Constraints on the Source Parameters of Low-Frequency Earthquakes in Parkfield and Cascadia

Amanda Thomas, Gregory C. Beroza, David R. Shelly, Michael Bostock, Allan M. Rubin, Genevieve Savard, & Lindsey Chuang

Submitted September 20, 2016, SCEC Contribution #6436, 2016 SCEC Annual Meeting Talk on 9/13 08:00 (PDF)

Low-frequency earthquakes (LFEs) are small repeating earthquakes that occur in conjunction with deep slow slip. Like typical earthquakes LFEs are thought to represent shear slip on crustal faults but when compared to earthquakes of the same magnitude LFEs have lower corner frequencies, implying longer durations, and are depleted in high-frequency content relative to earthquakes of similar magnitude. Along the San Andreas Fault, and in the Cascadia subduction zone, LFEs occur in rapid succession, forming tectonic tremor. Here we present results from LFE source studies in both Parkfield and Cascadia. In Parkfield, we use an empirical Green’s function approach to investigate which physical properties of the LFE source cause high-frequency depletion. We find that the M~1 LFEs have typical durations of ~0.2 s. Using the annual slip rate of the deep SAF and the average number of LFEs per year we estimate average LFE slip rates of ~0.24 mm/s. When combined with the LFE magnitude this number implies a stress drop of ~104 Pa, two to three orders of magnitude lower than ordinary earthquakes, and a rupture velocity of 0.35 km/s. Typical earthquakes are thought to have rupture velocities of ~80-90% the shear wave speed. In Cascadia, we correct LFE waveforms for path effects and use the resulting source time functions to calculate LFE duration and magnitude. We use these estimates to show that, like Parkfield, LFEs in Cascadia also have low stress drops, rupture and slip velocities. We also find that LFE duration displays a weaker dependence upon moment than expected for self-similarity, suggesting that LFE asperities are limited in dimension and that moment variation is dominated by slip. This behavior implies that LFEs exhibit a scaling distinct from both large-scale slow earthquakes and regular seismicity. Together the slow rupture velocity, low stress drops, and slow slip velocity explain why LFEs are depleted in high frequency content relative to ordinary earthquakes and suggest that LFE asperities represent areas capable of relatively higher slip speed in deep fault zones. Additionally, changes in rheology may not be required to explain both LFEs and slow slip; the same process that governs the slip speed during slow earthquakes may also limit the rupture velocity of LFEs.

Thomas, A., Beroza, G. C., Shelly, D. R., Bostock, M., Rubin, A. M., Savard, G., & Chuang, L. (2016, 09). Constraints on the Source Parameters of Low-Frequency Earthquakes in Parkfield and Cascadia. Oral Presentation at 2016 SCEC Annual Meeting.

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