Long-term Fault Slip Rates Research Articles

The Chaochou active fault in southern Taiwan: Characteristics and geomorphological significance as a reverse fault

The Chaochou Fault in southern Taiwan has long been recognized as an active fault, but its exact location is still uncertain. In this study, we focused on the middle part of the Chaochou Fault, where a flight of fluvial terraces was deformed and preserved. High-resolution aerial photos were first used to observe and map these terraces and their vicinity, followed by comprehensive field investigations including RTK-GPS surveys for high-resolution topographic profiles of the deformed terraces and OSL sample collections for obtaining the ages of the terraces. A series of active faults, consisting of Fault A (“FA”), Fault B (“FB”), and Fault Bb (“FBb”), are collectively named the Chaochou Active Fault Zone. The fault zone is recognized and characterized by range-facing scarps, frontal flexural scarps, and discontinuous slopes deformed by FA, FB, and FBb, respectively. The repeated activities of FA were recorded by the incremental fault scarp heights from young to old terraces. Considering the highest fault scarp height of 65.9 ± 6 m observed in T1 and its OSL age of 35.3 ± 4.3 ka, the long-term fault slip rate of FA is about 2 mm/yr.

Active deformation in the Makran region using geological, geodetic and stress direction data sets

SUMMARY Neotectonic flow of the Makran subduction zone is estimated using a kinematic modelling technique based on iterated weighted least-squares that fits to all kinematic data from both geological and geophysical sources. The kinematic data set includes 87 geodetic velocities, 1962 principal stress directions, 90 fault traces, 56 geological heave rates and velocity boundary conditions. Low seismicity of western Makran compared to its eastern part, may indicate that either the subduction interface is currently locked, accumulating elastic strain or aseismic slip (creep) occurs along this part of the plate boundary. Therefore, we define two different models to evaluate the possibility of creep in the western Makran. Models define a locked subduction zone versus a steady creeping subduction for the western Makran. The locking depth of the subducting fault is also investigated, and a locking between 14 and 40–45 km depth provided the best consistency with geodetic observations. The 2 kinematic models provide long-term fault slip rates. The models estimated the shortening rate of 16.6–22.5 mm yr−1 and the strike-slip movement of 0.2–6.0 mm yr−1 for six segments along the subduction fault. The steady creeping subduction model predicts a 1–2 mm yr−1 lower shortening rate than the locked model for the Makran subduction fault (MSF). To verify the results, the estimated fault slip rates are compared to slip rates based on the geodetical and geological studies, which have not been used as model inputs. Our estimated rates fall within the range of geodetic rates and are even more consistent with geological rates than previous GPS-based estimates. In addition, the model provides the long-term velocity, and distributed permanent strain rates in the region. Based on the SHIFT hypotheses, long-term seismicity rates are computed for both models based on the estimated strain rate. These maps were compared with seismic catalogues. The estimated seismicity rate for the western part of Makran from the creeping subduction model is more compatible with the observation. The results of two deformation models lead us to a coupling ratio of ∼0.1 for the western MSF.

Terrazas marinas levantadas por deformación tectónica costera activa a lo largo del este de Argel: implicaciones para la convergencia de las placas africana y europea y las curvas del nivel del mar

Marine terraces are geological features composed of elevated, flat surfaces and steep slopes. The evolution of these terraces is largely influenced by active coastal tectonic deformation. This study investigates the uplift of marine terraces along the east coast of Algiers and its implications for the African and European plate convergence and sea-level curves. The objective of studying marine terraces in the Zemmouri area is to gain a deeper understanding of the evolution of the Earth's coastlines and how they have been shaped over time by natural processes such as sea level changes, tectonic activity, geomorphic parameters, and erosion. Marine terraces provide valuable information about the history of sea level changes and the rate of tectonic uplift or subsidence of the coastal areas. Secondary data sources, including qualitative information and high-resolution satellite imagery (SRTM 30 m and ALOS 12.5 m), were used to analyze the impact of sea level on marine terraces. Fieldwork using GPS and altimeter measurements at the site of the Mw=6.8 Zemmouri earthquake that occurred on May 21, 2003, revealed the presence of two well-preserved marine terraces, which serve as important markers for measuring the long-term fault slip rate. The analysis highlights the significance of active coastal tectonic deformation in shaping the coastlines. This research offers new insights into the ongoing processes of plate convergence and sea level change along the east coast of Algiers, contributing to our overall comprehension of coastal evolution and the potential for seismic hazards in the region.

