Seismic activity near the Moriyoshi-zan volcano in Akita Prefecture, northeastern Japan: implications for geofluid migration and a midcrustal geofluid reservoir

Abstract

The 2011 off the Pacific coast of Tohoku (Tohoku-oki) earthquake caused increased seismicity in many inland areas in Japan. A seismic cluster north of the Moriyoshi-zan volcano in Akita prefecture, Tohoku District, is of interest in light of the contribution of geofluids to seismic activity. We observed a seismic cluster characterized by the migration of seismicity and reflected/scattered phases. We relocated hypocenters of the cluster using data from temporal observations and the hypoDD location technique, which significantly increased the hypocentral accuracy. We interpreted a complex spatiotemporal variation of seismicity in the cluster as the migration of pore fluid pressure from multiple pressure sources. The hydraulic diffusivity of the cluster was in the range of 0.01 to 0.7 m 2 /s and increased with time, implying that the migration of hypocenters accelerated after a pathway for fluids was formed by fracturing of the wall rock during the initial stage of seismic activity. A prominent feature of the seismograms is a reflected/scattered phase observed at stations around the volcano. We regard the phase as S-to-S scattered waves and estimated the location of the scatterers using a back-projection method. The scatterers are inferred to be located about 5 km northwest of the Moriyoshi-zan volcano, at an approximate depth of 13 km. The Moriyoshi-zan area is one of the source areas of deep low-frequency earthquakes that have been interpreted as events generated by the migration of geofluids. The depth of the scatterers is close to the upper limit of the depth at which low-frequency earthquakes occur. Thus, we interpret the observed scatterers to be a reservoir of geofluid that came from the uppermost mantle accompanying contemporaneous low-frequency earthquakes.

Complex microseismic activity and depth-dependent stress field changes in Wakayama, southwestern Japan

“…Nakajima and Hasegawa (2007) pointed out that high seismicity in this study area is probably related to locally concentrated fluids. High seismicity attributed to fluids is also pointed out elsewhere (e.g., Kosuga 2014;Terakawa et al 2013). Due to the influence of fluid, pore pressure is considered high enough to weaken the faults in this study area.…”

Remote triggering of seismicity at Japanese volcanoes following the 2016 M7.3 Kumamoto earthquake

“…We also estimate the peak dynamic stress changes associated with the passage of Rayleigh and Love waves in the triggered regions from the amplitudes of the surfacewave ground velocities (e.g., Peng et al 2009). Assuming plane wave propagation for teleseismic waves, the peak dynamic stress σ d is proportional to Gu'/V ph (Jaeger and Cook 1979), where G is the shear modulus, u' is the peak particle velocity, and V ph is the phase velocity. Using a nominal G value of 30 G Pa, V ph = 4.1 km/s for the Love waves, and V ph = 3.5 km/s for the Rayleigh waves, we estimate dynamic stress change values.…”

Special issue ‘Geofluid processes in subduction zones and mantle dynamics’

“…The former, having a diameter of 10 to 20 km, resides in the lower crust. Kosuga (2014) observed the migration of seismicity for a seismic cluster near the Moriyoshi volcano in the Northeast Japan arc and identifies distinct seismic scatters above low-frequency earthquakes in the lower crust. He argues that the observed migration is associated with geofluids supplied from the uppermost mantle.…”