Recurrence Interval Of Earthquakes Research Articles

Geomorphological Evidence of Active Faulting in Low Seismicity Regions—Examples From the Valley of Gobi Lakes, Southern Mongolia

Active intraplate deformation as a far-field effect of the India-Eurasia convergence has led to four Mw∼8 earthquakes in western and southern Mongolia in the past century. Palaeoseismological and morphotectonic studies have shown that these earthquakes are characteristic events along transpressive fault systems with cumulative offsets. The tectonically active Gobi Altai and Hangay mountains are separated by the seismically quiescent Valley of Gobi Lakes, which consists of major perennial rivers draining into endorheic lakes. Despite the scarcity of recorded earthquakes, Quaternary deposits in the Valley of Gobi Lakes are crosscut by multiple fault scarps with significant, landscape-altering displacements. To assess past earthquakes and the potential seismic hazard of this area, we apply remote sensing, tectono-morphometric techniques and cosmogenic nuclide dating to estimate the amount of deformation the faults in the Valley of Gobi Lakes are accommodating, and to determine the effect of these faults on local landscape evolution. The Tuyn Gol (gol = Mongolian for river) is crosscut by four E-W to NE-SW trending fault scarps that display variable fault kinematics due to scarp orientation differences relative to a stable NE-SW principle stress direction. Mapping of the >40–90 km long Valley of Gobi Lake faults shows that they can accommodate M ∼ 7 earthquakes. Offset measurements of the Tuyn Gol deposits allow Middle Pleistocene to modern vertical slip rate estimates and M ∼ 7 earthquake recurrence intervals of 0.012 ± 0.007–0.13 ± 0.07 mm/yr and 5.24 ± 2.61–81.57 ± 46.05 kyr, respectively. Cumulative vertical displacement amounts to 0.27 ± 0.08 mm/yr, which is similar to that of major tectonic structures such as the Bogd fault in the Gobi Altai. This implies that the total active deformation accommodated by southern Mongolian faults may be larger than previously expected and distributed across more faults between the Hangay and the (Gobi) Altai mountain ranges. Geomorphological observations and surface exposure dating indicate that the Tuyn Gol drainage system developed over four to five 100 kyr climate cycles, during which active deformation played an important role in drainage reorganization. Our results demonstrate the dominant role of tectonics on local landscape dynamics, indicating the importance of studying regional geomorphology to adequately estimate the earthquake potential of faults that were considered inactive.

Regional probabilistic tsunami hazard assessment associated with active faults along the eastern margin of the Sea of Japan

We analyze the regional tsunami hazard along the Sea of Japan coast associated with 60 active faults beneath the eastern margin of the Sea of Japan. We generate stochastic slip distribution using a Monte Carlo approach at each fault, and the total number of required earthquake samples is determined based on convergence analysis of maximum coastal tsunami heights. The earthquake recurrence interval on each fault is estimated from observed seismicity. The variance parameter representing aleatory uncertainty for probabilistic tsunami hazard analysis is determined from comparison with the four historical tsunamis, and a logic-tree is used for the choice of the values. Using nearshore tsunami heights at the 50 m isobath and an amplification factor by the Green’s law, hazard curves are constructed at 154 locations for coastal municipalities along the Sea of Japan coast. The highest maximum coastal tsunamis are expected to be approximately 3.7, 7.7, and 11.5 m for the return periods of 100-, 400-, and 1000-year, respectively. The results indicate that the hazard level generally increases from southwest to northeast, which is consistent with the number and type of the identified fault systems. Furthermore, the deaggregation of hazard suggests that tsunamis in the northeast are predominated by local sources, while the southwest parts are likely affected by several regional sources.

