Modulation of seismic noise near the San Jacinto fault in southern California: origin and observations of the cyclical time dependence and associated crustal properties

We examine the cyclic amplitude variation of seismic noise recorded by continuous three-component broad-band seismic data with durations spanning 91-713 d (2008-2011) from three different networks: Anza seismic network, IDA network and the transportable seismic array. These stations surround the San Jacinto Fault Zone (SJFZ) in southern California. We find the seismic noise amplitudes exhibit a cyclical variation between 0.3 and 7.2 Hz. The high-frequency (>= 0.9 Hz) noise variations can be linked to human activity and are not a concern. Our primary interest is signals in the low frequencies (0.3-0.9 Hz), where the seismic noise is modulated by semi-diurnal tidal mode M-2. These long-period (low-frequency) variations of seismic noise can be attributed to a temporal change of the ocean waves breaking at the shoreline, driven by ocean tidal loading. We focus on the M-2 variation of seismic noise at f = 0.6 Hz, travelling distances of similar to 92 km through the crust from offshore California to the inland Anza, California, region. Relative to the shoreline station, data from the inland stations show a phase lag of similar to-12 degrees, which we attribute to the cyclic change in M-2 that can alter crustal seismic attenuation. We also find that for mode M-2 at 0.6 Hz, the amplitude variations of the seismic quality factor (Q) depend on azimuth and varies from 0.22 per cent (southeast to northwest) to 1.28 per cent (northeast to southwest) with Q = 25 for Rayleigh waves. We propose the direction dependence of the Q variation at 0.6 Hz reflects the preferred orientation of subfaults parallel to the main faulting defined by the primarily N45 degrees W strike of the SJFZ.

Automated summary

The study focuses on the cyclic amplitude variation of seismic noise in southern California, showing patterns between 0.3 and 7.2 Hz. The modulation of seismic noise by semi-diurnal tidal mode M-2 is observed in low-frequency signals. Inland stations exhibit a phase lag relative to shoreline stations, possibly due to cyclic changes in M-2 and crustal seismic attenuation.