset image. The total length of the identified fault is about 170 km. We estimate the source parameters using a seven-segment fault model in a homogeneous elastic half-space. We first use a uniform slip model to estimate the slip, width, dip and rake for each segment, resulting in a maximum slip of 5.5 m with a depth of 11 km on the fourth segment. The average dip of the uniform slip model is about 93° northward and the average rake is about −2°. We then use a distributed slip model to estimate the pure strike-slip and oblique slip distribution, respectively. In the distributed slip model, the fault plane is discretized into 225 patches, each of them 4 km × 4 km. We fix the optimal geometric parameters and solve for the slip distribution using a bounded variable least-squares (BVLS) method. We find a geodetic moment of 1.91 × 1020 Nm (Mw7.5), of which almost 68% released in the uppermost 8 km and 82% in the uppermost 12 km. For all the models used in this study, the synthetic profiles along strike show asymmetric displacements on the opposite sides of the fault, which are in agreement with the observations. This suggests that a linear elastic model with variable and non-vertical dips is also reasonable for the mechanism of the Manyi earthquake.
The September–October 1997 seismic sequence in the Umbria–Marche regions of Central Italy (main shocks on September 26, Mw 5.7 and 6.0, and on October 14, Mw 5.6) left significant ground effects, which were mainly concentrated in the Colfiorito intermountain basin. These effects included surface faulting, ground cracks and settlements, rock falls, slides, hydrological and gas anomalies. The distribution and size of ground effects has proved useful for (1) defining the epicentral area and the location of the causative fault; (2) complementing the intensity pattern from damage distribution (this can be very useful in poorly inhabited zones); (3) integrating or testing the intensity assessment of many historical events, in order to obtain a better evaluation of the magnitude from intensity data. Of special interest was the observation of surface ruptures generated along segments of a system of normal faults already mapped as capable, with end-to-end lengths of 12 km and maximum displacements of 8 cm. Many pieces of evidence confirm that coseismic slip was not a secondary, gravity-induced, phenomenon, but had a tectonic origin. Detailed descriptions of surface faulting for moderate earthquakes are not common, being easily missed or misinterpreted; however, in this paper we emphasize that surface faulting due to the 1997 event can be used to infer the threshold magnitude for surface faulting in Central Apennines, allowing to calibrate palaeoearthquake size from fault offsets as seen in trench investigations.
This paper aims to review the main scientific achievements which were obtained in the first phase (1997–2003) of the Global Geodynamics Project (GGP) consisting of a worldwide network of superconducting gravimeters (SG) of about 20 instruments. We show that the low noise levels reached by these instruments in various frequency bands allow us either to investigate new signals of very small amplitude or to better determine other signals previously seen. We first report new results in the long-period seismic band with special emphasis on the detection of the 2S1 normal mode and the splitting of the fundamental spheroidal mode 2S0 after the magnitude 8.4 Peru earthquake in 2001. We also discuss briefly the ‘hum’, which consists of a sequence of fundamental normal modes existing between 2 and 7 mHz even in the lack of any seismic excitation, and was first discovered on the Syowa (in Antarctica) instrument in 1998. We will comment on the search for the Slichter mode 1S1 of degree 1 which is associated with a translational motion of the inner core inside the liquid core. Atmospheric effects are reviewed from the local to the global scale and the improvement due to pressure loading computations on residual gravity signals is shown. An interesting study exhibiting the gravity consequence due to sudden rainfall and vertical mass motion in the atmosphere (without ground pressure change) is presented. The precision of the SGs leads to some convincing results in the tidal domain, concerning the fluid core resonance effect (free core nutation (FCN)) on diurnal tides or various loading effects (linear, non-linear) from the oceans. In particular, SGs gravity measurements are shown to be useful validating tools for ocean tides, especially if they are small and/or confined to coastal regions. The low instrumental drift of the SGs also permits to investigate non-tidal effects in time-varying gravity, especially of annual periodicity. Hydrology has also a signature which can be seen in SG measurements as shown by several recent studies. At even lower frequency, there is the Chandler motion of 435-day period which leads to observable gravity changes at the Earth's surface. We finally report on the progress done in the last years in the problem of calibrating/validating space satellite data with SG surface gravity measurements.