Active crustal deformation model of the Fen–Wei rift zone, North China: Integration of geologic, geodetic, and stress direction datasets

The Fen–Wei rift zone (FWRZ) of North China is an important zone of active crustal deformation representing a transition from extrusion tectonics related to the Tibetan Plateau to subduction tectonics related to the potential far-field influence of the west Pacific plate. In this study, we determined the kinematic constraints of active crustal deformation in the FWRZ, which are fundamental for forecasting seismicity. NeoKinema, a kinematic finite-element model, was employed to estimate the long-term fault slip rates, distributed crustal deformation field, and on- and off-fault strain-rate fields in the FWRZ by fitting updated geological fault slip rate, geodetic GPS velocity, and principal compressive stress direction datasets. Our results show that the FWRZ is a characteristic low-strain kinematic setting, with most active faults exhibiting slip rates of less than 1 mm/a. The total sinistral shear rate from the southern Ordos block to the Qinling Mountains is approximately 1 mm/a, indicating limited tectonic extrusion along the EW-trending Qinling Mountains. Additionally, the central Shanxi rift exhibits prominent dextral shear of ∼0.5 mm/a that decreases toward its north and south ends, corresponding to crustal extension of 1.1–1.2 mm/a in the Datong and Yuncheng basins, respectively. However, this significant crustal extension cannot be solely attributed to terminal effects caused by dextral shear in the central Shanxi rift. A comparison between predicted seismicity and historical earthquake records reveals some remarkable seismic gaps, particularly in the Datong, Hancheng, and Yuncheng basins, indicating higher seismic potential in these locations. This study provides insights into the long-term crustal deformation processes and regional seismic potential of the FWRZ.

Asymmetric Interseismic Strain across the Western Altyn Tagh Fault from InSAR

As the northern boundary of the Tibetan Plateau, the long Altyn Tagh fault (ATF) controls the regional tectonic environment, and the study of its long-term fault slip rate is key to understanding the tectonic evolution and deformation of the northern Tibetan Plateau. In this paper, we measure the fault slip rate of the western segment of the ATF using InSAR observations between 2015 to 2020. The Multi-Temporal Interferometric InSAR analysis is applied to obtain the two-dimensional fault-parallel and vertical displacement fields. The spatially dense InSAR observations clearly illustrate the asymmetrical pattern of displacement fields across the fault. Constrained by our InSAR observations, the fault slip rate and locking depth of the western segment of the ATF are inverted using four different models in a Bayesian framework. The two-layer viscoelastic model incorporating lateral heterogeneity of rheology in the lower crust indicates that the fault slip rate of the western ATF is estimated to be 9.8 ± 1.1 mm/yr (at 83.8°E across the ATF) and 8.6 ± 1.1 mm/yr (at 85.1°E), respectively, and the locking depth is 15.8 ± 4.3 km and 14.8 ± 4.9 km. Our new estimates generally agree with the previous estimates of fault slip rate constrained by GPS observations. We conclude that the contrast between the thickness of the elastic layer and the shear modulus of the Tibetan plateau and the Tarim basin jointly contribute to the asymmetric interseismic strain accumulation on the ATF.

