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

圣安德烈亚斯深度断层观测站(SAFOD)项目的一个关键目标是钻探小地震重复发生的破裂带。破裂补丁的大小约为数十米；因此，以相当的精度了解其位置对SAFOD的成功至关重要。Pi的瑟伯和罗克以前在这一领域所做的工作，包括对在钻探现场附近部署密集的临时地震台网(Paso-UNO和Paso-DOS)的数据的分析，结果表明，虽然目标地块的震中与之前的研究结果一致，但深度比任何人之前假设的都要浅得多；差异足以需要重新考虑最初的钻探计划。许多试验反演的结果，包括应用两种不同的层析成像技术，表明这一结果是稳健的。深度的不确定性估计约为200米，虽然比以前的努力有所改善，但仍大于成功指导南大西洋渔业发展组织钻探的三个阶段所需的精度(第一阶段和第二阶段需要~100米的精度，而第三阶段需要~10米的精度)。该项目正在应用最近开发和升级的分析技术，既增加了现有的数据量，又显著降低了噪音水平。还利用了一种新的双差分断层成像技术，该技术已被证明比用标准技术获得的事件位置和波速图像都有显著改善。此外，重要的新数据现已从2003年11月的一系列地震实验中获得。这是一个关键的新数据集，它的分析和纳入建模将导致对模型和地震位置的重大改进。预计到2004年夏季第一阶段钻探开始时，这些措施将使不确定因素减少到100米以下。一旦第一阶段钻井完成，临时地震台网(Paso-tres)将安装并运行12个月，重新占用Paso-DOS站点中的12个。除了与钻孔和该地区永久台站中的仪器同时记录周围的地震活动外，还将记录来自10个小(~2磅)炮击的信号，以便通过将井下传感器视为“虚拟地震”来校准波速和估计绝对定位精度。如果采用与PASO-DOS相同的分析技术，将主动和被动的PASO-TRES数据与钻探现场附近的永久测站(HRSN和USGS)的数据和井下传感器相结合，将使震源位置的不确定性进一步减少到指导第二阶段钻探所需的水平。

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.