Full-resolution photomosaics of trench walls from the 2006 paleoseismic study of the San Andreas Fault at Coachella, California

The Coachella paleoseismic site is located on the San Andreas Fault along the northeast edge of the Coachella Valley in southern California, southwest of the intersection of Dillon Road and Avenue 44. In 2006, three benched trenches were excavated across the fault zone as part of an Alquist-Priolo fault investigation study by a private geotechnical firm (see APSI_003361 at https://maps.conservation.ca.gov/cgs/informationwarehouse/apreports/). These trenches exposed a thick section of latest Holocene lacustrine, alluvial, and shoreline deposits. Only the central and eastern trenches exposed faulting so paleoseismic investigations were confined to those two trenches. In the central trench (T1), we photographed and logged in detail both walls of about 70 m of the trench where it spanned several zones of complex faulting which form a 15-m-wide depression. After carefully cleaning the trench walls, we put up a 1- by 0.5-m string- and nail-grid. We photographed each 1- by 0.5-m panel individually and then logged features directly on these unrectified photos. The photos were digitally rectified later to remove distortion caused by irregularities in the trench walls and to correct the slight distortion introduced by the camera lens. The rectified photos were spliced together to make photomosaics of the trench walls. The assembled interpreted mosaics were previously published in an Open File report (Philibosian et al., 2009) and the data were fully analyzed, interpreted, and discussed in a journal article (Philibosian et al., 2011). However, due to file size constraints the resolution of the photomosaic images in the Open File report was reduced significantly from the original photographs. Given recent renewed interest in the coupled earthquake and lake chronology in the region, we publish herein the full-resolution trench wall photomosaics. Each bench riser is broken into a series of files with each file typically showing a 14-m-long section of wall. Also included are additional images showing alternative views of a few areas where the wall was cut back or with faults accentuated by carving grooves into the wall with a tool. These alternative views were used along with the others to produce the interpretations, but the images were not incorporated into the published photomosaics. Some of these additional images remain unrectified. Finally, a few overview photos showing sections of wall extending a few meters beyond the ends of the logged sections are provided to give further context to the logged sections. For the eastern trench (T2), we took a set of overview photographs of the full length (about 200 m) of each wall. These photographs were taken from the top of the trench towards the opposite wall. Because the photographs were taken at a downward angle, there is significant distortion. We logged directly on these photos in the field, recording significant contacts, primarily between lacustrine and subaerial deposits, along with descriptions. The Open File report (Philibosian et al., 2009) included low-resolution photomosaics made by splicing together these unrectified overview photos, overlain with our interpretations. Herein we publish the original full-resolution overview photographs (each covering a section of wall 5?10 m long), as well as composite photomosaics of both walls that are slightly lower resolution than the original photos but significantly higher resolution than the previously published photomosaic.

科切拉古地震遗址（Coachella Paleoseismic Site）坐落于加利福尼亚南部科切拉谷东北缘的圣安德烈亚斯断层（San Andreas Fault）之上，位于迪伦路（Dillon Road）与44大道（Avenue 44）交汇处西南侧。2006年，一家私营岩土工程公司（geotechnical firm）为开展阿尔奎斯特-普里奥洛（Alquist-Priolo）断层勘察研究，在该断层带开挖了3处台阶式探槽（benched trenches），相关报告编号为APSI_003361，详见https://maps.conservation.ca.gov/cgs/informationwarehouse/apreports/。这些探槽揭露了一套厚层的全新世晚期湖相（lacustrine）、冲积相（alluvial）及岸线沉积（shoreline deposits）。仅中部探槽与东部探槽揭露了断层构造，因此古地震研究仅针对这两处探槽展开。在中部探槽（T1）中，我们对横跨15米宽凹陷带的约70米探槽壁的两侧进行了详细拍照与编录。清理探槽壁面后，我们布设了1米×0.5米规格的绳钉测网（string- and nail-grid）。随后对每一块1米×0.5米的测网面板分别拍照，并直接在未校正照片（unrectified photos）上标记编录特征。后续我们对照片进行了数字校正（digitally rectified），以消除探槽壁面不平整带来的畸变，同时修正相机镜头引入的轻微失真。校正后的照片被拼接为探槽壁全景拼接影像（photomosaics）。此前，经整合解译的拼接影像已发表于公开档案报告（Open File report，Philibosian等，2009），相关数据的完整分析、解译与讨论则见于期刊论文（Philibosian等，2011）。然而受文件大小限制，公开档案报告中的全景拼接影像分辨率相较原始照片大幅降低。鉴于近期学界对该区域地震与湖沼年代耦合研究（coupled earthquake and lake chronology）的再度关注，本文首次发布高分辨率的探槽壁全景拼接影像。每一处探槽台阶竖板（bench riser）均被拆分为若干文件，每份文件通常对应一段14米长的探槽壁区域。本次发布还包含部分补充图像，展示了探槽壁局部经修整或通过工具刻槽突出断层构造的替代视角。这些替代视角图像与其他影像一同用于解译工作，但并未纳入已发表的全景拼接影像中，其中部分图像仍未经过校正。此外，还提供了少量概览照片（overview photographs），展示了编录段两端延伸数米的探槽壁区域，为编录段提供更多背景信息。针对东部探槽（T2），我们对两侧探槽壁的全长（约200米）拍摄了一组概览照片，拍摄视角为从探槽顶部朝向对面探槽壁。由于拍摄时采用向下倾角，照片存在显著畸变。我们在野外直接在这些照片上进行编录，记录了主要的接触界面（以湖相与陆相沉积（subaerial deposits）间的接触为主）并附上描述。公开档案报告（Philibosian等，2009）曾将这些未校正的概览照片拼接为低分辨率全景拼接影像，并叠加了解译标注。本文首次发布原始高分辨率概览照片（每份照片对应一段5~10米长的探槽壁区域），同时还提供了两侧探槽壁的复合全景拼接影像——其分辨率略低于原始照片，但远高于此前发表的拼接影像。