Collaborative Research: Seismic Tomography and Location Calibration Around the SAFOD Site

圣安德烈亚斯断层深度观测站（SAFOD）项目的一个关键目标是钻穿一次小型重复地震的破裂带。破裂斑块的大小在几十米左右；因此，以相当的精度了解其位置对SAFOD的成功至关重要。PI的Thurber和Roecker先前在该地区的工作结果，包括对钻探地点附近密集的临时地震台站网络（PASO-UNO和PASO-DOS）部署的数据进行分析，结果表明，虽然目标斑块的震中与先前的研究结果一致，但深度明显比任何人之前想象的要浅；不同之处在于需要重新考虑最初的钻井计划。许多试验反演的结果，包括两种不同层析成像技术的应用，表明这一结果是稳健的。深度的不确定性估计在200米左右，虽然与之前的工作相比有所改善，但仍然大于成功指导SAFOD钻井三个阶段所需的精度（第一阶段和第二阶段要求~100米精度，而第三阶段要求~10米精度）。该项目采用了最新开发和升级的分析技术，既增加了当前可用的数据量，又显著降低了噪声水平。一种新的双差分断层扫描技术也被使用，与标准技术相比，该技术可以显著改善事件位置和波速图像。此外，从2003年11月的一组地震实验中获得了重要的新数据。这是一个重要的新数据集，它的分析和纳入建模将导致模型和地震位置的重大改进。预计到2004年夏季第一阶段钻井开始时，这些步骤将使不确定性减少到100米以下。一旦第一阶段钻井完成，一个临时地震网络（PASO-TRES）将安装并运行12个月，重新占用12个PASO-DOS站点。除了使用钻孔中的仪器和该地区的永久站点同时记录周围的地震活动外，还将记录来自10个小（约2磅）的信号，以便通过将井下传感器视为“虚拟地震”来校准波速并估计绝对定位精度。PASO-TRES数据，包括主动和被动数据，与钻井现场附近的永久站点（HRSN和USGS）和井下传感器的数据相结合，当采用与PASO-DOS相同的分析技术时，将进一步减少震源位置的不确定性，使其达到指导第二阶段钻井所需的水平。

A key objective of the San Andreas Fault Observatory at Depth (SAFOD) project is to drill through the rupture patch of a small repeating earthquake. The size of the rupture patch is on the order of tens of meters; hence knowing its location to a comparable degree of accuracy is vital to SAFOD's success. Results from previous work in this area by PI's Thurber and Roecker, which involved the analysis of data from the deployment of dense, temporary networks of seismic stations (PASO-UNO and PASO-DOS) in the vicinity of the drill site, show that while the epicenter of the target patch is consistent with the results of a previous study, the depth is significantly shallower than anyone previously supposed; different enough to require rethinking the original drilling plan. Results from a number of trial inversions, including the application of two different tomographic techniques, indicate that this result is robust. The uncertainty in depth estimated to be on the order of 200-m, which, while an improvement over previous efforts, is still larger than what would be required to successfully guide the three phases of SAFOD drilling (Phases I and II require ~100-m accuracy, while Phase III requires ~10-m accuracy). This project is applying recently developed and upgraded analysis techniques to both increase the amount of data currently available and significantly reduce the level of noise. A new double-differencing tomography technique is also being utilized that has been shown to dramatically improve both event locations and wavespeed images over those obtained with standard techniques. In addition, important new data is now available from a set of November 2003 seismic experiments. This is a critical new dataset, and its analysis and incorporation into the modeling will lead to significant improvements to the model and earthquake locations. These steps are expected to yield a reduction in uncertainties to less than 100-m by the time Phase I drilling begins in summer 2004. Once Phase I drilling is complete, a temporary seismic network (PASO-TRES) will installed and operated for 12 months, reoccupying 12 of the PASO-DOS sites. In addition to recording ambient seismicity simultaneously with the instruments in the drill hole and permanent stations in the area, signals from 10 small (~2 lb) shots will be recorded in order to calibrate wavespeeds and estimate absolute location accuracy by treating the downhole sensors as "virtual earthquakes." The combination of PASO-TRES data, both active and passive, with those from the permanent stations near the drill site (HRSN and USGS) and downhole sensors, when subjected to the same analysis techniques as those employed on PASO-DOS, will allow a further reduction in the uncertainties in hypocenter location to the level required to direct the Phase II drilling.