Large Historical Earthquakes Research Articles

Seismicity assessment in and around Syria based on instrumental data: application of Gumbel distributions and Gutenberg-Richter relationship

An instrumental earthquake catalog covering the time span between 1903 and 2007 and for the area bounded by 32°N–38°N and 35°E–43°E has been compiled in this research. The catalog has a magnitude of completeness (Mc) with 3.5. Least squares and statistical probability Gumbel’s techniques with different approaches have been applied on the instrumental events in order to assess the average recurrence time periods for different earthquake magnitudes. The constants a and b of Gutenberg-Richter and the average recurrence times have been computed firstly for the study area and secondly for the central and northern parts of Dead Sea fault system. The different statistical computations using Knopoff and Kagan formalism are generally in agreement and suggest an average recurrence time of 203 years for an earthquake of magnitude 7 for the region. The occurrence of large well-documented historical earthquakes in Lebanon and western Syria, the existence of active fault segments, the absence of large earthquakes during the study period, the increasing number of the low-magnitude earthquakes, and the continued accumulation of the strain since 1900 indicate therefore the probability of an earthquake occurrence of a large magnitude. This should be permanently taken into consideration in seismic hazard assessment on the local and regional scales.

Imaging the nonvolcanic tremor zone beneath the San Andreas fault at Cholame, California using station-pair double-difference tomography

Nonvolcanic tremors have been detected deep beneath the San Andreas fault (SAF) near Cholame, California. The tremors are modulated by small stress changes induced by Earth tides and regional and teleseismic earthquakes, implying elevated pore fluid pressure in the tremor zone. In this area, tremors lie beneath the seismogenic zone, limiting the usefulness of local earthquakes for resolving seismic velocity properties in the tremor zone. Station-pair double-difference tomography that uses differential tremor arrival times is used, for the first time, to provide new detailed information about the complexity of the lower-crust and upper-mantle transition and the occurrence of deep tremors in zones of localized low shear wave velocity anomalies. The anomalies may represent zones containing high-pore pressure fluids that migrate into the tremor zone episodically from the southwest and that are ultimately derived from dehydration of deep serpentinized mantle wedge material. These results improve our understanding of the tremor process, why tremors are concentrated in specific zones along the SAF, and possible relationships between tremors and large historic earthquakes.

High S-wave attenuation anomalies and ringlike seismogenic structures in the lithosphere beneath Altai: Possible precursors of large earthquakes

This paper addresses inhomogeneities in the short-period S-wave attenuation field in the lithosphere beneath Altai. A technique based on the analysis of the amplitude ratios of Sn and Pn waves is used. High S-wave attenuation areas are identified in the West Altai, which are related to the source zones of recent large earthquakes, viz., the 1990 Zaisan earthquake and the 2003 Chuya earthquake. Associated with the Chuya earthquake, a large ringlike seismogenic structure had been formed since 1976. It is also found that ringlike seismogenic structures are confined to high S-wave attenuation areas unrelated to large historical earthquakes. It is supposed that processes paving the way for strong earthquakes are taking place in these areas. The magnitudes of probable earthquakes are estimated using the earlier derived correlation dependences of the sizes of ringlike seismogenic structures and the threshold values of magnitudes on the energy of principal earthquakes with prevailing focal mechanisms taken into consideration. The sources of some earthquakes are likely to occur near to the planned gas pipeline route from Western Siberia to China, which should be taken into account. The relationship of anomalies in the S-wave attenuation field and the ringlike seismogenic structures to a high content of deep-seated fluids in the lithosphere is discussed.

Current microseismicity and generating faults in the Gyeongju area, southeastern Korea

