Characterizing variations in magnitude-frequency distribution and b-value using relative magnitudes

地震的“震级-频率分布”（MFD）描述了不同震级的地震在特定地区发生的频率，大地震的频率远低于小地震。科学家从MFD获得的小地震与大地震的比例用“b值”表示。这个数量用于从频繁的小地震记录中推断出预计发生更罕见的大地震的频率，并标记出可能发生大地震的地区。对b值的可靠估计需要大量的地震数据收集（或“目录”），其中所有震级都以相同的方式估计。黄博士和她的团队将通过重新计算两个目录的地震震级来表征MFD和b值的变化：一个来自南加州（大多数地震自然发生），另一个来自俄克拉荷马州（大多数地震是由石油和天然气生产废水的注入引发的）。利用新的mfd，她和她的团队将评估在前震、大地震和余震期间这两个地区的b值如何变化；以及（如适用）附近注水井的操作。该项目将为定量地震危害分析提供重要信息，这是更新建筑规范和其他公共安全措施所必需的；并将促进对地震物理学的科学理解。PI和她的团队还将开发和分享一个免费的在线教育工具，使学生、教师和公众能够访问和分析地震仪数据，并估计地震震级。目前的MFD和b值分析受到小地震震级不确定性和地震目录中混合震级类型的强烈影响。最近的研究表明，在低星等范围内，mfd可能偏离指数分布，并且由于高分辨率星表中mfd的不连续，b值可能随着完整性的大小而发生很大变化。该项目将利用重新计算的震级差异，为小地震和大地震编制具有一致震级的地震目录，这将能够系统地分析南加州和俄克拉荷马州mfd和b值的时空变化。在重新计算震级后，本项目将研究mfd可能偏离指数分布及其对b值估计的影响。该项目还将量化在前震、主震和余震期间b值是否有显著变化，以及为诱发地震活动而进行注水井的操作。系统的MFD和b值分析将为地震危险性分析提供更可靠的参数，并揭示b值变化是否表明断层应力状态的变化。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The “magnitude-frequency distribution” (MFD) of earthquakes describes how often earthquakes of different magnitudes occur in a particular region, with large earthquakes being far less frequent than small ones. The proportion of small to large earthquakes, which scientists obtain from the MFD, is represented using the “b-value”. This quantity is used to infer from recordings of frequent, small earthquakes how often rarer, large earthquakes are expected, and to flag regions that may be at risk for major earthquakes. Robust estimates of b-value require enormous collections (or ‘catalogues’) of earthquake data, in which all magnitudes were estimated the same way. Dr. Huang and her team will characterize the variations in MFD and b-value by recalculating earthquake magnitudes for two catalogs: one from southern California (where most earthquakes occur naturally) and one from Oklahoma (where most earthquakes are triggered by injection of wastewater from oil and gas production). Using the new MFDs, she and her team will assess how b-values may vary in these two regions during foreshocks, large earthquakes, and aftershocks; and (where applicable) operation of nearby wastewater injection wells. This project will provide vital information for quantitative seismic hazard analysis, which is required for updating building codes and other public safety measures; and will advance scientific understanding of earthquake physics. The PI and her group will also develop and share a free online educational tool that will enable students, teachers, and the general public to access and analyze seismograph data, and estimate earthquake magnitudes.Current MFD and b-value analyses are strongly affected by magnitude uncertainties of small earthquakes and mixed magnitude types in earthquake catalogs. Recent studies have shown that MFDs can deviate from exponential distribution at low-magnitude ranges and b-value can vary considerably with the magnitude of completeness due to the discontinuity of MFDs in high-resolution catalogs. The project will produce earthquake catalogs with consistent magnitudes for small and large earthquakes using recalculated magnitude differences, which will enable a systematic analysis of the spatial-temporal variations in MFDs and b-value in Southern California and Oklahoma. After the magnitudes are recalculated, the project will investigate the possible deviations of MFDs from exponential distribution and their effects on the estimation of b-value. The project will also quantify whether there are significant changes of b-values during foreshocks, mainshocks, and aftershocks as well as the operation of injection wells for induced seismicity. The systematic MFD and b-value analysis together will produce more robust parameters for seismic hazard analysis and unveil whether b-value variations manifest changes in fault stress state.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.