June 26, 2008
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On June 14, at 08:43 (JST) there was a M7.2 earthquake at a depth of approximately 10km in the southern inland area of Iwate prefecture. This event had maximum seismic intensity 6 Upper in Iwate and Miyagi prefectures, and caused some damage. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. This event occurred at a shallow location within the crust. Seismic activity is a main shock-aftershock sequence. Most of the aftershocks occurred within the area that extends approximately 45km in length and 15km in width in a NNE-SSW direction. In a broad view, the locations were distributed dipping downwards to the west. The largest aftershock as of June 25 was a M5.7 earthquake at 09:20 on June 14 that had maximum seismic intensity 5 Lower. The aftershock activity has started to generally decline.
* Associated with this event, accelerations of 3,866gal on the vertical component and 4,022gal on a three-component vector summation, were recorded on the accelerometer located on the ground surface at the Ichinoseki-nishi station, immediately above the focal region. This is the first recording of over 4G (1G = 980gal; acceleration due to gravity). Also on the accelerometer situated at a depth of approximately 260 meters at the same site, accelerations of 640gal on the vertical component and 1,078gal on the three-component vector summation were recorded.
* According to the GPS data, there was an uplift of 2.1m and a horizontal displacement of 1.5m to the southeast at the Kurikoma-2 station, immediately above the focal region. From the analysis of acceleration waveforms, an uplift of 1.4m and a horizontal displacement of 0.6m to the northeast were observed. The results of the SAR interferometry analysis showed that the area with large displacements within the focal region extends approximately 30km in length and 10km in width. On the eastern margin, there is surface deformation which is thought to be the surface fault found in the field survey.
Judging from those crustal movements, it is estimated that the major slip on the fault has a length of approximately 30km and the fault plane is dipping to the west.
* From the analysis of the focal process of the main shock, the rupture is thought to have propagated toward the SSW, and the major slip area is estimated to be at a shallow location to the south of the rupture initiation.
* Field surveys so far have found surface deformations with uplift on the northwest side and vertical displacements including a maximum of about 50cm, located over a length of approximately 6km in a NNE-SSW direction. The sites of the deformed surfaces are to the southwest of the active fault (Dedana fault) which is located in the southern part of the Kitakami-Teichi-Seien Fault Zone. In regards to mapped active faults, it is possible that the locations are on an unrecognized active fault.
* Aftershocks were distributed in a larger area than the region where major slip occurred. There is a possibility that in the northern part of the aftershock area, events occurred on a deeper extension of the Dedana fault.