Improved Scaling Relationships for Seismic Moment and Average Slip of Strike-Slip Earthquakes Incorporating Fault-Slip Rate, Fault Width, and Stress Drop

ABSTRACT We develop a self-consistent scaling model relating magnitude Mw to surface rupture length (LE), surface displacement DE, and rupture width WE, for strike-slip faults. Knowledge of the long-term fault-slip rate SF improves magnitude estimates. Data are collected for 55 ground-rupturing strike-slip earthquakes that have geological estimates of LE, DE, and SF, and geophysical estimates of WE. We begin with the model of Anderson et al. (2017), which uses a closed form equation for the seismic moment of a surface-rupturing strike-slip fault of arbitrary aspect ratio and given stress drop, ΔτC. Using WE estimates does not improve Mw estimates. However, measurements of DE plus the relationship between ΔτC and surface slip provide an alternate approach to study WE. A grid of plausible stress drop and width pairs were used to predict displacement and earthquake magnitude. A likelihood function was computed from within the uncertainty ranges of the corresponding observed Mw and DE values. After maximizing likelihoods over earthquakes in length bins, we found the most likely values of WE for constant stress drop; these depend on the rupture length. The best-fitting model has the surprising form WE∝logLE—a gentle increase in width with rupture length. Residuals from this model are convincingly correlated to the fault-slip rate and also show a weak correlation with the crustal thickness. The resulting model thus supports a constant stress drop for ruptures of all lengths, consistent with teleseismic observation. The approach can be extended to test other observable factors that might improve the predictability of magnitude from a mapped fault for seismic hazard analyses.

Chlorine-36∕beryllium-10 burial dating of alluvial fan sediments associated with the Mission Creek strand of the San Andreas Fault system, California, USA

Abstract. We apply cosmogenic-nuclide burial dating using the 36Cl-in-K-feldspar∕10Be-in-quartz pair in fluvially transported granitoid clasts to determine the age of alluvial sediment displaced by the Mission Creek strand of the San Andreas Fault in southern California. Because the half-lives of 36Cl and 10Be are more different than those of the commonly used 26Al∕10Be pair, 36Cl∕10Be burial dating should be applicable to sediments in the range ca. 0.2–0.5 Ma, which is too young to be accurately dated with the 26Al∕10Be pair, and should be more precise for Middle and Late Pleistocene sediments in general. However, using the 36Cl∕10Be pair is more complex because the 36Cl∕10Be production ratio varies with the chemical composition of each sample. We use 36Cl∕10Be measurements in samples of granodiorite exposed at the surface at present to validate calculations of the 36Cl∕10Be production ratio in this lithology, and then we apply this information to determine the burial age of alluvial clasts of the same lithology. This particular field area presents the additional obstacle to burial dating (which is not specific to the 36Cl∕10Be pair, but would apply to any) that most buried alluvial clasts are derived from extremely rapidly eroding parts of the San Bernardino Mountains and have correspondingly extremely low nuclide concentrations, the majority of which most likely derive from nucleogenic (for 36Cl) and post-burial production. Although this precludes accurate burial dating of many clasts, data from surface and subsurface samples with higher nuclide concentrations, originating from lower-erosion-rate source areas, show that the age of upper Cabezon Formation alluvium is 260 ka. This is consistent with stratigraphic age constraints as well as independent estimates of long-term fault slip rates, and it highlights the potential usefulness of the 36Cl∕10Be pair for dating Upper and Middle Pleistocene clastic sediments.

