关键词: 郯庐断裂带, 安丘-莒县断裂, 水平位错量, 特征位移

Abstract: The Tanlu fault zone(TLFZ)is the largest strike-slip fault system in eastern China, which is composed of five main faults in Shandong and Jiangsu Provinces. Among them, the Anqiu-Juxian Fault(AJF)is the only fault with obvious activity since the late Quaternary, and it is also the seismogenic structure of the Anqiu M7 earthquake in 70BC. It is of great significance to understand the tectonic activity of the TLFZ by analyzing the co-seismic displacement of this earthquake and studying the long-term activity behavior of the fault. According to the spatial distribution characteristics and seismic activity, the northern segment of the AJF between Juxian and Changyi(NAJF)is divided into four sub-segments, which are, from south to north, the Juxian-Mengyan segment, the Qingfengling segment, the Anqiu-Mengtong segment and the Changyi-Nanliu segment, respectively. However, paleoearthquake studies in the NAJF are not ideal, and only suggested that this segment was active in the Holocene. In addition, there is also no competent evidence of coseismic displacement in the previous researches.
In this study, we interpreted the geomorphic trace of the fault through remote sensing images and found that there were a large number of gullies where dextral horizontal dislocations are discovered, which are concentrated in the Anqiu-Mengtong segment and Qingfengling segment. Later, we used the high-resolution UAV-SfM photogrammetry technology to map the typical geomorphic areas from Anqiu to Juxian in the field investigation, and obtained the DEM of areas with offset gullies. Then we measured the offsets of the gullies by the measurement software, LaDiCao_v2, and acquired 79 horizontal dislocations. Combined with 5 measurement results from the previous research, we finally obtained 84 horizontal dislocations, including 26 data in the Anqiu-Mengtong segment and 58 in the Qingfengling segment. According to the statistical results of the cumulative offset probability distribution(COPD), the horizontal displacements in the Anqiu-Mengtong segment mainly concentrated in 5 intervals with the peak values of 5m, 10.4m, 15.5m, 20.6m and 25m, respectively; the horizontal displacements in the Qingfengling segment mainly concentrated in 4 intervals with the peak values of 5m, 9.7m, 16m and 19.7m, respectively. The bigger data is of less statistical significance due to large time span and small amount. The smallest dextral horizontal displacements of gullies on these two segments are both about 5m, and the larger offsets are also multiples of 5m. In addition, as the increase of the interval peak value, the number of gullies in the interval decreases. Therefore, the minimum dislocation of 5m should represent the latest activity event of these two secondary faults and be the coseismic displacement of the earthquake; the large dislocations represent the cumulative displacements of multiple seismic events, which reveal the characteristic displacement of about 5m for the two secondary faults. However, due to the unclear paleoearthquake sequence, it is also unclear whether these sub-segments were active at the same time. In addition, based on the statistical analysis on the strike-slip seismic events, there are a series of empirical formulas among the coseismic displacement, magnitude, and surface rupture length about the strike-slip faults. We used the coseismic displacement of 5m to infer the magnitude and surface rupture length of the Anqiu earthquake, and the results show that the earthquake magnitude mostly ranges from 7.5 to 7.7 and the surface rupture length is about 100km. According to previous historical records, when the 70BC Anqiu earthquake struck, the quake was felt strongly in the city of Xi 'an, hundreds of kilometers away. Therefore, combined with the calculation results and the fact that only the 70BC Anqiu earthquake was recorded in the NAJF, if the coseismic displacement of 5m was caused by the Anqiu earthquake, its magnitude may be undervalued, and the actual magnitude should be above 7.5. At the same time, the latest paleoearthquake event on Juxian-Mengyan segment is(2 140±190)a BP ago, close to the Anqiu earthquake in 70BC. Therefore, due to the calculation results of the surface rupture length of 100km, the Anqiu earthquake may have caused the cascade rupture of Anqiu-Mengtong, Qingfengling, and Juxian-Mengyan segments. Or the characteristic displacement of 5m indicates another paleoearthquake event, and the seismogenic fault of the 70BC Anqiu M7 earthquake is the Changyi-Nanliu segment, because there are more evidences of Holocene activity observed in this segment. However, since there has been no strong earthquake in this segment for more than 2 000a and various evidences have indicated that this segment has the ability of generating strong earthquake, high attention should be paid to the seismic risk in this area in the future.

Key words: Tan-Lu fault zone, Anqiu-Juxian Fault, horizontal dislocation, characteristic slip