Sanriku-oki Earthquake on 7th December, 2012

launched: December 13, 2012

Japanese

An earthquake with magnitude of 7.3 occurred at Sanriku-oki (offshore, east of Sendai). This earthquake is considered to have occurred at outer rise (offshore 2011 Tohoku Earthquake).

[Note: Research/analysis outcomes gained by individual researchers in our institute will be posted on this website. All information publicized here do not indicate any official opinion as the Earthquake Research Institute.]


Focal Mechanism derived by W-phase Source Inversion

Following is the focal mechanism derived by W-phase Source Inversion using waveform data recorded in global scale.. This method is developed by Kanamori and Rivera (2008). However, since the analysis system is completely automated, the solution may be differed according to the further analysis.

 (Hiroshi Tsuruoka)


Strong motion seismogram recorded in ERI

This figure shows acceleration waveforms recorded at the ERI surface station (located near the storehouse) using strong motion seismometer. NS, EW, and UD component waveforms are shown from above, and horizontal axis shows 9minutes. Instrumental seismic intensity was 3.3.

(Koji Miyakawa)



The animation of Seismic wave propagation of magnitude 7.3 earth quake on 7th

Visualized image of the seismic wave propagation of the earthquake on 7th, recorded by K-NET/KiK-net from NIED (National Research Institute for Earth Science and Disaster Prevention).

(click the image to see animation)

(Teito Takemoto, Takuto Maeda, Masaru Todoriki, Takashi Furumura)


A reverse fault revealed by the polarity of long-period ground motion

 (click images to enlarge)

Fig. (a) A spatial pattern of P-wave first motion polarity, expected from the reverse fault mechanism of the two earthquakes in Global CMT catalogue.
Fig. (b) A spatial pattern of P-wave first motion polarity, expected from the solution of F-net mechanism.
Color scale beneath the figures indicates for both A and B

Fig.(c) and (d), is the waveform comparison of vertical ground motion, broadband (5-50sec) record, observed by the Hi-net of NIED in Hokkaido and Honshu. The instrumental response is corrected using the time-domain digital filter by Maeda et al. (2011), and the amplitude is standardized by the maximum value of each trace. Also, velocity is reduced by 7.8km/s in this figure. The P wave initial motion polarity is “up”(red arrow) in Hokkaido, and is obviously turning “down”(blue arrow) in Honshu. This is concordant with the radiation pattern of the reverse fault mechanism, and is supporting the possibility of 2 earthquakes: one with normal fault, and the other with reverse fault.

[ref.]Maeda, T., K. Obara, T. Furumura, and T. Saito, Interference of long-period seismic wavefield observed by dense Hi-net array in Japan, J. Geophys. Res., 116, B10303, doi:10.1029/2011JB008464, 2011.

(Takuto Maeda)


Estimated focal mechanism derived from teleseismic body-wave

The estimation of focal mechanism was attempted using the program of Kikuchi and Kanamori (2003) with 73 teleseismic body-wave records collected from the IRIS-DMC. As we set 2 triangular pulses, a reverse fault was derived for initial rupture, and a normal fault was derived for 20 seconds after the initial rupture. The total seismic moment was 8.8×1019Nm(Mw7.2): 5.9×1019Nm(Mw7.1, depth:56km ) for the reverse fault, and 7.8×1019Nm(Mw7.2, depth:6km)for the normal fault. However, this result may change according to the further analysis.

(Tomoya Harada, Satoko Murotani, Kenji Satake)