Shallow Seismic Profiles Research Articles

SEDIMENTARY CHARACTERISTICS AND EVOLUTIONARY HISTORY OF THE DISTAL PART OF THE TIDAL SAND RIDGES IN THE SOUTH YELLOW SEA, OFF THE JIANGSU COAST

Abstrct:The core SYS-0702was recovered in the distal part of the tidal sand ridges in the South Yellow Sea,off the Jiangsu coast at a water depth of 32m,and the uppermost 18.64mof the core corresponds to the distal deposits of the sand ridges.Based on lithologic features,grain-size distribution pattern,AMS14C dating data and 210Pb data of the core sediments,in combination with the high-resolution shallow seismic profile passing through the core site,this paper reveals the sedimentary characteristics of the distal sand ridges,formation time,and the influence of the old Yellow River on the sedimentation of the sand ridges when the river discharged into the South Yellow Sea from the Jiangsu coast during 1128to 1855.The research results show that the distal sand ridges have been keeping active since about 2.5cal.kaBP,and their internal reflections dip to the east and northeast downlapping onto the basal boundary of the sand ridges.The dip angles of the internal foresets are generally less than 0.2°,mostly convex in shape and tangent to the erosive surface of the sand ridges.The core section of the distal sand ridges roughly coarsens upward,and in the sandy sediments the saltation load is higher than 80%and the suspension load less than 20%,with a good sorting.During 1128—1855when the old Yellow River entered the South Yellow Sea from the Jiangsu coast,sedimentation rates in the distal sand ridges increased significantly,from 0.28～0.77cm/a before the river shifting up to 1.10cm/a during the event.But the rate decreased to 0.23cm/a after the old Yellow River going back to the Bohai sea in 1855.According to previous researches,the paleo-Yangtze river estuary and Dongtai were supposed to be the vertex of the radial tidal sand ridges in about 8kaBP and 6kaBP,respectively.The progradation of the sand ridges to the core site from the paleo-estuary during 8～2.5kaBP and from Dongtai during 6～2.5kaBP are about 61.7m/a and 65.9m/a,respectively.The progradation further towards the sea from the distal ridge,however,has reduced to about 4m/a since about 2.5cal.kaBP,which is considered to be caused by weakened tidal action and declined sediment supply in the deeper water.

Spatial distribution and controlling factors of sedimentary bodies in Jiaozhou Bay and Adjacent Sea Areas, Qingdao

The distributions of thickness of unconsolidated Quaternary sedimentary layers in Jiaozhou Bay and Qingdao offshore area were studied by using 1079-km high-resolution shallow seismic profiles and drilling core data, and the factors controlling the Quaternary evolution were discussed. The results show that such thickness distributions resulted from the coactions of geologic structures and marine hydrodynamic conditions since the Holocene. The geologic structures controlled the slope deposit, proluvial and fluvial fillings since the late Pleistocene. Holocene marine hydrodynamics eroded away sediments at the bay mouth, and tides carried these eroded materials to the sides of the bay mouth and released them there, forming channel-ridge-alternating geomorphic features. During transgressive processes, the sea level rose rapidly, and insufficient sediment supply and tidal actions yielded the relict sediments in the east of Qingdao offshore area.

Bathymetry, shallow seismic profiling and sediment coring in Sermilik near Helheimgletscher, South-East Greenland

The Greenland ice sheet is one of the most significant contributors to the rising global sea level with a contribution of 0.5 mm per year (Rignot & Kanagaratnam 2006). Evidence is emerging that rising temperatures of subsurface ocean currents play a vital role in the recent acceleration of large fast flowing glaciers such as Jakobshavn Isbræ in West Greenland (Holland et al. 2008) and Helheimgletscher in South-East Greenland (Straneo et al. 2010). Important questions are whether these incursions of warmer water are part of a recurrent phenomenon and indeed exactly how they influence the glaciers. The Geocenter Denmark project SEDIMICE (Linking sediments with ice-sheet response and glacier retreat in Greenland) investigates past ice fluctuations in the Helheimgletscher region in South-East Greenland with regard to magnitude, possible causes and effects. One of the main tasks in this project is to analyse sedimentary deposits in the main fjord Sermilik (Fig. 1), which is influenced by the tidally affected Helheimgletscher that has a short floating tongue. By combining sediment studies with modern climate studies we aim to extrapolate meteorological data back in time.

