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Abstract

The Hojedk area in the southeastern Iran experienced three earthquakes during 12 days in December 2017. These events occurred in proximity to a region that has experienced several moderate and large events in the past. In this paper, the role of Coulomb stress changes in occurrence of the Hojedk cluster was investigated, and also the produced stress changes due to these events on the surrounding and optimally oriented faults was calculated. Previous earthquakes (events in the Golbaf–Sirch region, the Shahdad slip, and the Dahuiyeh–Zarand earthquake) imparted positive stress changes on the fault planes of the Hojedk earthquakes, especially on the ruptured plane of the first main shock. The Hojedk first main shock triggered the second one by imparting maximum stress changes of about 11.7 MPa on the fault plane of this event, and these two events brought the third main shock to failure by producing more than 6.0 MPa stress changes on its ruptured plane. Among the major active faults in the surrounding area, the middle part of the Nayband fault, northern part of the Golbaf–Sirch fault, and southern part of the Kuhbanan fault received positive stress changes and southen parts of the Ravar and all parts of the Chatrud faults received negative stress changes due to the Hojedk earthquakes. Furthermore, our results for correlation between Coulomb stress changes and seismicity distribution showed that following the Dahuiyeh–Zarand earthquake, the majority of the seismicity located on the positive stress area and Coulomb stress changes have a controlling role on the spatial distribution of seismicity even after a decade. The occurrence of the Hojedk first main shock affects the spatial distribution of aftershocks until the occurrence of the next main hocks. By triggering the next one, the stress patterns change in the area, and seismicity follows from the stress patterns of the last event.

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... High seismic activity in areas that have undergone positive stress coulomb changes indicates the high stress of the Earth's crust that is scattered in the northern part of Sumatra (5). This case has already occurred in several regions in Indonesia such as Aceh (4)(6), Halmahera (7), Pidie Jaya (8), Papua (9), Palu (10), China (11)(12) (13), and Iran (14). Primary sources of seismic, geodetic, and tsunami observations indicate that for the six model sources tested, there was an average increase of 47% in the positive pressure aftershock mechanism of the earthquake (1997-10 March 2011) (15). ...
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1] Coulomb stress maps are produced by computing the tensorial stress perturbation due to an earthquake rupture and resolving this tensor onto planes of a particular orientation. It is often assumed that aftershock fault planes are ''optimally oriented''; in other words, the regional stress and coseismic stress change are used to compute the orientation of planes most likely to fail and the coseismic stress is resolved onto these orientations. This practice assumes that faults capable of sustaining aftershocks exist at all orientations, an assumption contradicted by the observation that aftershock focal mechanisms have strong preferred orientations consistent with mapped structural trends. Here we systematically investigate the best planes onto which stress should be resolved for the Landers, Hector Mine, Loma Prieta, and Northridge earthquakes by quantitatively comparing observed aftershock distributions with stress maps based on optimally oriented planes (two-and three-dimensional), main shock orientation, and regional structural trend. We find that the best model differs between different tectonic regions but that in all cases, models that incorporate the regional stress field tend to produce stress maps that best fit the observed aftershock distributions, although not all such models do so equally well. Our results suggest that when the regional stress field is poorly defined, or in structurally complex areas, the best model may be to fix the strike of the planes upon which the stress is to be resolved to that of the main shock but allow the dip and rake to vary.
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We present a new three-dimensional inventory of the southern San Francisco Bay area faults and use it to calculate stress applied principally by the 1989 M = 7.1 Loma Prieta earthquake and to compare fault seismicity rates before and after 1989. The major high-angle right-lateral faults exhibit a different response to the stress change than do minor oblique (right-lateral/thrust) faults. Seismicity on oblique-slip faults in the southern Santa Clara Valley thrust belt increased where the faults were unclamped. The strong dependence of seismicity change on normal stress change implies a high coefficient of static friction. In contrast, we observe that faults with significant offset (>50-100 km) behave differently; microseismicity on the Hayward fault diminished where right-lateral shear stress was reduced and where it was unclamped by the Loma Prieta earthquake. We observe a similar response on the San Andreas fault zone in southern California after the Landers earthquake sequence. Additionally, the offshore San Gregorio fault shows a seismicity rate increase where right-lateral/oblique shear stress was increased by the Loma Prieta earthquake despite also being clamped by it. These responses are consistent with either a low coefficient of static friction or high pore fluid pressures within the fault zones. We can explain the different behavior of the two styles of faults if those with large cumulative offset become impermeable through gouge buildup; coseismically pressurized pore fluids could be trapped and negate imposed normal stress changes, whereas in more limited offset faults, fluids could rapidly escape. The difference in behavior between minor and major faults may explain why frictional failure criteria that apply intermediate coefficients of static friction can be effective in describing the broad distributions of aftershocks that follow large earthquakes, since many of these events occur both inside and outside major fault zones.