Coseismic deformation and triggered landslides of the 2016 Mw 6.2 Amatrice earthquake in Italy

AbstractThe Central Apennines in Italy have had multiple moderate‐size but damaging shallow earthquakes. In this study, we optimize the fault geometry and invert for fault slip based on coseismic GPS and interferometric synthetic aperture radar (InSAR) for the 2016 Mw 6.2 Amatrice earthquake in Italy. Our results show that nearly all the fault slip occurred between 3 and 6 km depth but extends 20 km along strike. There was less than 4 cm static surface displacement at the town Amatrice where the most devastating damage occurred. Landslides triggered by earthquake ground shaking are not uncommon, but triggered landslides with submeter movement are challenging to be observed in the field. We find evidence of coseismically triggered deep‐seated landslides northwest and northeast of the epicenter where coseismic peak ground acceleration was estimated >0.5 g. By combining ascending and descending InSAR data, we are able to estimate the landslide thickness as at least 100 and 80 m near Monte Vettore and west of Castelluccio, respectively. The landslide near Monte Vettore terminates on the preexisting fault Monte Vettore Fault (MVEF) scarp. Our results imply that the long‐term fault slip rate of MVEF estimated based on paleoseismic studies could potentially have errors due to triggered landslides from nearby earthquake events.

Slip rate depth distribution for active faults in Central Italy using numerical models

Slip rate is a critical parameter for describing geologic and earthquake rates of known active faults. Although faults are inherently three-dimensional surfaces, the paucity of data allows for estimating only the slip rate at the ground surface and often only few values for an entire fault. These values are frequently assumed as proxies or as some average of slip rate at depth. Evidence of geological offset and single earthquake displacement, as well as mechanical requirements, show that fault slip varies significantly with depth. Slip rate should thus vary in a presumably similar way, yet these variations are rarely considered.In this work, we tackle the determination of slip rate depth distributions by applying the finite element method on a 2D vertical section, with stratification and faults, across the central Apennines, Italy. In a first step, we perform a plane-stress analysis assuming visco-elasto-plastic rheology and then search throughout a large range of values to minimize the RMS deviation between the model and the interseismic GPS velocities. Using a parametric analysis, we assess the accuracy of the best model and the sensitivity of its parameters. In a second step, we unlock the faults and let the model simulate 10kyr of deformation to estimate the fault long-term slip rates.The overall average slip rate at depth is approximately 1.1mm/yr for normal faults and 0.2mm/yr for thrust faults. A maximum value of about 2mm/yr characterizes the Avezzano fault that caused the 1915, Mw 7.0 earthquake. The slip rate depth distribution varies significantly from fault to fault and even between neighbouring faults, with maxima and minima located at different depths. We found uniform distributions only occasionally. We suggest that these findings can strongly influence the forecasting of cumulative earthquake depth distributions based on long-term fault slip rates.

Long term fault slip rates, distributed deformation rates and forecast of seismicity in the Iranian Plateau

In this paper, the long-term crustal flow of the Iranian Plateau is computed using a kinematic finite-element model (NeoKinema software). Based on the iterated weighted least squares method, the models are fitted to the newest data set of Iran including updated fault traces, geologic fault offset rates, geodetic benchmark velocities, principal stress directions, and velocity boundary conditions. We are successful to find the best kinematic model, in which geological slip rates, geodetic velocities, and interpolated stress directions are fitted at levels of 0.35, 1.0, and 1.0 datum standard deviation, respectively. The best fitted model, for the first time, provides long-term fault slip rates, velocity, and anelastic strain rate field in the Iranian Plateau from all available kinematic data. In order to verify the model, the estimates of fault slip rates are compared to slip rates from merely analyzing geodetic benchmark velocities or paleoseismological studies or published geological rates which have not been used in the model. Our estimated rates are all in the range of geodetic rates and are even more consistent with geological rates than previous GPS-based estimates. Using the selected model, long-term average seismicity maps and long-term moment rates are produced on the basis of the SHIFT hypothesis and previous global calibrations. Our kinematic model also provides a new constraint on ratio of seismic deformation to total deformation for different seismic zones of Iran. The resulting slip rates and the proposed seismic fraction of deformation provide the necessary input data for future time-dependent hazard studies in Iran. Moreover, spatial distribution and total number of strong (M > 6) and major (M > 7) earthquakes, which dominate the seismic hazard, are all compatible with the regional seismic catalog.