Gravimetry has the potential to provide important data, in combination with GPS, for detecting vertical surface motions and subsurface mass changes. Here, we focus on the first results of joint gravity and GPS studies in order to understand better the vertical component of the postseismic deformations of the 1999 earthquakes along the western North Anatolian Fault. We investigate the relationship between gravity changes and GPS motions during the period 2003–2005. The changes in this period constitute a snapshot of the nonlinear movements that were not studied before in the Marmara Region. The first observations evaluated here demonstrate that the joint analysis of GPS and gravity data help to constrain the 3D postseismic deformations and hence expand our knowledge of the geophysical process in the Marmara Region. We identify what appear to be different crustal properties in the western and eastern parts of the region. Furthermore, the GPS results indicate that the western extension of the 1999 İzmit rupture area presently has low strain accumulation. To the extent that this behaviour continues through the earthquake cycle, it reduces the moment release of the expected future earthquake in the eastern Marmara seismic gap. In contrast, the western part of Marmara region has important strain loading. While our results are not sufficiently accurate for detailed interpretation, the observed strain accumulation implies the potential for a significant earthquake in the western Marmara region. Generally, possible fault creep extending west of the İzmit fault break following the İzmit earthquake is very important to understand the future seismic hazard in the Marmara region because it reduces the amount of strain accumulation during the earthquake cycle which will either delay the onset of future events or produce smaller future earthquakes.
The Granada basin, located in the central sector of the Betic Cordillera, is one of the most seismically active zones of the Iberian Peninsula. For the first time, a geodetic network along the Padul fault and two levelling profiles (Genil and Viznar) crossing the Granada fault were operated to detect crust microdeformations in this area. Three years of terrestrial geodetic measurements are analysed to characterize the behaviour of these faults in a low to moderate strain rate environment. After the comparison of these three campaigns, we can conclude that there is no significant short-term movement of the Padul fault. Granada fault measurements show significant differences between 1999 and 2001 campaigns in Viznar profile. However, more data are needed to correlate this displacement to the tectonic activity.
Since 1989, hydrological, geochemical, and isotope investigations are accomplished at springs of the Upper Vogtland (Germany). The results give evidence of a fluidal response of the mineral spring ‘Wettinquelle’ (formerly: ‘Radonquelle’) Bad Brambach to the seismogenic processes in the swarm quake area of Nový Kostel (distance: 10 km), the most seismically active area in the region. In 1999, the measuring network was extended to groundwater drillings around the Wettinquelle, and multiparameter stations were installed at mofettes SW and S of the Nový Kostel area. Beginning at the end of August 2000, the strongest swarmquake series compared with the 1985/86 swarms occurred in the area of Nový Kostel. Hydrological and hydrochemical measurements show an anomalous behaviour of several parameters such as groundwater level, hydrostatic pressure, and free gas flow beginning at the end of July 2000, and partially, of a duration up to the first strong events of the autumn 2000 swarm. These precursor phenomena were the result of an increased fluidal pressure in the ground before the earthquakes. They were visible as higher static water levels in a gauge well and a stronger degassing of the mineral spring ‘Wettinquelle’. The anomalies confirm our co-seismic hydrogeological observations of the last 10 years at Bad Brambach, as well as our model conception of anomaly generation due to fluidal pressure pulses [Pure. Appl. Geophys. 157 (2000) 1621]. Furthermore, co- and post-seismic effects of hydrological and geoelectrical parameters were recorded in the mofette area of Soos and Bublák (Czech Republic).