A study of microseismicity in a 15×20km2 subregion of Gyeongju, southeastern Korea, establishes a direct link between minor earthquakes and known fault structures. The study area has a complex history of tectonic deformation and has experienced large historic earthquakes, with small earthquakes recorded since the beginning of modern instrumental monitoring. From 5years of continuously recorded local seismic data, 311 previously unidentified microearthquakes can be reliably located using the double-difference algorithm. These newly discovered events occur in linear streaks that can be spatially correlated with active faults, which could pose a serious hazard to nearby communities. At-risk infrastructure includes the largest industrial park in South Korea, nuclear power plants, and disposal facilities for radioactive waste. The current work suggests that the southern segment of the Yeonil Tectonic Line and segments of the Seokup and Waup Basin boundary faults are active. For areas with high rates of microseismic activity, reliably located hypocenters are spatially correlated with mapped faults; in less active areas, earthquake clusters tend to occur at fault intersections. Microearthquakes in stable continental regions are known to exist, but have been largely ignored in assessments of seismic hazard because their magnitudes are well below the detection thresholds of seismic networks. The total number of locatable microearthquakes could be dramatically increased by lowering the triggering thresholds of network detection algorithms. The present work offers an example of how microearthquakes can be reliably detected and located with advanced techniques. This could make it possible to create a new database to identify subsurface fault geometries and modes of fault movement, which could then be considered in the assessments of seismic hazard in regions where major earthquakes are rare.

Possibility of the Late Holocene uplift in the southeastern coast of Yeongil Bay, Korean Peninsula: Evidence from an exposed sea cave

Marine terraces have developed intermittently along the southeastern shore of Yeongil Bay on the Korean Peninsula. These terraces have been classified into four levels: 4–5, 14–15, 25–27, and 35m above sea level. Three sea caves formed in the cliffs of the 26-m marine terrace on the headland. One of these caves is filled with sediments >3.4-m-thick, consisting of well-sorted beach pebbles and sand or disorganized ignimbrite breccia. The uppermost sediments dating to 1680–1820 at ca. 4m above the present sea level may indicate a former shoreline. According to the historical earthquake catalog of the Korean Peninsula, the three largest earthquakes (Mw 6.2) were recorded in 1643 and 1681. The catalog suggests that two earthquakes in 1643 were felt in the southern area of the Korean Peninsula, in locations such as Hapcheon, Daegu, Andong, Kimhae, and Ulsan. The 1681 Yangyang earthquake caused shaking over the entire Korean Peninsula, and is thought to have been Korea’s largest historical earthquake. We concluded that 4-m uplifting of the sea cave beach sediments could be explained by the highest seismicity and the three largest historical earthquakes in the 1600s. It is believed that the 1681CE main earthquake, including its foreshocks and aftershocks, resulted in reactivation of the Ocheon fault, which may be the main cause of sea cave uplifting on the Masan-ri coast.

Normal faulting in the Simav graben of western Turkey reassessed with calibrated earthquake relocations

AbstractWestern Turkey has a long history of large earthquakes, but the responsible faults are poorly characterized. Here we reassess the past half century of instrumental earthquakes in the Simav‐Gediz region, starting with the 19 May 2011 Simav earthquake (Mw 5.9), which we image using interferometric synthetic aperture radar and regional and teleseismic waveforms. This event ruptured a steep, planar normal fault centered at 7–9 km depth but failed to break the surface. However, relocated main shock and aftershock hypocenters occurred beneath the main slip plane at 10–22 km depth, implying rupture initiation in areas of low coseismic slip. These calibrated modern earthquakes provide the impetus to relocate and reassess older instrumental events in the region. Aftershocks of the 1970 Gediz earthquake (Mw 7.1) form a narrow band, inconsistent with source models that invoke low‐angle detachment faulting, and may include events triggered dynamically by the unilateral main shock rupture. Epicenters of the 1969 Demirci earthquakes (Mw 5.9, 6.0) are more consistent with slip on the south dipping Akdağ fault than the larger, north dipping Simav fault. A counterintuitive aspect of recent seismicity across our study area is that the largest event (Mw 7.1) occurred in an area of slower extension and indistinct surface faulting, yet ruptured the surface, while recent earthquakes in the well‐defined and more rapidly extending Simav graben are smaller (Mw <6.0) and failed to produce surface breaks. Though our study area bounds a major metamorphic core complex, there is no evidence for involvement of low‐angle normal faulting in any of the recent large earthquakes.