Amino acid analysis of pre-Holocene foraminifera from Kriegers Flak in the Baltic Sea

The Geological Survey of Denmark and Greenland (GEUS) and the Institute of Baltic Sea Research in Warnemünde (formerly the Institut für Meereskunde of the DDR) have co-operated for more than two decades on unravelling the history of the south-western part of the Baltic Sea, mainly based on shallow seismic profiling, sampling of sediment cores and analyses of core samples (Jensen et al. 2002). Here we report on some results from one of the latest joint cruises with the German research vessel Maria S. Merian.

Estimation of near-surface geotechnical parameters using seismic measurements at the proposed KACST expansion site, Riyadh, KSA

Eight shallow seismic refraction profiles were conducted at the proposed KACST expansion site, northwest of Riyadh, to estimate the near-surface geotechnical parameters for construction purposes. Both compressional (P) and shear (S) waves were acquired, processed, and interpreted using “time-term” technique which is a combination of linear least squares and delay time analysis to invert the first arrivals for a velocity section. The most important geotechnical near-surface parameters such as stress ratio, Poisson’s ratio, material index, concentration index, N value, and foundation material-bearing capacity are calculated. The results of these seismic measurements were compared with the results of borehole report in the project area in terms of number of layers, the lithological content, thicknesses, and N values of rock quality designation. A good matching between the results was observed particularly at the sites of boreholes.

Shallow seismic reflection profiles over Permian strata affected by gypsum dissolution in NE England

Abstract In NE England, dissolution of Permian gypsum beds has caused subsidence at many localities. We report results from trial 2D profiles at five sites to evaluate the seismic reflection method as a tool for site investigation. At all five sites, the seismic profiles provided potentially useful information. Gypsum beds are imaged at depths in the range 30–70 m, and show evidence of rugose upper surfaces caused by contortion of the strata during rehydration and/or dissolution. The migrated data also reveal structures on the metre scale within the strata overlying the gypsum beds. These structures are interpreted as evidence of foundering as a result of the dissolution of gypsum. Many of them have no expression in the topography at the ground surface. Where gypsum is present at rockhead, the seismic results are generally poor, probably because of the weak contrast in acoustic impedance between hydrated gypsum and unconsolidated superficial deposits. Even this negative result is of value because it indicates that the subsurface may be unstable. As a general strategy, 2D profiles should be acquired at an early stage in site investigation and, if the results are satisfactory, a 3D seismic reflection survey can then be commissioned to provide a geological model of the site.

Postglacial sedimentary regime around northern Sylt, South-eastern North Sea, based on shallow seismic profiles

During the past 5 years the coastal zone offshore the northern part of the island of Sylt, has been investigated by sparker seismics and high-resolution subbottom profiling. The North Sea sector of the area is characterized by northward-directed sediment bypass as a result of strong long-shore (tidal) currents heading towards the Lister Tief. The southern part of Lister Tief is characterized by the presence of a major dune field with up to 8 m high compound dunes with wavelengths up to 350 m indicative of easterly-directed sediment transport (flood dominated). Further to the northwest, in the outer delta, a dune field suggesting westerly-directed sediment transport is identified. The two dune fields appear to be separated by minor indefinite bed forms. The northern part of the Lister Tief is characterised by the presence of a channel with a depth of 20-30 mbsf (meter below sea level) (30-40 msec TWT (Two Way Traveltime)) and displaying erosion towards the north. At the western termination of the channel the sediment transport pathway appears to be divided into two directions; one towards the outer delta, and one towards the sediment depocenter SW of Rømø. The Lister Ley is characterized by the presence of minor dunes, indicative of flood-current control, apart from a central zone where ebb-tide is responsible for northward-directed sediment transport. Sparker seismic and Chirp III data are consistent with processes having been persistent throughout the Holocene. Based on the study a model for the current direction and sedimentation regime is suggested.

