EES Guest Speaker - Dr. Peter Shearer

This week’s seminar speaker is Dr. Peter Shearer!  Please join us Thursday September 5th, 2024 at 3pm in room 351 Nat Sci.

Title:
Adventures in Measuring Variations in High-Frequency Radiation for Small to Moderate Earthquakes

Abstract:
Earthquake stress drop is a key parameter for understanding fault ruptures and estimating strong ground motions, but stress-drop estimates vary widely among different groups. Traditional methods for measuring earthquake stress drop for small earthquakes (M < 5) rely on analysis of P- and S-wave spectra. Similar-sized earthquakes vary in the strength of their high-frequency radiation and various modeling assumptions can be used to translate these differences into stress-drop estimates. Based on our recent work documenting hard-to-resolve tradeoffs between absolute stress drop, stress-drop scaling with moment, high-frequency fall-off rate, and empirical corrections for path and attenuation terms, we adopt a new approach in which the corner frequencies of the smallest earthquakes in each region are fixed to a constant value. This removes any true coherent spatial variations in stress drops among the smallest events but ensures that any spatial variations seen in larger-event stress drops are real and not an artifact of inaccurate path corrections. Applying this approach across southern California, we document spatial variations in stress drop for M 1.5 to 4 earthquakes that agree with previous work, such as lower-than-average stress drops in the Salton Trough, as well as small-scale stress-drop variations along many faults and aftershock sequences. However, our results are unable to independently determine the average stress drop for small earthquakes, highlighting the limitations of purely empirical approaches to spectral analysis for earthquake source properties and the importance of determining shallow crustal attenuation models.