Paleoseismological Studies Research Articles

Potentially large post-1505 AD earthquakes in western Nepal revealed by a lake sediment record

According to paleoseismological studies, the last earthquake that ruptured the Main Frontal Thrust in western Nepal occurred in 1505 AD. No evidence of large earthquakes has been documented since, giving rise to the concept of a seismic gap in the central Himalaya. Here, we report on a new record of earthquake-triggered turbidites from Lake Rara, western Nepal. Our lake-sediment record contains eight possibly moderate-to-large earthquake-triggered turbidites during the last 800 years, three of which overlap in age with previously reported Mw ≥ 7 events in western Nepal. Shaking intensity modelling, together with instrumental records, suggests that near-field earthquakes (≤15 km) should have a minimum Mw 5.6, and regional earthquakes (≤80 km) a Mw > ~ 6.5, to trigger turbidites. We present a likely scenario that western Nepal may be as seismically active as central Nepal; however, more data are needed to revaluate the seismic risk in the central Himalaya.

Evolution of views on the structure of sources of strong earthquakes
 at the end of XX and beginning of XXI centuries

The paper addresses the evolution of scientific views on the structure of the sources of strong earthquakes at the end of the 20th and beginning of the 21th century in Russia. The scientific concepts that emerged in the main developed countries initially typically lacked a clear and consistent understanding of the structure of sources of the strongest seismic events. In the 1950s, at the Schmidt Institute of Physics of the Earth of the USSR Academy of Sciences, G.A. Gamburtsev formulated a hypothesis of a long-term (a few hundred years) stability of seismic regime of a system of seismic sutures. The recently studied earthquakes have their sources in the regions of the large faults. The earthquakes of larger magnitudes have more extended and structurally more complex sources. Some sources in the considered cases are relatively simple to reconstruct (they encompass the fault planes of the large faults, e.g., the Spitak source, M = 6.8). Other sources are more complex; they are formed in the disjunctive nodes or encompass the crustal blocks. For example, the seismic source of the Altai earthquake (M = 7.3) has a volumetric structure and is developed along the boundaries of the large seismogenic blocks. The Wenchuan earthquake (M = 7.9) has a most complicated source which looks as a three-dimensional (3D) structure composed of a few crustal blocks framed by two extended northeast striking faults and separated by the northwesterly trending transverse fault. The structurally different sources differently manifest themselves in the pattern of seismic dislocations on the surface and in the distribution of aftershock hypocenters at depth. The anomalously low velocity “pockets” identified by local seismic tomography in the source areas of the Spitak and Altai earthquakes which accompany the main and secondary faults at depth are likely to be the zones of dynamic control of these faults. The breaked near-fault zones abundant with cracks and fractures are the severely looze inclusions in the crustal rocks hampering the propagation of seismic waves. Therefore, the P-waves in these pockets propagate at lower velocities than in the undamaged geological medium. The paleoseismological studies of seismic faults in trenches have shown that the strong earthquakes have occurred in the same sources in the past and the recurrence period of the strongest seismic events ranges from a few hundred to a few thousand years. Thus, the combined studies of the source zones of the strongest earthquakes conducted in the past decades in the different regions of Eurasia have shown that Gamburtsev’s hypothesis has remained relevant.

Paleoseismic evidence of a major earthquake event(s) along the hinterland faults: Pinjore Garden Fault (PGF) and Jhajra Fault (JF) in northwest Himalaya, India

Despite identification of numerous active faults in hinterland of the Himalaya, most of the paleoseismological studies are confined to the frontal fault – Himalayan Frontal Thrust (HFT). Barring few examples, no attempt has been made to understand the spatial and temporal pattern of earthquakes occurred along the hinterland faults. In this article we present new data on the coseismic slip and timing of the latest faulting event along the two hinterland faults: the Pinjore Garden Fault (PGF) and Jhajra Fault (JF) in northwest Himalaya. Trench investigations suggest at least two earthquakes along PGF and JF, with the most recent event (MRE) subsequent to 1283 CE–1443 CE and preceded 1600 CE, and around 1223 CE–1442 CE respectively, with observed single event and multi-event slip of 4.26 m and 12.7 m on PGF. The timing of the latest event deduced from the previous trenches (Hajipur to Rampur Ganda) indicates striking coherence with those along the hinterland faults (PGF and JF). We propose that the hinterland faults might have been reactivated by an independent earthquake event between 1283 CE and 1600 CE, just like the 1905 Kangra, 2005 Kashmir, 2015 Nepal earthquakes, which failed to rupture the front but produced surface deformation along the hinterland faults. The loading of the un-ruptured not defined portion of MHT with the residual stress along with constant convergence between the plates during the medieval period must have facilitated another earthquake, rupturing the frontal thrust within 100–300 years of time frame. Our results support the evidence that northwest Himalaya must have hosted multiple large magnitude earthquakes between 1200 CE–1600 CE but seismic quiescence now raises an alarm to the society and the government for the earthquake preparedness.