Tectonic-geomorphology of the Litang fault system, SE Tibetan Plateau, and implication for regional seismic hazard

The Litang fault system (LTFS) in the eastern Tibetan Plateau has generated several large (7.5>M>7) historical earthquakes and has exhumed granitic peaks rising >1700m above the mean elevation of the plateau, despite being located within a tectonic block surrounded by highly active faults. We study horizontally offset moraine crests from the Cuopu basin and a vertically offset alluvio-glacial fan from the eastern Maoya basin. We determine a left-lateral rate of 0.09±0.02mm/yr along a slowly slipping secondary fault at Cuopu, while the main active fault at present is the normal range-front N Cuopu fault, along which we determined a left-lateral rate of 2.3±0.6mm/yr since 173ka. At Maoya fan, matching the vertical 12±1m cumulative offset with the 21.7±4.2ka fan age yields a vertical (normal) rate of 0.6±0.1mm/yr. This rate is very similar to that recently determined at the same location using low-temperature thermochronology (0.59±0.03mm/yr since 6.6±0.5Ma). Left-lateral rates along the main faults of the LTFS range between 0.9 and 2.3mm/yr at all timescales from a few years to ~6Ma. The facts that the LTFS is highly segmented and that at present, the Cuopu, Maoya and South Jawa segments are mostly normal (while the Litang and Dewu segments are left-lateral/normal), could prevent the occurrence of M>7.5 destructive earthquakes along the LTFS, as is generally assumed. However, motion on the normal faults appears to be linked with motion on the strike–slip faults, potentially allowing for exceptional larger earthquakes, and implying that the area is not experiencing pure ~NS extension but rather NW–SE left-lateral transtension.

Seismic tomography and anisotropy of the Helan‐Liupan tectonic belt: Insight into lower crustal flow and seismotectonics

AbstractWe determined detailed 3‐D P and S wave velocities (Vp and Vs) and Poisson's ratio (σ) images as well as P wave azimuthal anisotropy in the crust and uppermost mantle beneath the Helan‐Liupan tectonic belt (HLTB) and adjacent regions. The data set used in this study consists of 38,880 P wave and 35,117 S wave arrival times from 5028 local earthquakes recorded by 66 seismic stations in the study area during 1980 to 2014. Obvious low‐Vp and low‐σ anomalies are revealed in the lower crust beneath the Qilian Orogenic Belt and Western Qinling, which we interpret as a weakened zone mainly caused by water and capable of ductile flow on a geological timescale. Our P wave anisotropy results suggest that the flow direction in the lower crust is nearly parallel to the direction of the geodetic crustal motion and that of the upper mantle flow beneath the study region. Most of the 26 large historical earthquakes (1125–1954) in the study region occurred in the boundary zones where Vp, Vs, and σ change drastically over a short distance. Beneath the source areas of the large historical earthquakes, fluid‐related low‐velocity zones exist widely in the lower crust. The fluids result from dehydration of hydrous minerals in the deeper crust and uppermost mantle beneath the northeastern Tibetan Plateau. When the fluids migrate upward to the active faults, the fault zone friction is reduced and so large crustal earthquakes can be triggered. Our present results shed new light on the seismogenesis and geodynamics of the northeastern Tibetan Plateau and adjacent areas.

Upper-plate controls on co-seismic slip in the 2011 magnitude 9.0 Tohoku-oki earthquake.

The March 2011 Tohoku-oki earthquake was only the second giant (moment magnitude Mw ≥ 9.0) earthquake to occur in the last 50 years and is the most recent to be recorded using modern geophysical techniques. Available data place high-resolution constraints on the kinematics of earthquake rupture, which have challenged prior knowledge about how much a fault can slip in a single earthquake and the seismic potential of a partially coupled megathrust interface. But it is not clear what physical or structural characteristics controlled either the rupture extent or the amplitude of slip in this earthquake. Here we use residual topography and gravity anomalies to constrain the geological structure of the overthrusting (upper) plate offshore northeast Japan. These data reveal an abrupt southwest-northeast-striking boundary in upper-plate structure, across which gravity modelling indicates a south-to-north increase in the density of rocks overlying the megathrust of 150-200 kilograms per cubic metre. We suggest that this boundary represents the offshore continuation of the Median Tectonic Line, which onshore juxtaposes geological terranes composed of granite batholiths (in the north) and accretionary complexes (in the south). The megathrust north of the Median Tectonic Line is interseismically locked, has a history of large earthquakes (18 with Mw > 7 since 1896) and produced peak slip exceeding 40 metres in the Tohoku-oki earthquake. In contrast, the megathrust south of this boundary has higher rates of interseismic creep, has not generated an earthquake with MJ > 7 (local magnitude estimated by the Japan Meteorological Agency) since 1923, and experienced relatively minor (if any) co-seismic slip in 2011. We propose that the structure and frictional properties of the overthrusting plate control megathrust coupling and seismogenic behaviour in northeast Japan.