Effects of Earthquake Recurrence on Localization of Interseismic Deformation Around Locked Strike‐Slip Faults

AbstractLocalized geodetic deformation of arctan shape around locked strike‐slip faults is widely reported, but there are also important exceptions showing distributed deformation. Understanding the controlling mechanism is important to hazard assessment and geodynamic analysis. Here we use simple finite element viscoelastic earthquake cycle models to investigate the basic mechanics of this process. Our models feature a vertical strike‐slip fault in an elastic layer overlying a viscoelastic substrate of Maxwell or Burgers rheology, with or without a low‐viscosity shear zone representing deeper extension of the fault. We demonstrate that the primary control on the localization of interseismic deformation is the recurrence interval of past earthquakes. Given viscosity, shorter recurrence leads to greater localization, regardless of the rheological model used. The presence of a low‐viscosity deep fault does not change this conclusion, although it tends to lessen localization by promoting faster postseismic stress relaxation. Distributed deformation, although less reported, is a natural consequence of very long recurrence and in theory should be as common as localized deformation. We think that the apparent propensity of the latter is likely associated with the much greater quantity and better quality of geodetic observations from higher‐rate and shorter‐recurrence faults. Our results also show the important role of nearby earthquakes along the same fault. For faults of relatively short recurrence, frequent ruptures of nearby segments, modeled using a migrating rupture sequence, further enhance localization. For faults of very long recurrence, faster near‐fault deformation induced by a recent earthquake may give a false impression of localized interseismic deformation.

Complexity in the Earthquake Cycle Increases with the Number of Interacting Patches

Numerical simulations were performed to examine the effects of heterogeneity of frictional properties on earthquake cycles. In the model, rate- and state-dependent friction was assumed at a planar fault in an infinite uniform elastic medium. Simulated earthquake cycles were compared at model faults with one to four circular patches with velocity-weakening frictional properties. The characteristic slip distances for the patches were varied so that the effects of interactions between patches with different frictional properties could be assessed. Simple periodic, multiperiodic, and aperiodic earthquake cycles were observed in the simulation results. The frequencies of the multiperiodic and aperiodic cycles increased with the number of velocity-weakening patches, suggesting that more complex cycles may occur in a fault as the frictional properties become more heterogeneous. The simulated earthquake cycles were divided into several classes according to the combinations of ruptured patches in the earthquakes. Complex aperiodic earthquake cycles tended to occur when the values of the parameters lay between the parameter ranges for the different patterns. The frequency distribution of simulated earthquake recurrence intervals became more continuous and smooth as the number of patches increased. The simulation results suggest that interactions between many patches with different frictional properties form part of the explanation of realistic complex earthquake cycles.

Palaeoseismic records in lacustrine sediments—A case study of the Daqingshan piedmont fault and Hasuhai Lake in Inner Mongolia, China

AbstractAs a traditional method for palaeoseismic studies, trenching can be combined with dating techniques to identify palaeoseismic events and the earthquake recurrence interval. However, when using trenches to study palaeoearthquakes, factors such as the active tectonic background of the earthquake‐caused structure, the lithology on both sides of the fault, the geomorphology location and type and the samples and methods for dating will affect the location of the trench. Thus, trenches should be carefully selected and used to identify the impact of ancient earthquakes. The results have substantial uncertainties and limitations. In recent years, scholars have made considerable progress in using other methods to reveal the palaeoseismic information of faults. Moreover, the history of fault activity may have been recorded in the lacustrine sediment adjacent to the fault. Hasuhai Lake is adjacent to the middle segment of the Daqingshan piedmont fault in Inner Mongolia. Since the Holocene, the region has experienced a temperate continental semi‐arid climate with little interference, and Hasuhai Lake and peripheral waters present weak hydrodynamic conditions that provide an ideal location for the study of palaeoseismic records in lacustrine sediments. Sediment samples and samples for dating were collected from three trenches excavated on the periphery of the Hasuhai Lake. Their variations in grain size and magnetic susceptibility revealed that wind and flowing water jointly produced the sedimentary conditions of Hasuhai sediments. The 14C dating results and variations in the grain size distribution, grain size components and magnetic susceptibility of sediments caused by seismic events obtained in this study were compared with those caused by a series of palaeoseismic events at the middle segment of the Daqingshan piedmont fault reported by previous studies using trenches, knickpoints and palaeosol records. The results identified seven palaeoseismic events recorded near Hasuhai Lake since 12,000 years. The combined use of lacustrine sediment variation characteristics and dating techniques is an effective method for studying palaeoseismic events.