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The Iranian plateau accommodates the 35 mm/yr convergence rate between the Eurasian and Arabian plates by strike-slip and reverse faults with relatively low slip rates in a zone 1000 km across. Although these faults have only locally been the subject of paleoseismological studies, a rich historical and archeological record spans several thousand years, long enough to establish recurrence intervals of 1000 to 5000 yr on individual fault segments. Several clusters of earthquakes provide evidence of interaction among reverse and strike-slip faults, probably due to adjacent faults being loaded by individual earthquakes. The Dasht-e-Bayaz sequence of 1936 to 1997 includes earthquakes on left-lateral, right-lateral, and reverse faults. The Neyshabur sequence of four earthquakes between 1209 and 1405 respected the segment boundary between the Neyshabur and Binalud reverse fault systems. The two pairs of earthquakes may have ruptured different faults in each segment, similar to the 1971 and 1994 San Fernando, California, earthquakes. The 1978 Tabas reversefault earthquake was preceded by the 1968 Ferdows earthquake, part of the Dasht-e-Bayaz sequence. The North Tabriz fault system ruptured from southeast to northwest in three earthquakes from 1721 to 1786; a previous cluster may have struck this region in 855 to 958. The Mosha fault north of Tehran ruptured in three earthquakes in 958, 1665, and 1830. Five large earthquakes struck the Tehran region from 743 to 1177, but only two that large have struck the area since 1177. Other earthquakes occurred in pairs in the Talesh Mountains near the Caspian Sea (1863, 1896), the Iran-Turkey border (1840, 1843), and the Nayband-Gowk fault system (both in 1981). Other historical events did not occur as parts of sequences. The historic seismic moment release in Iran accounts for only a small part of the plate convergence rate, which may be due to aseismic slip or to the Iranian historical record, long as it is, being too short to sample long-term deformation across the plateau. No historic earthquakes of M ≧ 8 have struck Iran. However, several long, straight strike-slip faults (Doruneh, West Neh, East Neh, and Nayband) have not sustained large historical earthquakes, raising the possibility that these long faults could produce earthquakes of M ≧ 8, thereby removing at least part of the apparent slip deficit. An increased understanding of Iran's seismic hazard could be obtained by an extensive paleoseismology program and space-geodetic arrays, supplementing the abundant historical and archaeological record.
Article
The 26 December 2003 (Mw 6.6) Bam earthquake is one of the most destructive events in the recorded history of Iran. Good variable-slip fault models and precise aftershock data enabled us to calculate Coulomb stress changes on the surrounding faults, optimally oriented faults, and the aftershock nodal planes to learn whether they were brought closer to failure. We also test the role of secondary stress on the aftershocks triggering. Calculated stress on the surrounding faults showed that the southern part of the Gowk fault and middle part of the Dalfard and Bam faults received positive stress changes. Stress also increases at the ends of the ruptured fault plane to expand it. Our calculations showed that deeper part of the Bam fault planes received positive stress changes due to slipping on the shallow depth. It can lead to the occurrence of aftershocks and enhance the probability of large events in the future that represent a substantial remaining seismic hazard to the reconstructed city of Bam. By imparting stress on optimally oriented faults, the correlation between Coulomb stress changes and aftershock distribution is investigated. Obtained results showed good correlation between coulomb stress change and seismicity. From 331 well located aftershock hypocenters, about 270 events (more than 81%) are located in the positive stress changes area. The imparted stress due to the main shock and secondary aftershocks on the nodal planes of the 158 aftershocks is calculated. The results show that the imparted stress due to main shock encouraged about 60.1% of nodal planes and 77.8% of the chosen nodal planes of aftershocks and added secondary aftershock encouraged about 81.6% of chosen nodal planes. In other word by adding secondary triggering stress Coulomb Index for chosen nodal planes increased from 0.778 to 0.816 and about 80% of aftershocks receive greater positive stress changes. Keywords: Coulomb stress change; Secondary stress triggering; Aftershock; Coulomb Index; Bam Earthquake.
Article
A triplet of M w ∼6 earthquakes on 2017 December 1-12 occurred ∼50 km north of Kerman, Iran, in an area of mountainous topography where several major right-lateral strike-slip fault systems - the Gowk, Nayband, Lakar Kuh and Kuh Banan faults - converge. Here we assess their source parameters and surficial expression using regional and teleseismic waveforms and arrival times, synthetic aperture radar interferometry, optical satellite image correlation and field observations. All three main shocks occurred on shallow reverse faults associated with the southern termination of the Lakar Kuh right-lateral strike-slip fault. The first two main shocks on 1 December and 12 December (08:43 UTC) likely ruptured and reruptured a previously unrecognized, blind, NE-dipping fault beneath the Mian Kuh range. Slip in both earthquakes extends much further along strike than down dip, hinting at structural or stratigraphic controls on rupture dimensions. The third main shock on 12 December (21:41 UTC) is perhaps the most interesting of the three events. It ruptured a conjugate SW-dipping thrust in the hangingwall of the first fault, generating a sinuous fault scarp in the alluvial plain north of the Mian Kuh range, consistent with its unusually shallow centroid depth of ∼2 km. Its high ratio of net surface slip (average ∼1.5 m and maximum ∼2.5 m) to length (∼7 km) and its narrow down-dip width (∼6 km) implies a very high stress drop. The surface rupture aligns along-strike with larger scarps that contain uplifted and incised fan surfaces in their hangingwalls, but this subtle expression of active faulting had not been fully recognized prior to these earthquakes. The clustering in space and time of large, shallow earthquakes on hidden faults is of broad concern for seismic hazard assessment in mountainous parts of Iran and in other collisional settings. © The Author(s) 2019. Published by Oxford University Press on behalf of The Royal Astronomical Society.
