Testing Predictions from the Cascade and Pre-Seismic Slip Models for Foreshock Occurrence with the High Precision Catalog for Northern California

使用北加州高精度目录测试级联和震前滑动模型对前震发生的预测

• 批准号: 0943771
• 负责人: David Schaff
• 金额: $ 23.84万
• 依托单位: Columbia University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-04-01 至 2014-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0943771&HistoricalAwards=false
• 关键词: Testing; Predictions; Cascade; Pre; Seismic

An unanswered question is how do large earthquakes initiate and can their nucleation process be distinguished from small earthquakes? The existence of foreshocks before some large earthquakes is one of the best documented observables preceding mainshocks and therefore may offer clues into these processes. A related question is whether foreshocks are different than any other earthquakes? Foreshocks are the most obvious and well recorded precursors to some large earthquakes. It is essential to understand the physical mechanisms for their occurrence. If a single triggering law can explain the observed behavior for foreshocks, mainshocks, and aftershocks, then foreshocks have no more predictive power than any other earthquake. If, however, they occur as a byproduct and indirect evidence of an aseismic nucleation phase that scales with eventual mainshock magnitude they may have some forecasting ability, in and among themselves, or provide confirmation and support of the pre-seismic slip model predicted by experimental and theoretical research to occur before large earthquakes. Other methods of detecting this nucleation phase may then perhaps be developed. The implications of this research will bear directly on the question of earthquake predictability, thus having broad impact and interest to the general public, but especially among hazard forecasting analysts, governmental agencies, and policy makers. This proposal will test two competing models for the occurrence of foreshocks. The first is the cascade model where foreshocks are a successive series of failures that push the mainshock closer to failure. The second, pre-seismic slip, is one where foreshocks are indirect evidence of a slow, aseismic nucleation phase that is predicted to scale with mainshock magnitude. Both predictions can be evaluated with the high resolution catalog we have developed for northern California (up to three orders of magnitude improvement). Static stress change calculations with improved source parameters and locations can determine if this is a viable mechanism for enhancing or destressing mainshock failure in the cascade model. Conversely measurements of the foreshock zone radius and rupture may show a correlation with mainshock magnitude as predicted in the pre-seismic slip model. Interpretation of these measurements has been difficult before in most prior studies because the location errors were greater than the expected size of the nucleation region. Our preliminary results on a larger data set of 162 foreshock sequences show that there is a negative correlation of foreshock occurrence with normal stress manifested in both the lithostatic load with depth and with regional tectonic stress indicated by the orientation of mainshock slip. Our proposed work will examine if a single, unifying triggering law on theoretical grounds can explain the observations in our data. Clear well-defined predictions of two competing models for foreshock occurrence can now be tested with better quality data and much more of it to determine meaningful statistics and reliable conclusions. These scientific hypotheses were essentially untestable before due to location errors, but now with a large, 300,000 event complete high- precision catalog of northern California, both the static stress triggering mechanism of the cascade model and the aseismic nucleation phase of the pre-seismic slip model can independently be examined if they are consistent with the observed data. It is still not known how large earthquakes initiate and therefore it is important to gain knowledge and insight into the physical processes for immediate foreshocks.

一个悬而未决的问题是，大地震是如何开始的，它们的成核过程能否与小地震区分开来？在一些大地震之前，前震的存在是在主震之前记录的最好的观测之一，因此可能为这些过程提供线索。一个相关的问题是，前震是否与其他地震不同？前震是一些大地震最明显、记录最充分的前兆。了解它们发生的物理机制是至关重要的。如果一个单一的触发规律可以解释观测到的前震、主震和余震的行为，那么前震的预测能力并不比其他地震强。然而，如果它们是地震成核阶段的副产品和间接证据，与最终的主震震级成比例，它们本身或它们之间可能具有一定的预测能力，或者为实验和理论研究预测的地震前滑动模型在大地震之前发生提供证实和支持。然后可能会发展出检测这种成核阶段的其他方法。本研究的意义将直接关系到地震可预见性问题，因此对普通公众，特别是灾害预测分析师、政府机构和政策制定者具有广泛的影响和兴趣。这一提议将检验两个相互竞争的前震发生模型。第一种是级联模型，其中前震是一系列连续的故障，使主震更接近故障。第二种是震前滑动，在这种滑动中，前震是缓慢的地震成核阶段的间接证据，预计该阶段与主震震级成核。这两种预测都可以用我们为北加州开发的高分辨率目录进行评估（提高了三个数量级）。改进震源参数和位置的静态应力变化计算可以确定这是否是级联模型中增强或减轻主震破坏的可行机制。相反，前震带半径和破裂的测量结果可能与震前滑动模型中预测的主震震级有关。在之前的大多数研究中，由于定位误差大于成核区的预期尺寸，对这些测量结果的解释一直很困难。对162个前震序列的初步研究结果表明，前震的发生与正应力呈负相关关系，这既表现为静岩载荷与深度的负相关关系，也与主震滑移方向所指示的区域构造应力呈负相关关系。我们提出的工作将检查是否一个单一的，统一的触发定律在理论基础上可以解释我们的数据中的观察结果。对两种相互竞争的前震发生模型进行明确定义的预测，现在可以用质量更好的数据进行测试，并且可以获得更多的数据，以确定有意义的统计数据和可靠的结论。由于定位误差，这些科学假设在以前基本上是无法检验的，但现在有了一个庞大的、30万事件完整的北加州高精度目录，无论是级联模型的静态应力触发机制还是震前滑动模型的地震成核阶段，如果它们与观测数据一致，都可以独立检验。目前还不知道地震是如何开始的，因此获得知识和洞察直接前震的物理过程是很重要的。