安宁河断裂带小相岭段古地震的新证据及最晚事件的年代

NEW EVIDENCE OF PALEOEARTHQUAKES AND DATE OF THE LATEST EVENT ON THE XIAOXIANGLING MOUNTAIN SEGMENT OF THE ANNINGHE FAULT ZONE

沿安宁河断裂带小相岭段发现了古地震的地表破裂遗迹。在两个场地开挖了横跨主断裂的探槽 ,以研究这里的晚全新世古地震。结果表明 ,在最近的大约 2 30 0a中 ,该断裂段至少发生过 4次伴有地表破裂的古地震事件 ,平均复发间隔为 6 0 0～ 70 0a。这里的最晚事件很可能发生在公元 15世纪 ,并且有可能发生在公元 14 80年。最晚事件沿地表断层产生了 3m的平均左旋位错 ,这相当于一次矩震级为 7 5± 0 3的走滑型大地震的同震平均位错量。

Surface rupture traces of several large paleoearthquakes are found along the Xiaoxiangling Mountain segment of the Anninghe fault zone. Trenches are excavated across the main fault at two sites to investigate later Holocene paleoearthquakes of this fault segment. The results show that at least 4 paleoseismic events with surface ruptures occurred on the fault segment during the last 2 300 years, suggesting an average recurrence interval of 600 to 700 years. Through carrying out the dendrochronologic calibrations of the 14 C ages of 4 samples taken from micro stratum-units lian over and under the latast event's horizon, and analyzing the probability distributions for the calendric dates from the calibrations, the date of the latest event here is bracketed sometime in the 15th century.Further,combining the fragmentry documented record of an ancient earthquake for a nearby town,it is believed that the latest event occurred possibly in the year of A.D. 1480. This event produced an average left lateral offset as much as 3 meters along the surface fault. Such an amount of mean coseismic offset is comparable with those yielded by large strike-slip earthquakes with moment magnitude 7.5±0.3.