ACTIVE TECTONIC AND PALEOSEISMOLOGIC CHARACTERISTICS OF THE YENİCE-GÖNEN FAULT, NW TURKEY, IN LIGHT OF THE 18 MARCH 1953 YENİCE-GÖNEN EARTHQUAKE (Ms=7.2)

The Yenice-Gönen Fault (YGF) is located in the central part of the Biga Peninsula between Gönen and Yenice and is a right-lateral strike-slip active fault with general trending N65°E. On 18 March 1953 there was an earthquake on the YGF that died 263 people (Ms=7.2) and a 70 km surface rupture developed during this earthquake. In this study measurements of slip distribution were performed on the surface rupture of the Yenice-Gönen Earthquake (YGE) and the annual slip rate on the YGF was calculated. Right-lateral displacements of between 1.70±0.1 and 3.20±0.2 metres were measured on the surface rupture of the YGE. The maximum displacements were in an area close to the central part of the YGF (between Çakır and Karaköy village) and these values decreased relatively in NE and SW directions, documenting that was the bilaterally surface rupture propogation during the YGE. In the last 300.000 years, a total offset of 800±50 metres was measured on Gönen River and annual slip rate of 2.65±0.15 mm was calculated along the YGF. Current GPS studies show right-lateral strike-slip faults in the South Marmara region have slip rates of 6-8 mm/year. This slip rate is assessed as being partitioned between the YGF, Sarıköy Fault and Çan-Biga Fault zones which represent extensions of the North Anatolian Fault system within the South Marmara region. Paleoseismology studies determined that in the last 6200 years 6 earthquakes resulting in surface ruptures have occurred on the YGF, including the 1953 earthquake. According to the 14C and OSL dating results, the variable and irregular earthquake recurrence interval of YGF is ranging from 505 to 1793 years. In this study average earthquake recurrence interval of YGF was determined as 1180 years.

Tectonic geomorphology and Quaternary landscape development in the Albania - Montenegro border region: An inventory

Young and recent tectonic activity can be discerned from instrumentally recorded seismic data as well as numerous geomorphological phenomena in the transition between the Dinarides and Hellenides fold-and-thrust belts. We introduce a multiparametric approach using field, remote sensing, and seismic data from the region SW of Shkodra Lake between the towns of Bar (Montenegro) and Lezha (Albania), an area that offers unique opportunities of relating landscape development with tectonic deformation. Particularly, we focus on windgap development on anticlines bound by Pliocene-Quaternary active faults that allow statements on relative timing and rates of tectonic movement during the Quaternary. We propose an overall orogen-perpendicular (NE-SW) gradient from enhanced uplift and high tectonic activity in the hinterland to lower activity in the foreland. An orogen-parallel (NW-SE) gradient is added by the (far field) effects of Shkodra-Peja Transverse Fault Zone (SPFZ). These overall tendencies are valid despite a complex small-scale mosaic of local differential uplift and subsidence. Recent GPS studies and plate movement reconstructions are sparse, providing rough estimates of 4–5 mm/y horizontal and ca. 1 mm/y vertical movement. Geomorphological and paleoseismological studies are crucial to particularize recent and to estimate long-term relative and absolute ground movement rates for this area.

