Hisanori Kimura

This article describes improvement of the rapid earthquake analysis system, accurate and quick analysis system for source parameters (AQUA), which has automatically determined the location, magnitude, and fault type of M < ∼7 earthquakes with good accuracy by analyzing long-period waveform data, but could not process the 2011 Off Tohoku giant earthquake (M9.0) due to instrumental and program limitations. Here, we address these problems by incorporating strong-motion seismic data and changing processing parameters accordingly, which extends the upper limit of applicable magnitudes to M9. In a simulated process, this earthquake is properly determined as thrust faulting with M8.6 offshore northeast Japan within 431 s of the first phase arrival. We also evaluate its performance. After improvements, four recent large earthquakes (M7.0–7.9) were properly processed in real time. The successful determination ratio is 100% for events with Japanese seismic intensity ≥6 Lower, and 100% for events with M>7 except nearly simultaneous events. Source parameters with and without fixed horizontal locations are provided within 100–250 and 100–600 s of the first P arrival, respectively. This system is valuable for disaster mitigation because it rapidly provides information for use in tsunami warning and early damage assessment.

We detect time-lapse changes in seismic velocity associated with the 2011 Tohoku earthquake, Japan (M_w 9.0), by applying the moving time window cross-correlation analysis to repeating earthquake records registered by Hi-net borehole seismograms. The phase delay curve of the repeating earthquake records demonstrates up to 0.2 per cent apparent velocity reduction for S-wave time windows, while the reduction is only 0.1 per cent at the maximum for P-wave time windows. The apparent velocity reductions for S-wave time windows are especially large for offshore region near the large slip area of the Tohoku earthquake. To investigate the sensitivity of the phase delay curve to partial velocity change, we perform a finite difference (FD) wave propagation simulation using short-wavelength random inhomogeneous media. We found that a 1 per cent velocity reduction at the top 150 m depth (shallow zone) is the most possible model to explain the observed phase delay curve. The evaluated velocity reduction at the shallow zone is consistent with previous studies that detected velocity changes due to strong ground motion. Through the FD simulation, we also found that displacement of the source location after the Tohoku earthquake is not likely the primary cause of the observed apparent velocity change. The average velocity reduction at depths from 150 m to 80 km (deep zone) is evaluated to be much smaller than 0.1 per cent. This small velocity reduction at the deep zone can be caused by a static strain change due to the Tohoku earthquake and its post-seismic deformation.

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.

Slow slip events (SSEs) are another mode of fault deformation than the fast faulting of regular earthquakes. Such transient episodes have been observed at plate boundaries in a number of subduction zones around the globe. The SSEs near the Boso Peninsula, central Japan, are among the most documented SSEs, with the longest repeating history, of almost 30 y, and have a recurrence interval of 5 to 7 y. A remarkable characteristic of the slow slip episodes is the accompanying earthquake swarm activity. Our stable, long-term seismic observations enable us to detect SSEs using the recorded earthquake catalog, by considering an earthquake swarm as a proxy for a slow slip episode. Six recurrent episodes are identified in this way since 1982. The average duration of the SSE interoccurrence interval is 68 mo; however, there are significant fluctuations from this mean. While a regular cycle can be explained using a simple physical model, the mechanisms that are responsible for the observed fluctuations are poorly known. Here we show that the latest SSE in the Boso Peninsula was likely hastened by the stress transfer from the March 11, 2011 great Tohoku earthquake. Moreover, a similar mechanism accounts for the delay of an SSE in 1990 by a nearby earthquake. The low stress buildups and drops during the SSE cycle can explain the strong sensitivity of these SSEs to stress transfer from external sources.

Strong ground motions from the 2011 off the Pacific coast of Tohoku Earthquake, the most powerful earthquake to have occurred in and around Japan after the installation of a modern seismic network,...

Quake Control: Large earthquakes occur at the margins of two colliding plates, where one plate subducts beneath the other at a shallow angle. These megathrust earthquakes often cause destructive tsunamis owing to the displacement of large volumes of water at the fault along the plate boundary. Two related studies of the seismic structure of subduction zones attempt to reveal the underlying mechanisms of megathrust earthquakes (see the Perspective by Wang ). Kimura et al. (p. 210 ) compared seismic reflection images and microearthquake locations at the Philippine Sea plate where it subducts obliquely beneath Japan. The locations of repeating microearthquakes correspond to active transfer of material from the subducting plate to the continent—a process only previously assumed from exhumed metamorphic rocks. Dean et al. (p. 207 ) observe an expansive structure in the sea-floor sediment near the location of the 2004 and 2005 Sumatra earthquakes in Indonesia that suggests sediment properties may influence the magnitude of megathrust ruptures and their subsequent tsunamis.