Pre-existing Geological Structures Research Articles

Research Catalog of Inland Seismicity in the Southern Korean Peninsula from 2012 to 2021 Using Deep Learning Techniques

Abstract A seismicity catalog spanning 2012–2021 is proposed for the inland and near-coastal areas of the southern Korean Peninsula (SKP). Using deep learning (DL) techniques combined with conventional methods, we developed an integrated framework for compiling a comprehensive seismicity catalog. The proposed DL-based framework allowed us to process, within a week, a large volume of data (spanning 10 yr) collected from more than 300 seismic stations. To improve the framework’s performance, a DL picker (i.e., EQTransformer) was retrained using the local datasets from the SKP combined with globally obtained data. A total of 66,858 events were detected by phase association using a machine learning algorithm, and a DL-based event discrimination model classified 29,371 events as natural earthquakes. We estimate source information more precisely using newly updated parameters for locations (a 1D velocity model and station corrections related to the location process) and magnitudes (a local magnitude equation) based on data derived from the application of the DL picker. Compared with a previous catalog, the proposed catalog exhibited improved statistical completeness, detecting 21,475 additional earthquakes. With the newly detected and located earthquakes, we observed the relative low seismicity in the northern SKP, and the linear trends of earthquakes striking northeast–southwest (NE–SW) and northwest–southeast (NW–SE) with a near-right angle between them. In particular, the NE–SW trend corresponds to boundaries of major tectonic regions in the SKP that potentially indicates the development of fault structures along the boundaries. The two predominant trends slightly differ to the suggested optimal fault orientations, implying more complex processes of preexisting geological structures. This study demonstrates the effectiveness of the DL-based framework in analyzing large datasets and detecting many microearthquakes in seismically inactive regions, which will advance our understanding of seismotectonics and seismic hazards in stable continental regions.

Polyphase rock slope failure controlled by pre-existing geological structures and rock bridges

Even after decades of intensive research, assessing rock slope stability remains a challenge. One reason for this is the spatial variability of rock bridges (RBs) related to non-persistent, pre-existing geological structures, especially as the detection of RBs is generally limited to the post-failure period. Thus, the identification and classification of RBs and their inclusion in numerical studies are demanding, yet essential, since even small quantities of RBs can be decisive for rock slope stability. In our study, we demonstrate how brittle RB failure and pre-existing geological structures control the mechanisms of a polyphase rock slope failure. Therefore, we present a case study in the Austrian Alps, where three rock falls with a failure volume of 30,000 m3 occurred in 2019. Based on detailed process reconstructions, high-resolution terrain models, and comprehensive geological and rock mechanical investigations, we derived high-quality input for our distinct element model (DEM). By applying asymmetric Voronoi tessellation in the DEM, we modelled the coalescence of pre-existing geological structures by brittle RB failure. As a result, we identified toppling as the predominant failure mechanism at the study site. Distinctive geological structures decisively affected the failure mechanism. However, the toppling failure was only reproducible by incorporating RBs in the DEM in their pre-failure position. Finally, we found that joint persistence, and consequently the presence of potential RBs, controls which initial rock fall failure mechanism was developed. In conclusion, we state that the initial toppling failure of the Hüttschlag rock falls is controlled by non-persistent geological structures in interplay with RBs.

Groundwater control for construction projects in chalk

Abstract Construction excavations and tunnels in chalk can encounter groundwater challenges, including high water flow rates, instability of excavations in weathered chalk and basal instability in overlying aquitards caused by high groundwater pressures in deeper chalk aquifers. In hydrogeological settings where the chalk has been exposed to periglacial weathering during the Quaternary Period the upper zones may be degraded to structureless chalk which can potentially be of very low hydraulic conductivity (putty chalk) or very high hydraulic conductivity (chalk bearings). In deeper, structured chalk groundwater flow tends to be concentrated along fissures associated with pre-existing geological structures such as bedding planes, flint beds or faults. A range of groundwater control strategies can be deployed, including open pumping, pre-drainage pumping, shallow and deep cut-off walls, ground treatment and, for tunnels and shafts, application of fluid counter pressures to exclude groundwater. The strategy appropriate to a given site must be selected based on a thorough understanding of the hydrogeological setting and chalk weathering profile. This requires a ground investigation of appropriate scope, using suitable techniques to characterize the chalk. Borehole geophysics can play a key role in identifying discrete zones of inflow.

