Engineering Geology

The objectives of this research were to make accurate measurements of thermal diffusivity at normal and elevated temperatures on eight well-characterized rocks and to investigate the effects of water saturation and heating upon thermal diffusivity. The eight rock types studied were Barre Granite, Dresser Basalt, St. Cloud Granodiorite, Westerly Granite, Berea Sandstone, Holston Marble, Salem Limestone, and Sioux Quartzite. A summary geologic description and chemical analysis of each rock type is included. The thermal diffusivity measurements were made at one atmosphere pressure using the laser flash method. In the natural, air-dried state at 300 K the values of diffusivity ranged from about 0.280 cm2 sec−1 for the quartzite to 0.0114 cm2 sec−1 for the limestone. Thermal diffusivity varies inversely with temperature and at 800 K the value is 50 to 75% lower than at room temperature. At room temperature, rocks saturated with water increase in diffusivity as much as 24 percent. Several qualitative trends were observed relating the thermal diffusivity of rock to its composition, texture and structure.

The year was particularly eventful in terms of the recognition of improved technologies impinging on the duties of the Engineering Geologist. National economies were strained to the limit under the pressure of the new world terrorist attacks and burgeoning populations. much of which stemmed from escapes from oppressive national regimes to the free world. The impacts of terrorism range from increased danger and costs of field work, to special design considerations and postponed and delayed project work.On the positive side, continued progress has been made in the means of recognition and professional development of our practitioners and in the general call for their work. Competent engineering geologists are in consistent demand, but the working conditions are not improved. A general deterioration of university funding, aggravated by bureaucratic excesses among administrators, have tended to make life miserable for dedicated faculty and a general move is afoot to cut back on the number of funded geology departments in North America and Europe. This situation has also been worsened by the general withdrawal of the mineral industries and by retraction of the petroleum companies from all but their most favored campuses.

On 13 May 1995 a strong earthquake of Ms=6.6 struck the cities of Kozani and Grevena in northwestern Greece. This region is characterized by low seismicity. In the same area, three hydroelectric dams have been operating for the last 30 years. One of them, the Polyphyto dam, is located only 40 km from the epicenter of the 13 May earthquake.In the present work all available seismological aspects of the main event, such as focal properties and source parameters together with foreshock and aftershock characteristics of the earthquake sequence are considered in order to examine whether the water level changes behind the Polyphyto dam induced the unusual seismic activity in this area. Thus, a detailed examination of the seismic activity is made and this is compared to the seismotectonic regime of the region and the reservoir loading from 1976 to 1995. The results show that there is no obvious correlation between seasonal or sharp fluctuations in the water level and the seismicity of the region (except once during 1989). Moreover, comparison with other previous cases of induced seismicity in Greece and in other countries shows no similarities to the Kozani–Grevena earthquake sequence.It is concluded that the Kozani–Grevena earthquake is therefore an event in the framework of the regional seismicity rather than an event triggered by the impounding of the Polyphyto artificial Lake.

The effects of the Catalan earthquake of February 2, 1428, between 8 h and 9 h local time, have been studied by reevaluating the information gathered by Fontseré and Iglesies (1971). MSK-intensities have been assigned to 45 localities to obtain an isoseismal map. A maximum intensity of IX has been deduced. The most probable location is the area around the village of Queralps (42.4° N 2.2° E) at a depth of about 8 km. The attenuation seems to be similar to that found for earthquakes in Central Europe, which considerably differs from that in southern Spain.

Strong motion records from the South Hyogo earthquake of January 17, 1995, are overviewed. The discussion is focused on (1) the characteristic of near field records including the predominant direction of motion versus the fault strike and their spectral characteristic, and (2) the orientation error of seismometers and ground motion amplification at borehole array stations. Two numerical techniques for seismic response analysis of ground are applied for the array stations. The one is the frequency-dependent equi-linearized technique for the frequency domain analysis, and the other is the effective stress-based liquefaction analysis method. The numerical calculations are compared with the records at four borehole array stations.