Relationships between fault geometry, slip rate variability and earthquake recurrence in extensional settings

Field observations and modelling indicate that elastic interaction between active faults can lead to variations in earthquake recurrence intervals measured on timescales of 102-104 yr. Fault geometry strongly influences the nature of the interaction between adjacent structures as it controls the spatial redistribution of stress when rupture occurs. In this paper, we use a previously published numerical model for elastic interaction between spontaneously growing faults to investigate the relationships between fault geometry, fault slip rate variations and the statistics of earthquake recurrence. These relationships develop and become systematic as a long-term consequence of stress redistribution in individual rupture events even though on short timescales earthquake activity appears to be stochastic. We characterize fault behaviour using the coefficient of variation (CV) of earthquake recurrence intervals and introduce a new measure, slip-rate variability (SRV) that takes into account the size and time ordering of slip events. CV generally increases when the strain is partitioned on more than one fault but the relationship between long-term fault slip rate (SRmean) and CV is poorly defined. In contrast, SRV increases systematically where faulting is more distributed and SRmean is lower. To first order, SRV is inversely proportional to SRmean. We also extract earthquake recurrence statistics and compare these to previously published probability density functions used in earthquake forecasting. The histograms of earthquake recurrence vary systematically as a function of fault geometry and are best characterized by a Weibull distribution with fitting parameters that vary from site to site along the fault array. We explain these phenomena in terms of a time-varying, geometrical control on stress loading of individual faults arising from the history of elastic interactions and compare our results with published data on SRV and earthquake recurrence along normal faults in New Zealand and in the Italian Apennines. Our results suggest that palaeoseismic data should be collected and analysed with structural geometry in mind and that information on SRV, CV and SRmean should be integrated with data from earthquake catalogues when evaluating seismic hazard.

Present-day crustal deformation along the Philippine Fault in Luzon, Philippines

The Philippine Fault results from the oblique convergence between the Philippine Sea Plate and the Sunda Block/Eurasian Plate. The fault exhibits left-lateral slip and transects the Philippine archipelago from the northwest corner of Luzon to the southeast end of Mindanao for about 1200km. To better understand fault slip behavior along the Philippine Fault, eight GPS surveys were conducted from 1996 to 2008 in the Luzon region. We combine the 12-yr survey-mode GPS data in the Luzon region and continuous GPS data in Taiwan, along with additional 15 International GNSS Service sites in the Asia-Pacific region, and use the GAMIT/GLOBK software to calculate site coordinates. We then estimate the site velocity from position time series by linear regression. Our results show that the horizontal velocities with respect to the Sunda Block gradually decrease from north to south along the western Luzon at rates of 85–49mm/yr in the west–northwest direction. This feature also implies a southward decrease of convergence rate along the Manila Trench. Significant internal deformation is observed near the Philippine Fault. Using a two dimensional elastic dislocation model and GPS velocities, we invert for fault geometries and back-slip rates of the Philippine Fault. The results indicate that the back-slip rates on the Philippine Fault increase from north to south, with the rates of 22, 37 and 40mm/yr, respectively, on the northern, central, and southern segments. The inferred long-term fault slip rates of 24–40mm/yr are very close to back-slip rates on locked fault segments, suggesting the Philippine Fault is fully locked. The stress tensor inversions from earthquake focal mechanisms indicate a transpressional regime in the Luzon area. Directions of σ1 axes and maximum horizontal compressive axes are between 90° and 110°, consistent with major tectonic features in the Philippines. The high angle between σ1 axes and the Philippine Fault in central Luzon suggests a weak fault zone possibly associated with fluid pressure.