Late Quaternary transpressional earthquakes on a long-lived intraplate fault: A case study of the Southern Yangsan Fault, SE Korea

The NNE–SSW-striking Yangsan Fault in southeastern Korea has been regarded as one of the most prominent seismogenic structures in the Korean Peninsula on the basis of instrumental and historical seismicity, and paleoseismic records along the fault zone. Its seismic behavior is, however, still uncertain due to long recurrence intervals of strong earthquakes and insufficient historical and geologic records. We conducted a detailed paleoseismic investigation, including an 8 m-deep excavation, in order to understand recent earthquake-faulting events along the Southern Yangsan Fault. Our observations indicate that at least two and possibly four dominantly strike-slip surface-faulting events have occurred along a subsidiary fault in the eastern boundary of the fault valley since the late Pleistocene. Stratigraphic features and OSL ages of the stratigraphy exposed in the trenches indicate that the most recent rupture(s) occurred after 29.2 ± 1.4 ka and the timing of earlier ruptures are constrained to between 70.0 ± 3.7 ka and 29.2 ± 1.4 ka. Using a contact of the oldest dated unit that has at least 7.5 m of vertical separation, we calculate a minimum vertical slip rate during the late Quaternary of 0.11 mm/yr. The minimum horizontal slip rate is presumed to be two times that of the vertical slip rate based on striations observed on clasts next to the fault core. We also propose that the late Quaternary earthquake-faulting kinematics along the Yangsan Fault, expressed as contractional dextral slip with east-side-up geometry, is strongly dependent on a pre-existing fault zone architecture with a strike of N10–20°E that dips to the east, and the direction of neotectonic maximum horizontal stress (ENE–WSW to E–W).

Title Paleoseismological and Neotectonic Characteristics of Çaldıran Fault Around Çaldıran District

Sağ yanal atımlı Çaldıran Fayı Türkiye’nin Doğu Anadolu bölgesindeki önemli aktif faylarındandır. Fayın 24 Kasım 1976 tarihinde ürettiği yıkıcı deprem önemli hasar ve can kaybına neden olmuş ve 50-55 km yüzey kırığı oluşturmuştur. Çaldıran ilçesi dolayında yüzey deformasyonları ve paleosismolojik kazı esasına dayalı olarak gerçekleştirilen bu çalışmada son depremde oluşan yüzey kırıklarının korunduğu kesimlerin olduğu saptanmıştır. Yüzey ve paleosismolojik çalışmalar Holosen’de bölgede yüzey kırığı oluşturan (M≥7) en az üç depremin olduğunu göstermektedir. Çaldıran dolayında, son depremde 1,5 m sağ yanal ve 30-40 cm düşey ötelenme saptanmıştır. Ayrıca,1696’da Çaldıran bölgesinde meydana gelen depremi [18] Çaldıran Fayı’nın ürettiği kabul edilerek tekrarlanma aralığının 280 yıl; kısa dönem yatay kayma hızının 5,36 mm/yıl-1 düşey kayma hızının ise 1,43 mm/yıl-1 olduğu saptanmıştır.