Basin morphology and seismic stratigraphy of Lake Donggi Cona, north-eastern Tibetan Plateau, China

Basin morphology and depositional stratigraphy of Lake Donggi Cona on the north-eastern Tibetan Plateau has been studied with echo depth sounding and shallow seismic sub-bottom profiling. The basin is a pull-apart structure situated at the highly active Kunlun fault. It is characterised by a 92 m deep graben structure in the western part of the lake basin, and a shallow eastern lake part, the latter filled up with fluvial sediments from a large alluvial plain. Three prominent morphological levels were identified at 24 m, 39 m and 57 m below present lake level. They were partly created by basin subsidence, while the deposition of prograding delta sediments primarily formed the morphological steps. The inherited tectonic structures control on-going neotectonic activity, which seems to have only minor influence on present basin morphology. The basin is filled by sediments of at least 30 m thickness in the depocentre. Three major depositional units can be distinguished. In comparison with lithological changes in a sediment core, they give evidence of pronounced lake-level fluctuations and dramatic changes in lake volume. A numerical age datum of ∼19 cal ka BP at the base of the sediment core allows a tentative reconstruction of Late Glacial to Holocene lake development. A very low base level preceded a time of delta formation during higher lake level, i.e. ∼26 m lower than present during Last Glacial Maximum, creating a lake of only 18% of its present size. At the transition to Holocene the lake-level rose very quickly changing to a deep lake environment. Thus, transgression may have promoted maximum lake stands.

Danian cool-water coral reefs in southern Scandinavia localised over seafloor highs

Danian (Paleocene) reefs formed by ahermatypic scleractinian corals in relatively deep water are known in a few localities in southern Scandinavia. Reflection and shallow seismic profiles, and samples from drilling and scuba diving in bridge pier excavations in Øresund, the strait between Denmark and Sweden, for the first time allow interpretation of the factors that controlled the localisation of the reefs. After the mass-extinction at the K/T boundary, reef-building scleractinian corals were absent in the Boreal early Danian in the Danish basin. Faunal evolution after the biotic crisis and a rise in relative sea level are interpreted to have favoured formation of deep-water coral reefs in the Øresund region in mid-Danian time. The reefs were 6–20 m high and 20–200 m long on the seafloor and have a patchy distribution within a bryozoan mound-dominated setting in an area of about 10 km 2. They are composed of the framework-building ahermatypic scleractinian coral Dendrophyllia candelabrum with minor occurrences of bryozoans, echinoderms, gastropods and bivalves. It has been suspected that the reefs were located over contemporaneous seafloor highs but this notion was only based on comparison with similar modern deep-water reefs offshore Norway. The data from Øresund indicate that the Danian reef complex was formed over a late Maastrichtian palaeo-seafloor high, the Saltholm-Malmø High, supporting this interpretation and thus adding fundamentally to the understanding of the factors controlling the formation and localisation of these deep and cool-water coral reefs. Individual reefs were initiated and grew on eroded crests and steep south-dipping flanks of bryozoan mounds and were predominantly situated on the southern part of the high and are interpreted as growing towards NW-flowing bottom currents rich in particulate nutrients.

Shallow Crustal Structures and Buried Active Faults Revealed by Seismic Reflection Profiles in the Northwest of Beijing Plain

AbstractGeological and geophysical studies suggest that there are many buried active faults and buried Quaternary basins below the Beijing plain. In order to investigate shallow crustal structures, geometry of buried faults, fault activity and the relationship between shallow and deep structures in this area, a mid‐deep seismic reflection profile with 60‐fold and several shallow seismic reflection profiles across buried faults were completed in the northwestern region of Beijing plain in 2006. The results show that the depth of crystalline basement is about 3~6 km along the profile. The seismic reflection section shows a set of strong reflected phases with good lateral continuity above the crystalline basement, which should be the sedimentary rock systems of Cenozoic, Mesozoic, and Paleozoic era. Beneath the crystalline basement, there is a series of weak reflected events with poor continuity possibly representing the crystallized rock system with comparatively strong metamorphism or other non‐layered rock systems. The faults revealed by mid‐deep and shallow seismic reflection profiles present consistent upper‐lower relationship and have been significantly active since Quaternary, which play an important role in controlling the rift valley‐horst like basin and range tectonics as well as the thicknesses of Cenozoic strata in this region. The study results can further improve the understanding of the neotectonic activity in the Beijing area, and the used exploration method and technology can serve as a reference for the survey of deep‐shallow structures in other areas.

