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

在本研究项目的3年期间，我们访问了菲律宾10次，进行航片解释，野外填图，菲律宾吕宋岛断裂带的古地震槽，以及吕宋岛东南部晚第四纪火山喷发的填图。我们已经准备了一个1/50000比例尺的活动断层图的研究区域的基础上解释约1/30000比例尺的航空照片。然后，我们进行了活动断裂的构造地貌研究。我们能够映射以前未识别的痕迹的圣何塞和圣曼努埃尔故障，以及获得新的见解到分支模式的PFZ的活动断层。根据断裂带的几何形状和历史地震活动，吕宋岛中部PFZ可分为3个地震段，它们彼此独立破裂。为了测试这个分割模型，我们在PFZ的各个部分挖掘了多个战壕。在过去的2000年里，迪格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，似乎是由加巴尔登断层的滑动引起的。圣何塞断层在过去2500年中至少发生过3次破裂，最近的一次破裂事件可能与1796年的地震有关。因此，至少在上一个地震周期，三个部分分别破裂，支持所提出的分割模型。这表明，仔细观察断裂带的几何形状，特别是分支模式提供了重要的线索，以确定故障，同时破裂，在个别地震。吕宋岛东南部马荣火山和伊罗辛火山口晚第四纪火山碎屑的地层学和年代学研究表明，这些火山碎屑可作为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.