Ancient and Medieval Events and Recurrence Interval of Great Kanto Earthquakes along the Sagami Trough, Central Japan, as Inferred from Historiographical Seismology

Abstract To know the Ancient and Medieval events and the recurrence interval of the great Kanto earthquakes such as the 1923 and 1703 ones generated by the subduction of the Philippine Sea plate along the Sagami trough, central Japan, I examined the latest dataset of historical records. I used only rank-A (contemporary) materials in the Online Database of Historical Documents on Japanese Earthquakes and Eruptions in the Ancient and Medieval Ages. Among destructive earthquakes in the Kanto district before the sixteenth century, the 878 Gangyo earthquake, which has been suspected an inland event generated from the Isehara fault, is considered the oldest candidate of the Kanto earthquake based on the large-scale disaster and intense aftershock activity, though tsunami is not mentioned. The 1293 Showo (Einin) Kamakura earthquake can be regarded as a great Kanto earthquake, because of severe damage in the Kanto district and remarkable aftershock activity. During the fifteenth century, both of the 1433 Eikyo and the 1495 Meio earthquakes are regarded as candidates of the Kanto earthquake; the former caused severe damage around Kamakura on the northern coast of Sagami Bay with high-aftershock activity and presumably caused tsunami, and the latter seems to have caused large tsunamis at Kamakura and the west coast of Sagami Bay. Although further investigation is necessary to clarify which one is the Kanto earthquake, we can say that an interplate earthquake probably occurred in the fifteenth century. Intervals between successive events, in 878, 1293, 1433 or 1495, 1703, and 1923, range from 140 to 270 yr except for the first interval of 415 yr. It is very difficult to confirm whether a great interplate earthquake occurred or not during this interval by means of historiographical seismology, because the Kanto district was in the worst situation of poor historical records in those days.

Seismicity and Active Faulting around the Metropolitan Area of Athens, Greece

ABSTRACTThe existence of active faults near large cities poses significant risk to the life and property of its inhabitants as well as to its public infrastructure. Here, we investigate the interplay between seismicity, active faulting, and interseismic strain accumulation within a radius of ∼50 km from the metropolitan area of Athens, the capital of Greece. We find that during the period 2011–2018, a total of 4722 earthquakes were recorded, the majority of which had local magnitudes <3.0 with only four events being of moderate magnitude (ML 4.1–4.3). Precise relative locations with horizontal and vertical errors of ≤1 and 2 km, respectively, were obtained for 2666 of these events using the double-difference algorithm. Earthquake relocation was compared to the surface traces of 31 active and 49 “less-active” normal faults drawn from high-resolution (∼5 m pixel size) digital elevation models and complemented by analysis of geodetic data from 30 permanent Global Positioning System (GPS) stations. Joint analysis of these datasets suggests that microseismicity mostly clusters along the “less-active” faults, whereas the faults associated with impressive postglacial scarps (indicating recent activity) and historic seismicity are mostly quiet. Interestingly, GPS data indicate that both fault types currently accumulate elastic strain that ranges from 0.5 to 2.3 mm/yr. Based on their estimated rupture area, more than half of the recorded faults (N=54) are capable of generating earthquakes with moment magnitudes between 6.0 and 6.6. Although some of these sources are characterized by impressive postglacial scarps, many others have long earthquake recurrence intervals (i.e., have not ruptured during the past ∼16 ka) and are associated with intense microseismicity and elastic strain accumulation, calling for future investigations on their seismogenic potential.

Geomorphic and paleoseismological evidence of active Kachchh Mainland Fault, Kachchh, India

