Testing and Reconciling EGF Methods for Estimating Corner Frequency and Stress Drop from P-wave Spectra

Peter M. Shearer, Rachel E. Abercrombie, Daniel T. Trugman, & Wei Wang

Submitted August 9, 2018, SCEC Contribution #8365, 2018 SCEC Annual Meeting Poster #075

Researchers have long used empirical Green’s function (EGF) approaches to correct body-wave spectra for attenuation and estimate corner frequencies and Brune-type stress drops. Recently, however, it has become clear that there are strong tradeoffs among parameters in the model fits, which may explain why stress-drop estimates of specific earthquakes show relatively poor agreement between different studies. We examine this issue by analyzing a tight 6-km-wide cluster of over 3000 M 0.5–5.5 aftershocks of the 1992 Landers earthquake, recorded by many SCSN stations. Because the cluster dimension is small compared to the station distances, we assume that attenuation and path effects are constant for each station, thus simplifying the analysis. We begin by computing P-wave spectra from unclipped vertical-component data using the multi-taper approach of Trugman and Shearer (2017) and retain spectra with good signal-to-noise between 2.5 and 25 Hz. We then apply and compare two different analysis and modeling methods: (1) the spectral decomposition and global EGF fitting approach (e.g., Shearer et al., 2006; Trugman and Shearer, 2017), and (2) the more traditional EGF method of estimating target-event corner frequencies (fc) using spectral ratios with smaller nearby events (e.g., Abercrombie et al., 2016). We find that spectral decomposition yields event terms that are consistent with stacks of spectral ratios for individual events. The main source of fc differences between the two methods comes from the modeling approach to estimate the EGF. The global EGF-fitting approach suffers from parameter tradeoffs among the absolute stress drop, the stress-drop scaling with moment, and the high-frequency falloff rate, but has the advantage that the relative stress drops among the different events in the cluster are well-resolved even if their absolute levels are not. Because the spectral-ratio approach solves for a different EGF for each target event and does not impose any constraint on the corner frequencies of the smaller events, it can produce biased results for target-event corner frequencies. For the Landers cluster, the performance of the spectral-ratio method can be improved by constraining the average small-event fc to a fixed value, in which case the two methods yield very similar results. We plan to apply these insights to devise optimal strategies for spectral analysis and stress-drop estimation for large datasets of distributed seismicity.

Key Words
stress drop, EGF methods, P-wave spectra

Citation
Shearer, P. M., Abercrombie, R. E., Trugman, D. T., & Wang, W. (2018, 08). Testing and Reconciling EGF Methods for Estimating Corner Frequency and Stress Drop from P-wave Spectra. Poster Presentation at 2018 SCEC Annual Meeting.


Related Projects & Working Groups
Seismology