Takanori Matsuzawa

Slow Earthquakes Linked Along Dip in the Nankai Subduction Zone

Three types of temporally linked slow earthquakes may limit nearby buildup of stress. We identified a strong temporal correlation between three distinct types of slow earthquakes distributed over 100 kilometers along the dip of the subducting oceanic plate at the western margin of the Nankai megathrust rupture zone, southwest Japan. In 2003 and 2010, shallow very-low-frequency earthquakes near the Nankai trough as well as nonvolcanic tremor at depths of 30 to 40 kilometers were triggered by the acceleration of a long-term slow slip event in between. This correlation suggests that the slow slip might extend along-dip between the source areas of deeper and shallower slow earthquakes and thus could modulate the stress buildup on the adjacent megathrust rupture zone.

The Boso slow slip events in 2007 and 2011 as a driving process for the accompanying earthquake swarm

Near the Boso Peninsula, central Japan, slow slip events (SSEs) accompanied by earthquake swarms repeatedly occur every 4 to 7 years. We apply a Network Inversion Filter to tilt change and Global Navigation Satellite Systems displacement data simultaneously in order to obtain the spatiotemporal slip evolution of the Boso Peninsula SSEs in 2007 and 2011. Slip initiates on the eastern offshore area in both of the events and propagates to the northwestward in 2007, whereas the 2011 slip propagates to the west. These slip propagations correlate well both spatially and temporally with the migration of the accompanying seismicity. This indicates that the Boso slow slip is a major driving process for earthquake swarm activities, and that monitoring of interplate slip has the potential to be used in assessing the possibility of an earthquake in near real time.

Source Parameters and Rupture Velocities of Microearthquakes in Western Nagano, Japan, Determined Using Stopping Phases

We use an inversion method based on stopping phases (Imanishi and Takeo, 2002) to estimate the source dimension, ellipticity, and rupture velocity of microearthquakes and investigate the scaling relationships between source parameters. We studied 25 earthquakes, ranging in size from M 1.3 to M 2.7, that occurred between May and August 1999 at the western Nagano prefecture, Japan, which is characterized by a high rate of shallow earthquakes. The data consist of seismograms recorded in an 800-m borehole and at 46 surface and 2 shallow borehole seismic stations whose spacing is a few kilometers. These data were recorded with a sampling frequency of 10 kHz. In particular, the 800-m-borehole data provide a wide frequency bandwidth with greatly reduced ground noise and coda wave amplitudes compared with surface recordings. High-frequency stopping phases appear in the body waves in Hilbert transform pairs and are readily detected on seismograms recorded in the 800-m borehole. After correcting both borehole and surface data for attenuation, we also measure the rise time, which is defined as the interval from the arrival time of the direct wave to the timing of the maximum amplitude in the displacement pulse. The differential time of the stopping phases and the rise times were used to obtain source parameters. We found that several microearthquakes propagated unilaterally, suggesting that all microearthquakes cannot be modeled as a simple circular crack model. Static stress drops range from approximately 0.1 to 2 MPa and do not vary with seismic moment. It seems that the breakdown in stress drop scaling seen in previous studies using surface data is simply an artifact of attenuation in the crust. The average value of rupture velocity does not depend on earthquake size and is similar to those reported for moderate and large earthquakes. It is likely that earthquakes are self-similar over a wide range of earthquake size and that the dynamics of small and large earthquakes are similar.

Very low frequency earthquakes off the Pacific coast of Tohoku, Japan

We found very low frequency earthquakes (VLFEs) at a shallow subduction zone close to the Japan Trench off the Pacific coast of Tohoku, Japan. Centroid moment tensor solutions of VLFEs showed reverse fault mechanisms with a compression axis in the east-west direction. A cross-correlation analysis of seismograms with template events between 2005 and 2013 revealed three major VLFE clusters and their temporal evolution. A VLFE cluster in the central off-Tohoku region located in the large slip area of the 2011 Tohoku earthquake was detectable only before the Tohoku earthquake. However, VLFEs in the northern and southern off-Tohoku regions at the rim of the large slip area were activated after the Tohoku earthquake. The change in the activity may reflect the stress redistribution by the coseismic and/or afterslip processes of the Tohoku earthquake.

Possible shallow slow slip events in Hyuga‐nada, Nankai subduction zone, inferred from migration of very low frequency earthquakes

We investigated the spatiotemporal evolution of a shallow very low frequency earthquake (sVLFE) swarm linked to the 2009/2010 long-term slow slip event (SSE) in the Bungo channel, southwestern Japan. Broadband seismograms were analyzed using a cross-correlation technique to detect sVLFEs having similar waveforms to template sVLFEs, and their relative locations were estimated. The sVLFEs exhibit clear migration over a distance of 150 km along the Nankai trough, similar to nonvolcanic tremors and deep very low frequency earthquakes (dVLFEs) accompanied by short-term SSEs on the downward extension of the seismogenic zone. This similarity between sVLFEs and dVLFEs suggests that SSEs occur in both deeper and shallower extensions of the seismogenic zone. The analyzed sVLFEs were likely caused by a shallow SSE that occurred from January to March 2010 following the initiation and acceleration of the long-term SSE. This temporal evolution may be caused by stress interaction between the shallow SSE and the long-term SSE.

Average slip rate at the transition zone on the plate interface beneath the Kii Peninsula, Japan, estimated from deep low-frequency tremors

We investigated the average slip rate at the transition zone on the plate interface beneath the Kii Peninsula, Japan, using an empirical method based on a proportional relation of sizes between short-term slow slip events and nonvolcanic deep low-frequency tremors. The estimated average slip rates are 3.0±0.4 cm/yr, 2.6±0.4 cm/yr, and 2.4±0.4 cm/yr, in the northern, central, and southern Kii Peninsula, respectively. Values in the northern and the central areas compensate for the difference between the slip deficit rate at the transition zone and the convergence rate of the subducting Philippine Sea plate. The discrepancy among those rates is, however, large in the southern area, suggesting a lateral variation in the proportional relation or the existence of a steady, or quasi-steady, slip.