The goal of this paper is to present especially the source and the type of liquefaction-related failures of those buildings which satisfied the structural codes of buildings and practices but experienced settlement, tilting and overturning in Adapazari during the Turkey earthquake of August 17, 1999. In this context, the Adapazari region is first evaluated in terms of geological setting, tectonics, seismicity and liquefaction susceptibility based on in situ and laboratory test data. In addition, the number of stories, types of buildings and their related failures have been mapped accordingly. Furthermore, laboratory model tests are conducted to enforce and/or confirm the type of building's failures with particular reference to their geometrical shape and the number of stories assuming that the models representing buildings satisfy the structural codes of buildings and practices.

Landslides are common natural hazards in the seismically active North Anatolian Fault Zone of Turkey. Although seismic activity, heavy rainfall, channel incisions, and anthropogenic effects are commonly the main triggers of landslides, on March 17, 2005, a catastrophic large landslide in Sivas, northeastern of Turkey, the Kuzulu landslide, was triggered by snowmelt without any other precursor. The initial failure of the Kuzulu landslide was rotational. Following the rotational failure, the earth material in the zone of accumulation exhibited an extremely rapid flow caused by steep gradient and high water content. The Agnus Creek valley, where Kuzulu village is located, was filled by the earth-flow material and a landslide dam was formed on the upper part of Agnus Creek. The distance from the toe of the rotational failure down to the toe of the earth flow measured more than 1800 m, with about 12.5 million m3 of displaced earth material. The velocity of the Kuzulu landslide was extremely fast, approximately 6 m/s. The main purposes of this study are to describe the mechanism and the factors conditioning the Kuzulu landslide, to present its environmental impacts, and to produce landslide-susceptibility maps of the Kuzulu landslide area and its near vicinity. For this purpose, a detailed landslide inventory map was prepared and geology, slope, aspect, elevation, topographic-wetness index and stream-power index were considered as conditioning factors. During the susceptibility analyses, the conditional probability approach was used and a landslide-susceptibility map was produced. The landslide-susceptibility map will help decision makers in site selection and the site-planning process. The map may also be accepted as a basis for landslide risk-management studies to be applied in the study area.

One of the most significant effects of the 17 January, 1994 Northridge, California earthquake (M=6.7) was the triggering of thousands of landslides over a broad area. Some of these landslides damaged and destroyed homes and other structures, blocked roads, disrupted pipelines, and caused other serious damage. Analysis of the distribution and characteristics of these landslides is important in understanding what areas may be susceptible to landsliding in future earthquakes. We analyzed the frequency, distribution, and geometries of triggered landslides in the Santa Susana 7.5′ quadrangle, an area of intense seismic landslide activity near the earthquake epicenter. Landslides occurred primarily in young (Late Miocene through Pleistocene) uncemented or very weakly cemented sediment that has been repeatedly folded, faulted, and uplifted in the past 1.5 million years. The most common types of landslide triggered by the earthquake were highly disrupted, shallow falls and slides of rock and debris. Far less numerous were deeper, more coherent slumps and block slides, primarily occurring in more cohesive or competent materials. The landslides in the Santa Susana quadrangle were divided into two samples: single landslides (1502) and landslide complexes (60), which involved multiple coalescing failures of surficial material. We described landslide morphologies by computing simple morphometric parameters (area, length, width, aspect ratio, slope angle). To quantify and rank the relative susceptibility of each geologic unit to seismic landsliding, we calculated two indices: (1) the susceptibility index, which is the ratio (given as a percentage) of the area covered by landslide sources within a geologic unit to the total outcrop area of that unit; and (2) the frequency index [given in landslides per square kilometer (ls/km2)], which is the total number of landslides within each geologic unit divided by the outcrop area of that unit. Susceptibility categories include very high (>2.5% landslide area or >30 ls/km2), high (1.0–2.5% landslide area or 10–30 ls/km2), moderate (0.5–1.0% landslide area or 3–10 ls/km2), and low (<0.5% landslide area and <3 ls/km2).

