We conducted an S-wave shallow seismic reflection profiling to reveal subsurface structure deformed by a blind thrust across the Ayasegawa fault in the northern area of the Omiya upland, central Japan. The NW-SE trending and SW dipping Ayasegawa fault is located in the southeastern part of the Kanto-heiya-hokuseien fault zone along the northwestern margin of the Kanto Plain, and is one of the nearest active faults to the Tokyo metropolitan area. This seismic profiling consisted of two survey lines which are Line 1 with a length of about 0.5km and Line 2 with that of about 0.9km. An automated S-wave plank hammering system (Hanshin Consultants Co., Ltd.) was used as seismic source. Both the standard shot intervals and group intervals of geophones were 2m. The common mid-point seismic reflection data was acquired by a digital telemetry recording system (JGI, Inc.). Folded strata, which were showing the deformation zone, were clearly recognized in the obtained post-stack migrated, depth converted seismic section. The width of the deformation zone with anticlinal uplift is about 0.8km along the seismic section. The relative uplift rate in the southwestern side of the Ayasegawa fault is 0.12-0.14mm/yr at the crest of the anticlinal uplift inside the deformation zone, and is 0.08-0.09mm/yr outside the deformation zone.
We investigated the spatial distribution, temporal change and some statistical features of the swarm activity in Hakone volcano after the 2011 Off the Pacific Coast of Tohoku earthquake (hereafter, the 2011 Tohoku earthquake). Though overall spatial distribution of the activity was not much different from that observed at swarm activities in recent years, its temporal change was quite different: Contrary to recent activities in which burst-like earthquake occurrence was observed repeatedly, the activity after the 2011 Tohoku earthquake declined rather rapidly and monotonously according to an inverse power law of the elapsed time from the 2011 Tohoku earthquake. This feature was most clearly seen in the change of daily number of earthquake clusters. Another notable feature of the activity was that the b value was significantly smaller than the values of recent swarm activities. These characteristics suggest that the swarm activity was induced by the sudden increase of static stress caused by the 2011 Tohoku earthquake on March 11.
We reported seismic waveform analysis results of the source process of the 2011 mega-thrust Tohoku-oki earthquake. The Tohoku-oki earthquake is the first mega-thrust earthquake in Japan since the initiation of modern and multi-channel seismic observation. Many researchers have performed source inversion using seismic waveforms observed at near-source or/and global seismic networks, and presented their seismic source models. As pointed out by some researchers, however, seismic source models for the 2011 Tohoku-oki earthquake are different from one another. The discrepancy has prevented us to understand the nature of mega-thrust earthquake in Tohoku-oki region, which may be originated from data processing manner, assumption of error structure, strength of constraints, and setting of source model. In this paper, we endeavor to describe detailed analysis procedure for each study. Common feature in slip distributions obtained by all studies is that the huge seismic slip located off the coast of Miyagi (Miyagi-oki) where huge slip deficit was detected by GPS studies. Many studies obtained the large rupture near Japan Trench or hypocenter during 45-70 sec after initial break, which may be a key of understanding this earthquake. At present, it is difficult to discuss the detailed rupture process of the 2011 Tohoku-oki earthquake. The future studies using multi-channel data analysis with proper error structure model will reveal the nature of mega-thrust earthquake.
The 2011 off the Pacific coast of Tohoku Earthquake (Mw9.0) occurred on March 11, 2011, caused strong ground motion around northeastern Japan, and generated devastating tsunami, which killed more than 16,000 people. Before the strong ground motion hit cities, the Japan Meteorological Agency (JMA) issued Earthquake Early Warning (EEW) announcements to the general public of the Tohoku district and then the warning was automatically broadcast through TV, radios and cellular phone messages. JMA also issued the first tsunami warnings/advisories based on hypocentral parameters, i.e., location, focal depth and magnitude, at 14:49 (Japan Standard Time), which was about three minutes after the occurrence of the earthquake, and then upgraded them using sea-level observation data. This paper reports the performance of the EEW and the tsunami warnings/advisories, lessons learned from the earthquake, and direction for the improvement of the warning systems.
The 2011 M 9.0 Tohoku-Oki earthquake is the largest earthquake that occurred in and around Japan since the beginning of the recorded history. This megathrust event initiated approximately 100km off-shore Miyagi prefecture, in northeast Japan, and its rupture extended 400-500km along the subducting Pacific plate. This is the first M 9-class earthquake that has been closely recorded by a dense seismograph network. Strong motions of this earthquake are characterized by large seismic intensities and peak ground accelerations (PGA), long durations, and wideness of the area that experienced intense shaking. The ground motions were recorded by 1223 K-NET and KiK-net stations. The PGA exceeded gravity at 20 sites; the largest PGA, of 2933 gals, was observed at the K-NET Tsukidate station (MYG004). The attenuation of the recorded peak values shows a possibility of saturating strong ground motion amplitude with the magnitude. The complex features of the accelerograms and velocity waveforms are discussed in connection with the source processes estimated using long and short period waveform data. Due to the large ground motions and tsunamis associated with this event, more than 16 thousand people were killed and more than 360 thousand houses and buildings were totally or partially destroyed. Although the tsunamis were the primary cause of damage, the strong shaking, liquefaction and landslides also brought serious destruction. However, it was reported that the damage ratios of houses and buildings directly due to shaking were not as high as for the former earthquakes having comparable seismic intensities and PGA. The recorded ground motions at most stations where the seismic intensities and PGA were large had dominant periods shorter than 0.5s and relatively poor power in the 1-2s period range which has strong influence on the damage of few-stories wooden houses. The main reason for the short-period predominance is the amplification due to the low-velocity superficial layer and can be roughly explained by empirical amplification factors for 0.1-0.5s periods. Long-period ground motions were also observed. The velocity response spectra (5% dumping) for periods of 4-20s reached around 100cm/s at many stations, mainly in the Tohoku and Kanto regions. This level may be considered not very large taking into account that the Tohoku-Oki earthquake was an M 9-class event. In the Kanto district, at epicentral distances of 300-400km, liquefaction widely occurred at the artificially reclaimed land in Tokyo and Chiba bay areas and the basin of major rivers, such as the Tone and Ara. The damage (e.g., cutoff of lifelines and differential settlement of house-basements) due to liquefaction was very severe. In this paper we summarize the strong motion characteristics associated with the M 9.0 Tohoku-Oki earthquake and review the latest results from the viewpoint of strong motions.