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

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.