Crustal structure of the Alps as seen by attenuation tomography

We develop a simple tomographic approach exploiting the decay rate of coda waves to map the absorption properties of the crust in a region delimited approximately by the Rhine Graben to the North, the Apennines to the South, the Massif Central to the West and the Dinarides to the East. Our dataset comprises 40 000 coda records of about 2000 weak to moderate crustal earthquakes, with magnitude ranging from 2.8 to 6 and recorded by broad-band, accelerometric and short-period stations. After proper choice of a coda window minimizing the effects of variable epicentral distances, we measure the coda quality factor Qc in five non-overlapping frequency windows covering the 1–32 Hz band for all available source station pairs. These measurements are subsequently converted into maps of absorption quality factor (Qi) using a linearized, approximate relation between Qc and Qi. In practice the following procedure is applied in each frequency band: (1) we divide the target region into 40×40 km cells; (2) for each source-station pair, we assign the measured Qc value to each pixel intercepted by the direct ray path; (3) the results are averaged over all paths and subsequently smoothed with a 3×3 pixels moving window. Our approach is consistent with the high sensitivity of Qc to the value of Qi between source and station.Our tomographic approach reveals strong lateral variations of absorption with length scales ranging from 100 km to 1000 km. At low frequency (∼1 Hz), the correlation with the surface geology is clear, Cenozoic and Mesozoic sedimentary basins (resp. crystalline massifs) being recognized as high (resp. low)-absorption regions. Furthermore the Qi map delineates finer geological features such as the Ivrea Body, the Rhône Valley, or felsic intrusions in the central Alps. At high-frequency (>16 Hz), only the thickest Cenozoic sedimentary deposits show up as high-attenuation regions and a north/south dichotomy is apparent in the absorption structure. The limit between low-attenuation regions to the North and high-attenuation region to the South correlates geographically with the location of the Periadriatic Lineament (PL), a major late-alpine crustal- to lithospheric-scale structure. Furthermore, the attenuation structure seems to prolong the PL to the West along a line marked by large historical earthquakes. The Apennines orogenic belts exhibit a distinct frequency behavior, with high attenuation at low-frequency and low-attenuation at high-frequency. Low-frequency absorption may likely be explained by the relatively thick cover of Cenozoic sedimentary materials, as well as by shallow geothermal activity. We hypothesize that the frequency dependence of the attenuation structure, in particular in the Apennines, is caused by a change of the wavefield composition which accentuates the sensitivity of the coda to the deeper parts of the medium as the frequency increases.

Spatial variations in late Quaternary slip rates along the Doruneh Fault System (Central Iran)

The Doruneh Fault System (DFS) is one of the major active strike-slip faults in the Arabia-Eurasia collision zone. Despite its geological activity, no large (M ≥ 6.5) historical or instrumental earthquakes have been recorded along it. To date, the rate and distribution of slip, as well as the seismic behavior of the DFS, have been unknown. We reconstructed 67 geomorphic offsets recorded by three successive alluvial abandonment surfaces (Q1, Q2, and Q3) displaced along the western (WFZ) and central (CFZ) fault zones. The determined ages of ~12, ~36, and ~120 ka, using in situ-produced 10 Be and 36 Cl cosmogenic nuclides for theses surfaces, allowed to estimate three sets of individual left-lateral slip rates and consequently to describe the spatiotemporal distribution of slip along the CFZ and WFZ. The slip rates averaged over time intervals of ~36 and ~120 ka reveal variable slip rates along length but similar slip rates at a point with a maximum rate of ~8.2 mm/yr. During the Holocene, however, the fault slip behavior appears more complex, with a maximum rate of ~5.3 mm/yr. The CFZ is divided into two ~4 km apart segments, with symmetrical slip distributions relative to a persistent boundary, which has not been ruptured over the last ~12 ka. The maximum length of seismic fault segments varies from 70 to 100 km, which could produce earthquakes with a magnitude of M w 7.2–7.4. This emphasizes the necessity of segmentation models for long strike-slip faults that may not necessarily rupture along their whole length during a single earthquake.