Book
Cambridge Core - Structural Geology, Tectonics and Geodynamics - The Mechanics of Earthquakes and Faulting - by Christopher H. Scholz
Article
By using slip model from USGS and focal mechanism and aftershocks distribution from Iranian Seismological Center (IRSC) for Sarpol-e-Zahab earthquake (Mw 7.3) in the 2017 November 12 we investigated the correlation between Coulomb stress changes and aftershocks distribution. In this study, we selected about 500 aftershocks with magnitude larger than 2.5 and azimuthal gap less than 180 degrees. Calculated Coulomb stress changes on the optimally oriented faults showed that most of the seismicity occurred in regions of increased stress and the majority of them concentrated on the ruptured plane, especially in west and south parts. Also, nodal planes of the selected 11 aftershocks received positive Coulomb stress changes. So there is a good correlation between Coulomb stress changes and aftershocks distribution in Sarpol-e-Zahab event. Furthermore calculated static stress on the surrounding faults showed that middle part of the High Zagros Fault (HZF), the northern part of the Main Recent Fault (MRF), and the northern part of the Zagros Foredeep Fault (ZFF) are located in the positive stress change area.
Article
The north Tehran fault (NTF) is known to be one of the most drastic sources of seismic hazard on the city of Tehran. In this study we provide broadband (0–10 Hz) ground motions for the city as a consequence of probable M7.2 earthquake on the NTF. Low frequency motions (0–2 Hz) are provided from spectral element dynamic simulation of 17 scenario models. High frequency (2–10 Hz) motions are calculated with a physics based method based on S-to-S backscattering theory. Broadband ground motions at the bed-rock level show amplifications, both at low and high frequencies, due to the existence of deep Tehran basin in the vicinity of the NTF. By employing soil profiles obtained from regional studies, effect of shallow soil layers on broadband ground motions is investigated by both linear and nonlinear analyses. While linear soil response overestimate ground motion prediction equations, nonlinear response predicts plausible results within one standard deviation of empirical relationships. Average PGAs at the northern, central and southern parts of the city are estimated about 0.93 g, 0.59 g and 0.4 g, respectively. Increased damping caused by nonlinear soil behavior, reduces the soil linear responses considerably, in particular at frequencies above 3 Hz. Nonlinear de-amplification reduces linear spectral accelerations up to 63 per cent at stations above soft thick sediments. By performing more general analyses, which exclude source-to-site effects on stations, a correction function is proposed for typical site classes of Tehran. Parameters for the function which reduces linear soil response in order to take into account nonlinear soil de-amplification are provided for various frequencies in the range of engineering interest. In addition to fully nonlinear analyses, equivalent-linear calculations were also conducted which their comparison revealed appropriateness of the method for large peaks and low frequencies, but its shortage for small to medium peaks and motions with higher than 3 Hz frequencies.
Article
A complete set of closed analytical expressions is presented in a unified manner for the internal displacements and strains due to shear and tensile faults in a half-space for both point and finite rectangular sources. These expressions are particularly compact and systematically composed of terms representing deformations in an infinite medium, a term related to surface deformation and that is multiplied by the depth of observation point. Several practical suggestions to avoid mathematical singularities and computational instabilities are also presented. The expressions derived here represent powerful tools both for the observational and theoretical analyses of static field changes associated with earthquake and volcanic phenomena.
