Youichi Asano

Stress before and after the 2011 great Tohoku‐oki earthquake and induced earthquakes in inland areas of eastern Japan

[1] Stress fields in inland areas of eastern Japan before and after the Tohoku-oki earthquake were estimated by inverting focal mechanism data. Before the earthquake, s1 axis was oriented EW in Tohoku but NW-SE in Kanto-Chubu. The stress fields changed after the earthquake in northern Tohoku and in southeastern Tohoku near Iwaki city, where the orientations of the principal stresses became approximately the same as the orientations of the static stress change associated with the earthquake. This indicates that differential stress magnitudes in these areas before the earthquake were smaller than 1 MPa. The stress field did not change in central Tohoku, even though the stresses loaded after the earthquake had nearly reversed orientations, which indicates that the differential stress magnitudes there were significantly larger than 1 MPa. In Kanto-Chubu, stresses having nearly the same orientations as the background stresses were loaded after the earthquake, and the stress fields did not change as expected. This may have caused very high induced seismicities in Kanto-Chubu. Citation: Yoshida, K., A. Hasegawa, T. Okada, T. Iinuma, Y. Ito, and Y. Asano (2012), Stress before and after the 2011 great Tohoku-oki earthquake and induced earthquakes in inland areas of eastern Japan, Geophys. Res. Lett., 39, L03302,

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.

Spatiotemporal distribution of very-low frequency earthquakes in Tokachi-oki near the junction of the Kuril and Japan trenches revealed by using array signal processing

We have developed a signal-processing method for array data recorded at densely and widely distributed tiltmeter stations in Japan and analyzed the data recorded during the last 5 years. Filtered seismograms were analyzed to detect coherent seismic waves and estimate the epicenters. In order to identify very-low frequency earthquakes (VLFEs) with only low-frequency energies only, we selected only events not listed in the earthquake catalogue provided by the Japan Meteorological Agency from all of the detected events. The epicenters of the obtained VLFEs were estimated to be distributed not only along the Nankai Trough, where the events occurred on the reverse faults in the well-developed accretionary prism, but also in Tokachi-oki without accretion. This result suggests that even in such a region, VLFEs can occur on a shallower portion of the plate boundary than the seismogenic region of ordinary earthquakes and/or in a deformed area of the overriding plate along the subduction zone without accretion.

Aftershock distribution and 3D seismic velocity structure in and around the focal area of the 2004 mid Niigata prefecture earthquake obtained by applying double-difference tomography to dense temporary seismic network data

A destructive large earthquake (the 2004 mid Niigata prefecture earthquake) sequence occurred in the central part (Chuetsu district) of Niigata prefecture, central Japan on October 23, 2004. We have deployed a temporary seismic network composed of 54 stations for aftershock observation just above and around the focal area of the earthquake for about a month. Using travel time data from the temporary seismic network and surrounding routine stations, we obtained precise aftershock distribution and 3D seismic velocity structure in and around the fault planes of the earthquake and four major (M ≥ 6) aftershocks by double-difference tomography. The results clearly show three major aftershock alignments. Two of them are almost parallel and dipping toward the WNW. The shallow and deep aftershock alignments correspond to the fault plane of the mainshock and that of the largest aftershock (M6.4), respectively. The third alignment is almost perpendicular to the WNW-ward dipping planes and perhaps corresponds to the fault plane of the M6 aftershock on October 27. General feature of the obtained velocity structure is that the hanging wall (western part of the focal area) has lower velocity and the footwall (eastern part of the focal area) has higher velocity. Major velocity boundary seems to shift westward in comparison to in northern and southern parts at a location near the central part of the focal area, where the main shock rupture started. Some parts of the fault planes were imaged as low velocity zones. This complex crustal structure would be one of possible causes of the multi-fault rupture of the 2004 mid Niigata prefecture earthquake sequence.

Migrating tremor off southern Kyushu as evidence for slow slip of a shallow subduction interface

Silent slip events get shallow Clues to help better predict the likelihood of devastating earthquakes and tsunamis may be embedded in a more gentle type of rumbling. Using oceanbottom seismometers, Yamashita et al. report rare observations of migrating tremors in the shallow part of a subduction zone off southern Kyushu, Japan. The tremors appear to be linked to a very low-frequency earthquake and seem to migrate to the region where big earthquakes are generated. The tremors may be tracing how and where stress gets concentrated onto the earthquake-producing portion of the fault. Science, this issue p. 676 Earthquake and tsunami hazard forecasts may benefit from shallow observations of seismic tremor migration in subduction zones. Detection of shallow slow earthquakes offers insight into the near-trench part of the subduction interface, an important region in the development of great earthquake ruptures and tsunami generation. Ocean-bottom monitoring of offshore seismicity off southern Kyushu, Japan, recorded a complete episode of low-frequency tremor, lasting for 1 month, that was associated with very-low-frequency earthquake (VLFE) activity in the shallow plate interface. The shallow tremor episode exhibited two migration modes reminiscent of deep tremor down-dip of the seismogenic zone in some other subduction zones: a large-scale slower propagation mode and a rapid reversal mode. These similarities in migration properties and the association with VLFEs strongly suggest that both the shallow and deep tremor and VLFE may be triggered by the migration of episodic slow slip events.