Late Quaternary Large Earthquakes on the Western Branch of the Xiaojiang Fault and Their Tectonic Implications

AbstractThe Xiaojiang fault is a major active left‐lateral fault along the southeastern margin of the Tibetan Plateau. The largest historical earthquake in Yunnan Province, with a magnitude 8 and a mean coseismic left‐lateral displacement of ∼ 6.9 m, occurred on the western branch of the Xiaojiang fault Studying this fault is important in understanding current deformation and kinematic characteristics of the Tibetan Plateau. Activities and stretches have been well undertaken on the Xiaojiang fault, while paleoseismic research work is always the weak link on this fault. To investigate the paleoseismic history and large earthquake activity of the Xiaojiang fault, we opened a large trench at the northern edge of Caohaizi sag pond on the western branch of the Xiaojiang fault. Six paleoseismic events have been identified, and named E1 through E6 from the oldest to the youngest Charcoal and woods are abundant 20 samples were dated to constrain the ages of the paleoseismic events at 40 000–36 300 BC, 35 400–24 800 BC, 9 500 BC–AD 500, AD 390–720, AD 1120–1620 and AD 1750–present We associate the youngest event E6 with the 1833 M8 earthquake. Events E4, E5 and E6 show a continuous record of the western strand of the Xiaojiang fault in the late Holocene, with a average recurrence interval of 370–480 yr. Large earthquake recurrence in the late Holocene is far less than the recurrence of 2000–4000 yr posed in previous studies. Thus, the seismic hazard on the Xiaojiang fault should be reevaluated. Furthermore, the irregular recurrence of large earthquakes on the Xiaojiang fault and other faults in the Xianshuihe‐Xiaojiang system, indicates the uneven southeastward extrusion of the Sichuan‐Yunnan block along the southeastern margin of the Tibetan Plateau.

Tectonic setting and general features of coseismic rupture of the 25 April, 2015 <italic>M</italic><sub>w</sub> 7.8 Gorkha, Nepal earthquake

An earthquake of M w 7.8 occurred on 25 April, 2015 in Nepal Himalaya, with a focal mechanism of low-angle thrust, determined unanimously by multiple research agencies. More than 100 aftershocks occurred thus far, including strong ones with magnitude larger than M s7.0, triggered normal-mechanism swarms hundreds of kilometers northeast of the epicenter in Tibet. The mainshock, located in central Himalayan orogenic belt and within the seismic gap between the ruptured zones of the 1934 Bihar M w~8.1 and 1505 Lo Mustang M w~8.2 earthquakes, is the largest one on the Main Himalaya Thrust since 1950 Assam M w~8.4 earthquake. To help a better understanding of the earthquake, we provide in this paper a review on the tectonic setting and geometry of the Himalayan fault system, large historical earthquakes and their rupture areas, pattern of modern microseismicity, general models of seismic cycle in the Nepal Himalaya, and spatial pattern of surface deformation resulted from interseismic loading and coseismic slip. Combining the overall features of coseismic rupture of the mainshock from teleseismic finite fault solutions, we comment on this earthquake regarding to its relationship with the characteristic (typical topography-building) Himalayan events, the comparison with the 2008 Wenchuan M w 7.9 earthquake of similar magnitude, the general condition for low-angle thrust-type rupture reaching surface, and its implication to regional seismic hazard.

Sedimentary records of past earthquakes in Boraboy Lake during the last ca 600 years (North Anatolian Fault, Turkey)