Main features of the August–December 2000 earthquake swarm which occurred in the major focal area of the North-West Bohemia / Vogtland swarm region are presented. Seismograms from four stations of WEBNET were automatically processed to get arrival times, first motion amplitudes and hypocentre coordinates of a representative set of events. Altogether 7017 microearthquakes in the magnitude range of ML=0–3.3 were identified. It is shown the decay of activity of individual swarm phases followed the modified Omori law, which points to a partial similarity with aftershock sequences of tectonic earthquakes. The space-time distribution of a subset of 2913 events with low location residuals shows a strong space clustering of the earthquake hypocentres and their pronounced migration between individual swarm phases. Most of the activity took place along an elliptical, nearly vertically dipping, 6 km long N-S oriented fault plane in depths ranging from 10.5 to 6.5 km. The P and T axes were estimated by FOCMEC for the 782 strong events and three groups of earthquakes with similar faulting type were distinguished. In contrast to the normal and strike-slip faulting events that created the prevailing portion of the swarm and were distributed uniformly within the focal area, the reverse events were clustered in time and space.
We report here crustal shear-wave anisotropy, ranging from 1% to 10.76% with an average of 2.4% in the aftershock zone of the 2001 Bhuj earthquake, Gujarat, India, from a study of leading shear-wave polarization directions (LPSDs), which vary on average from NNW–SSE to E–W with a delay of 0.07–0.14 s. The delays in the NNW–SSE to NE–SW directions observed at seven stations, near the seismogenic fault, suggest cracks parallel to the direction of the maximum horizontal regional compressional stress prevailing in the region, suggesting a dilatancy-induced anisotropy resulting from approximately stress-aligned parallel vertical micro-cracks. In contrast, the LPSDs at Ramvav, Rapar and Vondh stations, away from the seismogenic fault, are fault parallel, approximately E–W and almost orthogonal to the stress-aligned polarizations inferred elsewhere. The maximum average time delay of 0.14 s is observed at Lodai, where the fast polarization direction is found to be N338°W. This has been observed from anisotropic poro-elastic (APE) modelling and observations that these are 90° flips in shear-wave polarization, resulting from propagation through micro-cracks containing fluids at critically high pore-fluid pressure surrounding the hypocenter of the 2001 mainshock. The presence of high pore-fluid pressure in the seismogenic fault zone could also explain the observed scatter in shear-wave time delays. Further, the coincidence of the N–S trending intrusive bodies (as inferred from tomographic studies in the area) with the N–S direction of regional maximum horizontal compressional stress supports the interpretation of stress-aligned vertical extensive-dilatant anisotropic (EDA) cracks. The depth distribution of the estimated anisotropy (1–10.76%), b-values and stress drop values suggests an increase at 18–30 km depths, which could be attributed to high pore-fluid pressures resulting from a fluid-filled fractured rock matrix or open micro-cracks (characterized by high crack density and high porosity) coinciding with a low velocity zone (at 18–30 km depths) as delineated from tomographic studies in the area.
In view of an anomalous crust–mantle structure beneath the 2001 Bhuj earthquake region, double-difference relocations of 1402 aftershocks of the 2001 Bhuj earthquake were determined, using an improved 1D velocity model constructed from 3D velocity tomograms based on data from 10 to 58 three-component seismograph stations. This clearly delineated four major tectonic features: (i) south-dipping north Wagad fault (NWF), (ii and iii) south-dipping south Wagad faults 1 and 2 (SWF1, SWF2), and (iv) a northeast dipping transverse fault (ITF), which is a new find. The relocated aftershocks correlate satisfactorily with the geologically mapped and inferred faults in the epicentral region. The relocated focal depths delineate a marked variation to the tune of 12 km in the brittle–ductile transition depths beneath the central aftershock zone that could be attributed to a lateral variation in crustal composition (more or less mafic) or in the level of fracturing across the fault zone. A fault intersection between the NWF and ITF has been clearly mapped in the 10–20 km depth range beneath the central aftershock zone. It is inferred that large intraplate stresses associated with the fault intersection, deepening of the brittle–ductile transition to a depth of 34 km due to the presence of mafic/ultramafic material in the crust–mantle transition zone, and the presence of aqueous fluids (released during the metamorphic process of eclogitisation of lower crustal olivine-rich rocks) and volatile CO2 at the hypocentral depths, might have resulted in generating the 2001 Bhuj earthquake sequence covering the entire lower crust.
