Physical Model of Volcanic Deep Low-Frequency Earthquakes

Deep low-frequency earthquakes (LFEs) are small earthquakes at the depth of ~10–45 km that radiate low frequency seismic waves. One major type of LFEs is tectonic LFEs that locate around plate boundaries, and they are thought to be slip events. Another type of LFEs is volcanic LFEs that locate around the Moho mostly beneath active or Quaternary volcanoes, and their physical mechanism is not well established, but we found their interesting feature recently. We found that the volcanic LFEs in eastern Shimane in Japan show linear distribution and their major focal mechanism is the compensated linear vector dipole (CLVD) orientated to the direction of the line formed by the hypocenter distribution. We interpret them as a resonance oscillation within an old magma conduit. The next question is what excites the resonance. We suspect the effect of cooling magma as a trigger for the resonance oscillation of volcanic LFEs, in the idea that the thermal contraction of magma produces anisotropic strain rate and resultant some brittle failure would trigger oscillation. To verify this effect, we calculated produced strain rate both analytically and numerically.