Evaluation of Seismic Activity in Northern Miyagi Prefecture

It is thought that the seismic activity was a foreshock - main shock - aftershock sequence. The main shock was a shallow event in the crust with a focal mechanism that showed a reverse fault with a compression axis in an E-W direction.

As of 17:00 on August 7, the largest aftershock was a M5.3 event (maximum seismic intensity 6 Lower) at 16:56 on July 26 that occurred near the northern end of the aftershock area. The M4.8 event at 10:22 on July 26 and the M5.0 event at 4:08 on July 28 had a maximum seismic intensity 5 Lower.

* According to the GPS data and the result of leveling, there was 15cm of movement in a southeastern direction and 10cm of lift was observed at the Yamoto observation station (Yamoto-cho, Monou-gun, Miyagi prefecture) which is located close to the epicenter. Also according to the result of leveling, the largest uplift of 15cm was observed in the area from Naruse-cho, Monou-gun, Miyagi prefecture to Yamoto-cho. The earthquake fault determined from the focal mechanism of the main shock and the aftershock activity distribution, was a reverse fault with a N-S strike and dipping toward the west, which is consistent with the source model determined from the GPS and level data.

*So far, from the field studies in the area, no surface faulting has been found.

*From the analysis of the main shock focal process, it is estimated that there was a region with large slip that occurred on the shallow portion of the fault surface.

*Aftershock activity has been declining normally and it is estimated that there is about a 20 percent chance of occurrence of M4.5 or greater aftershocks, within the 3 days from 17:00 on August 7. If an aftershock with a magnitude of about 4.5 occurs, there will presumably be shaking with a maximum seismic intensity 5 Lower. Also it is estimated that aftershocks of M2.7 or greater (approximately corresponding to events with intensity equal to or greater than1) may occur about 1.5 times per day until the end of August.

• Earthquakes in Northern Miyagi Prefecture
• Earthquakes in Northern Miyagi prefecture - Aftershock Activity
• Aftershock Distribution by Time Periods (M>=2.0)
• Aftershock Distribution by Time Periods (M>=2.0) - Part 2
• Focal Mechanism Distribution of Earthquakes in Northern Miyagi Prefecture
• Aftershock Distribution from Data Logger Stations
• Graphs of Changes in Baseline Mizusawa - Yamoto
• Vertical Changes from Leveling Data in the Area from Sendai City to Yamoto
• Preliminary Fault Model for the July 26 07:13 Earthquake (Mj 6.2) in Northern Miyagi Prefecture
• Investigations of Surface Faulting for the Earthquake in Northern Miyagi Prefecture
• Field investigations of the July 26, 2003 Earthquake in Northern Miyagi Prefecture
• Field Investigtions of the July 26, 2003 Earthquake in Northern Miyagi Prefecture (Figure)
• Source Process of the July 26, 2003 07:13 Earthquake in Northern Miyagi Prefecture (preliminary) - Source Inversion from Near-field Recordings of the Earthquake
• 2003 Earthquake in Northern Miyagi Prefecture - Source Process, Strong Motion, Damage
• Source Inversion (July 26, 2003 07:13 JST)
• Analysis of Aftershocks (M >=2.7) of the Earthquake in Northern Miyagi Prefecture
• Comparison of Seismic Activity for the Recent Earthquake and the Period before the 1978 Offshore Miyagi Prefecture Earthquake (>=M5.5)
• Comparison of the Probabilistic Seismic Shaking Map and the Locations of Intensity 6 Lower, or Greater, from the Recent Damaging Earthquake