Article
The North Tehran Fault (NTF) represents an important source of seismic hazard to the city of Tehran, Iran, as it is known to be the causative fault of large historical earthquakes. In this study, the characteristics of long-period near-field ground-motions in the city of Tehran for M7.2 scenario earthquakes on the eastern segment of the NTF have been investigated using dynamic rupture simulations. Taking into account the NTF geometrical features, wave propagation properties and characteristics of rupture dynamics, realistic low frequency (0-2 Hz) ground motions are calculated using 2D spectral element simulations. Various self-similar initial stress distributions provide a wide range of rupture propagation complexities, rise-times and final slip scenarios, which are statistically consistent with empirical attenuation relationships for the region. Near-fault wave-field features, such as long-period pulses, are obtained from dynamic simulations and their effects on the ground-motions are discussed. The model takes into account the Tehran basin structure, which is composed of deep soft alluviums, by including a 2D shear-wave velocity model which comes from the most recent studies of the region. Our ground motion simulations reveal that the low-velocity sediments of the Tehran basin have major effects on the amplitude, frequency content and duration of the simulated time-histories. The simulated ground-motions from these scenarios are generally in agreement with the Next Generation Attenuation models (e.g. Boore and Atkinson, 2008) for shallow crustal earthquakes in active tectonic regions. The mean 2-s horizontal spectral acceleration in the northern parts of the city is about 0.46g, while near the midtown this value is up to ~0.26g above the deepest parts of the basin. The basin effects are higher on deeper parts and higher frequencies.
Article
The Makran subduction zone is one of the last convergent margins to be investigated using space-based geodesy. While there is a lack of historical and modern instrumentation in the egion, a sparse sampling of continuous and campaign measurements over the past decade has allowed us to make the first estimates of convergence rates. We combine GPS measurements from 20 stations located in Iran, Pakistan and Oman along with hypocentral locations from the International Seismological Centre to create a preliminary 3-D estimate of the geometry of themegathrust, along with a preliminary fault-coupling model for the Makran subduction zone. Using a convergence rate which is strongly constrained by measurements from the incoming Arabia plate along with the backslip method of Savage, we find the Makran subduction zone appears to be locked to a depth of at least 38 km and accumulating strain. We also find evidence for a segmentation of plate coupling, with a 300 km long section of reduced plate coupling. The range of acceptable locking depths from our modelling and the 900 km along-strike length for the megathrust, makes the Makran subduction zone capable of earthquakes up to Mw = 8.8. In addition, we find evidence for slow-slip-like transient deformation events on two GPS stations. These observations are suggestive of transient deformation events observed in Cascadia, Japan and elsewhere.
Article
This chapter deals with the long-term seismic pattern and active faulting behavior on the Iranian Plateau, showing: temporal active seismic cycles (clustered earthquake sequence) followed by a long period of seismic quiescence in a densely populated subparallel fault zone segmented rupturing and along-strike migration of seismicity of a single fault zone complete rupturing of multisegmented single fault within a short time period of decades to hundreds of years; seismic migration and interaction along multiple parallel faults interplay between strike-slip and reverse faults within the seismogenic layer, and transverse reverse faulting off the main longitudinal strike-slip; contrasting surface active faults and deeper seismogenic sources; slip partitioning onto separate reverse and strike-slip faulting; active conjugate strike-slip faulting; and aseismic slip on nearby thrust. The chapter also covers issues such as: post-Neogene change of sense of motion of active faults; tectonic reactivation of inherited structures; Late Neogene-quaternary migration of active faulting away from the mountain range-front to the piedmont area; topographic ridge-top strike-slip faulting; spatial shear strain partitioning; fault bounded block rotation; thrust fault earthquakes at the termination of strike-slip faults; decoupled earthquakes in the Zgros fold-and-thrust mountain belt and geomorphologically featureless active faults.
Article
The curved left-lateral strike-slip Xianshuihe–Xiaojiang Fault System (XXFS) in southwestern China extends at least 1400 km in the eastern margin of the Tibetan Plateau. Fieldworks confirm that the XXFS is one of the longest and most seismically active faults in China. The strain released by the slip motion on the XXFS is related to the convergence between the Indian and Eurasian plates. The entire fault system has experienced at least 35 earthquakes of M>6 in the recent 300 years and almost all segments of the system have been the locus of major historical earthquakes. Since the XXFS region is heavily populated (over 50 million people), understanding the migration of the large earthquakes in space and time is of crucial importance for the seismic hazard assessment in this region. We analyze a sequence of 25 earthquakes (M⩾6.5) that occurred along the XXFS since 1713, and investigate their influence on the 2008 Mw7.9 Wenchuan earthquake occurred on the adjacent Longmenshan fault. In our analysis, the relevant parameters for the earth crust are constrained by seismic studies. The locations and geometries of the earthquake faults as well as the rupture distributions are taken from field observations and seismological studies. Results from the Coulomb failure stress modeling indicate significant interactions among the earthquakes. After the 1713 earthquake, 19 out of 24 earthquakes occurred in the positive stress zone of the preceding earthquakes. The other 5 earthquakes located in the area without significant stress changes induced by the preceding events. In particular, we can identify 4 visible earthquake gaps with increasing seismic hazard along the XXFS, consistent with the findings from the paleo-seismological studies. The seismic activity and tectonic motion on the XXFS reduced the Coulomb stress accumulation at the hypocenter of 2008 Mw7.9 Wenchuan earthquake, implying that the Wenchuan earthquake might not be triggered directly by the seismic activities on the XXFS. On the other hand, the Coulomb failure stress induced by the Wenchuan earthquake has increased in a region of 125-km-long segment of the XXFS, northwest of Kangding City.
