海原断裂的古地震及特征地震破裂的分级性讨论

海原断裂高湾子地点三维探槽揭露了全新世5次事件的位移量。由老至新走滑位移量分别为：5．6±2．3,1．5±1．1,1．5±1．2,2±1和7±0．5m。古地震对比反映出,晚第四纪以来断裂带发生的14次事件中,仅有2～3次事件为贯穿全断裂的破裂事件,说明古地震并不都具有1920年海原大地震的强度。特征地震和段落存在分级性。破裂分级的现象不是孤立的,但多发生在走滑断裂上。

PALEOEARTHQUAKES ALONG HAIYUAN FAULTAND DISCUSSION OF GRADING ON RUPTURE OF LARGE EARTHQUAKES

Although the ideology and methodology of paleoearthquake research in China keepup with the advanced world level, there is still a certain insufficiency in some aspects.The main deficiency is that most of the previous work was not sufficient enough toensure the reliability of the obtained paleoearthquake data. For examples, 3-Dbenching has never been used so far to reveal the distribution of displacementproduced by paleoearthquakes along strike-slip fault. In thes paper, we will lay aspecial emphasis on the discussion of paleoseismology based on an exhaustive study.The Haiyuan strike-slip fault on the northeastern margin of Qinghai-Xizang Plateauhas been selected as a case for the study of paleoearthquakes and displacements bymeans of 3-D trenching , while grading on characteristic earthquakes and segments willbe discussied.A trench group had been excavated across the Haiyuan Fault at Gaowanzi village.The bench group consists of 11 trenches, each of which is 2.5-3.5m deep, and wasexcavated in a field of 42m in length and 20-25m in width. The benches have beenlogged on a scale of 1: 20 by using 100cm × 100cm added by 20cm × 20cm grids.The paleoearthquake events revealed by the trenches were discussed in 3-D space.The resultS have been compared with those obtained from other trenches along theHaiyuan Fault in order to get better understanding of the history of paleoseismic activ ity along this fault. Moreover, fault window or Progressive constraining method hasbeen used to compare paleoseismic events in the same segment and to confirm thesegmentation of the fault provided that the geometry of the fault and the displacementproduced by the events had been taken into account.The 3-D benching along Haiyuan Fault at Gaowanzi has revealed 7 Holocene Paleoseismic events (including the M= 8.6 earthquake in 1920). The sinistral displacements produced by the last 5 events have been determined to be 5.6 ± 2.3, 1.5±1．1,1.5 ± 1.2, 2 ± 1, and 7 ± 0.5m from old tO young, respectively . The amountS ofdisplacementS Produced by these events indicates that not all of the events were asstrong as the 1920 earthquake, and only one event was close to the latter . 14 eventscan be recognized when comparing the information obtained from 3-D benching withthat obtained from the other 7 trenches along the Haiyuan Fault . Among them, 3events had ruptured the whole fault (the master segment), while the others justruptured the secondary segments of the fault It shows that there are two grates of thecharacteristic earthquakes occurred on Haiyuan Fault Large characteristic earthquakes(M≥ 8.2) ruptured the whole fault (the master segment) and small characteristic earthquakes (7.5 ≥ M≥ 7) associated with the secondary segments.There are also the glading phenomena of characteristic earthquakes and segmentsalong Altyn Fault and Laohushan -Maomaoshan strike -slip fault except Haiyuan FaultBesides, the grading phenomena are hardly discovered along normal faults. Therefore,we think the grading phenomena of characteristic earthquakes and segments correlated mainly with strike-slip faults.