Paleoearthquakes in the Uimon basin (Gorny Altai)

Paleoseismological studies confirm that the Uimon basin is thrust by its northern mountain border along the active South Terekta fault. The latest motion along the fault in the 7-8th centuries AD induced an earthquake with a magnitude of Mw= 7.4-7.7 and a shaking intensity of I = 9-11 on the MSK-64 scale. The same fault generated another event (M > 7, I = 9-10), possibly, about 16 kyr ago, which triggered gravity sliding. The rockslide dammed the Uimon valley and produced a lake, where lacustrine deposition began about 14 ± 1 kyr ago, and a later M > 7 (I = 9-10) earthquake at ~6 ka caused the dam collapse and the lake drainage. Traces of much older earthquakes that occurred within the Uimon basin are detectable from secondary deformation structures (seismites) in soft sediments deposited during the drainage of a Late Pleistocene ice-dammed lake between 100 and 90 ka and in ~77 ka alluvium. The magnitude and intensity of these paleoearthquakes were at least M > 5.0-5.5 and I > 6-7.

Visualisation and analysis of shear-deformation bands in unconsolidated Pleistocene sand using ground-penetrating radar: Implications for paleoseismological studies

Deformation bands in unconsolidated sediments are of great value for paleoseismological studies in sedimentary archives. Using ground-penetrating radar (GPR), we investigated an array of shear-deformation bands that developed in unconsolidated Pleistocene glacifluvial Gilbert-type delta sediments. A dense grid (spacing 0.6 m) of GPR profiles was measured on top of a 20 m-long outcrop that exposes shear-deformation bands. Features in the radargrams could be directly tied to the exposure. The shear-deformation bands are partly represented by inclined reflectors and partly by the offset of reflections at delta clinoforms. 3-D interpretation of the 2-D radar sections shows that the bands have near-planar geometries that can be traced throughout the entire sediment volume. Thin sections of sediment samples show that the analysed shear-deformation bands have a denser grain packing than the host sediment. Thus they have a lower porosity and smaller pore sizes and therefore, in the vadose zone, the deformation bands have a higher water content due to enhanced capillary forces. This, together with the partially-developed weak calcite cementation and the distinct offset along the bands, are likely the main reasons for the clear and unambiguous expression of the shear-deformation bands in the radar survey. The study shows that deformation-band arrays can clearly be detected using GPR and quickly mapped over larger sediment volumes. With the 3-D analysis, it is further possible to derive the orientation and geometry of the bands. This allows correlation of the bands with the regional fault trend. Studying deformation bands in unconsolidated sediments with GPR is therefore a powerful approach in paleoseismological studies. Based on our data, we postulate that the outcrop is part of a dextral strike-slip zone that was reactivated by glacial isostatic adjustment.

Paleoseismological study of the southern Zongwulong Shan fault, Qilian Mountains, western China

The Tibetan plateau is prone to destructive earthquakes. A number of strong thrust events occurred in the recent past, seeming to migrate from south to north, to the Qilian Mountains area. Many active thrust faults are known in the Qilian Mountains, and most of them are capable of producing large earthquakes. Here we study the southern Zongwulong Shan (shan means mountain) fault that lacks any paleoseismology studies so far. This fault is the eastern boundary of the Qaidam and Qilian blocks, posing a threat to the populated area of Delingha in western China. In this paper, we document its Holocene rupture history based on paleoseismic trenches, and we assess the seismic risk in this area. We excavated two trenches across 4.5±0.5m high fault scarps in alluvial fans. We found ruptures that affect young alluvial units and almost penetrate the top thick loess unit. We show that four paleoearthquakes occurred in the Holocene with an average recurrence interval of 2300±500years. The most recent event happened between 272BCE and 572CE. Three older events probably occurred in the past 9.5±0.8ka.

On the paleoseismic evidence of the 1803 earthquake rupture (or lack of it) along the frontal thrust of the Kumaun Himalaya

The foothills of the Himalaya bordered by the Main Frontal Thrust (MFT) continue to be a locus of paleoseismological studies. One of such recent studies of trench stratigraphy near the central (Indian) Himalayan foothills (Malik et al., (2016) has reported multiple ruptures dated at 467–570, 1294–1587 and 1750–1932CE. The last offset has been attributed to the Uttarkashi earthquake of 1803 and the penultimate faulting, with lesser confidence to an earthquake in 1505CE. We tested these claims by logging an adjacent section on a shared scarp, and the new trench site, however, revealed a stratigraphic configuration partially in variance with from what has been reported in the earlier study. Our findings do not support the previous interpretation of the trench stratigraphy that suggested multiple displacements cutting across a varied set of deformed stratigraphic units leading up to the 1803 rupture. The current interpretation posits a single episode of a low-angle displacement at this site occurred between 1266 CE and 1636. Our results suggest a single medieval earthquake, conforming to what was reported from the previously studied neighboring sites to the east and west. The present study while reiterating a great medieval earthquake questions the assumption that the 1803 earthquake ruptured the MFT. Although a décollement earthquake, the 1803 rupture may have been arrested midway on the basal flat, and fell short of reaching the MFT, somewhat comparable to a suite of blind thrust earthquakes like the1905 Kangra and the 1833 Nepal earthquakes.