Panarea, characterized by gas unrest in 2002–2003, is the volcanic island with the least constrained structure in the eastern-central Aeolian Arc (Italy). Based on structural measurements, we define here its deformation pattern relative to the Arc. The main deformations are subvertical extension fractures (63% of data), normal faults (25%) and dikes (12%). The mean orientation of the extension fractures and faults is ∼N38°E, with a mean opening direction of N135° ± 8°, implying extension with a moderate component of dextral shear. These data, matched with those available for Stromboli volcano (pure opening) and Vulcano, Lipari and Salina volcanoes (predominant dextral motions) along the eastern-central Arc, suggest a progressive westward rotation of the extension direction and an increase in the dextral shear. The dextral shear turns into compression in the western arc. The recent unrest at Panarea, coeval to that of nearby Stromboli, may also be explained by the structural context, as both volcanoes lie along the portion of the Arc subject to extension.
A 2.5-month long gravity sequence, encompassing the starting period of the 2002–2003 Etna eruption and coming from a summit station only 1 km away from the new fractures, is presented and discussed. The sequence comprises four hours-long anomalies that have a great chance to reflect mass redistributions linked to the ensuing activity. In particular, the start of the eruptive activity on the northeastern flank was marked by a gravity decrease as strong as about 400 μGal, which reverted soon afterwards. This strong decrease/increase anomaly is interpreted as the opening, by tectonic forces, of a fracture system along the Northeastern Rift of Mt. Etna, followed by an intrusion of magma from the central conduit to the new fractures. They were used by the intruding magma as a path to the eruptive vents at lower elevations.
An earthquake (Mw = 6.4) occurred on May 1, 2003 in Bingöl province of the East Anadolu Region of Turkey. It was characterized by a shallow focal depth, an intense and prolonged aftershock activity and serious ground effects. This study presents main characteristics and geo-engineering evaluation of the earthquake based on site observations, strong ground motion records and geotechnical data. Although the earthquake caused rock falls and several landslides, and limited number of liquefaction-induced ground failures, no evident surface rupture occurred. However, site observations and distribution trend of the epicenters of the aftershocks suggested that the Sudügünü fault, which is a right-lateral strike slip fault striking in NW–SE direction, was the most probable causative fault. Based on the acceleration response spectra and natural periods of the structures in Turkey, it is concluded that the buildings with three to four stories in Bingöl should have been subjected to severe shaking. Amplification at the cliff sides due to topographical effects played an important role on structural damages. Since the limited number of liquefaction-induced ground failures occurred in rural areas, they did not cause any structural damage. However, one of these is a great of interest, because it occurred in soils, which resulted from weathered tuffs and its analysis also provided useful information about the horizontal ground acceleration necessary to initiate the liquefaction.