Many large-scale landslides induced by earthquakes have been reported in loess soils. They often cause catastrophes because they travel long distances at high velocity. To clarify the mechanism of these landslides, field survey was performed on the landslides triggered by the Haiyuan Earthquake (China, 1920), and a series of ring shear tests was conducted on the loess soils collected from a landslide. The field surveys revealed that most of the loess landslides triggered by the Haiyuan Earthquake occurred on concave slopes gentler than 15° with long runout distance, showing very small equivalent friction angle. Cyclic ring shear tests on saturated specimens in the undrained condition showed that when cyclic shear stress was applied to a loose loess specimen, pore pressure was built-up gradually before the failure, and after that, large pore pressure was quickly generated due to the failure of loess soil structure. Meanwhile, to investigate the mechanism that produces the high mobility of loess landslides, ring shear tests were conducted on loess specimens with large shear displacement of up to several meters under the undrained condition, and it was found that the initial normal stress has no effects, and overconsolidation ratio has little if any effects, on the apparent friction angle in the steady state for loess soils. Results of tests on soils at different saturation degrees revealed that the reduction in shear strength results more likely from the generation of pore water pressure, not of pore air pressure.

Earthquake-triggered landslides are a major geological hazard in Central Asia. In July 1949, the M7.4 Khait earthquake triggered many hundreds of landslides in a mountainous region near the southern limit of the Tien Shan Mountains, central Tajikistan. These landslides involved widespread rock-slope failure as well as large numbers of flowslides in loess that mantles the steep slopes of the region. In the Yasman valley hundreds of loess landslides coalesced to form a massive loess flow (est. vol. 245 Mm3) that travelled up to 20 km on a slope of only 2°. In an adjacent valley, the Khait landslide involved transformation of an earthquake-triggered rockslide into a very rapid flow by the entrainment of saturated loess into its movement. It travelled 7.41 km over a vertical distance of 1421 m with an estimated average velocity of ~30 m/s. We estimate its volume as 75 Mm3, an order of magnitude less that previously published estimates. The Khait landslide was simulated using DAN. The number of casualties due to earthquake-triggered landslides in the epicentral region was considerable. Approximately 4000 people were killed in the Yasman valley loess flow as 20 villages (kishlaks) were overwhelmed. In the Khait landslide alone we estimate ca. 800 people lost their lives when the villages of Khait and Khisorak were overrun by rapidly moving debris. Our data indicates that a total of approximately 7200 people were killed by earthquake-triggered landslides in the epicentral region of the Khait earthquake and that, in terms of loss of life, the 1949 Yasman valley loess flow was one of the most destructive landslides in recent history.

The Baastad landslide occurred within a large area of marine clay deposits where the clay had been transformed into quick clay from leaching. The landscape in the area is highly dissected from stream erosion and numerous slope failures and slides. The slide scar covers about 80,000 m2 of farmland. Three buildings were destroyed in the slide.According to observations and investigations, the whole area slid out almost simultaneously, i.e., the slide was of the so-called flake type. For all known documented slides of this category, conventional stability analysis based on ultimate effective stress parameters φ′ and c′, give values of the factor of safety which are too high. It is an unsafe procedure. The behaviour of quick-clays has been studied extensively at the Norwegian Geotechnical Institute over the last 20 years. These studies show that the strength properties of these clays should be based on undrained shear strength values rather than the parameters φ′ and c′.Back calculations of the Baastad slide based on undrained shear strength values have been carried out and were in good agreement with the theoretical value of the factor of safety.

