Micorstructural Analysis of Late Holocene Sediments Deformed by the San Andreas Fault at Point Arena, California

在活动断层（如北方加州的圣安德烈亚斯、海沃德和卡拉韦拉斯断层）中，通常不可能区分地震破裂相关（同震）结构和蠕变相关（蠕变）结构，但评估过去地震相关地表破裂的存在与否对于评估地震危险性至关重要。 识别跨断层开挖的沟槽中的单个地表破裂地震通常是解释场地古地震历史的基础，从而也是评估沿给定断层段沿着地震复发间隔的基础。然而，地震蠕变也使表层沉积物变形，并可在沟槽暴露中产生类似于同震破裂的特征。 区分这两种类型的断层特征的不确定性可能导致对地震时间和复发的错误解释。 这项研究扩展了传统的古地震调查，包括分析活动断层变形的沉积物样品中的微结构特征。桤木溪研究区位于1906年最后一次破裂的圣安德烈亚斯断层段上，提供了一个记录明确的同震破裂的地点的显微结构信息，并允许与早期对圣安德烈亚斯断层蠕动段的研究进行直接比较。基于计算机的图像分析是一种新工具，可用于测量断层砂样品的显微尺度特征。正在使用图像分析软件研究从圣安德烈亚斯断层采集的断层砂定向样品的扫描电子显微镜图像。 通过测量断层带样品的粒度变化程度和晶粒择优取向等特征，并与蠕滑断层带、液化喷口和断层带附近未变形砂体样品的粒度变化程度和晶粒择优取向等特征进行比较，研究内容包括：（1）表征同震应变在单个断层带上的局部化程度，描述高应变率下形成的显微构造（同震滑动）;（2）提供更好的理解断层带结构的多孔，不良的石化材料。(3)添加到一个小的，但关键的数据集，描述变形过程和应变率在数百至数万年的时间段，和（4）产生的信息，是直接适用于评估过去的表面断裂的存在或不存在的故障在整个旧金山弗朗西斯科湾地区。

Differentiating between earthquake rupture-related (coseismic) structures and creep-related (aseismic) structures in active faults such as the San Andreas, Hayward, and Calaveras faults in northern California is commonly not possible, yet assessing the presence or absence of past earthquake-related surface ruptures is critical for evaluating seismic hazard. Recognition of individual surface-rupture earthquakes in trenches excavated across faults is commonly the basis for interpreting the paleoearthquake history of a site and thus for evaluating earthquake recurrence intervals along a given fault segment. However, aseismic creep also deforms surficial deposits and can produce features in trench exposures that resemble those from coseismic rupture. Uncertainty in distinguishing these two types of fault features may result in an erroneous interpretation of earthquake timing and recurrence. This research is extending traditional paleoseismologic investigations to include analysis of microtextural features in sediment samples deformed by active faults. The Alder Cr. study area, on the segment of the San Andreas Fault that last ruptured in 1906, is providing microstructural information from a site that records unambiguous coseismic rupture and allowing direct comparison with an earlier study of the creeping segment of the San Andreas Fault.Computer-based image analysis is a new tool that can be used to measure microscopic-scale characteristics of faulted sand samples. Scanning electron microscope images of oriented sample of faulted sand collected from the San Andreas Fault are being studied using image analysis software. Features such as degree of grain size variation and preferred grain orientation of fault zone samples are measured, and then compared to those from creeping fault zones, from liquifaction vents, and from undeformed sand near the fault zone.This study is: (1) characterizing the degree to which coseismic strain is localized on individual fault strands and describing microstructures formed at high strain rate (coseismic slip); (2) providing a better understanding of fault zone architecture in porous, poorly lithified materials. (3) adding to a small but crucial data set describing deformation processes and strain rates in the time period of hundreds to tens of thousands of years, and (4) yielding information that is directly applicable to assessing the presence or absence of past surface ruptures on faults throughout the San Francisco Bay region.