Structural characterisation, internal deformation, and kinematics of an active deep-seated rock slide in a valley glacier retreat area

This study presents a multi-disciplinary approach for combining geological-geomorphological field mapping with different surface deformation monitoring techniques (i.e. extensometer, tachymetry, photogrammetry, airborne laser scanning (ALS), terrestrial laser scanning (TLS), uncrewed aerial vehicle (UAV)) to evaluate the impact of structural features on the formation process and deformation behaviour of the deep-seated Marzellkamm rock compound slide. The investigated rock slide is located in a high-alpine, glacier retreat environment in Northern Tyrol (Ötztal Alps, Austria) and formed in a well-foliated, fractured metamorphic rock mass. The total volume of the rock slide of approximately 13 Mm 3 was estimated based on geographic information system (GIS) analysis by comparing the surface topography and reconstructed geometry of the basal shear zone. In the upper part of the rock slide, annual mean velocities of up to 0.3 m/year were measured by a combined tachymetric-global navigation satellite system (GNSS) procedure. Based on multi-temporal terrain model analyses, the highest sliding velocities were obtained at the central slope foot area, reaching more than 1 m/year. Geological-geomorphological field mapping showed that the rock slide can be subdivided into two large rock slide systems with different geometry, formation age, and current activity. Furthermore, structural and geomorphological features (i.e. main and minor scarps, graben-structures, downhill- and uphill-facing scarps, tensile fractures, trenches), as well as surface deformation data, indicate the formation of six rock slide slabs. These slabs move downwards at different velocities and are separated by discrete shear zones, where slope displacement primarily accumulates. Large offsets along these shear zones indicate that internal slab deformation has only limited influence on the overall rock slide behaviour. At the Marzellkamm rock slide, pre-existing tectonic fault zones and planes were reactivated and used as weakness zones, playing a crucial role in the overall rock slide geometry and internal separation of the rock mass into slabs. Field survey and monitoring data suggest that a fully persistent, curved, non-circular basal shear zone has developed. Based on comprehensive analyses of mapping and deformation monitoring data, a geological-geometrical and kinematical model of the Marzellkamm rock slide was developed, providing a basis for site-specific hazard assessment and numerical modelling. • Two rock slide systems of six rock slide slabs of different deformation behaviour • Significant influence of pre-existing geological structures on rock slide formation • Deformation is controlled by the magnitude of shearing on separating shear zones. • Field survey indicates a fully persistent, curved, non-circular basal shear zone. • Two rock fall/avalanche events occurred at the lower part of the most active slabs.

The 31 March 2020 Mw 6.5 Stanley, Idaho, Earthquake: Seismotectonics and Preliminary Aftershock Analysis

AbstractWe report on the tectonic framework, seismicity, and aftershock monitoring efforts related to the 31 March 2020 Mw 6.5 Stanley, Idaho, earthquake. The earthquake sequence has produced both strike-slip and dip-slip motion, with minimal surface displacement or damage. The earthquake occurred at the northern limits of the Sawtooth normal fault. This fault separates the Centennial tectonic belt, a zone of active seismicity within the Basin and Range Province, from the Idaho batholith to the west and Challis volcanic belt to the north and east. We show evidence for a potential kinematic link between the northeast-dipping Sawtooth fault and the southwest-dipping Lost River fault. These opposing faults have recorded four of the five M≥6 Idaho earthquakes from the past 76 yr, including 1983 Mw 6.9 Borah Peak and the 1944 M 6.1 and 1945 M 6.0 Seafoam earthquakes. Geological and geophysical data point to possible fault boundary segments driven by pre-existing geologic structures. We suggest that the limits of both the Sawtooth and Lost River faults extend north beyond their mapped extent, are influenced by the relic trans-Challis fault system, and that seismicity within this region will likely continue for the coming years. Ongoing seismic monitoring efforts will lead to an improved understanding of ground shaking potential and active fault characteristics.