Abstract Multiproxy sedimentological analyses along 4.9 m-long sequence of Boraboy Lake, which is located on the central eastern part of the North Anatolian Fault (NAF), reveal the sedimentary traces of past large earthquakes in the region. The lake has a relatively large catchment area (10 km2) compared to its size (0.12 km2), which renders sedimentation sensitive to heavy rain/storm events. Accordingly, the background sedimentation, which is composed of faintly laminated reddish/yellowish brown clayey silt, is frequently interrupted by organic-rich intercalations probably due to heavy rain/storm events transporting terrestrial plant remains from the densely vegetated catchment. In addition to frequent organic-rich intercalations, the background sedimentation is interrupted by four mass-wasting deposits (MWD) of which thickness range is between 15 and 50 cm. High-resolution ITRAX μXRF data confirms higher homogeneity along the MWDs (E1–E4) compared to the background sedimentation. Based on 137Cs and 210Pbxs dating and radiocarbon chronology, three MWDs detected in Boraboy sequence (E2, E3 and E4) temporally correlate with large historical earthquakes along the NAF; the 1943 Tosya (Ms = 7.6) and/or 1942 Niksar-Erbaa (Ms = 7.1), the 1776 Amasya-Merzifon and the 1668 North Anatolian (Ms = 7.9) earthquakes. The youngest MWD in the sequence (E1), which is dated to early 2000s, does not correlate with any strong earthquake in the region. This MWD was probably a single mass-wasting event due to routine overloading and oversteepening on the delta front formed by the main inlet of the lake. In subaqueous paleoseismology, coevality of multi-location mass-wasting events is used as a criterion to assign a seismic triggering mechanism, and to rule out mass-wasting events due to routine overloading/oversteepening of subaqueous slopes. Within this context, Boraboy sequence provides a valuable example to discuss sedimentological imprints of single- vs. multi-source MWDs.

Late Cenozoic deformation of the Da’an-Dedu Fault Zone and its implications for the earthquake activities in the Songliao basin, NE China

The Da’an-Dedu Fault Zone is a major tectonic feature cutting through the Songliao Basin from south to north in NE China. Five earthquakes with magnitudes over 5 that occurred during the past 30years suggest the fault zone is a seismogenic structure with future seismic potential. The structural pattern, tectonic history, Quaternary activity and seismic potential have previously been unknown due to the Quaternary sedimentary coverage and lack of large historic earthquakes (M>7). In this paper, we use seismic reflection profiles and drilling from petroleum explorations and shallow-depth seismic reflections to study those problems. The total length of the Da’an-Dedu Fault Zone is more than 400km; modern seismicity delineates it into 4 segments each with a length of 90–100km. In cross-section view, the folds and associated faults form a complex structural belt with a width of more than 10km. Shallow-level seismic reflection across the Da’an-Dedu Fault Zone reveals that the Late Quaternary sediments were folded and faulted, indicating its present tectonic activity. The Da’an-Dedu Fault Zone and Songliao Basin have been subjected to three stages of tectonic evolution: a rifting stage characterized by normal faulting and extension (∼145–112Ma), a prolonged stage of thermal subsidence (∼112–65Ma), and a tectonic reversal that has been taking place since ∼65Ma. Our shallow-level reflection profiles show that the folding and reverse faulting have influenced the Late Quaternary sediments. The seismicity and moderate earthquakes suggest that the tectonic activity persists today. The deformation rate across the Da’an-Dedu Fault Zone, however, is measured to be very slow. In conjunction with the inference that most deformation in NE China may be taken up by the Yilan-Yitong Fault Zone bounding the Songliao Basin to the east, we suggest moderate earthquake potential and thus moderate seismic hazards along the Da’an-Dedu Fault Zone. The geological structures, which include anticlines and reverse faults, imply that the Da’an-Dedu Fault Zone has been a contractive structure since ∼65Ma, and this type of contraction dominates the tectonic deformation in the Songliao Basin and the entire NE China, although the reasons for these conditions need to be further studied.

Systematic deflection and offset of the Yangtze River drainage system along the strike-slip Ganzi-Yushu-Xianshuihe Fault Zone, Tibetan Plateau

The Ganzi-Yushu-Xianshuihe Fault Zone (GYXFZ) is a typical active strike-slip fault that has triggered many large historic earthquakes, including the 2010 Mw 6.9 Yushu earthquake in the central Tibetan Plateau. This fault zone extends for ca. 800km from the central Tibetan Plateau to its southeastern margin and varies in trend from WNW-ESE in the northwestern segment of the fault zone to NNW-SSE in the southeastern segment, having the geometry of an arc projecting northeastwards. In this study, we present evidence for the systematical sinistral deflection and/or offset of the Yangtze River and its branch stream channels and valleys along the GYXFZ. Topographic analysis of three-dimensional (3D) perspective images constructed using digital elevation model (DEM) data, 0.5m-resolution WorldView and GeoEye images, and 15m-resolution Landsat-Enhanced Thematic Mapper (ETM+) images, together with analysis of geological structures, reveals the following: (i) the main river channels and valleys of the Yangtze River drainage system show systematic sinistral deflections and/or offsets along the GYXFZ; (ii) various amounts of sinistral offset have accumulated on the tributary stream channels, valleys, and gullies of the Yangtze River along the fault, with a linear relation, D=aL, between the upstream length L from the deflected point and the offset amount D with a certain coefficient a; (iii) the maximum amount of sinistral offset is up to ca. 60km, which was accumulated in the past 13–5Ma; and (iv) the long-term average strike-slip rate is ca. 4.6–12mm/year. Geological and geomorphic evidence, combined with geophysical data, demonstrates that the GYXFZ is currently active as one of the major seismogenic faults in the Tibetan Plateau, dominated by left-lateral strike-slip motion. Our findings supply important evidence for the tectonic evolution of strike-slip faults in the Tibetan Plateau since the Eurasia-India continental collision.

