Establishing criteria for segmentation of strike-slip faults based on data from the Philippine fault

在本研究项目的3年时间里，我们先后10次访问菲律宾，进行航片解译、野外填图、吕宋岛菲律宾断裂带的古地震海沟填图和吕宋岛东南部晚第四纪Tephra的填图。在对约1/50000比例尺航片解释的基础上，编制了研究区1/30000比例尺活动断裂图。然后，我们对活动断裂进行了构造地貌研究。我们能够绘制出圣何塞和圣曼纽尔断层以前未被识别的痕迹，以及对PFZ活动断层的分支模式的新见解。根据断裂带的几何形状和历史地震活动，吕宋岛中部的PFZ可以划分为3个独立破裂的地震段。为了测试这个分段模型，我们在加工区的各个分段上挖掘了多条壕沟。在过去的2000年里，迪迪格断层已经破裂了4次，倒数第二次地震发生在1645年地震之前，这场地震似乎是由加巴尔登断层上的滑动引起的。在过去的2500年里，圣何塞断层至少破裂了3次，最近的一次断层事件可能与1796年的地震有关。因此，至少在上一次地震周期中，这三个部分分别破裂，支持所提出的分段模型。这表明，对断裂带几何形状的仔细观察，特别是分支模式的观察，为识别在个别地震中同时破裂的断层提供了重要线索。对吕宋岛东南部马荣火山和伊洛辛火山口晚第四纪火山灰的地层学和年代学调查表明，这些火山的火山灰可以作为未来吕宋岛第四纪研究的标志层。

During the 3 year period of this research project, we have visited the Philippine 10 times conducting aerial photograph interpretation, field mapping, paleoseismic trenching of the Philippine fault zone in the Luzon Island, and mapping of late Quaternary tephra in southeastern Luzon Island. We have prepared a 1/50000 scale active fault map of the study area based on interpretation of about 1/30000 scale aerial photographs. We then conducted tectonic geomorphic studies of the active faults. We were able to map previously unidentified traces of the San Jose and San Manuel faults as well as obtain new insights into branching pattern of the active faults of the PFZ. Based on fault zone geometry and historical seismicity, the PFZ in central Luzon Island can be divided into 3 earthquake segments that rupture independently of one another. In order to test this segmentation model, we have excavated multiple trenches across individual segments of the PFZ. The Digdig fault has ruptured 4 times during the past 2000 years and the penultimate earthquake preceded the 1645 earthquake that appears to have caused by slip on the Gabaldon fault. The San Jose fault has ruptured at least 3 times during the past 2500 years and the most recent faulting event may be correlated to the 1796 earthquake. Therefore, at least during the last earthquake cycle, the three segments ruptured separately, supporting the proposed segmentation model. This suggests that the careful observation of fault zone geometry, especially the branching pattern gives important clues to identify faults that rupture simultaneously during individual earthquakes. Stratigraphic and chronological survey of late Quaternary tephra from the Mayon Volcano and Irosin Caldera in southeastern Luzon Island suggest that tephra from these volcanoes can be used as marker horizons for Quaternary studies in the Luzon Island in the future.