Recurrence of strong seismic events in the area of the 2011-2012 Tuva earthquakes according to paleoseismological data

This paper presents the results of paleoseismological studies in the epicentral area of the Tuva earthquake of 2011 (M = 6.7) and 2012 (M = 6.8). Their seismotectonic position and seismic history over the last millennia has been studied. The results are of great importance because these earthquakes are the strongest in the history of seismological observations in Tuva and are thus the first well-studied strong earthquakes in this region. The data show that relatively weak events similar to the 2011-2012 Tuva earthquakes recurred every 300-500 years in the past millennium, while catastrophic earthquakes with M = 7.0-7.2 and higher occur approximately every thousand years.

Block-and-fault dynamics modelling of the Himalayan frontal arc: Implications for seismic cycle, slip deficit, and great earthquakes

A numerical block-and-fault dynamics model (BAFD) of the Himalayan frontal arc, India is developed to understand the long-term patterns of strain accumulation and occurrences of great earthquakes in the Himalaya. The morphostructural scheme outlines twelve major crustal blocks, and external driving motions are prescribed using GPS data. The BAFD model reproduces essential features of the geodynamics and seismicity of the Himalayan frontal arc. The locations of the large synthetic earthquakes and their maximum magnitudes are consistent with the information available from the instrumental and historical earthquake catalogues. We model the evolution of the slip deficit and seismic cycles for different sections across the Himalaya frontal arc. The modelled seismic cycles are found to be varying from 700 to 2100years and are in good agreement with the return periods estimates from the recent paleoseismological studies. We notice that the accumulation of the slip deficit depends not only on the rate of shortening, rheology and structure but also on the dynamics of the confining crustal blocks. Further, we observe that tectonic motions of the Shillong plateau and Assam basin microplates play a significant role in controlling the seismicity patterns of the Bhutan block, which resulted in the decreased seismic activity, and increased rate of aseismic displacement. Thus, we infer that the regional seismicity patterns are a consequence of dynamics of the entire regional fault-and-block system rather than dynamics of individual fault. Our BAFD modelling predicts the maximum earthquake hazard associated with future large/great earthquakes for the central Himalayan gap region, which lies between the 1905 Kangra and the 2015 Gorkha earthquake ruptures, but relatively less hazard in Kashmir and Assam.

Paleoseismological uncertainty estimation in the Acambay region, Central Mexico

Paleseismological studies provide valuable information of the earthquake rupture processes such as fault dimensions, average and maximum displacements, as well as recurrence times and magnitudes of events which took place in the geologic past. This information is based on observations of the geological record. Interpretation of geological observa-tions has a source of uncertainties inherent to the large number of hypothesis that explain the observed geological features. Information obtained from paleoseismic studies is im-portant in seismic hazard analyses, and particularly crucial for regions of low seismic activity where the recurrence period of major earthquakes reaches several thousand years. However, using this information in hazard analysis requires the systematic treatment of uncertainties. We estimated uncertainties of four paleoseismological studies conducted at three different faults of the Acambay graben region in Central Mexico. The method used is based on a logic-tree formalism that quantifies the cumulative uncertainties associated with the different stages of the paleoseismic studies together with a quantification of the entropy at each step and at the end of the process. The final uncertainty and its relative importance in seismic hazard analysis is expressed as the paleoseismic quality factor, which indicate 0.14, 0.40-0.50, and 0.41 for the Acambay-Tixmadejé, Pastores and San Mateo faults, respectively. These values can be incorporated in seismic hazard analyses for the region.