Liquefaction effects generated by the 1977 San Juan Province, Argentina, earthquake (Ms = 7.4) are described. The larger and more abundant effects were concentrated in the 60-km long band of the lowlands in the Valle del Bermejo and in an equally long band along the Rio San Juan in the Valle de Tulum. Fissures in the Valle del Bermejo were up to several hundred meters long and up to several meters wide. Sand deposits, from boils that erupted through the fissures, covered areas up to tens of square meters. Fissures generally parallelled nearby stream channels. Because the Valle del Bermejo is undeveloped, these large features caused no damage. Liquefaction in the Valle del Tulum caused important or unusual damage at several localities, including the following five sites: (1) At the Barrio Justo P. Castro, a subdivision of Caucete, liquefaction of subsurface sediments decoupled overlying, unliquefied stiff sediments, producing a form of ground failure called “ground oscillation”. The associated differential ground movements pulled apart houses and pavements in extension, while shearing curbs and buckling canal linings in compression at the same locality. (2) At the Escuela Normal, in Caucete, the roof of a 30-m long single-story classroom building shifted westward relative to the foundation. That displacement fractured and tilted columns supporting the roof. The foundation was fractured at several places, leaving open cracks, as wide as 15 mm. The cumulative width of the open cracks was 48 mm, an amount roughly equivalent to the 63 mm of offset between the roof and foundation at the east end of the building. The ground and foundation beneath the building extended (or spread) laterally opening cracks and lengthening the foundation while the roof remained in place. (3) The most spectacular damage to structures at the community of San Martin was the tilting of a 6-m high water tower and the toppling of a nearby pump house into a 1-m deep crater. Similarly, a small crater developed beneath a hand-pump in an open area and a large, 6-m diameter crater formed nearby. The following sequence of events created the craters and toppled the pump structures: During the earthquake, ground shaking generated excess pore pressures which were dissipated by upward flow of groundwater. Free drainage was restricted by an impermeable plastic-silt layer. Water apparently accumulated below the plastic-silt layer and then burst to the surface through several holes and cracks, including holes around well casings. (4) At the San Isidro winery, nine storage tanks tilted 2 to 5°. Five reinforced-concrete tanks were dismantled but four steel tanks were repaired by placing new footings and jacking the structures into an upright position. (5) At Escuela J.J. Pasos, differential settlement beneath building fractured several columns and walls. The largest settlements were about 60 mm and the maximum settlement of footings supporting columns was about 40 mm. In spite of the damage, the buildings were in no danger of collapse.

In 1984 a mass of Quaternary pyroclastic rock (est. vol. 0.74×106m3) slid from the western flank of Mount Cayley volcano in southwest British Columbia. The disintegrating rock mass entrained a further 0.20×106 m3 and formed a rock avalanche that travelled a horizontal distance of 3.46 km from its source over a vertical elevation difference of 1.18 km, equivalent to a fahrböschung of 19°. From the superelevation of the debris trimline in the mid-path, it is estimated that velocities reached at least 42 m/s; in the upper part of its path velocities may have approached 70 m/s. The rock avalanche was partially transformed into a distal debris flow that travelled a further 2.6 km down Turbid Creek in a narrow channelised path to the Squamish River, temporarily blocking it. The motion of the rock avalanche, including the production of a distal debris flow, was successfully simulated using a dynamic analytical model. Both the results of this analysis and field evidence indicate that the rock avalanche did not come to a halt in the upper part of its path as suggested by Cruden and Lu (1992), but travelled to its distal limit in one uninterrupted movement. This finding has important implications for landslide hazard assessment at Mount Cayley and similar sites. The landslide is typical of those which occur on the steep slopes of dissected volcanoes and is one of seven high-velocity rock avalanches that have occurred in the Garibaldi Volcanic Belt of southwest British Columbia since 1855.

