Detailed Research Of Paleoearthquakes At Several Typical Regions In China And Exploration Of Recurrence Behavior Of Large Earthquakes

The ideology and methodology of paleoearthquake research in China are oninternational level, but there is a certain insufficiency in some aspects. The largest oddsare that our work was not done enough meticulously so that the reliability ofpaleoearthquake data was influenced. For examples, we have not excavated 3-D trenchto expose displacement distribution of paleoearthquakes along strike-slip faults before,we have not enough samples to date events usually and we have not pay more attentionto recurrence behavior of large earthquakes in a region. This is a special ph. D thesis todiscuss paleoseismology, we want to get progress on some aspects, so the work isrequired to do detailing. We selected Haiyuan strike-slip fault which distributes alongthe northeastern bound of the Qing-Tibet plateau to study paleoearthquakes and theirdisplacement with 3-D trench method. We contrasted the active history ofpaleoearthquakes in segments, faults and a region in the Yanhuai basin where isextended structure area. We exposed paleoearthquakes along Xiadian fault which ishalf-burying fault in the eastern Beijing plain and is seismogenic fault of 1679 M 8earthquake. According to the data of the three areas, we discuss the recurrencebehavior of large earthquakes and study how to use information of paleoseismology forseismic hazard assessment.At Gaowanzi along Haiyuan fault, we excavated a trench group consisting of 11trenches in a field of 40m long and 20m wide. The trench is 2.5-3.5m deep. We loggedtrenches by the scale of 1:20 using grids of 100cm×100cm or 20cm×20cm.Paleoearthquaks were discussed in space of 3-D. Then, the history of paleoseismicactivity is discovered by comparing the obtained result with those from other trenchdata along Haiyuan fault. At area of normal faults in Yanhuai basin, we exploredpaleoevents and their distribution on three scales: on segments, on faults and in region.The data of 33 trenches are utilized to discuss paleoseismic history in Yanhuai basin,including 2 large trenches. A large trench was excavated along Xiadian fault alongwhich an earthquake of M 8 occurred in 1679. The trench is 25m long, 6m wide and 6mdeep. We analyze events and their displacements according to information of largetrench, and referring the form of the fault scarps and data from other trenches. Besides,the method of the fault window or progressive constraining is utilized to compare paleoevents in same segment, and to divide segments if the geometry of faults anddisplacement of events is thought out in comparing. The quantitative methods areaddressed to analyze recurrence behavior of paleoevents and we use these methods todiscuss distribution of paleoearthquakes at three research areas in the thesis.W get following main consequences after a plenty of geologic working andstudying of 43 trenches and analyzing of 144 samples(14C 52, TL 92),:(1) The research of 3-D trench at Gaowanzi on Haiyuan fault exposed the 7 eventsin Holocene(including 1920 M 8.6 earthquake), and displacements of the late fiveevents were obtained. The sinister displacements of them are 5.6±2.3m, 1.5±1.2m, 1.5±1.2m, 2±1m and 7±0.5m from old to young respectively. The data of displacementsshow that the all events are not as strong as 1920 earthquake. Only one closes 1920event and other are much smaller on magnitude. The fourteen events and their ageswere acquired as comparing information in 3-D trench with those from other 7 trenchesalong Haiyuan fault. Three of the 14 events ruptured the whole fault(master segment)and others just ruptured secondary segment. It is inequable for the recurrence intervalsof these events. The activity of paleoearthquakes is weak in early Holocene and strongduring middle-late Holocene. It is 100 years for the shortest interval between twoevents and more than 2000 years for the longest interval. The average interval of theevents that we have known is 1045±750 years for whole Haiyuan fault. It is closerPoisson distribution on recurrence model. The recurrence interval of events is1941±727 years on the west segment and 951±409 years on center segment. It is showsthat the recurrence behavior of the secondary segments follows "alternate model ofthort-long interval." The average interval of large events that magnitude are closed1920 earthquake is 5373±1046 years.