Late Quaternary paleoearthquakes along the northern segment of the Nantinghe fault on the southeastern margin of the Tibetan Plateau

The strong earthquake behaviors of faults are significant for learning crustal deformation mechanisms and for assessing regional seismic risk. To date, faults that bound tectonic blocks have attracted considerable concern and many studies; however, scant attention has been paid to faults within blocks that can also host devastating earthquakes. The Nantinghe fault is a left-lateral strike-slip fault within the Southwestern Yunnan Block, and it slips at ∼4mm/yr suggesting strong activity in the late Quaternary. Nevertheless, no earthquake greater than 6has ever been recorded along it, except for the 1941M ∼7 earthquake near the Myanmar-China border region. In contrast, many earthquakes have occurred in the near region, delineating a seismic gap near the Nantinghe fault. Although several studies have been conducted upon it, the activity of its northern segment is confusing, and whether this fault segment has loaded sufficient stress to fail remains debatable. Furthermore, previous work failed to conduct any paleoseismological studies bringing out great uncertainty in learning its activity and faulting behavior, as well as in assessing the regional seismic risk. To solve these problems, we mapped the fault traces utilizing high-resolution satellite images and aerial photographs, and conducted three paleoseismological trenches along the northern segment of the Nantinghe fault. The trench excavations revealed a ∼45,000-year incomplete paleoearthquake history and confirmed that this fault segment has been active since the late Pleistocene but was not ruptured during the 1941 earthquake. Additionally, at least five paleoearthquakes are identified with their respective age ranges of before 39,030BCE; 38,500–37,220BCE; 28,475–5445BCE; 3535BCE–800CE; and 1320–1435CE based on radiocarbon dating. Among the paleoearthquakes, the latest is suggested to have generated a surface rupture much longer than 14km with a magnitude likely up to Ms 7.0. Furthermore, based on the elapsed time since the latest paleoearthquake and the sinistral slip rate along the fault, it is proposed that the northern segment of the Nantinghe fault has accumulated a seismic energy equivalent to Ms 7.0, and it is in a high seismic risk along this fault segment and in the neighboring area.

Tectonic characteristics of the Lishan Piedmont Fault in the SE Weihe Graben (central China), as revealed by the geomorphological and structural analyses

The fault kinematics, slip rate and paleoseismicity along the approximate EW-trending Lishan Piedmont Fault (LPF) in the SE Weihe Graben, central China have been determined. The results of geomorphological analysis, field investigations and radiocarbon (14C) age dating indicate that: (i) slope angles of the observed triangular facets cluster around 20°; (ii) empirical correlations yield a vertical slip rate (throw rate) of ~0.11–0.25mm yr−1, which is lower than the neighboring active faults; (iii) the LPF is characterized by almost purely normal slip, and was active within the past ~18,000yr. These findings can provide a basis for the assessment of seismic risk in and around the densely-populated city of Xi'an (an old capital of China). However, we suggest more precise constraints on the fault slip rate and detailed paleoseismological studies on the timing of the most recent earthquake and the repeat time of paleo-earthquakes are needed in order to better understand the seismogenic behaviors of the active LPF.

A probabilistic seismic hazard assessment of the Trans-Mexican Volcanic Belt, Mexico based on historical and instrumentally recorded seismicity

The Trans-Mexican Volcanic Belt (TMVB) is an active volcanic chain being deformed by an intra-arc extensional fault network. Although several crustal earthquakes with magnitude>7 have originated in the TMVB since the 16th century, the background seismicity of this geological structure is very low and the region is usually considered of low seismic hazard. In this study, we present an updated probabilistic seismic hazard model of the TMVB. The seismicity catalog used here includes forty-three historically and instrumentally recorded earthquakes, from 1858 to 2014; five of these are large earthquakes that occurred in the TMVB during the XIXth century. Due to the lack of a statically representative sample, we propose, in a qualitative manner, the seismicity catalog is complete for M≥4 since 1964 and for M≥6 since 1858. Moreover, we introduce three different earthquake frequency-magnitude relations. The first one is a conventional Gutenberg Richter fit of the distribution of the instrumentally recorded earthquakes data. The other two are non-conventional, semi-parametric approaches that integrate the historical and the instrumental data to determine seismicity rates in the region. Our preferred model (seismicity model B) fits separately the instrumental and the historical data and merge the two fits into one curve. A uniform seismic hazard (USH) of the TMVB for a return period of 500 years was calculated considering three major sources of earthquakes: 1) Subduction thrust-faulting events in the Middle American Trench (MAT); 2) Earthquakes within the subducted Cocos plate and, 3) Shallow crustal earthquakes in the TMVB. According to the seismicity model B, the average recurrence time of a M≥7 earthqua-ke on the TMVB is approximately 150 years. In contrast, the recurrence time estimated from the instrumental catalog is 12,000 years. The results of this seismicity model, which is based on historical and instrumental data, agrees also with the return periods of prehistoric earthquakes, estimated for short segments of the fault system in the TMVB in paleoseismological studies. When comparing the results of our preferred seismicity model, the PGA estimated using only the instrumental seismicity are 18 to 56% smaller than those predicted by the model using the historical catalog.