3D modeling of the influence of a splay fault on controlling the propagation of nonlinear stress waves induced by blast loading

Risk assessment of the propagation of stress waves induced by blast loading is an important aspect engineering design and practice. Pre-existing geological structures affect the propagation of stress waves in the rock masses/strata. For homogenous materials, the wave energy scatters in the forms of wave reflection, refraction, diffraction, and mode conversion. The coefficient of absorption and scattering losses can help to understand the structural (joints, cracks, and large-scale faults) and physical properties (strength, lithology) of the rock masses/strata. Therefore, to explore the influence of interacting faults on the propagation of blast loading-induced stress waves, six 3D high-resolution numerical models were developed and run on with COMSOL Multiphysics software to test the influence of the Young's Modulus (E) of two pre-existing and interacting faults on the attenuation, superposition, and reflection of nonlinear stress waves. The results show that propagation of the stress waves through the domain in between the two faults amplified the stress energy. Moreover, and the stress wave intensity increased noticeably as they entered and left the tips of the interacting faults. The optimal propagation path of the stress waves lie within the materials whose E differ the least from that of the surrounding rocks. The fluctuation of the displacement along fault A was relatively large when the E of fault A was small (R = 0.1). The maximum strain energy density (SED) dropped by 84.2% from 3396 J/m3 to 536 J/m3 from the beginning tip to the middle domain of fault A; and further dropped by 28.9% as the waves traveled from the middle part to the tail of fault A. he reduction in the SED is in agreement to the power function (y = 3409x−0.406, R = 0.9994). The time interval (△FP) for having obtained the different △SED between the neighboring crest and trough of the frequency is significantly different. △FP yielded approximately 0.002, 0.0022 and 0.0325 s at an equal propagation distance from the beginning tip to the tail of the faults, respectively. This indicates that the energy loss during the first stage is large, and the stress wave decays quickly. This paper contributes significantly to our contemporary understanding of the attenuation, superposition, and reflection effects of the blast-loading induced stress wave propagation between the interacting tips of adjacent faults.

Rift-related magmatism on magma-poor margins: Structural and potential-field analyses of the Mesozoic Notre Dame Bay intrusions, Newfoundland, Canada and their link to North Atlantic Opening

The magma-poor Newfoundland margin formed following lithospheric stretching, rifting and breakup resulting in the separation of the Grand Banks from Iberia, and northern Newfoundland from Ireland. Mesozoic igneous rocks contemporaneous with rifting have been documented on- and offshore Newfoundland, with later offshore magmatism occurring post-breakup. The Notre Dame Bay Magmatic Province (NDBMP) is a Jurassic-Cretaceous, potentially rift-related occurrence of igneous rocks onshore Newfoundland that includes the gabbroic Budgell Harbour Stock (BHS) and marginal lamprophyre dykes, in addition to the nearby, potentially related, Dildo Pond Intrusion (DPI) and Leading Tickles Stock (LTS). Although the BHS is penetrated by wells, it is poorly exposed at surface, and the deeper structure remained unknown. Here, results of field-based mapping combined with inversion of Full Tensor Gradiometry (FTG) and aeromagnetic data covering the BHS are presented. Although the FTG data are of a higher resolution than the magnetic data, comparable results were obtained from these independent datasets. We found that the dykes form clusters at the ends of density and susceptibility anomalies, interpreted to be lobe-like magmatic conduits. Furthermore, potential-field analysis has revealed the irregular geometry and southward-dipping nature of the BHS, in addition to the presence of multiple, near surface anomalies that may correspond to dyke clusters or other igneous bodies. Structural analysis indicates that emplacement of the dykes was controlled by pre-existing geological structures and that they have probably been deformed post-intrusion, potentially via the reactivation of pre-rift faults. The potential-field investigations and observations of dyke margin lineations from field-based structural analysis indicate that multiple dyke sources and bodies similar to the BHS may also exist. Finally, this study demonstrates that magma-poor margins, such as the Newfoundland margin, may host significant volumes of extension-related magmatic rocks, challenging the concept of a distinction between magma-rich and magma-poor rifting.