The present work pertains to the stratigraphic evidences found during paleoseismic investigations on the trench walls opened across the Kachchh Mainland Fault (KMF). High-resolution satellite (CARTOSAT-1 DEM) data is used for mapping fault traces and carried out paleoseismic studies to identify paleo-earthquakes. We undertook geomorphological and paleoseismological investigations along the central segment of the KMF zone with the aim of estimating long-term slip rate and recurrence interval of strong earthquakes. Two N-S trenches were excavated across the E-W striking branches of the KMF, named as KMF-1 and KMF-2, revealing that the KMF is dipping towards the north. Detailed studies of the trench walls and optical dating of event horizons have confirmed four earthquakes occurred in the KMF zone between 5600 and 1000 years. The earthquake event I occurred between 5000 and 5500 years; event II occurred between 4000 and 4600 years. The event III occurred between 2000 and 2900 years, and the event IV occurred between 1023 and 1259 AD and 02 BC to 427 AD. Our studies further indicate that the central KMF zone is probably undergoing a quiescent period of ~ 1000 years accumulating the stress in the system to generate a large earthquake. The uplifted and truncated fluvial strath terrace deposits resulting from the Holocene displacements are preserved along the hanging wall block of the KMF. Dividing the elevation of the bedrock strath by their ages yields estimates of the vertical uplift rate of 0.37 to 1.48 mm/year of the hanging wall block of central KMF. With the help of displaced sedimentary units and optical ages, we estimated an average slip rate of 0.28 mm/year and uplift rate of 0.13 mm/year along KMF-1 over an interval of 1800–4000 years. However, the estimated slip rate along KMF-2 is 0.30 mm/year and the observed uplift rate is 0.17 mm/year. The results obtained from the trench investigations along the KMF not only help in understanding the dynamics of KMF but also aid seismic hazard evaluation in the Western Peninsular India.

Preliminary analysis of earthquake probability based on the synthetic seismic catalog

The analysis of seismic hazards relies on the statistical analysis of historical seismic data and the instrumental seismic catalog to obtain the regional earthquake recurrence interval and earthquake probability. The accuracy of analysis thus depends strongly on the completeness of the seismic data used. However, available seismic catalogs are too short or incomplete for the reliable analysis of the statistical characteristics of earthquakes. If a long-term synthetic seismic catalog can be generated using a physics-based numerical simulation, and the simulation results match the crustal deformation, seismicity, and other observations, then such a synthetic catalog helps us to further understand the characteristics of seismic activity and analyze the regional seismic hazard. In this paper, taking the northeastern Tibetan Plateau as a case study, we establish a three-dimensional visco-elastoplastic finite-element model to simulate earthquake cycles and the spatiotemporal evolution of earthquakes on the model fault system and obtain a seismic catalog on a time scale of tens of thousands of years. On the basis that the model satisfies the regional geodynamics of the northeastern Tibetan Plateau, we analyze seismicity on the northeastern Tibetan Plateau using the simulated synthetic earthquake catalog. The characteristics of earthquake recurrence at different locations and different magnitudes, and the long-term average probability of earthquake occurrence within the fault system on the northeastern Tibetan plateau are studied. The results are a reference for regional seismic hazard assessment and provide a basis for the physics-based numerical prediction of earthquakes.

The 2018 national seismic hazard assessment of Australia: Quantifying hazard changes and model uncertainties

Seismic hazard assessments in stable continental regions such as Australia face considerable challenges compared with active tectonic regions. Long earthquake recurrence intervals relative to historical records make forecasting the magnitude, rates, and locations of future earthquakes difficult. Similarly, there are few recordings of strong ground motions from moderate-to-large earthquakes to inform development and selection of appropriate ground-motion models (GMMs). Through thorough treatment of these epistemic uncertainties, combined with major improvements to the earthquake catalog, a 2018 National Seismic Hazard Assessment (NSHA18) of Australia has been undertaken. The resulting hazard levels at the 10% in 50-year probability of exceedance level are in general significantly lower than previous assessments, including hazard factors used in the Australian earthquake loading standard ( AS 1170.4–2007 (R2018)), demonstrating our evolving understanding of seismic hazard in Australia. The key reasons for the decrease in seismic hazard factors are adjustments to catalog magnitudes for earthquakes in the early instrumental period, and the use of modern ground-motion attenuation models. This article summarizes the development of the NSHA18 explores uncertainties associated with the hazard model, and identifies the dominant factors driving the resulting changes in hazard compared with previous assessments.