The Tschierva rock avalanche occurred on October 29, 1988 in the area of the Piz Morteratsch, Switzerland. Releasing a total volume of ~ 300,000 m3, the avalanche ran out over 1 km destroying a hiking trail before stopping on the Tschierva Glacier. We analyze the setting of this periglacial slope failure, combining geomechanical and cryosphere investigations to identify the primary factors contributing to the rock avalanche. An approach to slope stability assessment is presented that copes with existing data limitations in an inaccessible alpine terrain. Results from the analyses of morphology, geology, glaciation history, permafrost, hydrology, and meteorological data allowed preliminary inferences to be made regarding the influence of these factors on slope stability. Conceptual kinematic and numerical slope stability modeling critically analyzed the role of kinematic degrees of freedom, glacier retreat, and water infiltration from above the detachment zone. Results highlight the strong influence of discontinuity orientation with respect to the slope face, the role of a fault zone with increased joint density, and long-term progressive development of persistent discontinuities induced by glacier retreat and groundwater loading cycles in leading to the rock avalanche. The role of permafrost could not be clearly assessed, however observations and analyses indicate that permafrost had no dominant influence on the slope failure. Extraordinary precipitation prior to the event is suggested to have played a role in triggering the rock avalanche, especially in combination with observed superficial ice that could have sealed the rock face generating high water pressures. Our results emphasize the importance of analyzing multiple contributing factors when assessing alpine rock slope failures, with careful consideration of data limitations prevailing in such areas.

The 1989 Loma Prieta, California earthquake (moment magnitude, M=6.9) generated landslides throughout an area of about 15,000 km2 in central California. Most of these landslides occurred in an area of about 2000 km2 in the mountainous terrain around the epicenter, where they were mapped during field investigations immediately following the earthquake. The distribution of these landslides is investigated statistically, using regression and one-way analysis of variance (ANOVA) techniques to determine how the occurrence of landslides correlates with distance from the earthquake source, slope steepness, and rock type. The landslide concentration (defined as the number of landslide sources per unit area) has a strong inverse correlation with distance from the earthquake source and a strong positive correlation with slope steepness. The landslide concentration differs substantially among the various geologic units in the area. The differences correlate to some degree with differences in lithology and degree of induration, but this correlation is less clear, suggesting a more complex relationship between landslide occurrence and rock properties.

In 1992, a large magnitude earthquake (Ms = 7.3) hit the northern part of the Kyrgyz Tien Shan range where it triggered rockslides and many debris slides or flows. One of these mass movements occurred on the Chet–Korumdy ridge located in the Suusamyr Basin. It consists of a multi-rotational debris slump in its upper part that turned into a debris flow in its lower part. Involving arenitic material overlying silty clays, it has a volume of about 0.5 to 1.106 m3, a maximum thickness of 40 m and a run-out of 200 m. The field observations and measurements carried out on this slope suggest that local amplification effects could have contributed to the initiation of the seismic failure. To test this hypothesis in the lack of instrumental evidence of local ground-motion recordings, we conducted a sensitivity study of site effects based on a numerical analysis in the visco-elastic domain with a two-dimensional finite difference code. Varying the topography and the geology of the investigated slope, topographic site effects are found to be less important than geological site effects which are controlled by the contrast of impedance between the surface materials and the bedrock. The geometry of the low-velocity surface layer has also an influence on site effects, which is often difficult to be distinguished from pure topographic effects. Considering all modelling results, we conclude that site amplifications alone cannot have triggered the Suusamyr landslide during the 1992 earthquake. The static slope stability analyses done in previous studies revealed that the Suusamyr failure neither can have a purely static origin. Even if the water table is very high within the arenite layer, only a minor failure develops in the lower part of the slope. Therefore, we believe that the triggering of the Suusamyr landslide is a consequence of pore pressure build up in areas characterized by significant ground-motion amplifications.

The site amplification is estimated at five seismic stations of the Latur region using the horizontal to vertical spectral ratios of 33 aftershocks of the main Killari earthquake of September 29, 1993 (UTC). Spectral amplifications, ranging from a factor of 2–6 are found to vary with frequency at different places. Significant amplification is found at four sites within the Latur region, at Basavakalyan, Kasgi, Killari, and Mudgad Eakoji villages. Our results show a positive correlation between the site amplification and the damage pattern in area. The pattern and the nature of the site amplification estimated in the present study corroborates also with the analytical models and the borehole data indicating alternating layers of unconsolidated sediments and basaltic rocks.

