Region Paleoearthquake Research

The paleoearthquake research is a part of active tectonic research, it has great significance. We have obtained much paleoearthquake research data of a fault or segments, we have not studied recurrence behavior of paleoearthquakes in a region. The probabilistic seismic risk is to evaluate the earthquake that will occur in the same place.The recurrence interval intraplate is as long as several thousand years or over ten thousand years. The difference of recurrence interval intraplate is several hundred years or thousand years, forecasting using such data is insufficient.There are many earthquakes taking place in a region in the interval that two earthquakes occurred in the same place intraplate. The interval between two neighbor earthquakes in a region is greatly less than the intervals between two earthquakes in the same place, it means "shortening" recurrence interval and also reduces indetermination of recurrence interval. It is possible to find a new seismic hazard assessment model that is suitable to intraplate.The studying area locates in the northeastern margin of Tibet and the western margin of Huabei. Historic earthquakes and modem seismicity are very strong in thin area, five (M>7)earthquakes have occurred between A.D 1561 and A.D 1920, it is a real regional clustering example.Earthquake is a form of modern tectonic movement. The distribution of modern seismicity has been used to study modern crust movement, similarly, region paleoearthquake recurrence image can be used to study Holocene tectonic movement and make up shortage of geological and geomorgraphic methods.We got following main consequences based on a lot of field geologic working and trenching: (1)Nuishoushan—Luoshan fault was regarded as the boundary fault between Tibet and Huabei, and belonged to Tibet. Through studying the active characteristics of main faults since late Quaternary, we believed Luoshan fault is not linked with Nuishoushan fault but Huanghe fault, and belong to Huabei tectonic area now.(2)Haiyuan fault ruptured 10 times since 10,000 years B.P, average recurrence interval is 940a, the longest interval is 1582a,the shortest interval is 382a, the recurrence behavior has clustering characteristics. Two paleoearthquakes ruptured entire fault .which occurred 8534±80a B.P and A.D 1920, the interval is longer than 8,000 years.Five paleoevents ruptured western segment, average recurrence interval is 2116a, the longest interval is 2387a, the shortest interval is 1930a.Seven paleoevents ruptured eastern segment, average recurrence interval is 1410a, the longest interval is 1890a, the shortest interval is 845a.(3)Zhongwei—Tongxin fault ruptured only five times since 10,000 years B.P Three events ruptured eastern segments, two events ruptured western segments, there is not rupturing entire fault events. The longest recurrence interval is 3212a, the shortest recurrence interval is 1085a, average recurrence interval is 2090a.(4)Luoshan fault can't be divided into sub rupture segments. Four paieoevents ruptured entire fault since 10,000years B.P ,the longest recurrence interval is 3180a, the shortestrecurrence interval is 1670a, average recurrence interval is 2588a.(5)Helanshan fault also can't be divided into sub rupture segments. Five paleoevents ruptured entire faults, they occurred 82401170a, 6330±80a, 4760±80a, 2675±70a B.P and A.D 1739, average recurrence interval is 1570a.(6)The region paleoearthquake recurrence behavior is clustering. There are six clustering periods since 10,000years B.P , they can be divided into two types. The first continued about 300a, located beginning and last of the six clustering periods. Ecery fault rupture once and migration direction is not definite. The second sustained about 1000a, the paleoearthquake migration direction is definite, from Haiyuan fault to Zhongwei— Tongxin fault to Luoshan fault to Helanshan fault, then returned to Haiyuan fault. Every period, Haiyuan fault ruptured twice, other faults ruptured once.(7)Region paleoearthquake active image reflected the regional crust movement process: 8,000a B.P ,a strong moving force continued about 300years, subsequently, a stable and relatively week moving force continued about 7,000a B.P ,the recent strong moving force continued about 300a 400years B.P .The regional crust movement process studying by tectonic geography : the regional crust uplifted rapidly early Holocene, 7m high terrace formed; subsequently, the regional crust steadily uplifted, four terraces 3-4m high formed.It is consistent that the regional crust movement process studied by regional paleoearthquake images and tectonic geomorgraphy.(8)The important progresses :?proposing regional paleoearthquake recurrence model through real working; it is possible to find a new probabilistic seismic risk method which is suitabl to intraplate;?giving a relative perfact paleoearthquake catalogue in a region;(Dthe behavior of paleoevent recurrence is concerned with the fault texture .The fault texture is simple ,its paleoevent recurrence is quasi-period; the fault texture is complex, its paleoevent recurrence behavior is clustering; ? analysing the structure movement through regional paleoevent recurrence 10,000years B.P, the regional tectonic movement style studied from regional paleoevent recurrence behaviro and tectonic geomorgraphy is consistent since 10,000 years B.P.(9)Remaining problems: ? regional paleoearthquake recurrence begins recently .there are not practical region separation principled is needed to study different tectonic region; ?some fault segment paleoearthquake is not complete, it perhaps influenced the final results;