Geophysical investigation for shallow subsurface geotechnical problems of Mokattam area, Cairo, Egypt

Nine vertical electrical soundings of Schlumberger configuration were measured with AB/2 = 1–500 m. Manual and computerized interpretation were done to detect the subsurface stratigraphy of the study area. The results show that the subsurface section consists of alternated units of limestone, clay, marly limestone and dolomitic limestone and the thickness of clay unit ranged from 10 to 40 m. Nine dipole–dipole sections have also been constructed to give a clearer picture of the subsurface at the study area. The length of each dipole–dipole section is 235 m, with a electrode spacing ranging between 5 and 25 m. The Res2Dinv software was used for processing and interpretation of field data. The dipole–dipole sections at the upper plateau display high resistivity values at most parts of the plateau. Twelve shallow seismic refraction profiles are measured at selected locations for the dipole sections to define the interface between the fractured limestone and the upper surface of the clay layer. Each profile consists of 24 geophones with a geophone spacing of 2–3 m. Interpretation of seismic data indicates that the surface layer of the upper plateau consists of fractured limestone with a velocity range of 1.16–1.56 km/s and another layer of compacted clay with a velocity range of 1.38–1.88 km/s. Furthermore, the surface layer of the middle plateau consists of marl and marly limestone with a velocity about 2.1 km/s and its underlying layer consists of massive limestone with a velocity of 4.94 km/s.

Definition of soil characteristics and ground response at the northwestern part of the Gulf of Suez, Egypt

The area of interest represents the industrial part of Ain El-Sokhna new port, located in the northwestern part of the seismically active Gulf of Suez zone. The main objective of the current study is to estimate the site characteristics of the area of interest in terms of the fundamental frequency and the corresponding peak amplitude using noise measurements. The microtremor measurements were performed at 44 sites distributed over the study area in order to calculate the horizontal-to-vertical (H/V) spectral ratio. The standard spectral ratio (SSR) is used in addition to the numerical modelling of horizontal shear (SH) waves in soil at selected sites in order to have a comparison with the H/V spectral ratio. The required 1D soil models for the numerical modelling of SH-waves were derived from 220 P-wave shallow seismic refraction profiles in addition to 30 SH-wave profiles. Maps of the fundamental frequency (f0) and its corresponding H/V peak amplitude (A0) were provided, and a range of site conditions in the area were shown. The amplification factor results derived from the SSR technique are very similar to those derived from the H/V spectral ratio. In most cases, the H/V spectral ratio proved to be suitable for calculating the fundamental resonance. Results were found to be compatible with the surface geology of the area of interest.

Preliminary seismic hazard assessment, shallow seismic refraction and resistivity sounding studies for future urban planning at the Gebel Umm Baraqa area, Egypt

Gebel Umm Baraqa Fan, west Gulf of Aqaba, Sinai, is one of the most important tourism areas in Egypt. However, it is located on the active Dead Sea-Gulf of Aqaba Levant transform fault system. Geophysical studies, including fresh water aquifer delineation, shallow seismic refraction, soil characterization and preliminary seismic hazard assessment, were conducted to help in future city planning. A total of 11 vertical electrical soundings (1000–3000 m maximum AB/2) and three bore-holes were drilled in the site for the analysis of ground water, total dissolved solids (TDS) and fresh water aquifer properties. The interpretation of the one-dimensional (1D) inversion of the resistivity data delineated the fresh water aquifer and determined its hydro-geologic parameters. Eleven shallow seismic refraction profiles (125 m in length) have been collected and interpreted using the generalized reciprocal method, and the resulting depth–velocity models were verified using an advanced finite difference (FD) technique. Shallow seismic refraction effectively delineates two subsurface layers (VP ∼ 450 m s-1 and VP ∼ 1000 m s-1). A preliminary seismic hazard assessment in Umm Baraqa has produced an estimate of the probabilistic peak ground acceleration hazard in the study area. A recent and historical earthquake catalog for the time period 2200 BC to 2006 has been compiled for the area. New accurate seismic source zoning is considered because such details affect the degree of hazard in the city. The estimated amount of PGA reveals values ranging from 250 to 260 cm s-2 in the bedrock of the Umm Baraqa area during a 100 year interval (a suitable time window for buildings). Recommendations as to suitable types of buildings, considering the amount of shaking and the aquifer properties given in this study, are expected to be helpful for the Umm Baraqa area.