New findings on the sources of strong earthquakes in Kerch Peninsula based on paleoseismological data

This paper presents the results of the recent paleoseismological studies covering the entire Kerch Peninsula. The Yuzhnoberezhnaya fold-and-thrust belt extending along the southern coast of Crimea in the Black Sea was earlier considered the principal seismogenic structure. Our findings suggest that the seismic hazard on the Kerch Peninsula is largely related to potential strong onshore earthquake sources. The observed discrepancy between instrumental and paleoseismological data suggests that seismic quiescence exists in this region at present.

Tectonic geomorphology of the Kemalpaşa Basin and surrounding horsts, southwestern part of the Gediz Graben, Western Anatolia

The Kemalpaşa Basin is one of the Quaternary basins in Western Anatolia and represents the south-western branch of the Gediz Graben system in this extensional province. This basin has been formed under the NNE–SSW trending extensional tectonic regime. It is bounded by a major fault, the Kemalpaşa Fault, in the south and it is bounded by a number of downstepping faults, called as Spildağı Fault Zone, in the north. Both margin-bounding faults of the Kemalpaşa Basin are oblique-slip normal faults. In order to better understand the activities of these faults, we investigated the tectonic geomorphology of the Kemalpaşa Basin and interpreted the effect of tectonic activity on the geomorphological evolution using geomorphic markers such as drainage basin patterns, facet geometries and morphometric indices such as hypsometric curves and integral (HI), basin shape index (Bs), valley floor width-to-height ratio (Vf) and mountain front sinuosity (Smf). The morphometric analysis of 30 drainage basins in total and mountain fronts bounding the basin from both sides suggests a relatively high degree of tectonic activity. The mountain front sinuosity (Smf) generally varies from 1.1 to 1.3 in both sides of the basin suggesting the active fronts and facet slopes (12°–32°) suggest a relatively high degree of activity along the both sides of the Kemalpaşa Basin. Similarly, the valley floor width-to-height ratios (Vf) obtained from the both sides indicate low values varying from 0.043 to 0.92, which are typical values (<1) for tectonically active mountain fronts. The all values obtained are lower for the southern side. Therefore, we suggest that the tectonic activity of the Kemalpaşa Fault higher than the Spildağı Fault Zone. This difference that can be arised from the different uplift rates also reveals the typical asymmetric characteristics of the Kemalpaşa Basin. Additionally, the trapezoidal facets which have been observed on the southern side of the basin indicate that the Kemalpaşa Fault is evolutionally more active as compared to the Spildağı Fault Zone. The geomorphic indices indicate that the Quaternary landscape evolution of the Kemalpaşa Basin was governed by tectonic and erosional processes, and also the all results of morphometric analysis suggest a relatively high degree of tectonic activity along the faults bounding the Kemalpaşa Basin. Moreover, considering that active large normal faults with an average 15 km long can cause major earthquake, the earthquake hazard in the Kemalpaşa Basin should be investigated in detailed paleoseismological studies.

Integrated geophysical investigations in a fault zone located on southwestern part of İzmir city, Western Anatolia, Turkey

Integrated geophysical investigations consisting of joint application of various geophysical techniques have become a major tool of active tectonic investigations. The choice of integrated techniques depends on geological features, tectonic and fault characteristics of the study area, required resolution and penetration depth of used techniques and also financial supports. Therefore, fault geometry and offsets, sediment thickness and properties, features of folded strata and tectonic characteristics of near-surface sections of the subsurface could be thoroughly determined using integrated geophysical approaches. Although Ground Penetrating Radar (GPR), Electrical Resistivity Tomography (ERT) and Seismic Refraction Tomography (SRT) methods are commonly used in active tectonic investigations, other geophysical techniques will also contribute in obtaining of different properties in the complex geological environments of tectonically active sites. In this study, six different geophysical methods used to define faulting locations and characterizations around the study area. These are GPR, ERT, SRT, Very Low Frequency electromagnetic (VLF), magnetics and self-potential (SP). Overall integrated geophysical approaches used in this study gave us commonly important results about the near surface geological properties and faulting characteristics in the investigation area. After integrated interpretations of geophysical surveys, we determined an optimal trench location for paleoseismological studies. The main geological properties associated with faulting process obtained after trenching studies. In addition, geophysical results pointed out some indications concerning the active faulting mechanism in the area investigated. Consequently, the trenching studies indicate that the integrated approach of geophysical techniques applied on the fault problem reveals very useful and interpretative results in description of various properties of faulting zone in the investigation site.