Article
The 290 km long Nayband right-lateral fault cuts across a region seismically quiescent during the last few millennia. Chlorine-36 and optically stimulated luminescence (OSL) dating of cumulative geomorphic offsets between 9 ± 1 m and 195 ± 15 m with ages from 6.8 ± 0.6 ka to ∼ 100 ka allow deriving a slip rate of 1.8 ± 0.7mm yr-1. The paleoseismic record retrieved from the first trench excavated across the fault combined with 18 OSL ages demonstrates the occurrence of at least four large (Mw ∼ 7) earthquakes during the last 17.4 ± 1.3 ka and two older earthquakes (before ∼ 23 ka and 70 ± 5 ka). Sediments from the last ∼ 7 ka contain evidence of the three younger earthquakes. Penultimate and antepenultimate events occurred between 6.5 ± 0.4 ka and 6.7 ± 0.4 ka within at most 1 ka whereas the most recent earthquake occurred within the last millennium. Such an irregular earthquake occurrence may suggest seismic clustering. Therefore, the imminence of an earthquake along the fault cannot be discarded even if the most recent earthquake occurred within the last 800 years. This event went unnoticed in the historical records demonstrating the incompleteness of the historical seismic catalogs in Central Iran, challenging any assessment of seismic hazard without geologic information. Infrequent large earthquakes typify the slow-slipping strike-slip faults slicing Central and Eastern Iran. Also, the slip rates summed from the Iran Plateau up to the Afghan lowlands appear in fairly good agreement with the most recent GPS data.
Article
Changes in stress in southern California are modeled from 1812 to 2025 using as input (1) stress drops associated with six large (7.0≤M<7.5) to great (M≥7.5) earthquakes through 1995 and (2) stress buildup associated with major faults with slip rates ≥3 mm/yr as constrained by geodetic, paleoseismic, and seismic measurements. Evolution of stress and the triggering of moderate to large earthquakes are treated in a tensorial rather than a scalar manner. We present snapshots of the cumulative Coulomb failure function (ΔCFF) as a function of time for faults of various strike, dip, and rake throughout southern California. We take ΔCFF to be zero everywhere just prior to the great shock of 1812. We find that about 95% of those well-located M≥6 earthquakes whose mechanisms involve either strike-slip or reverse faulting are consistent with the Coulomb stress evolutionary model; that is, they occurred in areas of positive ΔCFF. The interaction between slow-moving faults and stresses generated by faster-moving faults significantly advanced the occurrence of the 1933 Long Beach and 1992 Landers events in their earthquake cycles. Coulomb stresses near major thrust faults of the western and central Transverse Ranges have been accumulating for a long time. Future great earthquakes along the San Andreas fault, especially if the San Bernardino and Coachella Valley segments rupture together, can trigger moderate to large earthquakes in the Transverse Ranges, as appears to have happened in the Santa Barbara earthquake that occurred 13 days after the great San Andreas shock of 1812. Maps of current ΔCFF provide additional guides to long-term earthquake prediction.
Article
On 2005 February 22, the Dahuiyeh-Zarand earthquake, Mw 6.5, struck one of the most seismically active regions in south-central Iran, east of Zarand City in Kerman province, causing more than 500 fatalities. The causative fault of the 2005 Dahuiyeh-Zarand earthquake, a reverse fault, striking nearly EW and dipping to the north, was located within a mountainous region and therefore more difficult to identify compared to the range-bounded faults. Its identification, after the 2005 event, is very important for both the estimation of seismic hazard as well as for the damage and fatality functions. We have inverted six three-component near-field strong motion waveforms to obtain the complete earthquake rupture history and slip distribution. Accelerograms are bandpass filtered with 0.2-1.0 Hz, and a length of 15-17 s of the waveforms is inverted. The lack of absolute timing has been successfully overcome by estimating, from the velocity model of the region, the propagation of P and S waves from the epicentre to the stations. The final fault slip model and the estimated source parameters are able to explain the observed waveforms. The rupture is found to be bilateral with a maximum slip of 2.4 m concentrated on two asperities in the west and east sides of the nucleation point at depths of 6-12 km. The western asperity is located to the east of Zarand City and beneath the Dahuiyeh village, which might explain why the Dahuiyeh village was totally destroyed by this earthquake.