Along the Rokko Mountains and Awaji Island, NE-SW to ENE-WSW oriented active faults exist in an en echelon arrangement. They constitute a portion of the Arima-Takatsuki tectonic line (ATTL) which extends from Kyoto, through Awaji Island, to the Median Tectonic Line. The ATTL is also correlated to lineaments linking the saddles and steep slopes of gravity (Bouguer) anomalies. The main shock of the 1995 magnitude 7.2 (M 7.2) Kobe earthquake was located at the mid-point of the ATTL. The main shock also created a 9-km long rupture on the ground surface along the NE-SW Nojima fault line in the northern part of Awaji Island. The earthquake aftershocks were distributed over a 40-km long zone along the central segment of the line. Surface ruptures and cracks accompanying the Kobe earthquake were scattered along a 40-km segment centrally located on the ATTL. Spatial correlation of the surface ruptures and aftershock distribution on the ATTL suggests that the Kobe earthquake was the result of a 40-km long rupture of the central segment of the ATTL.

Geomorphic effects observed in the Barranco (creek) de Arás basin are used to characterize the flood. Sediment features allow to qualify the flood as essentially a water flow. Using the critical section method, the peak flood discharge is estimated to be between 400 and 600 m3 s−1. Similar results were obtained using a paleohydraulic formula based on the size of the largest mobilized clasts. Using the rational method with available rainfall data, the discharge for a recurrence interval of 500 years is estimated to be between 150 and 200 m3 s−1. These results agree with predictions obtained using curves of peak discharge versus basin area based on regional data. Several trenches dug on the fan showed that the size of boulders mobilized by the event is larger that those left by previous floods at the same place. When the estimated peak flood discharge is related to the basin area, values between 20 and 30 m3 s−1 km−2 are obtained, demonstrating that the Barranco de Arás flood was most unusual.

On 28 October 1996, a landslide occurred on the Huashiban slope near the Liangjiaren Hydropower Station. The landslide was caused by a failure mechanism induced by an earthquake (Mw 7.0) at Lijiang on 3 February 1996, after which a tensile fracture zone formed on the Huashiban slope. The subsequent impact of several small earthquakes on the slope increased the tension crack connectivity and the tensile strength gradually decreased. Severe rainfall in the rainy season between May and October, especially in July and August, decreased the shear strength of the bottom sliding surface, resulting in a landslide on the Huashiban slope on 28 October 1996. The stability of the Huashiban slope is important for plant site selection. If a further landslide occurs on the slope, the Liangjiaren Hydropower Station project will be critically affected, so analysis of the current slope stability is extremely important. At present, the Huashiban slope can be divided into five subzones: (1) a landslide deposit zone; (2) a landslide zone; (3) a tensile fracture zone; (4) a crescent-shaped zone; and (5) an upper slide zone. Using historic landslide data for 28 October 1996, retrospective analysis of the landslide process revealed shear strength parameters for the Huashiban slope. The cohesion strength is 0.024 MPa, the friction angle at the bottom sliding surface is 14°, and the tension crack connectivity is 50%. These parameters were used to compute the stability of the Huashiban slope. Under the same conditions, the stability of the whole slope is better than that of part of the slope, and the overall stability is dominated by the tensile fracture zone. Under earthquake and rainfall conditions, it is possible that failure will occur in the tensile fracture zone. Thus, the Huashiban slope should be reinforced for construction and operation of the Liangjiaren Hydropower Station.