Quaternary slip rates along the northeastern boundary of the Arabia-Eurasia collision zone (Kopeh Dagh Mountains, Northeast Iran)

The Kopeh Dagh is accommodating a large portion of the northward motion of central Iran with respect to Eurasia, involving a major right-lateral strike-slip fault system (Bakharden-Quchan). This fault system corresponds to the northeastern boundary of the Arabia-Eurasia collision, and can be considered as a lithospheric-scale tectonic feature. We present a well-constrained estimation of late Quaternary slip rates along two major strike-slip faults (the Baghan and Quchan faults) in this fault system, using in situ-produced 36Cl nuclide to date two offset alluvial fan surfaces. Combining detailed satellite image and digital topographic data analyses complemented with geomorphic fieldwork allows quantifying the cumulative offset values of 940 ± 100 and 360 ± 50 m of the fan surfaces along the Baghan and Quchan faults, respectively. A total of 12 carbonate boulders from the fan surfaces were collected and dated. This yields minimum age of two episodes of fan abandonment at 280 ± 16 (Baghan fault) and 83 ± 4 ka (Quchan fault). Age estimates and measured offsets of the fans are consistent with respective maximum long-term fault slip rates of 2.8 ± 1.0 and 4.3 ± 0.6 mm yr−1 for the Baghan and Quchan faults over the Middle-Late Pleistocene. Applying the slip rates to cumulative post-folding offsets along the Baghan and Quchan faults indicates that strike-slip motion within the Kopeh Dagh may have started ∼4 Ma. This constrains the timing of a major tectonic reorganization in the Kopeh Dagh, previously recorded through Arabia-Eurasia collision between 3 and 7 Ma. At the regional scale, the sum of total cumulative strike-slip offsets is about 35–40 km, which implies a total maximum slip rate of about 9 ± 2 mm yr−1 in the Central-Eastern Kopeh Dagh. This is resolved to average northward and westward slip rates of ∼8 and ∼4 mm yr−1, respectively, for the Western Kopeh Dagh with respect to Eurasia. Our results also suggests that the localized strike-slip faulting in the Central Kopeh Dagh can be considered as an intercontinental movement between northeast Iran and Eurasia, accommodating about 80 per cent of northward motion of central Iran with respect to Eurasia.

Long-term crustal movement caused by the Chiuchiungkeng Fault in southwestern Taiwan: Constraints from luminescence dating

To investigate the long-term crustal movement of the Chiuchiungkeng Fault (CCKF), a proposed active fault located in the southwestern coastal plain of Taiwan, two boreholes and dissected alluvial fans were selected for determination of depositional ages using luminescence. The standard single-aliquot regenerative-dose (SAR) protocol was applied to large aliquots of 90–150 μm quartz, with dose rates based mainly on high-resolution gamma spectrometry. The laboratory growth curves show a relatively large saturation value, which should allow reliable ages to be determined back to ∼150 ka. To test whether a significant sensitivity change occurred during natural measurement, the single-aliquot regeneration and added dose (SARA) method was also used to estimate equivalent dose; this showed that a sensitivity change of ∼4% may be introduced during the routine SAR procedure. The age results, unaffected by significant sensitivity change, all lie within 60–105 ka, and an 80 ka synchronous surface is used to evaluate the long-term fault behaviour. Based on the offset across the hanging wall and footwall, a long-term fault slip rate of ∼1.5 m ka −1 is estimated for the near-surface CCKF. From the anomalously steep slope of an 80 ka abandoned fan surface observed in the hanging wall, a long-term tilt rate of ∼42 m km −1 can be also derived based on the assumption that the initial slope is given by the modern river bed. Finally, it is deduced that the slip rate of the CCKF along its deeper part must be greater than ∼1.5 m ka −1 since the apparent rate is reduced by folding of the hanging wall.