Article
The 11 March 2011 Tohoku Earthquake provides an unprecedented test of the extent to which Coulomb stress transfer governs the triggering of aftershocks. During 11-31 March, there were 177 aftershocks with focal mechanisms, and so the Coulomb stress change imparted by the rupture can be resolved on the aftershock nodal planes to learn whether they were brought closer to failure. Numerous source models for the mainshock have been inverted from seismic, geodetic, and tsunami observations. Here, we show that, among six tested source models, there is a mean 47% gain in positively-stressed aftershock mechanisms over that for the background (1997-10 March 2011) earthquakes, which serve as the control group. An aftershock fault friction of 0.4 is found to fit the data better than 0.0 or 0.8, and among all the tested models, Wei and Sladen (2011) produced the largest gain, 63%. We also calculate that at least 5 of the seven large, exotic, or remote aftershocks were brought >= 0.3 bars closer to failure. With these tests as confirmation, we calculate that large sections of the Japan trench megathrust, the outer trench slope normal faults, the Kanto fragment beneath Tokyo, and the Itoigawa-Shizuoka Tectonic Line, were also brought >= 0.3 bars closer to failure.
Article
A damaging and widely felt moderate earthquake (Mw 6.4) hit the rural, mountainous region of southwestern Pakistan on October 28, 2008. The main shock was followed by another earthquake of identical magnitude (Mw 6.4) on the next day. The spatial distribution of aftershocks and focal mechanism revealed a NW–SE striking rupture with right-lateral strike-slip motion which is sympathetic to the NNW–SSE striking active mapped Urghargai Fault. The occurrence of strike-slip earthquakes suggests that along with the thrust faults, strike slip faults too are present beneath the fold-and-thrust belt of Sulaiman–Kirthar ranges and accommodates some of the relative motion of the Indian and Eurasian plates.To assess the characteristics of this sequence, the statistical parameters like aftershocks temporal decay, b-value of G–R relationship, partitioning of radiated seismic energy due to aftershocks, and spatial fractal dimension (D-value) have been examined. The b-value is estimated as 1.03 ± 0.42 and suggests the tectonic genesis of the sequence and crustal heterogeneity within rock mass. The low p-value of 0.89 ± 0.07 implies slow decay of aftershocks activity which is probably an evidence for low surface heat flow. A value of spatial fractal dimension of 2.08 ± 0.02 indicates random spatial distribution and that the source is a two-dimensional plane filled-up by fractures.The static coseismic Coulomb stress changes due to the foreshock (Mw 5.3) were found to increase stress by more than 0.04 bars at the hypocenter of the main shock, thus promoting the failure. The cumulative coseismic Coulomb stress changes due to the foreshock and mainshocks suggest that most of the aftershocks occurred in the region of increased Coulomb stress, and to the SE to the mainshock rupture.
Article
The magnitude of the stress drops that occur during frictional sliding on ground surfaces and on faults has been studied at confining pressures of as much as 5 kb. It was found that the stiffness of the loading system and the rate at which the load was applied had no effect on the magnitude of the sudden stress drops. Confining pressure and rock type were found to be the most important parameters. For example, sliding on fault surfaces in unaltered silicate rocks at confining pressure below 1 to 2 kb was stable; that is, stick slip was absent. At higher pressures, motion occurred by stick slip, and the magnitude of the stress drop during slip increased with pressure. Stick slip was absent at all pressures in gabbro, in dunite where the minerals are altered to serpentine, and in limestone and porous tuff. These results suggest that, if stick slip on a fault in the earth produces earthquakes, the earthquakes should become more abundant and increasingly severe with depth. Also, if a fault traverses various rock types, then over part of the fault elastic buildup of stress prior to sudden movement may occur at the same time as stable creeping motion elsewhere on the fault.
Article
The paucity of detectable seismicity shadows in the days/months following a main shock has raised the question as to whether dynamic rather than static triggering could be the main mechanism driving the seismicity at these timescales. The lack of correlation between the stress unloading of the main fault and the typically vigourous aftershock production taking place on it, however, suggests that the spatial heterogeneity of the coseismic stress change could also be a factor in the apparent suppression of seismicity shadows, at least on the main fault. Here we study whether this stress variability can indeed be an important aspect in the stress modeling of earthquake sequences. A rate-and-state friction model of seismicity is used, in conjunction with realistic levels of stress heterogeneity at the earthquake nucleation scale (1–10 m, as predicted from these friction laws), based on scale-invariant models of coseismic slip distribution. In this model, a relatively weak on-fault quiescence is delayed by months to years because of the high stress heterogeneity. Delayed quiescences due to slip heterogeneity are similarly predicted at distances of up to about half the rupture length away from the fault. We also postulate that off-fault seismicity can be significantly controlled by stress variability that originates from small-scale crustal heterogeneity and the complexity in fault geometries. Rather than mapping stress changes by providing a single stress value at every grid point, these results suggest that an estimate of the likely stress variability (acting at the nucleation scale) is also needed, especially when trying to account for the seismicity occurring at relatively short timescales.
Article
A consequence of the dilatancy/fluid-diffusion mechanism for shallow earthquakes is that considerable volumes of fluid are rapidly redistributed in the crust following seismic faulting. This is borne out by the outpourings of warm groundwater which have been observed along fault traces following some moderate (M5-M7) earthquakes. The quantities of fluid involved are such that significant hydrothermal mineralisation may result from each seismically induced fluid pulse, and the mechanism provides an explanation for the textures of hydrothermal vein deposits associated with ancient faults, which almost invariably indicate that mineralisation was episodic.