(2) There are 4 faults that are active and recorded paleoearthquakes in Yanhuaibasin during late upper Pleistocene and Holocene. The north boundary fault of Huailai-Zhuolu basin has recorded the 14 paleoearthquakes since 40ka BP and the averageinterval of event recurrence is 2967±1653 years. The fault consists of four segmentsand paleoearthquakes have been exposed along three segments. Shenzhuang-Changtang segments recorded 6 events and the recurrence interval of average is6980±4487. If we divide active history of paleoearthquakes as two stages, the averageinterval of events is 2700±707 years for Holocene and 8075±884 years beforeHolocene. The 4 events were exposed along Jimingyu-Huangtugang segment and theaverage interval of events is 11273±5785 years. Xihongzhan-Huangtuyao segmentrecorded the 4 events and the average interval of events is 9400±1892 years.The 7 events in past 32000 years were discovered on the north boundary fault ofFanshan basin, which consists of two segments. The 4 events were recorded on thenorthern segment and the average interval of the events is 9096±4226, The 3 events were exposed on southern segment and the average interval of the events is 10775±178years. The average interval of event recurrence is 5232±1625 years for the whole fault.The longest interval is 7450±years and the shortest interval is 3450±years. The lastevent of the fault is probably 1337 Langshan M 6 1/2 earthquake.The obtained data show that the north boundary fault of Yanqing basin hasoccurred 10 events since 30700 years BP on 5 geometric segments. Some of the eventsassociated with faulting of an independent segment, but three events accompany withfaulting of more than one geometric segment. A segment record 3-5 events commonlyand the recurrence interval of events is 4856-6450 years. For example, it is 6450±2093years onⅡsegment, 4846±1150 years onⅢsegment and 6431±473 years onⅣsegment. The average interval of event recurrence is 3359±1373 years for the wholefault. The longest interval is 4450±years and the shortest interval is 1150±years. Thelast event of the fault is probably 1484 Juyong M 6 3/4 earthquake.The fault of Xingpaoan-shacheng is half-bury fault, which is 32km long. The faultscarp that can see clearly is 2-3km long on the fault. Two trenches were excavated onthe fault and the 4 events occurred in past 23200 years were discovered. The averageinterval of the event recurrence is 7643±401 years. The last event on the fault isprobably 1720 Shacheng M 7 earthquake.The 4 faults of Yanhuai basin have occurred 34 events during past 40000 years.The average interval of event recurrence is 1153±990 years. Displacements of mostevents are 1.2-2.2m and three event's is more than 3m. Recurrence interval of them isabout 10000 years.(3) The four events in past 20000 years were discovered on the Xiadian fault onwhich 1079 M 8 earthquake has occuned Tow early events were exposed by indirectmarks, such as paleoliquefaction, fill-wedge. Two late events(including 1679 event)were determined by direct faulting. Ages of the events are about 20000, 13300, 7500and 317 years BP, respectively. The largest throw is about 1.75m for 1679 earthquakeand 1.4m for before 1679 event.(4) Characteristic earthquake and quasi-periodic recurrence are two mainbehaviors of large earthquakes in the studying areas. Paleoearthquake data show thatthere are two grades of characteristic earthquakes and two scale segments, mastersegment and secondary segment. Recurrence behavior of paleoearthquakes is obviouslydifferent among segments, faults and a region. The recurrence intervals ofpaleoearthquakes conform with quasi-periodic model and its variety "alternate model ofshort-long interval" on most segments. They follow "alternate model of short-longinterval" and "cluster model" on faults consisted of more than one segment.Distribution of paleoearthquakes is nearly stochastic Poison process in a region.Besides, the event distribution is variable with time, no matter what it is segments, faults and regions. It is that recurrence interval of large earthquakes has phasing feature.We have to think out grading of characteristic earthquakes and phasing of recurrenceinterval when paleoseismic data are used on seismic hazard assessment.(5)The regularity of paleoseismic recurrence relates to geometry of segments.Event recurrence has good regularity on segment which is simple in geometry and largein scale. On this kind of segments, large earthquakes follow usually quasi-periodic ortime-predicable model, such as Xihongzhan-Huantuyao segment of the north boundaryfault of Huailai-Zhuolu basin, Yanwanggou-Shugou segment of the north boundaryfault of Fanshan basin and Xinbaoan-Shacheng fault. Recurrence interval ofpaleoearthquakes is general unregularity on segment which is complex in geometry andsmall in scale. Events occurred on these segments follow usually alternate model ofshort-long interval or duster model like Shenzhuang-Changtang segment andJimingyu-Hangtugang segment of the north boundary fault of Huailai-Zhuolu basin.These reflect that recurrence intervals of large earthquakes relate with each other actionbetween segments and (or) faults.Either way, the three features of large earthquake behavior in intro-continent canbe learned through the detailed research of paleoearthquakes at the three areas. That isuniversality and grading of characteristic earthquakes, phasing of recurrence interval,and each other action between segments and (or) faults in complex structure area.