Mechanical deformation model of the western United States instantaneous strain-rate field

Summary We present a relationship between the long-term fault slip rates and instantaneous velocities as measured by Global Positioning System (GPS) or other geodetic measurements over a short time span. The main elements are the secularly increasing forces imposed by the bounding Pacific and Juan de Fuca (JdF) plates on the North American plate, viscoelastic relaxation following selected large earthquakes occurring on faults that are locked during their respective interseismic periods, and steady slip along creeping portions of faults in the context of a thin-plate system. In detail, the physical model allows separate treatments of faults with known geometry and slip history, faults with incomplete characterization (i.e. fault geometry but not necessarily slip history is available), creeping faults, and dislocation sources distributed between the faults. We model the western United States strain-rate field, derived from 746 GPS velocity vectors, in order to test the importance of the relaxation from historic events and characterize the tectonic forces imposed by the bounding Pacific and JdF plates. Relaxation following major earthquakes (Mγ 8.0) strongly shapes the present strain-rate field over most of the plate boundary zone. Equally important are lateral shear transmitted across the Pacific–North America plate boundary along ∼1000 km of the continental shelf, downdip forces distributed along the Cascadia subduction interface, and distributed slip in the lower lithosphere. Post-earthquake relaxation and tectonic forcing, combined with distributed deep slip, constructively interfere near the western margin of the plate boundary zone, producing locally large strain accumulation along the San Andreas fault (SAF) system. However, they destructively interfere further into the plate interior, resulting in smaller and more variable strain accumulation patterns in the eastern part of the plate boundary zone. Much of the right-lateral strain accumulation along the SAF system is systematically underpredicted by models which account only for relaxation from known large earthquakes. This strongly suggests that in addition to viscoelastic-cycle effects, steady deep slip in the lower lithosphere is needed to explain the observed strain-rate field.

DEM-based morphometry of range-front escarpments in Attica, central Greece, and its relation to fault slip rates

In this paper, we apply current geological knowledge on faulting processes to digital processing of Digital Elevation Models (DEM) in order to pinpoint locations of active faults. The analysis is based on semiautomatic interpretation of 20- and 60-m DEM and their products (slope, shaded relief). In Northern–Eastern Attica, five normal fault segments were recognized on the 20-m DEM. All faults strike WNW–ESE. The faults are from west to east: Thriassion (THFS), Fili (FIFS), Afidnai (AFFS), Avlon (AVFS), and Pendeli (PEFS) and range in length from 10 to 20 km. All of them show geomorphic evidence for recent activity such as prominent range-front escarpments, V-shaped valleys, triangular facets, and tilted footwall areas. However, escarpment morphometry and footwall geometry reveal systematic differences between the “external” segments (PEFS, THFS, and AVFS) and the “internal” segments (AFFS and FIFS), which may be due to mechanical interaction among segments and/or preexisting topography. In addition, transects across all five escarpments show mean scarp slope angles of 22.1°±0.7° for both carbonate and metamorphic bedrock. The slope angle equation for the external segments shows asymptotic behaviour with increasing height. We make an empirical suggestion that slope angle is a function of the long-term fault slip rate which ranges between 0.13 and 0.3 mm/yr. The identified faults may rupture up to magnitude 6.4–6.6 earthquakes. The analysis of the 60-m DEM shows a difference in fault patterns between Western and Northern Attica, which is related to crustal rheology variations.