Five key lessons gained from induced seismicity monitoring in western Canada

In response to induced seismicity observed in western Canada, existing public networks have been densified and a number of private networks have been deployed to closely monitor induced earthquakes associated with oil and gas operations in the region. Over the past three years, we generated an unprecedented volume of seismic data from monitoring induced seismicity for some of the most active operators in western Canada. This rich data set can be used to understand preexisting geologic structures, the activation mechanisms and probabilities, and seismological attributes of the resultant ground motions. Acknowledging that the primary goal of private networks is assisting operators in making operational decisions, these insights can play key roles in improving the accuracy of event magnitudes, ground-motion predictions, and hazard estimates, which successively can be used for developing effective risk management strategies.

Comprehensive evaluation of the stability of the left-bank slope at the Baihetan hydropower station in southwest China

The stability of the left-bank slope is a crucial geological engineering problem at the Baihetan hydropower station, China. Due to continuous excavations on the rock slope, different regions of the surrounding rock mass undergo varying degrees of unloading deformation. It is important to assess the stability of the rock slope from a macroscopic viewpoint by investigating its deformation characteristics and mechanisms. Therefore, in this work, microseismic (MS) monitoring was first employed to detect the progressive rock mass damage in the rock slope subjected to excavation, including the initiation, propagation, coalescence, and interaction of rock microfractures. Numerical modeling was subsequently performed to understand the deformation and failure mechanism of the rock slope. Moreover, traditional surveying approaches (i.e., multiple-point extensometers and inclinometers) and field observations were also used to analyze the deformation and failure characteristics of the rock slope. The MS monitoring results showed that spatiotemporal regularities in the evolution of seismic source locations were indicators of deformation failure and potential sliding surfaces. MS event clustering can be used to delineate activated pre-existing geological structures (i.e., LS331 and LS337). The simulation results show that the deformation and failure characteristics of the rock slope are mainly controlled by pre-existing weak structural planes (i.e., the intraformational faulted zones LS3319, LS331, and LS337 and fault F17). These results agree well with the results of geological data and conventional monitoring data. Our study reveals that an integrated approach combining MS monitoring, numerical modeling, traditional surveying, and field observations leads to a better understanding of the behavior of the rock slope under the influence of excavation as well as greater control of the working faces, ensuring safety under complex geological and excavation conditions.

Geological structures control on earthquake ruptures: The Mw7.7, 2013, Balochistan earthquake, Pakistan

AbstractThe 2013 Mw7.7 Balochistan earthquake, Pakistan, ruptured the Hoshab fault. Left‐lateral motion dominated the deformation pattern, although significant vertical motion is found along the southern part of the rupture. Correlation of high‐resolution (2.5 m) optical satellite images provided horizontal displacement along the entire rupture. In parallel, we mapped the ground rupture geometry at 1:500 scale. We show that the azimuth of the ground rupture distributes mainly between two directions, N216° and N259°. The direction N216° matches the direction of preexisting geologic structures resulting from penetrative deformation caused by the nearby Makran subduction. Hence, during a significant part of its rupture, the 2013 Balochistan rupture kept switching between a long‐term fault front and secondary branches, in which existence and direction are related to the compressional context. It shows unambiguous direct interactions between different preexisting geologic structures, regional stress, and dynamic‐rupture stress, which controlled earthquake propagation path.