Article
More than 2000 instrumentally recorded earthquakes occurring in the Iran region during the period 1918-2004 have been relocated and reassessed, with special attention to focal depth, using an advanced technique for 1-D earthquake location. A careful review of starting depths, association of teleseismic depth phases, and the effects of reading errors on these phases are made and, when necessary, waveforms have been examined to better constrain EHB focal depths. Uncertainties in EHB epicentres are on the order of 10-15 km in the Iran region, owing to the Earth's lateral heterogeneity and uneven station distribution. Uncertainties of reviewed EHB focal depth estimates are on the order of 10 km, as compared to about 4 km for long-period P and SH body-waveform inversions. Nevertheless, these EHB depth estimates are sufficiently accurate to resolve robust differences in focal depth distribution throughout the Iran region and, within their errors, show patterns that are in agreement with the smaller number of earthquakes whose depths have been confirmed by body-wave modelling or local seismic networks. The importance of this result is that future earthquakes with apparently anomalous depths can easily be identified, and checked, if necessary. Most earthquakes in the Iranian continental lithosphere occur in the upper crust, with the crustal shortening produced by continental collision accommodated entirely by thickening and distributed deformation. In the Zagros Mountains nearly all earthquakes are confined to the upper crust (depths <20 km), and there is no evidence for a seismically active subducted slab dipping NE beneath central Iran. By contrast, in southeastern Iran, where the Arabian seafloor is being subducted beneath the Makran coast, low-level earthquake activity occurs in the upper crust as well as to depths of at least 150 km within a northward-dipping subducting slab. Near the Oman Line, a region transitional between the Zagros and the Makran, seismicity extends to depths of up to 30-45 km in the crust, consistent with low-angle thrusting of Arabian basement beneath central Iran. In north-central Iran, along the Alborz mountain belt, seismic activity occurs primarily in the upper crust but with some infrequent events in the lower crust, particularly in the western part of the belt (the Talesh), where the South Caspian basin underthrusts NW Iran. Earthquakes that occur in a band across the central Caspian, following the Apscheron-Balkhan sill between Azerbaijan and Turkmenistan, have depths in the range 30-100 km, deepening northwards. These are thought to be connected with either incipient or remnant northeast subduction of the South Caspian basin basement beneath the east-west trending Apscheron-Balkhan sill. Curiously, in this region of genuine mantle seismicity, there is no evidence for earthquakes shallower than 30 km.
Article
Two sets of conjugate strike slip faults of various sizes and displacements occur in a tightly deformed area in Sistan district, southeast Iran. The smalles angle between the smaller faults of the two sets, measured across the compressional direction, is 40°, but between the larger faults of the two sets the angle is as wide as 145°. This angular spread is explained by Cloos's suggestion that strike slip faults (like any other shear planes) rotate outward from their original position during the deformation.
Article
The island of Taiwan is affected by intense seismic activity, which includes large events as the disastrous 1999 Chi-Chi earthquake. To improve seismic hazard assessment in this area, we estimate the effect of both interseismic loading and major events since 1736 on the state of stress of major active faults. We focus our approach on western Taiwan, which is the most densely populated part of Taiwan. We pay a specific attention to faults geometry and to both interseismic and coseismic slip distributions. Our results suggest that both earthquakes and interseismic loading before 1999 increase the Coulomb stress in the north-western part of the Chelungpu fault, a region which experienced the highest coseismic slip during the Chi-Chi earthquake. More importantly our results reveal a Coulomb stress increase in the southern part of the Changhua thrust fault, below a densely populated area.
Article
Many aspects of earthquake mechanics remain an enigma as we enter the closing years of the twentieth century. One potential bright spot is the realization that simple calculations of stress changes may explain some earthquake interactions, just as previous and on going studies of stress changes have begun to explain human-induced seismicity. This paper, which introduces the special section "Stress Triggers, Stress Shadows, and Implications for Seismic Hazard," reviews many published works and presents a compilation of quantitative earthquake interaction studies from a stress change perspective. This synthesis supplies some clues about certain aspects of earthquake mechanics. It also demonstrates that much work remains before we can understand the complete story of how earthquakes work.
