July 11, 2008
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

The Iwate-Miyagi Nairiku Earthquake in 2008
(Mainly about Surface Deformation)

* On June 14, at 08:43 (JST) there was a M7.2 earthquake at a depth of approximately 10km in the southern onshore area of Iwate prefecture. This event had maximum seismic intensity 6 Upper in Iwate and Miyagi prefectures, and caused some damage. The largest aftershock until July 10 is the M5.7 event at 09:20 on June 14 (maximum seismic intensity 5 Lower). The aftershock activity has been generally declining. (see the 184th Evaluation of the Earthquake Research Committee "The Iwate-Miyagi Nairiku Earthquake in 2008" presented on June 26)

* According to the GPS data, there was shortening in a WNW-ESE direction observed in the area centered on the focal region, which is thought to be postseismic movement.

* From the analysis of the focal process of the main shock, it is estimated that regions with large slip were concentrated in the shallow region to the south of the rupture initiation. The fault model that is estimated from the GPS data and the results of the SAR interferometry analysis, shows a concentrated area with similar amounts of slip.

* Field studies so far have found surface deformations located over a length of approximately 20km in a NNE-SSW direction. In these studies, it has been confirmed that surface deformations of uplift on the northwest side with a maximum vertical displacement of about 50cm, were distributed over a length of at least approximately 8km. Those deformations are located along a geologic boundary fault which is shown on the geologic map.

* From the interpretation of an aerial photograph taken in 1976 and afterthe recent earthquake, it has been recognized that landforms which were thought to have been caused by an active fault, are intermittently located over a length of approximately 4km where surface deformations are seen. The following trench excavation surveys at the locations where ground surface deformations are seen, corresponding to those landforms recognized in the photograph, showed evidence of past activity. The recent earthquake is thought to be related to this active fault.