Offset Channels May Not Accurately Record Strike‐Slip Fault Displacement: Evidence From Landscape Evolution Models

AbstractSlip distribution, slip rate, and slip per event for strike‐slip faults are commonly determined by correlating offset stream channels—under the assumption that they record seismic slip—but offset channels are formed by the interplay of tectonic and geomorphic processes. To constrain offset channel development under known tectonic and geomorphic conditions, we use numerical landscape evolution simulations along a theoretical strike‐slip fault with uniform and steady uplift, erosion, and diffusion. We investigate the influence of four tectonic parameters (fault zone width, earthquake recurrence interval, variance of the recurrence interval, and total slip relative to channel spacing) on offset channel development through multiple earthquake cycles. Analysis of >3,000 automatically measured offsets from >135 simulations suggests ~30% variability in individual measurements, but modeled displacement is recovered by averaging multiple measurements. However, the average of multiple offset measurements systematically underestimates modeled slip except when the fault zone is less than ~5 m wide, total slip is less than channel spacing, and offsets are measured shortly after an earthquake. In these simulations, postearthquake landscape evolution widens the geomorphic expression of the fault zone and modifies apparent channel offsets. We distinguish this “geomorphic fault zone” from the tectonic fault zone (zone of coseismic distributed deformation). This study highlights the capability of landscape evolution models to explore a range of conditions not easily defined in natural examples and the importance of averaging multiple measurements. Our results verify that paleoseismic studies must consider how geomorphic change has modified offset markers and use caution interpreting slip histories with multiple earthquakes.

Modelling fluid flow in complex natural fault zones: Implications for natural and human-induced earthquake nucleation

Pore fluid overpressures in active fault systems can drive fluid flow and cause fault weakening and seismicity. In return, deformation accommodated by different modes of failure (e.g. brittle vs. ductile) also affects fault zone permeability and, hence, fluid flow and pore fluid pressure distribution. Current numerical simulation techniques model how fluid flow controls fault reactivation and associated seismicity. However, the control exerted by pore fluid pressure on the transition from slow aseismic fault sliding to fast seismic sliding, during the earthquake nucleation phase, is still poorly understood. Here, we model overpressured, supercritical CO2 fluid flow in natural faults, where non-linear, complex feedback between fluid flow, fluid pressure and fault deformation controls the length of the nucleation phase of an earthquake and the duration of the interseismic period. The model setup is an analogue for recent seismic source events in the Northern Apennines of Italy (e.g. Mw 6.0 1997-98 Colfiorito and Mw 6.5 2016 Norcia earthquakes). Our modelling results of Darcy fluid flow show that the duration of the nucleation phase can be reduced by orders of magnitude, when realistic models of fault zone architecture and pore pressure- and deformation-dependent permeability are considered. In particular, earthquake nucleation phase duration can drop from more than 10 years to a few days/minutes, while the seismic moment can decrease by a factor of 6. Notably, the moment of aseismic slip (M0=109Nm) obtained during the nucleation phase modelled in our study is of the same order as the detection limit of local strain measurements using strain meters. These findings have significant implications for earthquake early warning systems, as the duration and moment of the nucleation phase will affect the likelihood of timely precursory signal detection. Interestingly, aseismic slip has been measured up to a few months before some recent large earthquakes, although in a different tectonic context than the model developed here, rekindling interest in the nucleation phase of earthquakes. In addition, our results have important implications for short and long term earthquake forecasting, as crustal fluid migration during the interseismic period may control fault strength and earthquake recurrence intervals.

Late quaternary slip rate of the east segment of the Yushu fault in the central-eastern Tibetan Plateau

The Yushu fault is a mainly left-lateral strike slip fault with some normal component within the Ganzi-Yushu fault system in central-eastern Tibetan Plateau. In 2010, the catastrophic Yushu Ms 7.1 earthquake occurred along this fault, causing approximately 3000 casualties and widespread damages; however, the late Quaternary slip rate of this fault was not tightly constrained due to poor research. Based on the interpretation of satellite images, field measurement of the offset of terrace risers, optically stimulated luminescence (OSL) and radiocarbon (14C) dating of terrace ages, the late Quaternary slip rate of the east segment of the Yushu fault is estimated to be 6.8–8.2 mm/yr. In consideration of the partitioning effect of the Batang fault south of Yushu, the late Quaternary slip rate of the entire Yushu segment is approximately 9–12 mm/yr. Given the characteristic offset of about 6–8 m, the recurrence interval of large earthquakes along the Yushu fault is estimated to be approximately 750–1150 yr. This research provides basic parameters for the long-term seismic hazards assessment of the Yushu area and implies the limited southeastward extrusion rate (~10 mm/yr) of Qiangtang block relative to the Bayan Har block.