The horizontal pendulums of the Grotta Gigante (Giant Cave) in the Trieste Karst, are long-base tiltmeters with Zöllner type suspension. The instruments have been continuously recording tilt and shear in the Grotta Gigante since the date of their installation by Prof. Antonio Marussi in 1966. Their setup has been completely overhauled several times since installation, restricting the interruptions of the measurements though to a minimum. The continuous recordings, apart from some interruptions, cover thus almost 40 years of measurements, producing a very noticeable long-term tiltmeter record of crustal deformation. The original recording system, still in function, was photographic with a mechanical timing and paper-advancing system, which has never given any problems at all, as it is very stable and not vulnerable by external factors as high humidity, problems in power supply, lightning or similar. In December 2003 a new recording system was installed, based on a solid-state acquisition system intercepting a laser light reflected from a mirror mounted on the horizontal pendulum beam. The sampling rate is 30 Hz, which turns the long-base instrument to a very-broad-band tiltmeter, apt to record the tilt signal on a broad-band of frequencies, ranging from secular deformation rate through the earth tides to seismic waves. Here we describe the acquisition system and present two endline members of the instrumental observation, the up to date long-term recording, and the observation of the great Sumatra-Andaman Islands earthquake of December 26, 2004, seismic moment magnitude Mw = 9.1–9.3 [Lay, T., Kanamori, H., Ammon, C.J., Nettles, M., Ward, S.N., Aster, R.C., Beck, S.L., Bilek, S.L., Brudzinski, M.L., Butler, R., DeShon, H.R., Ekström, G., Satake, K., Sipkin, S., 2005. The Great Sumatra-Andaman Earthquake of 26 December 2004. Science. 308, 1127–1133.]. The secular-term observations indicate an average tilting over the last four decades towards NW of 23.4 nrad/year. We find evidences that this tilting is regional and has been going on since at least 125 ka. The recent earthquake of December 26, 2004 was well recorded by the pendulums. We show that the free oscillation modes were activated, including the lowest modes as e.g. 0T2, 0T3, 0T4, 0T5 and 2S1, 0S3, 0S4, 1S2.
Coseismic gravity change, correlated with the 2004 off the Kii peninsula earthquakes (∼M7), was detected with a superconducting gravimeter installed at the Inuyama station of Nagoya University, central Japan, where distances from epicenters were ∼250 km. The influence of precipitation in the vicinity of the site just before the earthquakes was included in the observed temporal gravity variation. Therefore the precipitation effect was removed using a simple tank model with the Radar-AMeDAS precipitation data. As a result, the magnitude of the coseismic gravity decrease was ∼10 nm s−2 (1 μGal). The gravity changes were calculated using a dislocation model with two fault models those defined by GPS estimates. These theoretical values corresponded to the observed value in the order of 1 μGal.
Since 1997, two quartz tube strainmeters at the Geodynamic Observatory Moxa, located 30 km south of Jena, are used to observe long-period horizontal deformation signals. Both strainmeters are 26 m long with orientations NS and EW and are installed in a gallery. To this system a third component was added in 1999, which connects the ends of the quartz tubes diagonally. This component is realised as a laser strainmeter, running through a 38 m long horizontal borehole. The first data analyses show high signal-to-noise ratios for the tidal frequencies and also the free oscillations caused by the Sumatra earthquake in December 2004 are clearly detectable.It can be shown that the quartz strainmeter extending in EW direction generally contains significant more noise induced by barometric pressure than the NS-component. The laser strainmeter record shows strong influences of changing barometric pressure, due to the fact that the beam does not run in a vacuum. This influence is reduced in the higher frequencies by sealing the ends of the horizontal borehole with high quality glass. In addition, the observations are clearly temperature dependent and the influence of rainfall could be verified by two irrigation experiments.
Which rule of mixture is the best for predicting the overall elastic properties ofpolyphase rocks based on the elasticities and volume fractions of their constituents? In order toaddress this question, we sintered forsterite–enstatite polycrystalline aggregates with a variedforsterite volume fraction (0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0). Elastic properties of thesesynthesized composites were measured as a function of pressure up to 3.0 GPa in aliquid-medium piston cylinder apparatus using a high precision ultrasonic interferometrictechnique. The experimental data can be much better described by the shear-lag model than bythe commonly used simple models such as Voigt, Reuss and Hill averages, Hashin–Shtrikmanbounds, Ravichandran bounds, Halpin–Tsai equations, and Pauls calculations. We interpret thisas an indication that the elastic interaction and stress transfer between phases are neglected in allmodels except the shear-lag model. In particular, the present study suggests that olivine in themantle may be more abundant than previously inferred from the comparison between seismicEarth model velocities and calculated overall elastic properties of mantle mineral compositesaccording to the Hill average or Hashin–Shtrikman bounds.
