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.