Insights in Low Frequency Earthquake Source Processes from Observations of Their Size-Duration Scaling

Low frequency earthquakes (LFE) are detected in association with volcanic and tectonic tremor signals as impulsive, repeated, low frequency (1-5 Hz) events originating from localized sources. While the mechanism causing this depletion of the high frequency content of their signal is still unknown, this feature may indicate that the source processes at the origin of LFE are different from those for regular earthquakes. Tectonic LFE are often associated with slip instabilities in the brittle-ductile transition zones of active faults and volcanic LFE with fluid transport in magmatic and hydrothermal systems. Key constraints on the LFE-generating physical mechanisms can be obtained by establishing scaling laws between their sizes and durations. We apply a simple spectral analysis method to the S-waveforms of each LFE to retrieve its seismic moment and corner frequency. The former characterizes the earthquake's size while the latter is inversely proportional to its duration. First, we analyze a selection of tectonic LFE from the Mexican "Sweet Spot" (Guerrero, Mexico). We find characteristic values of M ∼ 10¹³ N.m (M_w ∼ 2.6) and f_c ∼ 2 Hz. The moment-corner frequency distribution compared to values reported in previous studies in tectonic contexts is consistent with the scaling law suggested by Bostock et al. (2015): f_c ∼ M^-1/10 . We then apply the same source- parameters determination method to deep volcanic LFE detected in the Klyuchevskoy volcanic group in Kamtchatka, Russia. While the seismic moments for these earthquakes are slightly smaller, they still approximately follow the f_c ∼ M^-1/10 scaling. This size-duration scaling observed for LFE is very different from the one established for regular earthquakes (f_c ∼ M^-1/3) and from the scaling more recently suggested by Ide et al. (2007) for the broad class of "slow earthquakes". The scaling observed for LFE suggests that they are generated by sources of nearly constant size with strongly varying intensities. LFE then do not exhibit the self-similarity characteristic of regular earthquakes, strongly suggesting that the physical mechanisms at their origin are different. Moreover, the agreement with the size-duration scaling for both tectonic and volcanic LFE might indicate a similarity in their source behavior.