Implications of Seismic Velocity Structure at the Junction of Kuril‐Northeastern Japan Arcs on Active Shallow Seismicity and Deep Low‐Frequency Earthquakes

Abstract

We estimate the velocity structures of P (Vp) and S waves (Vs) in southern Hokkaido, Japan, where a large number of crustal low‐frequency earthquakes (LFEs) have occurred. To investigate the relationship between structure and crustal seismicity, we compare the obtained velocity heterogeneity between the crustal LFE regions, which are characterized by active volcanoes, Quaternary volcanoes, and seismic swarms. The tomographic results show that low‐velocity zones (LVZs) with high Vp/Vs are located around the Moho discontinuity beneath active volcanoes, and the LVZs extend horizontally from beneath the active volcanoes to other LFE regions. In southern Hokkaido, crustal LFEs also appear inside and below the upper crust as well as around the Moho discontinuity. These intermediate‐depth and deep LFEs are located around the LVZs with a high Vp/Vs ratio, indicating the contribution of partial melt to the LFEs genesis, even at a distance from the active volcanoes. At intermediate depths of the crust, LFEs are distributed even in the high‐velocity zones, adjacent to the LVZs. This indicates that the partial melt migrations within the crust affect the activity of LFE at the intermediate depths. Below the seismic swarms in the upper crust, LVZs with crustal LFEs are distributed, probably owing to upward migrations of fluid originating from the solidification of the partial melt; this indicates a link between the shallow and deep crustal seismicity in southern Hokkaido.

Deep Low‐Frequency Earthquakes Associated With the Eruptions of Shinmoe‐dake in KirishimaVolcanoes

“…These two low‐velocity zones roughly correspond to the depths of DLF earthquakes. Furthermore, the observations are consistent with the idea that DLF earthquakes occur in or around low‐velocity zones in many volcanoes (Hasegawa et al, ; Nakajima, ; Shiina et al, ). We may interpret that hypocenters of DLF earthquakes at Kirishima volcanoes may be concentrated at many small sources such as small magma reservoirs located in or around the magma sills in the low‐velocity zones (Figure ).…”

The 2018 Hokkaido Eastern Iburi earthquake (MJMA = 6.7) was triggered by a strike-slip faulting in a stepover segment: insights from the aftershock distribution and the focal mechanism solution of the main shock

“…The velocity of the S wave (V s ) was calculated at each depth as V p /1.73 with the assumption that the V p /V s ratio is 1.73. Recently several authors presented a 3-D tomographic model of V p and V s in this region (Katsumata et al 2006;Yoshida et al 2007;Kita et al 2012;Shiina et al 2018); thus, we assumed a 3-D velocity structure as shown in Additional file 1. The study area was divided into grids every 10 km in both the longitude and latitude directions on each plane at depths of 0, 5, 10, 22.5, 35, and 50 km.…”

“…The study area was divided into grids every 10 km in both the longitude and latitude directions on each plane at depths of 0, 5, 10, 22.5, 35, and 50 km. Values of V p and the V p /V s ratio at depths shallower than 10 km were based on Yoshida et al (2007), and those at depths equal to and deeper than 10 km were based on Shiina et al (2018).…”

Three-dimensional S-wave attenuation structure in and around source area of the 2018 Hokkaido Eastern Iburi Earthquake, Japan

“…Although validities of estimated Qs would be declined, these low-Qs areas beneath the volcanoes seem to extend towards the backarc region with depths. It would represent the upwelling flow in the mantle wedge which has been retrieved as low-velocity (e.g., Nakajima et al 2001;Kita et al 2014;Shiina et al 2018) and high-attenuation (Nakamura and Uetake 2004;Takahashi 2012;Nakajima et al 2013;Liu et al 2014) anomalies in P and S waves in the Hokkaido and the Tohoku district, Japan. At the forearc side from the volcanoes, the high-Qs zones are widely estimated at the depths of 30-40 km or deeper.…”

“…This suggests that S-wave attenuation in the Pacific slab and mantle wedge beneath the forearc region tend to be weak. It is known that attenuation effects can be modeled by the scattering attenuation and the intrinsic absorption (e.g., Hoshiba 1993; Takahashi et al 2009;Sato et al 2012). Carcolé and Sato (2010) mentioned characteristics in the two types of Qs using the multiple lapse time window analysis.…”