Collaborative Research: Comparing Deformation Rates in Wrench Borderlands from Geodetic and Geologic Data to Evaluate the Permanent and Recoverable Components

利用全球定位系统（GPS）进行大地测量，可以很好地描述与加利福尼亚西部板块构造运动相关的当前速度场。在活跃变形带中大地测量记录的确切性质仍然是一个重大问题，因为该技术记录了扳手边界地带（主要走滑断层附近的块）的可恢复（弹性）和永久（准塑性）应变。本提案的目标是通过同时研究圣安德烈亚斯断层系统的两个不同部分，评估扳手边界变形中可恢复分量和永久分量的相对量级。加利福尼亚中部的爬行段：由于断层滑动事件之间的间隔很长（50年），单靠大地测量通常无法区分可恢复变形和永久变形。相比之下，在圣安德烈亚斯断层蠕动的中部，两次滑动的间隔时间很短（几周到几个月）。通过在爬行段的短地震周期内收集大地测量数据，PI将能够评估断层附近位移场的可采分量和永久分量的相对大小。他们建议将目前累积的永久测量值的估计值与由地质和古地磁数据确定的长期平均值进行对比。先前的研究表明，在扳手边缘地带中新世和更年轻的沉积物的古地磁信号是顺时针旋转的。记录整个区域的面积范围、区域分布和旋转量将使他们能够计算出永久变形的长期平均值。提案的这一部分涉及永久性GPS站和监测、运动式GPS、古地磁和地质测绘。Durmid Hill， Salton槽，南加州：在这个地区，以前的工作已经确定了永久应变的数量。PI计划对变形的沉积岩进行一步一步的三维逆变形。为了将这些增量应变数据转换为变形速率的估计，必须对变形时间有精确的了解。在这个极好的暴露区域，与0.76 Ma Bishop凝灰岩相关的灰层的存在提供了变形时间的一个方面。记录沉积过程中磁场的古磁学技术将提供更多信息。利用不同沉积层的不同年龄来提供不同的垂直轴旋转，PI可以通过确定哪些单元受到影响来确定小型构造（如节理、裂缝和小褶皱）的时间。通过比较大地速率（包括可恢复和永久应变分量）和地质速率（只记录永久应变分量），他们可以评估该地区可恢复应变积累的数量。提案的这一部分涉及地质测绘、古地磁和运动式GPS。通过结合我们两个地区的地质、大地测量和古地磁调查结果，PI将评估扳手边缘地区可恢复（弹性）和永久（准塑性）应变的相对贡献。这个问题的解决对于地震力学、边界变形的地质意义以及主要断层的潜在滑动震级都具有根本性的意义。

The current velocity field associated with plate tectonic motion in western California is well characterized by geodetic measurements using Global Positioning Systems (GPS). The exact nature of what geodetic measurements record in actively deforming zones remains a significant problem, as this technique records both recoverable (elastic) and permanent (quasi-plastic) strains in wrench borderlands (blocks adjacent to major strike-slip faults). The goal of this proposal is to evaluate the relative magnitudes of the recoverable and permanent components in the deformation of wrench borderlands, by simultaneously studying two different sections of the San Andreas fault system. Creeping segment, central California: Geodetic measurements alone are generally unable to distinguish between recoverable and permanent deformation, because of the long (50 yr) interval between slip episodes on faults. In contrast, intervals between slip episodes are short (weeks to months) in the creeping central section of the San Andreas fault. By collecting geodetic data throughout the short seismic cycle in the creeping segment, the PI's will be able to evaluate the relative sizes of the recoverable and permanent components of the displacement field in the vicinity of the fault. They propose to check the estimates of currently accumulating permanent measurements against long-term averages determined from geologic and paleogmagnetic data. Prior work suggests clockwise rotation of the paleomagnetic signal of Miocene and younger sediments in the wrench borderlands. Documenting the areal extent, regional distribution, and amount of rotation throughout this area will allow them to calculate a long-term average for permanent deformation. This part of the proposal involves permanent GPS stations and monitoring, campaign-style GPS, paleomagnetism, and geologic mapping. Durmid Hill, Salton Trough, southern California: In this area, previous work has determined the amount of permanent strain. The PI's propose to complete a step-wise, three-dimensional retro-deformation of deformed sedimentary rocks. In order to convert these incremental strain data into estimates of deformation rates, one must have precise knowledge of time of deformation. In this region of excellent exposure, one aspect of deformation timing is provided by the presence of an ash layer correlated with the 0.76 Ma Bishop tuff. Additional information will result from paleomagnetic techniques that record the magnetic field during deposition. Utilizing the variety of ages of the different sedimentary layers to provide differential vertical axis rotations, The PI's can determine the timing of minor structures (e.g., joints, fractures, and small folds) by determining which units are affected. By comparing geodetic rates (which include both recoverable and permanent strain components) with geologic rates (which record only a permanent strain component) they can assess the amount of recoverable strain accumulation in this area. This part of the proposal involves geologic mapping, paleomagnetism, and campaign-style GPS. By combining the results of geological, geodetic, and paleomagnetic investigations from our two field areas, the PI's will assess the relative contribution of recoverable (elastic) and permanent (quasi-plastic) strains in wrench borderlands. Resolution of this issue has fundamental implications for earthquake mechanics, geological implications of borderland deformation, and potential slip magnitudes on major faults.