Engineering applications of seismic refraction method: A field example at Wadi Wardan, Northeast Gulf of Suez, Sinai, Egypt

Fourteen shallow seismic refraction profiles were conducted in Wadi Wardan, northeast Gulf of Suez, Sinai to delineate some of the shallow soil engineering characteristics for construction purposes. Both compressional (P) and shear (S) waves were acquired and interpreted using Generalized Reciprocal Method (GRM) then the resulted velocity-depth model is verified using a Finite Difference (FD) method to improve the final velocity-depth model. A number of engineering parameters such as Concentration Index, Material Index, Density Gradient, and Stress Ratios are calculated. The seismic velocity values, engineering, consolidation, and strength parameters show that the bedrock at northern and southern parts of the study area is characterized by less competent rock quality, while the central part is characterized by more competent rock quality. Hence, the central part is suggested for construction activities.

Late quaternary seismic stratigraphy and active faults of the Gulf of İzmit (NE Marmara Sea)

The Upper Quaternary seismic stratigraphy and active faults of the Gulf of Izmit were investigated by means of high-resolution shallow seismic profiling data in the source region of 1999 Izmit earthquake. High-resolution seismic reflection data correlated with borehole data indicate that the stratigraphy of Izmit Bay consists of three distinct depositional sequences formed in response to middle Pleistocene-Holocene sea-level changes. Reflector R, separating the pre-Holocene sequences (1 and 2) from the Holocene sequence (3), represents an erosional unconformity produced by the subaerial fluvial erosion of the continental shelves at the time of the last glacial maximum. Occasional, anomalous reflections (acoustic turbidity) observed within the Holocene sequence are interpreted as gas accumulations. The maximum thickness of the Holocene sediments is found to be about 25 m. The isopach map of Holocene sediment implies that the thickness of the Holocene decreases from the east towards the central and western basins of Izmit Bay. Two distinct fault systems are interpreted in Izmit Bay. The main fault system extending roughly in an E-W direction along the Gulf of Izmit is an active right lateral strike slip fault with a normal component. The secondary faults are normal faults striking in different directions and these are identified as being both active and inactive. In addition, prominent compressive features are identified in the seismic cross-sections of some profiles acquired to the east of Hersek Peninsula where the focal mechanisms of the aftershocks of the 1999 Izmit earthquake also reveal predominantly reverse faulting mechanisms, as identified by a local dense seismic network.

Xác định cấu trúc hình thái của châu thổ cửu long trên thềm lục địa ven bờ Cà Mau trên cơ sở tài liệu địa chấn nông phân giải cao

Determination of Cuu Long delta's morphology in the Ca Mau continental shelf area by applying high resolution shallow seismic profile data

Sedimentary features and processes in the Nazaré and Setúbal submarine canyons, west Iberian margin

Here we present part of the first complete sidescan sonar dataset of the Nazaré and Setúbal Canyons, west Iberian margin, which, in combination with multibeam bathymetry, shallow seismic profiles and precise piston coring of intra-canyon targets, are used to characterise the sedimentary dynamics of these deep-sea settings. The results show that Nazaré and Setúbal Canyons are highly complex environments. They display a range of sedimentary features and processes that reflect changes in downslope canyon geometry and a transition from erosive proximal to more depositional distal sections. The proximal (upper) sections of both canyons are characterised by a deeply incised, narrow, V-shaped thalweg, flanked by small gullies and terraces. Numerous small and localised intra-canyon landslides and rock avalanches occur in this section, triggered by instability processes that are preconditioned by the steep topography. Sequences of stacked thin-bedded, fine-grained turbidites occur locally on intra-canyon terraces, and are interpreted to be the result of small-volume, possibly river flood-generated turbidity currents that do not appear to reach the lower canyon. In Nazaré Canyon these turbidites are associated with abundant coalified organic fragments. Part of the upper section in Nazaré Canyon has acted as an apparent depocenter through the Holocene, with very high sedimentation rates related to enhanced nepheloid layer activity. Much larger slope failures are sourced from the shelf break and canyon head and are the result of occasional releases of large volumes of sediment, likely related to earthquake activity. These failures rapidly evolve into large-volume, high-energy, sand-rich turbidity currents that flush the entire canyon and dominantly deposit in the lower canyon and distal abyssal plains. This episodic turbidity current activity results in highly heterogeneous sediment distribution across the distal (lower) U-shaped floor of both canyons, with abundant erosional scours and depositional bedforms such as sediment waves. Our results highlight the complex interplay of sedimentary processes operating within major canyon systems, and have potential implications for efficient characterisation of hydrocarbon reservoirs, accurate spatial and temporal prediction of geohazards, and the distribution of benthic ecosystems in canyon environments.