Article
We used seismic body waves, radar interferometry and field investigation to examine the source processes of the destructive earthquake of 2005 February 22 near Zarand, in south-central Iran. The earthquake ruptured an intramountain reverse fault, striking E-W and dipping north at ~60° to a depth of about 10 km. It produced a series of coseismic scarps with up to 1 m vertical displacement over a total distance of ~13 km, continuous for 7 km. The line of the coseismic ruptures followed a known geological fault of unknown, but probably pre-Late Cenozoic, age and involved bedding-plane slip where the scarps were continuous at the surface. However, any signs of earlier coseismic ruptures along this fault had been obliterated by the time of the 2005 earthquake, probably by land sliding and weathering, so that the fault could not reasonably have been identified as active beforehand. The 2005 fault is at an oblique angle to the range-bounding Kuh Banan strike-slip fault, and may represent a splay from that fault, related to its southern termination. Other intramountain reverse faulting earthquakes have occurred in Iran, but this is the first to have produced a clear, mapped surface rupture, and to have been studied with InSAR. Faults of this type represent a serious seismic hazard in Iran and are difficult to assess, because their geomorphological expression is much less clear than the range-bounding reverse faults, which are more common and have been better studied.
Article
The complicated tectonics of the Mediterranean region, dominated by the subduction of the African plate under Eurasia, affects the whole of Greece. A significant extension rate across the Aegean sea is estimated from satellite geodetic observations, while intense seismicity is observed in parts of the Hellenic arc, manifested by strong earthquakes (Ms > 6) of intermediate depth that take place along it.In Western Greece, the Ionian Islands are situated in a transitional zone (from the Hellenic subduction to the Adriatic collision), characterised by a high crustal deformation rate as revealed by the high seismicity of this zone, the highest in Greece, and the GPS velocity field estimated for the region. In this part of the Aegean plate, transcurrent fault systems dominate, one of which is the Kephalonia Transform Fault (KTF), located offshore the Kephalonia and Lefkada Islands, with a right-lateral slip of the order of 3 cm/year.In the present work an attempt is made to assess the Coulomb stress change associated with well documented earthquake activity, from 1973 to 2003, in the Ionian Island of Lefkada. The results of this study suggest that the early 1973 event did not influence any subsequent moderate earthquakes in the area. On the other hand, the 1994 earthquake may have triggered the north segment of the 2003 event, while the 2003 earthquake ruptured two segments with the north one initiating rupture on the south segment.
Article
EARTHQUAKES induce changes in static stress on neighbouring faults that may delay, hasten or even trigger subsequent earthquakes1–10. The length of time over which such effects persist has a bearing on the potential contribution of stress analyses to earthquake hazard assessment, but is presently unknown. Here we use an elastic half-space model11 to estimate the static stress changes generated by damaging (magnitude M≥5) earthquakes in southern California over the past 26 years, and to investigate the influence of these changes on subsequent earthquake activity. We find that, in the 1.5-year period following a M≥5 earthquake, any subsequent nearby M≥5 earthquake almost always ruptures a fault that is loaded towards failure by the first earthquake. After this period, damaging earthquakes are equally likely to rupture loaded and relaxed faults. Our results suggest that there is a short period of time following a damaging earthquake in southern California in which simple Coulomb failure stress models could be used to identify regions of increased seismic hazard.
Article
A network of 27 GPS sites was implemented in Iran and northern Oman to measure displacements in this part of the Alpine-Himalayan mountain belt. We present and interpret the results of two surveys performed in 1999 September and 2001 October. GPS sites in Oman show northward motion of the Arabian Plate relative to Eurasia slower than the NUVEL-1A estimates (e.g. 22 +/- 2 mm yr-1 at N8°+/- 5°E instead of 30.5 mm yr-1 at N6°E at Bahrain longitude). We define a GPS Arabia-Eurasia Euler vector of 27.9°+/- 0.5°N, 19.5°+/- 1.4°E, 0.41°+/- 0.1° Myr-1. The Arabia-Eurasia convergence is accommodated differently in eastern and western Iran. East of 58°E, most of the shortening is accommodated by the Makran subduction zone (19.5 +/- 2 mm yr-1) and less by the Kopet-Dag (6.5 +/- 2 mm yr-1). West of 58°E, the deformation is distributed in separate fold and thrust belts. At the longitude of Tehran, the Zagros and the Alborz mountain ranges accommodate 6.5 +/- 2 mm yr-1 and 8 +/- 2 mm yr-1 respectively. The right-lateral displacement along the Main Recent Fault in the northern Zagros is about 3 +/- 2 mm yr-1, smaller than what was generally expected. By contrast, large right-lateral displacement takes place in northwestern Iran (up to 8 +/- mm yr-1). The Central Iranian Block is characterized by coherent plate motion (internal deformation
Article
This chapter discusses the recent developments in understanding how earthquakes interact with each other. The new theoretical framework for earthquake cycles is based on calculating the stress changes caused by one event and assessing where and what mechanism of earthquakes these changes may promote. For studying such stress interaction, the computation of the stress field outside a rupturing fault is analyzed. This is different from investigating the dynamic rupture growth requiring the reconstruction of the spatiotemporal evolution of the stress on the fault plane. The chapter discusses the theoretical background of earthquake sequences and reviews some of the simple examples that allowed stress coupling concepts to be accepted. The success of simple stress modeling led to the introduction of several modifications, adaptations, and refinements of the ideas.