The Andes between 36°30′ and 37°S represent a Cretaceous fold and thrust belt strongly reactivated in the late Miocene. Most of the features that absorbed Neogene shortening were already uplifted in the late Cretaceous, as revealed by field mapping and confirmed by previous fission track analysis. This Andean section is formed by two sectors: a western-inner sector generated by the closure of the upper Oligocene-lower Miocene intra-arc Cura Mallín basin between the middle and late Miocene (Guañacos fold and thrust belt), and an eastern-outer sector, where late Triassic-early Jurassic extensional depocenters were exhumed in two discrete phases of contraction, in the latest early Cretaceous and late Miocene to the Present, respectively (Chos Malal fold and thrust belt). Late Miocene deformation has not homogeneously reactivated Cretaceous compressive structures, being minimal south of 37°30′S through the eastern-outer sector (southern continuation of the Chos Malal fold and thrust belt). The reason for such an inhomogeneous deformational evolution seems to be related to the development of a late Miocene shallow subduction regime between 34°30′ and 37°45′S, as it was proposed in previous studies. This shallow subduction zone is evidenced by the eastward expansion of the arc that was accompanied by the eastern displacement of the orogenic front at these latitudes. As a result, the Cretaceous fold and thrust belt were strongly reactivated north of 37°30′S producing the major topographic break along the Southern Central Andes.
Ar/³⁹Ar dating of biotite from felsic orthogneiss recovered from the −3890-foot level of the Island Beach State Park (IBSP) well beneath the outer New Jersey Coastal Plain was accomplished using CO2 laser incremental-heating techniques. Over 75% of the Ar released from the incremental-heating experiment form a well-behaved plateau with a calculated age of 243.98 ± 0.10 Ma. The new 244 Ma biotite age reported here is a cooling age younger than the metamorphic event that crystallized or reheated the biotite. We consider reheating of older biotite to be unlikely because the concordant ⁴⁰Ar/³⁹Ar spectrum upon repeated incremental laser heating showed a well-developed plateau. Thus, biotites from the IBSP gneiss are interpreted as having crystallized during a single thermal event, followed by cooling to below 300 °C.
The main goal of our study is to investigate 3D topography of the Moho boundary for the area of the northern Red Sea including Gulf of Suez and Gulf of Aqaba. For potential field data inversion we apply a new method of local corrections. The method is efficient and does not require trial-and-error forward modeling. To separate sources of gravity and magnetic field in depth, a method is suggested, based on upward and downward continuation. Both new methods are applied to isolate the contribution of the Moho interface to the total field and to find its 3D topography. At the first stage, we separate near-surface and deeper sources. According to the obtained field of shallow sources a model of the horizontal layer above the depth of 7 km is suggested, which includes a density interface between light sediments and crystalline basement. Its depressions and uplifts correspond to known geological structures. At the next stage, we isolate the effect of very deep sources (below 100 km) and sources outside the area of investigation. After subtracting this field from the total effect of deeper sources, we obtain the contribution of the Moho interface. We make inversion separately for the area of rifts (Red Sea, Gulf of Suez and Gulf of Aqaba) and for the rest of the area. In the rift area we look for the upper boundary of low-density, heated anomalous upper mantle. In the rest of the area the field is satisfied by means of topography for the interface between lower crust and normal upper mantle. Both algorithms are applied also to the magnetic field. The magnetic model of the Moho boundary is in agreement with the gravitational one.