Seismites as an indicator for determination of earthquake recurrence interval: A case study from Erciş Fault (Eastern Anatolia-Turkey)

The southern part of the Eastern Anatolian Plateau is a key region for understanding the tectonic activity and related deformation patterns of the Arabian–Eurasian collision zone. The geological record of seismic events collected from lacustrine deposits of Lake Van Basin is a critical requirement for seismic hazard assessment and seismic risk reduction. Our paleoseismic investigations revealed well-preserved seismites at five locations in late Pleistocene lacustrine deposits. The seismites were dated using an optically stimulated luminescence dating method and were arranged according to their ages. Their type, such as convolute, dish-and-pillar, flame, and ball-and-pillow structures, as well as their locations, ages, and stratigraphic distribution enable us to identify the responsible fault and the earthquake recurrence interval. On the basis of these properties, the Erciş Fault has been recognized as the structure responsible for creating the seismites. The time span between the seismites indicates an apparent earthquake recurrence interval of 125–250 years for large earthquakes (M ≥ 5) related to the Erciş Fault. The method applied here enriches our knowledge on the seismic hazards present in the northern Lake Van region.

Impulsive Source of the 2017 MW=7.3 Ezgeleh, Iran, Earthquake.

On 12 November 2017, a M W=7.3 earthquake struck near the Iranian town of Ezgeleh, at the Iran‐Iraq border. This event was located within the Zagros fold and thrust belt which delimits the continental collision between the Arabian and Eurasian Plates. Despite a high seismic risk, the seismogenic behavior of the complex network of active faults is not well documented in this area due to the long recurrence interval of large earthquakes. In this study, we jointly invert interferometric synthetic aperture radar and near‐field strong motions to infer a kinematic slip model of the rupture. The incorporation of these near‐field observations enables a fine resolution of the kinematic rupture process. It reveals an impulsive seismic source with a strong southward rupture directivity, consistent with significant damage south of the epicenter. We also show that the slip direction does not match plate convergence, implying that some of the accumulated strain must be partitioned onto other faults.

The effect of sub-seismic fault slip processes on the isotopic signature of clay minerals – Implications for K-Ar dating of fault zones

Clay minerals are abundant in fault gouges and isotope dating of the radiogenic noble gases that they contain may be used to constrain timing of fault initiation, reactivation and to estimate recurrence intervals of earthquakes. The potential influence of fault slip and ambient temperature on the isotope signature of authigenic clay minerals, however, remains unknown. This study reports measurements of the K-Ar isotope signature of samples that have been sheared experimentally at sub-seismic slip rates within a range of ambient temperatures, as well as of samples undergoing standardised clay heating degas experiments in the investigation and comparison of the influence of sub-seismic frictional slip and external heating on clay gouge. The age data confirm that the sub-seismic frictional slip of clay gouge affects the clay microstructures and releases radiogenic 40Ar. The experimental data indicate that radiogenic 40Ar release due to external heating can be disregarded until ~350 °C, which is supported by Ar diffusion modelling. Frictional slip at sub-seismic slip rates and at room temperature results in a significant radiogenic 40Ar loss of ~56% and the remaining clay will inherit ~44% of radiogenic 40Ar. Sub-seismic frictional slip experiments at imposed elevated temperatures from 150 to 450 °C increase the amount of radiogenic 40Ar loss and decrease the amount of inherited radiogenic 40Ar. At a relatively high temperature of 450 °C, about 16% of radiogenic 40Ar will be present in the remaining sample portion, reinforcing the importance of rigorous clay grain size separation processes required prior to fault gouge dating.

