The Full spectrum of the response of Low Frequency Earthquakes to periodic stresses near Parkfield, California

The response of seismicity to periodic stress perturbations over a wide range of periods is not well understood, however, the occurrence of low-frequency earthquakes (LFEs) near Parkfield, California has been found to be strongly correlated with solid earth tides. Laboratory experiments and theoretical analyses show how the amplitude and period of imposed forcing affect the sensitivity to triggering, but frequency-dependent triggering has not been quantitatively explored in the field. Tidal forcing, seismic waves and annual loads act over a wide range of frequencies, therefore the sensitivity to those periodic loadings provides a good probe of the physical mechanisms affecting earthquake generation. In this study, we consider the LFEs near Parkfield, California since 2001. We find the occurrence of LFEs can be independently modulated by groups of tidal constituents at semi-diurnal, diurnal and fortnightly frequencies, and the strength of the response of LFEs to the different tidal constituents varies between LFE families. However, the correlation of the LFEs with periodic stressing from seismic waves and annual hydrological loads is not significant. This suggests that the period of the applied forcing plays an important role in the triggering process, and the interaction of periods of loading history and source region properties, such as friction, effective normal stress and pore fluid pressure, produces the observed frequency-dependent tidal triggering of LFEs.