Cooling magma model for deep volcanic long‐period earthquakes

Abstract

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 À14 s À1and produce a total strain of 2 × 10 À4. 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.

Seismic evidence for a cold serpentinized mantle wedge beneath Mount St Helens

“…6). DLP events are commonly observed in volcanically active regions, including beneath many of the volcanoes in the Cascade Range34, and these events are thought to be caused by the movement or cooling of magmatic fluids3536. The DLP at Mount St Helens are located near the edge of the low velocity column observed in the iMUSH tomography (Fig.…”

Triggered tectonic tremor in various types of fault systems of Japan following the 2012 Mw8.6 Sumatra earthquake

“…Thus, if we consider deep volcanic LFEs in central Hokkaido identified by JMA (i.e., orange circles in Figure ) as one type of tectonic tremor [ Aso et al , , ; Aso and Ide , ], observations by JMA confirm our assumption that ambient tremor occurs in this region. However, the differences between tectonic LFEs in the Nankai tremor zone and deep volcanic LFEs in central Hokkaido have not been thoroughly studied [ Obara , ; Aso and Tsai , ]. In contrast, in southeastern Kyushu, regardless of whether the triggered tremor source is correlated with the offshore shallow VLF earthquake [ Obara and Ito , ; Ito et al , ] and shallow tremor [ Obana and Kodaira , ; Suzuki and Yamashita , ] or the inland tectonic tremor [ Idehara et al , ], our observations have further confirmed that triggered tremor could be a useful tool for inferring background tremor.…”

Deep Long‐Period Earthquakes Beneath Mount St. Helens: Their Relationship to Tidal Stress, Episodic Tremor and Slip, and Regular Earthquakes

“…With presence of magma/fluid and their movement, the physical mechanisms of these DLPs might be shear faulting, since high fluid pressure due to compressive tidal stress would reduce effective normal stress on faults, which would weaken the strength of the fault or fault network. The presence of magma/fluid does not necessarily rule out the magma cooling model (Aso & Tsai, ), considering possible existence of fluid during long‐term cooling process of the magma.…”

“…Understanding the physical mechanism of DLPs could provide insights on (1) the magma/fluid migration process (e.g., Shapiro et al, ), (2) forearc mantle wedge dynamics (Vidale et al, ), and (3) volcanic eruption forecasting (e.g., Chouet, ). Based on observations, several hypotheses have been proposed to interpret the source process of DLPs, including dehydration embrittlement of already‐serpentinized mantle material (e.g., forearc DLPs in western Oregon by Vidale et al, ), magma and/or fluid movement (e.g., Shapiro et al, ; Ukawa & Ohtake, ), thermal strain from magma cooling (Aso & Tsai, ), as well as others (e.g., Aki & Koyanagi, ; Chouet, ).…”