Microseismic monitoring and stability evaluation for the large scale underground caverns at the Houziyan hydropower station in Southwest China

The stability of underground caverns and the excavatability of rock masses are important for geotechnical engineering practices during the design and construction stages. The risks associated with underground caverns at the Houziyan hydropower station in Southwest China are growing due to excavation-induced unloading. To assess the instability of underground caverns and resolve the complex subsurface conditions of the highly fractured rock mass, a high-resolution microseismic monitoring system was established in deep underground caverns. This system was used to determine the relationship between the measured microseismic activities and the excavation damage zones of the surrounding rock mass. The excavation damage zones and potential risk regions in the underground caverns were identified by analyzing the tempo-spatial distribution of microseismic activities. In addition, the correlation between microseismic activities and pre-existing geological structures was determined, and traditional monitoring results were analyzed. To validate the correlation between seismicity and the excavation damaged zones of the underground caverns, a numerical model was employed to further evaluate the deformation and stability of the surrounding rock mass. The monitoring results demonstrate that microseismic events mainly occurred in high-stress-concentration regions, corresponding with the results obtained from the numerical analysis. Therefore, this comprehensive method, which incorporates microseismic monitoring, numerical analysis, traditional monitoring and field observations, is promising for predicting the deformation and instability of surrounding rock masses in the underground caverns subjected to excavation.

Thirty years of precise gravity measurements at Mt. Vesuvius: an approach to detect underground mass movements

<p>Since 1982, high precision gravity measurements have been routinely carried out on Mt. Vesuvius. The gravity network consists of selected sites most of them coinciding with, or very close to, leveling benchmarks to remove the effect of the elevation changes from gravity variations. The reference station is located in Napoli, outside the volcanic area. Since 1986, absolute gravity measurements have been periodically made on a station on Mt. Vesuvius, close to a permanent gravity station established in 1987, and at the reference in Napoli. The results of the gravity measurements since 1982 are presented and discussed. Moderate gravity changes on short-time were generally observed. On long-term significant gravity changes occurred and the overall fields displayed well defined patterns. Several periods of evolution may be recognized. Gravity changes revealed by the relative surveys have been confirmed by repeated absolute measurements, which also confirmed the long-term stability of the reference site. The gravity changes over the recognized periods appear correlated with the seismic crises and with changes of the tidal parameters obtained by continuous measurements. The absence of significant ground deformation implies masses redistribution, essentially density changes without significant volume changes, such as fluids migration at the depth of the seismic foci, i.e. at a few kilometers. The fluid migration may occur through pre-existing geological structures, as also suggested by hydrological studies, and/or through new fractures generated by seismic activity. This interpretation is supported by the analyses of the spatial gravity changes overlapping the most significant and recent seismic crises.</p>

Co-seismic Riedel shear structures produced by the 2010 M w 6.9 Yushu earthquake, central Tibetan Plateau, China

Riedel shear structures are often developed within strike-slip fault zones. This study focuses on the Riedel shear structures within the co-seismic strike-slip surface rupture zone produced by the 2010 M w 6.9 Yushu earthquake in the central Tibetan Plateau, China. Field surveys and structural analysis of the surface ruptures reveal that the co-seismic Riedel shear structures are characterized by (i) tension cracks (T fractures); (ii) compression structures, mainly mole tracks (P shears); and (iii) discrete shear faults (R and Y shears) developed in unconsolidated alluvial deposits along the pre-existing left-lateral strike-slip Ganzi–Yushu Fault Zone. The T fractures generally show a right-stepping en echelon pattern. In contrast, the P and R shears indicate a left-stepping en echelon pattern. The Riedel shears are inclined to the general trend of the co-seismic surface rupture zone at mean counterclockwise angles of 41° (T fractures) and 21° (R shears), and a mean clockwise angle of 35° for the P shears. The present results show that (i) the co-seismic Riedel shear structures indicate a left-lateral strike-slip sense for the seismogenic fault, and (ii) the T fractures, and P and R shears are the primary Riedel shear structures formed during the early stages of the evolution of a strike-slip fault constrained by pre-existing geologic structures.