From 1997 to 1999, a huge number of slides, often turning into extremely rapid debris-earth flows, repeatedly affected the late Quaternary volcaniclastic deposits mantling the carbonate slopes of Campania region, Italy. The Sorrento Peninsula was the epicentral district of the 1997 regional slope-instability crisis. Some hundred shallow mass movements took place during January 1997 in this area. These were the last episode of a long series of slope failure events dating back to mid-18th century. Results from geological and geomorphologic surveys are presented. Landslide mechanism and triggering factors are analysed for the most important mass movement, which occurred during the January 9–11, 1997, regional event. On January 10, 1997, at about 8:15 PM, a rainfall-induced debris slide-debris flow occurred at Pozzano (province of Naples), mainly affecting the 79 AD pyroclastic products. Following a J-path, the landslide destroyed a private house and invaded the State Road no. 145. This event resulted in four deaths, 22 persons injured and road closure for about 2 months. There was less than 200 mm of rainfall in the 72-h period prior to the landslide, although intense precipitation had occurred during a preceding 4-month period. However, the slope failure event was not preceded by an extreme short-term antecedent rainfall, as already noticed in previous landslides of this type in Campania. Finally, following a preliminary geotechnical characterization of volcaniclastic soils, a slope-stability back analysis was carried out, which adopted the classical infinite slope scheme. This analysis gave further evidence of the role played by pore pressure in reducing the overall shear strength of pyroclastic soils.

During the July 9, 1997 Cariaco earthquake, the small town of Cariaco (located 10 km SW from the epicenter) and Cumaná (capital of the State of Sucre, located about 80 km west from the epicenter) were the most affected towns. The damage in Cariaco was essentially restricted to one-century-old dwellings in the downtown area, but also three rather modern buildings collapsed. A maximum intensity of VIII (MMI) was determined for the epicentral area with a clear orientation of the major damage along the strike of the El Pilar fault in east–west direction. The induced effects associated with this event are dominated by liquefaction phenomena and lateral spreading on soft sedimentary lowlands (along the shoreline of the Cariaco Gulf and riverbeds), as well as sliding at unstable slopes. Site studies were carried out in Cariaco, involving the geotechnical analysis of boreholes, seismic refraction studies and microtremor measurements to determine the characteristics of the Quaternary sediment fill in the area. From seismic refraction surveys, an interface separating sediments with S-wave velocity lower than 700 m/s from stiffer ones was located at 60–90 m in depth in the southern part of Cariaco. Further north it is supposed to exceed 90 m. Predominant periods of soil, derived from microtremor observations in Cariaco, vary between 0.6 and 1.2 s. The high percentage of damage in the center of Cariaco can be attributed to the poor quality of the dwellings combined with the presence of thick, poorly consolidated soils, and, in some particular cases, to liquefaction phenomena.

A flank collapse occurred at the Casita volcano in Nicaragua on 30 October 1998 during Hurricane Mitch. The collapse transformed into a disastrous lahar that completely buried two small towns 6 km downstream (killing about 2500 people), destroyed small settlements and disrupted the Pan American Highway.Based on the knowledge acquired during previous investigations with additional unpublished field data and observations, this study provides a comprehensive review of all previous studies (published and unpublished) on the 1998 Casita lahar and new insights into the initial flank collapse and the current stability. The knowledge on pre- and post-failure geometry, geology, lithology, tectonics and stratigraphy of the scarp area is improved and a summary of available geotechnical data is provided. Lithological characteristics that were significant in the initial flank collapse and failure mechanism were identified. The location of the failure surface was more precisely defined and the number and sequence of stages in the initial failure presented in previous studies have been confirmed. Slope stability analyses were carried out using limit-equilibrium methods.Geological interpretations, analysis of digital elevation models and geotechnical back-analyses confirmed that the flank collapse took place in three stages involving both the northern and southern areas of the scarp and occurred continuously during a time interval of seconds to a few minutes. In the first stage, failure initiated in a highly fractured and altered volcanic breccia in the northern area of the scarp which released a volume of 260 000 m3. The flow that developed from this failure removed colluvium deposits at the toe of the slope in the southern part in not more than 40 s. This rapid removal of the colluvium triggered a second stage which comprised 640 000 m3 and consisted in the failure of the southern part of the scarp by the sliding of a fractured volcanic breccia over a unit of clay-rich pyroclastic deposits. The third and final stage consisted in a failure of the remaining breccia and the overlying fractured lavas in the northern area and involved a volume of 690 000 m3.The fact that future events can affect some remaining settlements and the segment of the Pan American Highway, between Chinandega and León, motivated an analysis of the stability of the remaining slope using parameters calibrated in the back-analyses of the 1998 flank failure. The results indicated that the remaining slope is stable as long as groundwater levels in the northern and southern area are deeper than 65 m and 90 m respectively (relative to the points of maximum elevation on the analysed section) and the colluvium deposits in the southern area are not removed from the toe of the slope.

