History,status and trend about the research of paleoseismology

Paleoseismology was originated in the late nineteen century, but the modern paleoseismology formed at the end of the 1970s. Three stages (appearance, development and maturity) can be identified for paleoseismology in China. Great progress has been made in the aspects of trench technology, recognition signs, dating method and theoretical model, however,they are not developed to such a level at which the highly reliable data can be provided for the probability prediction of earthquakes. The main questions further concerned in the future include the study of geological evidence, chronology and displacement, the theoretical model about reliability, paleoearthquake feature in the region and large-earthquake reoccurrence, and new branches related with the paleoseismology in the stable continent region, usage of new technology and frontier sciences.

The Seismic Induced Soft Sediment Deformation Structures in the Middle Jurassic of Western Qaidamu Basin

Intervals of soft-sediment deformation structures are well-exposed in Jurassic lacustrine deposits in the western Qaidamu basin. Through field observation, many soft-sediment deformation structures can be identified, such as convoluted bedding, liquefied sand veins, load and flame structures, slump structures and sliding-overlapping structures. Based on their genesis, soft-sediment deformation structures can be classified as three types： seismic induced structures, vertical loading structures, and horizontal shear structures. Based on their geometry and genesis analysis, they are seismic-induced structures. According to the characteristics of convoluted bedding structures and liquefied sand veins, it can be inferred that there were earthquakes greater than magnitude 6 in the study area during the middle Jurassic. Furthermore, the study of the slump structures and sliding- overlapping structures indicates that there was a southeastern slope during the middle Jurassic. Since the distance from the study area to the Altyn Mountain and the Altyn fault is no more than 10km, it can be also inferred that the Altyn Mountain existed then and that the AItyn strike-slip fault was active during the middle Jurassic.

Paleoseismological Analysis Along the Astara Fault System(Talesh Mountain, North Iran)

The Astara Fault System（AFS） is located in the northwest Alborz, east of Talesh Mountain（TM） and west of the South Caspian Basin（SCB）. The AFS is one of the basement rock faults in Iran that is heavily involved in seismotectonic activity of the Talesh region, and to which subsidence of the SCB is attributed. There is little information available concerning previous AFS seismic activities and its properties. In order to elucidate the seismic behavior and activities of the AFS, we conducted a research study on paleoseismology of the fault. Based on paleoseismic evidence, two scenarios could be taken into consideration, one of which has three and another has four seismic events with magnitudes Mw in the range of 6.7 to 7.2. Evidence of these seismic events is within sedimentary succession as they have occurred during the past 3 ka（this age is determined based on the deposition rate of the region）. Six carbon samples were taken for C^14 age determination tests, the results of which clearly demonstrated that the EvIV（scenario A） and EvⅢ（scenario B） had occurred before 27,444 cal BP, while other events occurred in the time period between 27,444 cal BP and 3 ka ago. If we consider the occurrence of three or four seismic events（based on the two scenarios） to be between 27,444 cal BP and 3 ka ago, the average recurrence interval is 7,119 ± 1,017, but evidence for these events has been removed. If we assume EvI to be the youngest event（in both scenarios）, the minimum elapsed time is therefore 3 ka.

Paleoseismological Study of the Late Quaternary Slip-rate along the South Barkol Basin Fault and Its Tectonic Implications,Eastern Tian Shan,Xinjiang

The easternmost Tian Shan lies in eastern Xinjiang, Central Asia. The South Barkol basin fault（SBF） in the northern part of the easternmost Tian Shan is a major tectonic structure in this orogenic region. The late Quaternary activity, paleoseismology, and deformation characteristics of the fault provide important clues for understanding the tectonic process of the eastern Tian Shan orogen and implementing seismic mitigation. Through interpretation of high-resolution satellite images, unmanned aerial vehicle measurements, and detailed geological and geomorphic investigations, we suggest that the fault exhibits clear left-lateral slip along its western segment. Paleoseismic trenches dug near Xiongkuer reveal evidence of six large paleoearthquakes. The four latest paleoearthquakes were dated： the oldest event occurred at 4663 BC–3839 BC. Data on the horizontal offsets along the probable 1842 Barkol earthquake coseismic rupture suggest clear multiple relationships between cumulative offsets and possible ~4 m of coseismic left-lateral slip per event. From the cumulative offsets and 14 C sample ages, we suggest an average Holocene left-lateral slip rate of 2.4–2.8 mm/a on the SBF, accounting for ~80% of lateral deformation within the entire eastern Tian Shan fault system. This result is comparable with the shortening rate of 2–4 mm/a in the whole eastern Tian Shan, indicating an equal role of strike-slip tectonics and compressional tectonics in this orogen, and that the SBF may accommodate substantial lateral tectonic deformation.

Paleoseismic ruptures and evolution of a small triangular pull-apart basin on the Zemuhe fault

Trenching is a primary technique on paleoseismology to reveal evidence of surface deformation produced by large earthquakes.A good trenching site requires completeness of geologic recording on paleoseismic events and corresponding reliable dating from radiocarbon samples.Based on three-dimension trenching,we show a structure of a small triangular pull-apart basin at the Daqingliangzi section on the Zemuhe fault,then explore interrelation between paleoseismic surface rupturing and evolution of the pull-apart basin,and give a corresponding identification model.Sedimentary boundary of the pull-apart basin is tightly bounded by two branch faults,which produced multiple paleoseismic events with deformation of some large fissures in sequence.Strata are thinner at north of the pull-apart basin,however thicker at south.These above characteristics show that evolution of the pull-apart basin is a continuous sedimentation process accompanying extensional deformation produced by multiple paleoseismic events.Small pull-apart basins are favorable sites for trenching and paleoseismic study on active strike-slip faults.