Investigation of Anomalous Seismic Radiation of Long Valley Earthquakes

长谷地震异常地震辐射的研究加州大学伯克利分校地震实验室道格拉斯·德雷格自1980年5月发生的4次M6地震事件显示出非双偶地震矩张量以来，加州东部长谷火山口（LVC）地区的地震事件是由构造还是火山（流体）控制的过程引起的，一直是研究的焦点。这些事件的NDC辐射可归因于多种可能的机制，包括1)高流体压力下的拉伸破坏，2)复杂剪切破坏，以及3)未建模的近源速度非均质性造成的偏差，但控制机制仍未解决。最近LVC地震活动的宽带记录显示了异常辐射，其特征可能是偏差非双偶和体积膨胀（NDCI）震源的组合（Dreger et al., 2000），这表明流体直接参与了某些事件的震源过程。NDCI辐射是由于岩浆注入还是水注入，还是由于气相和平流超压，还是1980年事件的机制是本研究的重点。为了解决这些问题并调查异常事件的广泛程度，我们将进行地震波形研究，以确定118个ML3.5 LVC事件的地震矩张量和源过程时间历史。此外，我们将进行三维数值模拟，以测试近源速度结构是否会影响矩张量反演结果。最后，将重新定位与所研究事件相关的微地震活动，以确定事件特定的断层结构、几何断层复杂性和地震活动可能的时间迁移，作为多个NDCI事件级联期间流体流动的证据。了解LVC地震辐射的性质是揭示该地区构造和岩浆/热液过程的重要一步，也是更好地表征相关地震和火山灾害的重要一步。

Abstract for proposal EAR0087147 (PH #8x)Investigation of Anomalous Seismic Radiation of Long Valley EarthquakesDouglas DregerUniversity of California, Berkeley, Seismological LaboratoryWhether seismic events in the Long Valley Caldera (LVC) region of eastern California are due to tectonic or volcanic (fluid) controlled processes has been the focus of research since a swarm of four M6 events in May of 1980 demonstrated non-double-couple (NDC) seismic moment tensors. The NDC radiation of those events was attributed to a number of possible mechanisms including 1) tensile failure under high fluid pressure, 2) complex shear failure, and 3) bias due to unmodeled near-source velocity heterogeneity, however the controlling mechanism has remained unresolved. Recently broadband recordings of LVC seismicity have displayed anomalous radiation that may be characterized as a combination of deviatoric non-double-couple and volumetric dilation (NDCI) seismic sources (Dreger et al., 2000), which indicates that fluids are directly involved in the source process of some events. Whether the NDCI radiation is due to magmatic or aqueous injection, due to gaseous phases and advective over-pressure, or the proposed mechanisms for the 1980 events is the focus of this research. To address these questions and to investigate how widespread the anomalous events are we will conduct seismic waveform studies to determine seismic moment tensors and source process time histories of 118 ML3.5 LVC events. Additionally, we will perform 3D numerical simulations to test whether near-source velocity structure can bias moment tensor inversion results. Finally, microseismicity associated with the studied events will be relocated to determine event specific fault structure, geometric fault complexity and possible temporal migration of seismicity as evidence of fluid flow during cascades of multiple NDCI events. Understanding the nature of seismic radiation in the LVC is an important step to unraveling the tectonics and magmatic/hydrothermal processes in the region, and to better characterize the related seismic and volcanic hazard.