Integrated Seismic-Hazard Analysis of the Wasatch Front, Utah

We examined the combined effects of different sources that influence earthquake hazard of the populated Wasatch Front, Utah. We first evaluated the fault-stress interaction of the two largest historic earthquakes of the Intermountain Seismic Belt (ISB): the 1959 Hebgen Lake, Montana ( M s 7.5) and the 1983 Borah Peak, Idaho ( M s 7.3) earthquakes, which experienced multisegment, normal-faulting ruptures. Estimates of the static-stress change for these events revealed an increase in Coulomb failure stress in areas of extended aftershocks. These observations suggested that fault-stress analysis is applicable in evaluating the spatial distribution of aftershocks after large, normal-faulting earthquakes in the same extensional-stress regime of the eastern Basin-Range, including the Wasatch Front, Utah, which encompasses the 370-km-long Wasatch Fault and surrounding faults. On the basis of this result, we applied the modeling technique to the historically seismically quiescent Wasatch Fault and examined the relation between the pattern of stress change and the space-time distribution of paleoearthquakes. Ages and locations of the Wasatch Fault paleoseismic data imply 17 single-segment or 11 multisegment ruptures in the past 5.6 kyr. We prefer the multisegment model because almost all of the large, historical, normal-faulting earthquakes in the ISB were multisegment. Fitting the along-fault displacements by an analytic half-ellipse function provides a first-order distribution of the displacement over an entire rupture length and allows new estimates of fault slip rates and seismic moment. With these data, we also estimated the occurrence rate of M w > 6.6 Wasatch paleoearthquakes and showed that the rate was about three times higher than that inferred by the historical seismicity. This result, along with new information from a “megatrench” on the Wasatch Fault near Salt Lake City, that revealed no scarp-forming earthquakes between ca. 9.0 and 15.5 ka, suggests that large Wasatch Fault events in the past 5.6 kyr may be clustered. The recurrence rate of large earthquakes estimated by the geodetic-measured strain, on the other hand, is three to four times higher than that estimated by the long-term fault-slip rate. This difference, together with the observation of a low regional strain rate from historical seismicity, suggests that cumulative, aseismic deformation may be significant on the Wasatch Fault. We also examined a major consideration for along-strike segmentation, namely, how stress “contagion” could affect the probability of failure of adjacent faults. Including paleoearthquake-derived fault-slip rates, global positioning system derived geodetic moment rates, and the effect of stress contagion in the earthquake-hazard estimation for a specific location in the Salt Lake Valley revealed an increase in the annual frequency of peak ground acceleration ≥ 0.25 g by a factor of 1.4, 4.0, and 5.4, respectively, compared with that derived from historical seismicity only. Manuscript received 31 May 2001.

Late Holocene earthquake history of the central Altyn Tagh fault, China

The Altyn Tagh fault accommodates sinistral motion between the Tibetan Plateau and the Tarim block within the India-Eurasia collision zone. We used well-preserved evidence for surface-rupturing earthquakes to reconstruct the earthquake history for the central Altyn Tagh fault. We identified three geometric fault segments bounded by left steps and a bend. Geomorphic offsets indicate that the most recent event had maximum surface displacement of ;5.5 m in the west (38.58N, 90.08E), ;7 m in the central part of our study area, and ;4 m in the east (38.88N, 91.58E). The 14 C dates and trench logs of disrupted sediments indicate that these offsets occurred either in a single earthquake with a surface- rupture length .240 km dated as 680 6 108 yr B.P. or as two events. If there were two events, the westernmost recent event occurred 518 6 268 yr ago, whereas the eastern event occurred 650 6 80 yr ago and had a surface rupture length .155 km. We find two events in the past 0.8-2.2 k.y. in the west and two or three events in the east, yielding recurrence intervals of 0.7 6 0.4 k.y. and 1.1 6 0.3 k.y., respectively. These recurrence rates for major earthquakes are lower than expected if the long-term fault slip rate is .20 mm/yr. Explanations for the discrepancy include an overdue major earthquake, or accelerated deformation elsewhere in the India-Eurasia orogen.

Predicting the areas of crustal faulting in the San Francisco Bay region

We investigated the mechanism of the distinct deformation styles in fault-bounded crustal blocks in the San Francisco Bay region using a two-dimensional map-view model. The model features a nonlinear, elastoplastic (Coulomb) upper crust, and it includes all the major faults in the region. In addition to predicting accurately the long-term fault-slip rates and stress orientations in the region, the weak-fault model predicts areas of crustal faulting in close agreement with field observations. The most striking feature is the distinct spatial domains of the minimum horizontal principal stress (σ2), which are spatially associated with the distinct deformation styles in the region. Crustal faulting is most likely to occur in areas with the least σ2 value.