Deformation characteristics of co-seismic surface ruptures produced by the 1850 M 7.5 Xichang earthquake on the eastern margin of the Tibetan Plateau

The eastern margin of the Tibetan Plateau is bounded by a N–S-trending active fault zone, including the Zemuhe Fault, which is an important tectonic boundary in studies of continental dynamics and deformation modes. Here, we present the kinematic mechanism and deformation features of co-seismic surface ruptures caused by a large historic earthquake that occurred upon the Zemuhe Fault. Field investigations and interpretations of aerial photographs reveal a NNW–SSE-trending surface rupture zone, less than 100 m wide, defined by en echelon fractures, mole tracks, and fault scarps over a length of 60 km. Trench excavations and radiocarbon age data confirm that the surface rupture zone was produced by the 1850 M 7.5 Xichang earthquake. Gullies preserved on the youngest alluvial fans and the lowest terrace risers in the area record systematic left-lateral offsets of 1.4–6.0 m and vertical offsets of 0.2–0.65 m resulting from the 1850 earthquake. Topographic and geologic evidence reveals that the spatial distribution of the 1850 co-seismic surface rupture was controlled by pre-existing geological structures of the Zemuhe Fault. The present results confirm that the Zemuhe Fault plays an important role as a major strike-slip fault in the southeastward motion of the southeastern Tibetan Plateau, and that the southern segment of the Zemuhe Fault has high seismic potential.

Links between topography, erosion, rheological heterogeneity, and deformation in contractional settings: Insights from the central Andes

Orogenic structure in the central Andes (15°S–34°S) systematically varies with mean annual precipitation, suggesting that erosional processes may be coupled to tectonic processes. We explore the range of possible interactions between deformation, erosional processes, changing geodynamic conditions, and preexisting geologic structures to assess the relative importance of each. Our review and synthesis indicates that the pre-orogenic geologic history and changing geodynamic conditions leave a lasting imprint on the modern structure of the mountain belt. In contrast, changes in precipitation that result from regional-scale atmospheric and oceanic circulation interact with local topography to control the climatic zoning across the central Andes. Changes in erosion, which likely correspond to changes in climate, rock uplift rate, and the exposure of different lithologies, may in turn affect the tectonic development of the region at the scale of each morphotectonic province. In this view, geologic and geodynamic factors set the stage for the nature and strength of coupling between erosional processes, tectonic deformation, and mountain belt structure in the different parts of the central Andes.

Controls on the geometry of a Holocene crater row: a field study from southwest Iceland

The Reykjanes Peninsula rift zone in southwest Iceland is a highly oblique segment of the Mid-Atlantic ridge system which accommodates NW–SE extension during rifting episodes that consist of eruptions and normal faulting, and E–W left-lateral shear strain along strike-slip faults during longer amagmatic periods. Dominant tectonic features on the peninsula are a series of generally NE-striking, sub-parallel eruptive fissures and normal faults, and a cross-cutting zone of N–S striking, right-lateral strike-slip faults. The last series of rifting episodes ended in 1227, and a proposed 1,000 year cyclicity predicts the start of a new series of eruptions within the next 200 years. In order to more accurately characterize the nature of eruptions on the Reykjanes Peninsula, we present a new, spatially accurate map of the ∼2,350 year old Sundhnukur crater row in the western part of the peninsula, which was examined in detail in order to determine the structural controls on crater row geometry and to understand the interactions that take place between eruptive fissures and pre-existing geological structures. Volcanism is sometimes influenced by small perturbations in the surroundings such as gravitational loading, topography, changes in crustal properties or the presence of fault zones, but there are few field examples showing how fissures are influenced by these pre-existing structures. We identify 27 fissure segments, ranging in strike from 006° to 053°, with varying spacing and overlap between them. Significant local variability in strike and stepping sense of segments occurs in proximity to topographic highs as well as within zones of faulting that pre-date the crater row. Strike also varies at the northern end of the crater row as it approaches a region of older crust at the rift margin. Our data support numerical and laboratory modeling results which show that local topography, pre-existing fractures and crustal properties influence the path taken by magma on its way through the shallow crust.

