Cooling magma model for deep volcanic long-period earthquakes

Deep long-period events (DLP events) or deep low-frequency earthquakes (deep LFEs) are deep earthquakes that radiate low-frequency seismic waves. While tectonic deep LFEs on plate boundaries are thought to be slip events, there have only been a limited number of studies on the physical mechanism of volcanic DLP events around the Moho (crust-mantle boundary) beneath volcanoes. One reasonable mechanism capable of producing their initial fractures is the effect of thermal stresses. Since ascending magma diapirs tend to stagnate near the Moho, where the vertical gradient of density is high, we suggest that cooling magma may play an important role in volcanic DLP event occurrence. Assuming an initial thermal perturbation of 400°C within a tabular magma of half width 41 m or a cylindrical magma of 74 m radius, thermal strain rates within the intruded magma are higher than tectonic strain rates of ~ 10^(−14) s^(−1) and produce a total strain of 2 × 10^(−4). Shear brittle fractures generated by the thermal strains can produce a compensated linear vector dipole mechanism as observed and potentially also explain the harmonic seismic waveforms from an excited resonance. In our model, we predict correlation between the particular shape of the cluster and the orientation of focal mechanisms, which is partly supported by observations of Aso and Ide (2014). To assess the generality of our cooling magma model as a cause for volcanic DLP events, additional work on relocations and focal mechanisms is essential and would be important to understanding the physical processes causing volcanic DLP events.