Estimation of Local Magnitude in Northeastern Sonora, Mexico, Using Empirical Relations Based on Recorded Duration

Research Article| March 11, 2015 Estimation of Local Magnitude in Northeastern Sonora, Mexico, Using Empirical Relations Based on Recorded Duration Gina P. Villalobos‐Escobar; Gina P. Villalobos‐Escobar Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), División Ciencias de la Tierra, Departamento de Sismología, Carretera Ensenada‐Tijuana No. 3918, Ensenada, Baja California, 22860 Méxicovillalobos@geociencias.unam.mx Search for other works by this author on: GSW Google Scholar Raúl R. Castro Raúl R. Castro Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), División Ciencias de la Tierra, Departamento de Sismología, Carretera Ensenada‐Tijuana No. 3918, Ensenada, Baja California, 22860 Méxicovillalobos@geociencias.unam.mx Search for other works by this author on: GSW Google Scholar Author and Article Information Gina P. Villalobos‐Escobar Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), División Ciencias de la Tierra, Departamento de Sismología, Carretera Ensenada‐Tijuana No. 3918, Ensenada, Baja California, 22860 Méxicovillalobos@geociencias.unam.mx Raúl R. Castro Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), División Ciencias de la Tierra, Departamento de Sismología, Carretera Ensenada‐Tijuana No. 3918, Ensenada, Baja California, 22860 Méxicovillalobos@geociencias.unam.mx Publisher: Seismological Society of America First Online: 14 Jul 2017 Online ISSN: 1938-2057 Print ISSN: 0895-0695 © 2015 by the Seismological Society of America Seismological Research Letters (2015) 86 (3): 870–875. https://doi.org/10.1785/0220140027 Article history First Online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Gina P. Villalobos‐Escobar, Raúl R. Castro; Estimation of Local Magnitude in Northeastern Sonora, Mexico, Using Empirical Relations Based on Recorded Duration. Seismological Research Letters 2015;; 86 (3): 870–875. doi: https://doi.org/10.1785/0220140027 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search Most of northern Mexico is tectonically and morphologically situated in the southern Basin and Range Province (Suter and Contreras, 2002). The region of Sonora is particularly interesting, because the largest historical earthquake (3 May 1887, Mw 7.5) occurred in this province. The town of Bavispe was destroyed during this earthquake (Aguilera, 1888), and the seismic source generated the longest recorded normal‐fault surface rupture (101.8 km) in historic time (Suter, 2006). The major fault zone of this earthquake includes (from south to north) three neighboring segments: Otates (length l=18.9 km), Teras (l... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geodynamics of the Indian Lithospheric Plate relative to the neighbouring Plates as revealed by Space Geodetic Measurements

Abstract. The Indian Plate is highly dynamic in nature which in turn makes the Indo-Eurassian collision zone the foci of most of the historic large magnitude earthquakes. Processing of positional information from continuously observing reference stations is one of the space based geodetic techniques used globally and nationally to understand the crustal dynamics. The present study evaluates the dynamic nature of the Indian plate relative to its adjoining plates using the permanent GPS data (2011 to 2013) of 12 International GNSS Service (IGS), which are spread across the Indian, Eurassian, Australian, Somaliyan and African plates. The data processing was carried out using GAMIT/GLOBK software. The results indicate that the average velocity for the two IGS stations on the Indian Plate (Hyderabad and Bangalore) is 54.25 mm/year towards NE in the ITRF-2008 reference frame. The relative velocity of various stations with respect to the Indian plate has been estimated using the Bangalore station and has been found that the stations in the Eurasian plate (Lhasa, Urumqi, Bishkek and Kitab) are moving with velocity ranging from 25 to 33 mm/year in the SE direction resulting in compressional interaction with the Indian plate. This study reveals and confirms to the previous studies that the Indian- Eurassian-Australian Plates are moving at different relative velocities leading to compressional regimes at their margins leading to seismicity in these zones.

