Advances and Overview of the Study on Paleo-earthquake Events: A Review of Seismites

The distribution and formation mechanisms of typical identified seismites are analyzed based on various factors from plate tectonic positions, types of sedimentary basins and properties of seismogenic faults to focal mechanisms.Especially, structural styles, reserved positions, activity times, formation mechanisms and dynamics of soft-sediment deformation structures triggered by seismic activity are systematically analyzed.According to the genetic types of seismites, we propose 5 categories, including liquefied deformation, thixotropic deformation, hydroplastic deformation, superimposed gravity driving deformation and brittle deformation.Further, based on the main genetic types, composition of sediments and deformation styles, we draw up 35 secondary classifications.To determine paleo-seismic sequences in different times, activities of seismogenic faults, high-resolution tectonic events in one main tectonic movement and paleo-tectonic settings, and to understand the inducing mechanisms of paleo-earthquakes and ecologic environment evolution, researching on seismites are of great significance.Combing multiple approaches to identify the paleoearthquake records, simulating experiments on various soft-sediment deformation structures triggered by different magnitudes of shocking, dating precisely on paleo-seismic events, impacting on paleogeography and biological environment and on energy and resources domain are the frontiers of paleoseismic research.

A Preliminary Study on the Soft–Sediment Deformation Structures in the Late Quaternary Lacustrine Sediments at Tashkorgan, Northeastern Pamir, China

This study identified soft-sediment deformation structures （SSDS） of seismic origin from lacustrine sediments in the late Quaternary paleo-dammed lake at Tashkorgan, northeastern Pamir. The observed deformation structures include sand dykes, liquefied diapir and convolute structures, gravity induced SSDS, and thixotropic pillar and tabular structures. We conducted a preliminary study on the morphology, formation and trigger mechanisms of pillar and tabular structures formed by liquefaction of underlying coarse sand and thixotropy of the upper silty clay. The regional tectonic setting and distribution of lacustrine strata indicate that the most probable trigger for the SSDS in lacustrine sediments was seismic activity, with an approximate earthquake magnitude of M〉6.0; the potential seismogenic fault is the southern part of the Kongur normal fault extensional system. AMS ^4C dating results indicate that the SSDS were formed by seismic events occurring between 26050±100 yrBP and 22710±80 yrBP, implying intense fault activity in this region during the late Pleistocene. This study provides new evidence for understanding tectonic activity and regional geodynamics in western China.

Revisiting Late Quaternary Slip-rate along the Maqu Segment of the Eastern Kunlun Fault, Northeast Tibet

The Late Quaternary slip rate along the Maqu segment of the eastern Kunlun Fault was estimated using a combination of high-resolution remote sensing imagery interpretation, field observations and differential Global Positioning System（GPS） measurements of offset river terraces, and 14 C dating of snail shells collected from offset risers. The results show that the left-slip rate along the segment is 3–5 mm/a, and that the vertical slip rate is 0.3–0.5 mm/a. Both the horizontal and vertical slips on the segment remain consistent over a distance of ~100 km. It means that no slip gradient as previously suggested occurred along the Maqu segment, and which thus might behave as an independent seismogenic fault. Judging from multiple relationships among young terrace offsets, we infer that co-seismic surface rupture produced by a characteristic earthquake with a magnitude of Ms7.0–7.5 on the Maqu fault could generate a horizontal slip of 4.5–5 m and a vertical slip of 0.45–0.5 m, with a corresponding ratio（Dh/Dv） of about 9. Two surface rupture events must have occurred over the past 3300 years, the latest one possibly between 1485 cal BP and 1730 cal BP.

Neogene Paleoseismic Events and the Shanwang Biota's Burial in the Linqu Area,Shandong Province,China

Several paleoseismic events are recorded in the Neogene Linqu Group, exposed in the Linqu area, Shandong Province, China. The events were interpreted on the basis of fleldwork and laboratory analysis, which showed the presence of seismites with plastically deformed soft-sediment deformation structures in the Shanwang Formation, and of seismic volcanic rocks in the Yaoshan Formation which show brittle deformation. The earthquake-triggered soft-sediment deformations in the seismites include load structures, ball-and-pillow structures, flame structures, piUow-like beds, boudinage structures, slump folds, syn-depositional faults, veins of liquefied sand, and dikes of liquefied sandy lime-mud. The seismic activity is also reflected in what might be called ＇brittle seismites＇; these originated when, under the influence of seismic vibrations, semi-consolidated conglomerate was shattered. Moreover, volcanic activity is related to intense earthquakes that affected basalts intercalated with sand layers; these successions are known as ＇seismic volcanic rocks＇, which are characterized by veins of liquefied sand intruding the basalts. AH above traces of paleoseismic activity were left from one single time span of 4 Ma with active seismicity that took place 14-10 Ma. This time span is known as ＇the Linqu Neogene Paleoseismic Active Period＇, which is divided into four paleoseismic episodes, which were responses to tectonic extension and basin rifting in this area. It even includes the activity of the Yishu Fault Zone during the Miocene and the Neogene. The ratios of trace elements in the seismites, w（La）/w（Sc） and w （La）/w（Th） are higher than the average value of the upper crust, but w（Th）/w（Sc） is lower; this is geochemical evidence for the basin rifting that resulted in a high sedimentation rate. The intense and frequent paleo-earthquakes are held responsible for the rapid burial of the Shanwang Biota. Secondary earthquake-induced processes （e.g. slumping of a lake shore and the strongly increased lacustrine sedimentation rate） contributed to the rapid burial of the biota.

Paleo-Earthquakes along the Zheduotang Fault,Xianshuihe Fault System,Eastern Tibet:Implications for Seismic Hazard Evaluation

The Kangding City in eastern Tibet is at high risk due to frequent strong earthquakes along the Xianshuihe sinistral strike-slip fault bounding the Chuandian Block to the northeast.The knowledge of paleo-seismicity recurrence along this fault system is key to the evaluation of earthquake hazards in this region;thus,more accurate paleoseismic information are required.We examined the paleo-seismicity along the Zheduotang fault in the central segment of the Xianshuihe fault system by applying the field investigation,trenching,and Quaternary dating methods(e.g.,OSL and ^(14)C).Field observations found~8.5 m offset of stream by sinistral slip along the Zheduotang fault.We trenched the central fault zone of the Zheduotang fault and found that the colluvial wedges and five buried,discontinuous,A-soil horizons progressively have been offset in the shallow graben on the SW-side of the main fault indicative of the paleo-earthquakes.The dating results of OSL and ^(14)C,in line with existing data,enable us to establish the paleo-seismic history of the Zheduotang fault.It shows at least eight surface ruptures in the last 7500 years identified from displaced buried soils,colluvial wedges and terraces.Our study reveals~100 years minimum paleo-earthquake recurrence,suggesting potential large earthquakes in the Kangding area in the future.