An active fault in southern Taiwan detected using shallow seismics and ground penetration radar

We conducted high-resolution shallow seismic exploration and ground penetrating radar (GPR) exploration to assess the earthquake hazard along the speculated north-south striking Chaujou Fault that is along the mountain foot at the western edge of the Central Range in the southern tip of Taiwan. Shallow seismic and GPR survey lines are approximately eastwest oriented, perpendicular to the speculated fault. All these survey lines run across the speculated fault. A fault plane of an 50° east-dipping angle is interpreted using shallow seismic profiles and one GPR profile. The interpreted fault locations are consistent with the scarp on the earth?s surface with an error of only a few meters, indicating that the scarp marks the Chaujou Fault?s surface location. The GPR profile even shows fault planes within a few meters depth, indicating fault rupturing within the past centuries. As Taiwan has humid climate with high erosion and deposition rate, these evidences imply that the Chaujou Fault really exists and is an active fault, that it has been displaced within the past centuries, and that it is a potential earthquake-inducing mechanism.

Morphologic expression of Quaternary deformation in the northwestern foothills of the Ysyk-Köl basin, Tien Shan

The Tien Shan is one of the most active intracontinental mountain belts exhibiting numerous examples of Quaternary fault-related folding. To provide insight into the deformation of the Quaternary intermontane basins, the territory of the northwestern Ysyk-Kol region, where the growing Ak-Teke Anticline divided the piedmont apron of alluvial fans, is studied. It is shown that the Ak-Teke Hills are a sharply asymmetric anticline, which formed as a result of tectonic uplift and erosion related to motions along the South Ak-Teke Thrust Fault. The tectonic uplift gave rise to the local deviation of the drainage network in front of the northern limb of the fold. Optical (luminescent) dating suggests that the tectonic uplifting of the young anticline and the antecedent downcutting started 157 ka ago. The last upthrow of the high floodplain of the Toru-Aygyr River took place 1300 years ago. The structure of the South Ak-Teke Fault is examined by means of seismologic trenching and shallow seismic profiling across the fault. A laser tachymeter is applied to determine the vertical deformation of alluvial terraces in the Toru-Aygyr River valley at its intersection with the South Ak-Teke Fault. The rates of vertical deformation and an inferred number of strong earthquakes, which resulted in the upthrow of Quaternary river terraces of different ages, are calculated. The study territory is an example of changes in fluvial systems on growing folds in piedmont regions. As a result of shortening of the Earth’s crust in the mountainous belt owing to thrusting, new territories of previous sedimentation are involved in emergence. The tectonic activity migrates with time from the framing ridges toward the axial parts of intramontane basins.

Kinematic analysis of the Pakuashan fault tip fold, west central Taiwan: Shortening rate and age of folding inception

The Pakuashan anticline is an active fault tip fold that constitutes the frontal most zone of deformation along the western piedmont of the Taiwan Range. Assessing seismic hazards associated with this fold and its contribution to crustal shortening across central Taiwan requires some understanding of the fold structure and growth rate. To address this, we surveyed the geometry of several deformed strata and geomorphic surfaces, which recorded different cumulative amounts of shortening. These units were dated to ages ranging from ∼19 ka to ∼340 ka using optically stimulated luminescence (OSL). We collected shallow seismic profiles and used previously published seismic profiles to constrain the deep structure of the fold. These data show that the anticline has formed as a result of pure shear with subsequent limb rotation. The cumulative shortening along the direction of tectonic transport is estimated to be 1010 ± 160 m. An analytical fold model derived from a sandbox experiment is used to model growth strata. This yields a shortening rate of 16.3 ± 4.1 mm/yr and constrains the time of initiation of deformation to 62.2 ± 9.6 ka. In addition, the kinematic model of Pakuashan is used to assess how uplift, sedimentation, and erosion have sculpted the present‐day fold topography and morphology. The fold model, applied here for the first time on a natural example, appears promising in determining the kinematics of fault tip folds in similar contexts and therefore in assessing seismic hazards associated with blind thrust faults.