Spatial distribution for moment tensor solutions of the 2003 Tokachi-oki earthquake (MJMA = 8.0) and aftershocks

The 2003 Tokachi-oki earthquake with Mw 7.9 is the largest interplate earthquake occurred ever since the high dense broadband seismometer network, the National Research Institute for Earth Science and Disaster Prevention (NIED) F-net, has been established over Japan. We determine the spatial distribution of moment tensor solutions and centroid depths of the mainshock and aftershocks. All aftershocks are divided to three groups: (1) the thrust fault type whose nodal plane is similar to the main shock; (2) the other thrust type with nodal plane different from the main shock; and (3) the normal fault type. The type (1) shows a depth distribution inclined to NW gently, coincident to the upper boundary of descending Pacific Plate. The active area of the type (1) does not overlap with the co-seismic slip area of the main shock at all. On the other hand, the type (2) shows no characteristic depth distribution with centroid depth scattered above and beneath the upper plate boundary. The type (3) are distributed, mainly, at about 40 km depth above the upper plate boundary. P axes of some aftershocks occurred above the plate boundary show the direction from ENE-WSW to ESE-WNW that suggests the effect of the Hidaka collision.

Detailed seismic attenuation structure beneath Hokkaido, northeastern Japan: Arc‐arc collision process, arc magmatism, and seismotectonics

In this study, we imaged a detailed seismic attenuation structure (frequency-independent Q−1) beneath Hokkaido, Japan, by merging waveform data from a dense permanent seismic network with those from a very dense temporary network. Corner frequency of each event used for t* estimation was determined by the S coda wave spectral ratio method. The seismic attenuation (Qp−1) structure is clearly imaged at depths down to about 120 km. For the fore-arc side of Hokkaido, high-Qp zones are imaged at depths of 10 to 80 km in the crust and mantle wedge above the Pacific slab. Low-Qp zones are clearly imaged in the mantle wedge beneath the back-arc areas of eastern and southern Hokkaido. These low-Qp zones, extending from deeper regions, extend to the Moho beneath volcanoes, the locations of which are consistent with those of seismic low-velocity regions. These results suggest that the mantle wedge upwelling flow occurs beneath Hokkaido, except in the area where there is a gap in the volcano chain. In contrast, an inhomogeneous seismic attenuation structure is clearly imaged beneath the Hokkaido corner. A broad low-Qp zone is located at depths of 0–60 km to the west of the Hidaka main thrust. The location almost corresponds to that of the seismic low-velocity zone in the collision zone. The fault planes of the 1970 M6.7 and 1982 M7.1 earthquakes are located at the edges of this broad low-Qp zone. Observations in this study indicate that our findings contribute to understanding the detailed arc-arc collision process, magmatism, and seismotectonics beneath Hokkaido.

Hypocenter and focal mechanism distributions of aftershocks of July 26 2003 M6.4 northern Miyagi, NE Japan, earthquake revealed by temporary seismic observation

We conducted a temporary seismic observation just after the occurrence of July 26, 2003, M6.4 northern Miyagi earthquake, in order to precisely locate aftershock hypocenters. Thirteen portable data-logger stations and one satellite communication telemetry station were installed in and around the focal area of the M6.4 event. Hypocenters of aftershocks were located by using data observed at those temporary stations and nearby permanent stations of Tohoku University, National Research Institute for Earth Science and Disaster Prevention (NIED) and Japan Meteorological Agency (JMA). Obtained aftershock distribution clearly delineates the fault plane of this M6.4 event in the depth range of 3–12 km. The fault plane dips westward at an angle of ~50 degree in the northern part of the aftershock area and northwestward at ~40 degree in the southern part. Data observed at dense temporary stations just above the focal area and nearby permanent stations allowed us to determine focal mechanisms of many aftershocks. The results show that focal mechanism of reverse fault type is predominant in this aftershock sequence. Directions of P-axes, however, varies mainly with locations of hypocenters, and are classified into three groups. Aftershocks with P-axis of NW-SE direction occurred mainly in the southern part of the aftershock area where the M5.6 foreshock and the main shock ruptures were initiated. Many aftershocks with P-axis of east-west direction took place in the central part of the aftershock area where large amount of fault slips by the main shock were estimated from waveform inversions. Many aftershocks in the northernmost part of the aftershock area have focal mechanisms with P-axis of NE-SW direction, similar to that of the M5.5 largest aftershock. A few aftershocks with normal fault type occurred close to convex regions of the main shock fault plane or outside of it.

Migration of low‐frequency tremors revealed from multiple‐array analyses in western Shikoku, Japan

[1] Multiple-array observation above a belt-like tremor zone was conducted to investigate the detailed location and migration of tremor activity in western Shikoku, Japan. In March 2007, an episodic tremor and slip event occurred, and highly coherent waveforms were recorded at three arrays. Multiple signal classification analysis for the data from each array enabled measuring precise arrival directions. The majority of tremor signals suggested relatively low slowness. The arrival directions of tremor signals were used to locate tremor sources by the grid search method. Tracking the tremor activity showed that the tremor migrated within several hours in the northeast-southwest direction over a distance of 12-15 km, and its migration velocity was 1-2.5 km h -1 . This migration velocity is more rapid than the mean velocity of 0.5 km h -1 over the whole tremor episode lasting several days. Such a short-timescale migration may represent fluctuation of slip acceleration during the slow slip event. Whenever a tremor migrates southwestward, very low frequency earthquakes occur in the vicinity of the tremor migration terminus. This indicates that the tremor migration is related to the occurrence of very low frequency earthquakes and slow slip events.

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.