Why the New Madrid Earthquakes are M 7-8 and the Charleston Earthquake is M 7

Estimates of magnitudes of large historical earthquakes are an essential input to and can seriously affect seismic‐hazard estimates. The earthquake‐intensity observations, modified Mercalli intensities (MMI), and assigned magnitudes M of the 1811–1812 New Madrid events have been reinterpreted several times in the last decade and have been a source of controversy in making seismic‐hazard estimates in the central United States. Observations support the concept that the larger the earthquake, the greater the maximum‐felt distance. For the same crustal attenuation and local soil conditions, magnitude should be the main influence on intensity values at large distances. We apply this concept by comparing the mean MMI at distances of 600–1200 km for each of the four largest New Madrid 1811–1812 earthquakes, the 1886 Charleston, South Carolina, earthquake, the 1929 M 7.2 Grand Banks earthquake, and the 2001 M 7.6 Bhuj, India, earthquake. We fit the intensity observations using the form MMI= A + C ×dist−0.8×log(dist) to better define intensity attenuation in eastern North America (ENA). The intensity attenuation in cratonic India differs from ENA and is corrected to ENA using both the above estimate and published intensity relations. We evaluate source, marine geophysical, Q , and stress‐drop information, as well as a 1929 Milne–Shaw record at Chicago to confirm that the 1929 Grand Banks earthquake occurred in ENA crust. Our direct comparison of mean intensities beyond 600 km suggests M 7.5, 7.3, 7.7, and 6.9 for the three New Madrid 1811–1812 mainshocks and the largest aftershock and M 7.0 for the 1886 Charleston, South Carolina, earthquake, with an estimated uncertainty of 0.3 units at the 95% confidence level (based on a Monte Carlo analysis). Our mean New Madrid and Charleston mainshock magnitudes are similar to those of Bakun and Hopper (2004) and are much higher than those of Hough and Page (2011) for New Madrid. Online Material: Tables of mean modified Mercalli intensity for 800–1200 and 600–1000 km distance ranges, and figures of least‐squares fit for all intensity measures used and for frequency‐dependent Q in easternmost Canada.

Seismic structure of the Helan–Liupan–Ordos western margin tectonic belt in North-Central China and its geodynamic implications

We study high-resolution three-dimensional P-wave velocity (Vp) tomography and anisotropic structure of the crust and uppermost mantle under the Helan–Liupan–Ordos western margin tectonic belt in North-Central China using 13,506 high-quality P-wave arrival times from 2666 local earthquakes recorded by 87 seismic stations during 1980–2008. Our results show that prominent low-velocity (low-V) anomalies exist widely in the lower crust beneath the study region and the low-V zones extend to the uppermost mantle in some local areas, suggesting that the lower crust contains higher-temperature materials and fluids. The major fault zones, especially the large boundary faults of major tectonic units, are located at the edge portion of the low-V anomalies or transition zones between the low-V and high-V anomalies in the upper crust, whereas low-V anomalies are revealed in the lower crust under most of the faults. Most of large historical earthquakes are located in the boundary zones where P-wave velocity changes drastically in a short distance. Beneath the source zones of most of the large historical earthquakes, prominent low-V anomalies are visible in the lower crust. Significant P-wave azimuthal anisotropy is revealed in the study region, and the pattern of anisotropy in the upper crust is consistent with the surface geologic features. In the lower crust and uppermost mantle, the predominant fast velocity direction (FVD) is NNE–SSW under the Yinchuan Graben and NWW–SEE or NW–SE beneath the Corridor transitional zone, Qilian Orogenic Belt and Western Qinling Orogenic Belt, and the FVD is NE–SW under the eastern Qilian Orogenic Belt. The anisotropy in the lower crust may be caused by the lattice-preferred orientation of minerals, which may reflect the lower-crustal ductile flow with varied directions. The present results shed new light on the seismotectonics and geodynamic processes of the Qinghai–Tibetan Plateau and its northeastern margin.