Structural, tectonic and glaciological controls on the evolution of fjord landscapes

The fjord landscape of South America, stretching ~ 1500 km between Golfo Corcovado (~ 43°S) and Tierra del Fuego (~ 56°S), is the largest continuous fjord landscape on Earth. This paper presents the results of new structural geological and geomorphological mapping of this landscape using optical satellite images and digital elevation models. First-order geological structures are represented by strike-slip faults forming lineaments up to hundreds of kilometres long. The strike-slip faulting has been active since Late Cretaceous times and is responsible for the presence of a conspicuous structural cleavage visible as lineaments up to ~ 10 km long. A detailed analysis of these second-order lineaments from digital image data was carried out in three sectors. In Sector 1, located northwest of the North Patagonian Icefield, there are three distinct mean orientations, characterized by a main nearly orogen-parallel orientation (az. ~ 145°) and two orogen-oblique secondary orientations (az. ~ 20° and az. ~ 65°). In Sector 2, located west of the South Patagonian Icefield, there are also three separate mean orientations, with most of the lineaments concentrated between azimuths 0° and 80° (mean at ~ 36°); and two other orogen-oblique means at azimuth ~ 122° and ~ 163°. In Sector 3, around the Cordillera Darwin, there is a single main orogen-parallel mean at ~ 100–115°. In all three sectors, mapped fjord orientations bear a striking similarity to the structural data, with fjords orientated preferentially in the same direction as structural lineaments. We infer that successive glaciations followed the same ice-discharge routes, widening and deepening pre-existing geological structures at the expense of the surrounding terrain to create the fjord landscape. This study has broader implications for ice sheet reconstructions and landscape evolution beneath ice sheets because we demonstrate that the primary control on fjord development in glaciated areas is geological and not glaciological.

Rupture Segmentation and Process of the 2001 Mw 7.8 Central Kunlun, China, Earthquake

Field investigations allow us to constrain the coseismic surface rupture zone of ∼400 km with a strike slip up to 16.3 m associated with the 2001 M w 7.8 Central Kunlun earthquake that occurred along one of the western segments of the Kunlun fault, northern Tibet. The rupture zone may be divided into four segments based on the geological structures, tectonic landform features, spatial displacement distributions obtained from field observations, and analysis of teleseismic waveforms. The deformational characteristics of the surface ruptures and focal mechanism solutions reveal that the earthquake had a nearly pure strike-slip mechanism. The inversion results from seismic data show that the rupture started from the west near the epicentral area bilaterally and rapidly extended to the east in an unilateral manner for 380 km and that the largest slip region was limited in the subsegment 150-280 km east of the epicenter, consistent with field observations. The average stress drop is estimated to be 7 MPa in the area where the largest displacements occurred, a value typical of intraplate earthquakes. The geologic evidence and the inversion results from seismic data clearly show that temporal and spatial displacement distributions and the rupture process are restricted by the pre-existing geological structures of the Kunlun fault.

Mechanisms of Earth Fissuring Caused by Groundwater Withdrawal

Earth fissures associated with groundwater withdrawal are complex products of both human activities and natural forces, and they occur in definable geological environments. In this paper, the authors first characterize the driving forces for earth fissures caused by groundwater withdrawal. Then, the effects of various factors, such as stresses and pre-existing geological structures, are examined using conceptual models. Numerical results show that the fissuring process is controlled not only by the induced movement at depth and pre-existing structures but also by the in situ stress field. In addition, the degree to which aquifer movement and pre-existing structures actually trigger fissuring depends greatly on the in situ stress field. The authors conclude that earth fissuring related to groundwater withdrawal is a multi-step process that is influenced by a multiplicity of factors, one being the aquifer movement. With groundwater withdrawal, hydraulic and gravitational forces tend to drive aquifer material to deform both horizontally and vertically. Cumulative deformation or strain results in movement. In turn, this aquifer movement results in differential displacements at depth along planes of weakness, such as pre-existing faults and material interfaces. This differential movement (both horizontal and vertical) then generates tensile zones at depth. Once formed, such a tensile zone may migrate upward, form a crack (fail) where the vadose zone is brittle, and eventually express itself as an earth fissure at the land surface in arid or semi-arid regions. In humid regions, the same tensile zone (lateral stretching) at depth will simply express itself as a transient sub-vertical plane of enhanced porosity within and crossing the vadose zone.