Source spectra of five slow slip events in the Cascadia subduction zone: Constraints from tremor, VLFEs, and GPS

Abstract:

In the Cascadia subduction zone, large slow slip events are observed geodetically every 12-15 months with low-frequency tremor at 1-10 Hz on seismic records. This phenomenon is called episodic tremor and slip (ETS). During ETSs, broadband seismograms also show very low-frequency earthquakes (VLFEs) at a period range of 10-100 s. Although the ETS source spectrum is the fundamental question for characterizing the source process, its estimation in the seismic range has been difficult due to noise and instrumental limitations. We have therefore developed a series of methods to evaluate the absolute values of the source spectrum at seismic period ranges of 20-100 s and 2-8 Hz. We first detected potential VLFEs below noise by stacking the waveforms at a period range of 20-100 s at peak times of tremor amplitudes (Takeo et al. 2010). The stack-averaged waveforms showed VLFE signals with average moment release of Mw ~3 for each time window with tremor. By assuming the moment release of Mw ~3 for each peak time of tremor we finally obtained source spectrum at the VLFE band of 30-100 s for the Mw 6.8 ETS in 2010 (Takeo and Houston 2015, International Workshop at Nagoya). In this study, we applied the same method to different ETSs in the Cascadia subduction zone in 2007, 2008, 2009, 2011 with Mw 6.5-6.8. The obtained source spectra have patterns similar to that for 2010. From the moments and spectral amplitudes in the VLFE band, we infer apparent corner frequencies corresponding to 0.5-1 day, which is much shorter than the duration of each ETS of ~1 month. The spectra appear proportional to the inverse of frequency above the corner frequency. These results indicate that the source process of ETS consisted mainly of slip pulses with typical durations shorter than a day, consistent with a strong tidal modulation of slip.