Recurrence Time Distributions of Large Earthquakes in Eastern Iran

The eastern part of the Iranian plateau is characterized by large and infrequent earthquakes with recurrence intervals of more than several hundred years. Given that previous observations and paleoseismological studies are insufficient for forecasting large earthquakes, we have developed a physics‐based synthetic seismicity model for the fault system in eastern Iran using the Virtual Quake simulator. The model is independent of the seismic catalogs, however, it is tuned to match available earthquakes records. We show that the modeled seismicity rates and empirical frequency–magnitude distributions are consistent within the uncertainty of the empirical relations. Furthermore, our synthetic catalog agrees with previous paleoseismological investigations. From the resulting catalog, we can obtain the statistical distributions of recurrence times and waiting times for large earthquakes in the region as a whole and for the individual faults. In agreement with previous earthquake simulator studies, we find that the Poisson (time‐independent) distribution best describes the recurrence times of large earthquakes in the region as a whole. Large earthquakes on the individual faults show quasi‐periodic behavior, and for most faults can be well represented by the Weibull distribution. We present the corresponding time‐dependent conditional probabilities for large earthquakes throughout the region and for a few selected individual faults. Long‐term simulations indicate that waiting times for large earthquakes on specific faults are strongly dependent on the fault system configuration. Faults confined by other active faults, or consisting of several segments, host large earthquakes with more distributed recurrence times. Moreover, the obtained long‐term synthetic seismic catalog shows that fault interactions clearly have a major effect on occurrence of large earthquakes, and hence should be taken into account for seismic‐hazard assessment.

Detection limits of tidal-wetland sequences to identify variable rupture modes of megathrust earthquakes

Recent paleoseismological studies question whether segment boundaries identified for 20th and 21st century great, >M8, earthquakes persist through multiple earthquake cycles or whether smaller segments with different boundaries rupture and cause significant hazards. The smaller segments may include some currently slipping rather than locked. In this review, we outline general principles regarding indicators of relative sea-level change in tidal wetlands and the conditions in which paleoseismic indicators must be distinct from those resulting from non-seismic processes. We present new evidence from sites across southcentral Alaska to illustrate different detection limits of paleoseismic indicators and consider alternative interpretations for marsh submergence and emergence. We compare predictions of coseismic uplift and subsidence derived from geophysical models of earthquakes with different rupture modes. The spatial patterns of agreement and misfits between model predictions and quantitative reconstructions of coseismic submergence and emergence suggest that no earthquake within the last 4000 years had a pattern of rupture the same as the Mw 9.2 Alaska earthquake in 1964. From the Alaska examples and research from other subduction zones we suggest that If we want to understand whether a megathrust ruptures in segments of variable length in different earthquakes, we need to be site-specific as to what sort of geological-based criteria eliminate the possibility of a particular rupture mode in different earthquakes. We conclude that coastal paleoseismological studies benefit from a methodological framework that employs rigorous evaluation of five essential criteria and a sixth which may be very robust but only occur at some sites: 1 – lateral extent of peat-mud or mud-peat couplets with sharp contacts; 2 – suddenness of submergence or emergence, and replicated within each site; 3 – amount of vertical motion, quantified with 95% error terms and replicated within each site; 4 – syncroneity of submergence and emergence based on statistical age modelling; 5 – spatial pattern of submergence and emergence; 6 – possible additional evidence, such as evidence of a tsunami or liquefaction concurrent with submergence or emergence. We suggest that it is possible to consider detection limits as low as 0.1–0.2 m coseismic vertical change. • Revised methodological framework of 6 criteria for coastal paleoseismic studies. • New evidence from southcentral Alaska to illustrate different detection limits. • No earthquake within last 4000 years had same rupture pattern as 1964 earthquake. • Possible to consider detection limits of 0.1–0.2 m of coseismic land-level change.