Temporally Constant Quaternary Uplift Rates and Their Relationship With Extensional Upper‐Plate Faults in South Crete (Greece), Constrained With36Cl Cosmogenic Exposure Dating

AbstractPreserved sets of marine terraces and palaeoshorelines above subduction zones provide an opportunity to explore the long‐term deformation that occurs as a result of upper‐plate extension. We investigate uplifted palaeoshorelines along the South Central Crete Fault and over its western tip, located above the Hellenic Subduction Zone, in order to derive uplift rates and examine the role that known extensional faults contribute to observed coastal uplift. We have mapped palaeoshorelines and successfully dated four Late‐Quaternary wave‐cut platforms using in situ36Cl exposure dating. These absolute ages are used to guide a correlation of palaeoshorelines with Quaternary sea level highstands from 76.5 to ~900 ka; the results of which suggest that uplift rates vary along fault strikes but have been constant for up to 600 ka in places. Correlation of palaeoshorelines across the South Central Crete Fault results in a throw‐rate of 0.41 mm/year and, assuming repetition of 1.1‐m slip events, a fault‐specific earthquake recurrence interval of approximately 2,700 years. Elastic‐half‐space modeling implies that coastal uplift is related to offshore upper‐plate extensional faults. These faults may be responsible for perturbing the uplift rate signals in the south central Crete area. Our findings suggest that where uplifted marine terraces are used to make inferences about the mechanisms responsible for uplift throughout the Hellenic Subduction Zone, and other subduction zones worldwide, the impact of upper‐plate extensional faults over multiple seismic cycles should also be considered.

Evidence of long recurrence times and low slip rate along the 1668 Tancheng earthquake fault

Fault slip rates and recurrence intervals of large earthquakes for active faults are important parameters for better understanding active faults and assessing their seismic hazard. The Tanlu fault, located in the densely populated and economically developed area of eastern China is thought to be the source of the M 7 Anqiu earthquake in 70 BC, and M 8½ Tancheng earthquake in 1668. Understanding the recurrence interval of paleoearthquakes and fault slip rates for the Tancheng earthquake fault is critical for assessing the potential for seismic risk and assessment for the Tanlu fault. However, the only published study used several trenches and a single trench does not completely record the sequence of paleoearthquake events, leaving a large uncertainty of seismic hazard. GPS observations in North China show that there is no significant velocity gradient across the Tanlu fault, yet the long-term slip rate of the Tancheng segment of the fault is estimated to be significantly higher, up to 2 mm/a. Therefore, it is critical to conduct further research on the paleoearthquake history and slip rates of this fault segment to reconcile these contrasting results. In this study, we utilize aerial photographs and high-resolution satellite imagery, combined with field investigations of a surface rupture formed by the 1668 earthquake. The earthquake rupture is ~80 m long and ~2.5 m high, located in the Huaqiao village of northeastern Tancheng County. A gully across the rupture reveals a right lateral dislocation of approximately 36 m. We excavated two trenches on both sides of the rupture. Trench 1 reveals a positive flower structure and is consistent with a right lateral strike-slip fault with a thrust component consistent with the modern regional stress field. Trench 2 reveals a colluvial wedge formed by the latest paleoearthquake with a fine-grained deposit overlapping the colluvial wedge. The latest paleoearthquake should occur prior to deposition of the fine-grained deposit. AMS-14C radiocarbon dating was used to determine the age of the event and is approximately 12.8 +4.0/−3.7 ka, indicating a recurrence interval of large earthquakes greater than 10 ka, consistent with the results from other segments of the Tanlu fault. Combined with an average slip of ~9 m for the 1668 Tancheng earthquake, the dextral slip rate along the Tancheng earthquake fault since the end of the late Pleistocene is estimated to be 0.7 mm/a, consistent with low interseismic strain accumulation based on geodetic observations across the North China region.