The Bovec basin, which is filled with glacial and fluvial sediments, has recently been struck by two strong earthquakes (1998 and 2004) which caused extensive damage (VII–VIII EMS-98). Strong site effects resulted in large variations in damage to buildings in the area, which could not be explained by the surface variations in Quaternary sediments. The microtremor horizontal-to-vertical-spectral ratio (HVRS) method was therefore applied to a 200 m dense grid of free-field measurements to assess the fundamental frequency of the sediments. Large variations in the sediment frequency (3–22 Hz) were obtained, with most of the observed values in the range 6–12 Hz. The observed frequencies cannot be related to the total thickness of Quaternary sediments (sand, gravel), but can be explained by the presence of conglomerate or lithified moraine at shallow depths. The results were compared also with the velocity structure derived from seismic refraction data. Microtremor measurements performed in several two and some three- and four-storey houses (masonry with RC floors), which prevail in the Bovec basin, have shown that the main building frequencies in the area are in the range 7–11 Hz. This indicates that damage to houses in both earthquakes in some parts of the basin was enhanced by site amplification and soil-structure resonance. Areas of possible soil-structure resonance were identified in the settlements Bovec–Brdo, Bovec–Mala vas, Čezsoča and Kal-Koritnica. Considerable changes in fundamental frequencies within short distances were established in the town of Bovec. Their values are as high as 22 Hz in the central part of the town, but diminish to 6–11 Hz in the adjacent Brdo and Mala vas districts. This is in agreement with the distribution of damage in both earthquakes, which was considerably higher in Brdo and Mala vas, although the houses in the central part of the town are older.

Heavy rainfall from 26 to 31 August 1998 triggered many landslides in Nishigo Village of southern Fukushima Prefecture, Japan. The Hiegaesi landslide, a long-runout landslide with travel angle of 11°, which occurred in loamy volcanic-ash/pumice layer and was deposited in a nearby rice paddy, was investigated. In an observation pit dug in the middle part of the landslide deposit, the sliding zone just above the deflected rice plants was observed, and it was confirmed that grain crushing occurred in the sliding zone. The triggering and sliding mechanisms of this landslide then were investigated by ring-shear tests in laboratory. For the triggering mechanism, one saturated naturally drained test (test A: torque-controlled test) and one saturated undrained test (test B: speed-controlled test) were conducted on the samples taken from the source area of the landslide. Even in the naturally drained test opening the upper drain valve of the shear box, a temporary liquefaction occurred. In the undrained test, excess pore-pressure was generated along with shearing, and “sliding-surface liquefaction” phenomenon was observed. The effective stress and shear resistance finally decreased to near zero. These results can explain the observed phenomenon of small friction resistance like a flow of liquid when the sliding mass slid out of the source area. For the sliding mechanism of the landslide in the rice paddy, saturated undrained test (test C: speed-controlled test) was performed on soil sample above the deflected rice plants. The apparent friction angle obtained in this test was 8°. In addition, the residual friction angle measured after test B and test C was the same value of 41°. Combining with the observation on the shear zone in the ring-shear box after test C, it is concluded that, during the sliding in rice paddy, the undrained shear strength of the soil layer itself mainly influenced the high mobility of the landslide, probably because the friction between rice plants and soils is greater than the undrained shear strength inside the soil mass.