In a previous work, cast in axisymmetric flat geometry, we have shown that a realistic composite rheology (linear plus nonlinear) for the Earth’s mantle reproduces the postglacial isostatic readjustment of Laurentia significantly better than a purely linear one. In this work we address the same problem in a 3D flat geometry allowing to apply the ice load described by the ICE-3G deglaciation history without manipulations. Computation is numerically carried out through finite elements as the nonlinear formulation prevents the use of spectral methods. The goodness of fit is quantitatively tested by comparing observed and computed Relative Sea Level (RSL) data at 29 North American sites for the last 8 kyr. The mixed rheology model still shows a slightly but significantly better fit (in the statistical sense) than the linear model.
Traditionally, 3D geodetic networks are established as unions of horizontal and vertical control networks. However, over the last 10 years or so, GPS (Global Positioning System) networks have gained more and more importance. After geodetic networks are monumented, relevant measurements are taken, and point coordinates for the control points are estimated by the method of least squares. However, the method of least squares does not give any information about the robustness of networks. To measure robustness of a network, the deformation of individual points of the network is portrayed by strain. This technique is independent of adjustment constraints and reflects only the network geometry and accuracy of the observations. Furthermore, threshold values are needed to quantify the robustness of the network. If the displacements of some points of the network are worse than the threshold values, this suggests that we should redesign the network by changing the configuration or improving the measurements until we obtain a network of acceptable robustness. This paper describes how to obtain the displacements at the individual points of a 3D network, employs the specifications of Geodetic Survey Division to derive the acceptable values and shows the results on two different GPS networks.
In order to quantify low amplitude deformations in sedimentary basins, fault offsets are modelled, restored and quantified in 3D. We studied a field example where such faults where blanketed by a small angular unconformity between Upper and Lower Cretaceous in the central area of the Allauch Massif (SE Provence, France) to which we applied sub-surface structural modelling techniques (Gocad). We gathered mapping, structural and sedimentologic data in the field. Fault and stratigraphic surfaces were interpolated with gOcad for constructing a 3D structural model of the Allauch Massif. In order to restore the original geometry of the erosional wedge and low amplitude fault offsets, we restored the post-unconformity Pyrenean-Alpine deformation. We obtained a 3D structural model of the angular unconformity at the pre-Upper Turonian times.The angular unconformity was due to a 5° fault-induced tilting of the Lower Cretaceous and underlying layers, which implied a 3° erosion wedge and a differential erosion of ∼140 m over 4 km in the Valanginian and Hauterivian layers. Bauxites were deposited during the time interval between Early and Late Cretaceous. The restored geometry suggests that the bauxite was preserved in topographic steps aligned parallel to the strike (∼NÑW-SSE) of the Lower Cretaceous. This strike is due to tilting synchronous with normal faults.
The application of geodetic techniques to study crustal deformations associated with the geodynamic activity of a region is considered as a fundamental tool in seismic risk mitigation and in earthquake prediction research. In principle, the crustal deformation analysis is a purely geodetic problem as it concerns alteration of the Earth shape, so that deformations of the crust are directly connected with geodetic observables. The Tornquist zone across Skåne in southern Sweden is a classical fault zone that separates the Precambrium gneisses of the Baltic shield in the north from Phanerozoic Europe to the south. In this region, a Global Positioning Network (GPS) was established to study possible crustal motions. The aim of this article is to improve on previous study in to estimate the possible crustal strains and dilation parameters by a finite element analysis. Results show that the areas with maximum shear strain and dilation are located exactly in the active fault zones and their intersections. However, further observations in a dense network as well as integration with geological and geophysical data are needed to fully explore the recent crustal motions over the Tornquist zone.
In this paper, the satellite geodetic methods are investigated in connection with the movement of tectonic plates. We suggest establishing a combined 3D network for two goals. The first is to study the geoid from the kinematic point of view and vertical datum problems among the Baltic, Adriatic and Black seas. The second is to monitor spatial movements of tectonic plates, especially in the Carpatho-Balkan Region (CBR).