Kumamoto earthquake: a complex earthquake sequence with large strike-slip ruptures

Koji Okumura

Submitted September 27, 2016, SCEC Contribution #6471, 2016 SCEC Annual Meeting Talk on 9/13 08:30 (PDF)

The Mw 7.3 mainshock of the Kumamoto earthquake on April 16 ruptured the Futagawa fault zone mostly as it was forecasted. However, the sequence of earthquakes, ruptures, and localized severe shaking were beyond the forecast demonstrating the complex nature.

The earthquake sequence began with a Mw 6.2 at 21:26 JST on April 14. This one ruptured deeper part of the Takano and Shirahata faults (TSF) in SW of the Futagawa fault. Strong shaking (JMA scale 7 out of 7) damaged many houses in a small area of Mashiki town-center to kill 9 people. Following another Mw 6.0 on the April 14 source area, the Mw 7.3 occurred at 01:25 JST on April 16. The mainshock induced several Mw 5.3 to Mw 5.9 earthquakes outside its source area, at 75 km NW, 45 km NW, and 40 km SW away from the epicenter. Those induced earthquakes and aftershocks occurred in a zone about 120 km long across Kyushu Island while the Mw 7.3 rupture was only 30 km long.

The 30 km long Mw 7.3 rupture mostly followed the previously mapped Futagawa fault zone with dextral offset up to 2 m. However, the surface offset of the TSF was only 0.3 m. Also, the TSF ruptured twice both on April 14 and on April 16. Assuming the two shocks ruptured the same fault plane, the small offset on April 16 could be due to the strain release on April 14, or to the rather short elapsed time of 1200 to 1500 years. The northeasternmost 5 km section, which also caused severe damages, cutting into the Aso caldera was not mapped before. Rapid erosion in this high-relief area probably erased or buried past surface ruptures. A 3.7 km long branch fault with ~1 m offset appeared from a restraining bend of the Futagawa fault to the Mashiki town-center with conjugate sinistral offset. These faults ruptured active alluvial plain.

The severest damages were concentrated in a small area of the Mashiki town-center where JMA intensity 7 was recorded twice during Mw 6.2 and Mw 7.0. The possible causes of the severe damages are the source process, site amplification, surface faulting, slope failure and lateral spreading, and so on. It is necessary to integrate analyses and models by different disciplines. From geologic points of view, the presumable complexity of shallow subsurface structures at the boundary between Holocene graben fill in south and upland-forming Pleistocene volcanics and gravels in north is a likely cause of the localized amplification. Further investigation is necessary to solve this significant hazard.

Key Words
Kumamoto earthquake, sequence, induced earthquake, surface rupture, offset, damage, soil condition

Citation
Okumura, K. (2016, 09). Kumamoto earthquake: a complex earthquake sequence with large strike-slip ruptures. Oral Presentation at 2016 SCEC Annual Meeting.


Related Projects & Working Groups
Earthquake Geology