Could the recent Japan earthquake trigger a large Osaka quake?

By Shinji Toda, Ph.D., IRIDeS, Tohoku University, and Ross Stein, Ph.D., Temblor

The 17 June 2018 M=6.1 earthquake (now upgraded from M=5.9 by JMA) that struck 15 km (9 mi) northeast of Osaka continues to produce aftershocks and raise concerns about whether it could trigger larger earthquakes on either of two major active faults. Ruptures on either fault could strongly shake Osaka, a vibrant city of 2.7 million inhabitants.

 

The aftershock pattern suggests that perhaps two perpendicular faults ruptured at once, which is not uncommon. The sense of motion for the M=6.1 mainshock includes both strike-slip (horizontal) and thrust (vertical) slip, consistent with an inference of a two-fault rupture.
The aftershock pattern suggests that perhaps two perpendicular faults ruptured at once, which is not uncommon. The sense of motion for the M=6.1 mainshock includes both strike-slip (horizontal) and thrust (vertical) slip, consistent with an inference of a two-fault rupture.

 

Two major faults in play

We made preliminary calculations of the stress imparted by the M=6.1 quake to surrounding faults, with each fault divided into patches so the variation of stress could be resolved. This reveals that parts of the right-lateral Takatsuki Fault (also known as the Arima-Takatsuki Tectonic Line), and the Uemachi thrust fault, were brought significantly closer to failure. The Takatsuki fault was also strongly loaded by the 1995 M=6.9 Kobe earthquake to the southwest. The Uemachi fault cuts right through Osaka, and so is the most dangerous to the City.

 

Calculated stress transferred by the M=6.1 earthquake to surrounding faults (assuming friction of 0.4) suggests that a 15 x 15 km patch of the Takatsuki Fault, and a 15 x 10 km patch of the Uemachi fault, were brought closer to failure. Calculations were made in the Toda et al. Coulomb 3.4 software.
Calculated stress transferred by the M=6.1 earthquake to surrounding faults (assuming friction of 0.4) suggests that a 15 x 10 km patch of the Uemachi fault (top panel), and a 15 x 15 km patch of the Takatsuki Fault (bottom panel), were brought closer to failure. Calculations were made in the Toda et al. Coulomb 3.4 software.

 

Faults loaded by the amount of stress transferred by the M=6.1 shock can rupture immediately, or they could remain locked for many decades. We can say only that the effects of the stress increase will fade over several decades, so its impact is greatest now. Stress increases of this amount generally produce aftershocks, and so aftershocks on adjacent faults would provide an independent test of the calculation, which is being pursued.

 

Why does this matter?

On 15 April 2016, a M=6.2 earthquake struck near the city of Kumamoto, Japan. Preliminary Coulomb stress calculations suggested that the Futagawa Fault to the northeast, and the Hinagu Fault to the southwest, were both loaded by an amount the same or perhaps twice the amount that occurred in Osaka. A little over a day later, both faults ruptured together in a M=7.0 shock that took 50 lives and injured 3,000 people. Severe damage occurred in the Kumamoto and Oita Prefectures, with numerous structures collapsing and catching fire. More than 44,000 people were evacuated from their homes due to the disaster.

 

Coulomb stress calculations for the 2016 Kumamoto M=6.2 foreshock (left) and M=7.0 mainshock (right).
Coulomb stress calculations for the 2016 Kumamoto M=6.2 foreshock (left) and M=7.0 mainshock (right).

 

We want to stress that no large earthquakes are interpreted or known to be imminent, but the possibility of larger shocks cannot be dismissed. So, this is a moment for situational awareness and thoughtful precautions by the residents of greater Osaka.

 

Osaka, Japan – November 20, 2015: Overlooking Osaka Castle Park at dusk. The castle dates from 1583 and the most recent reconstruction was completed in 1997.

Sources

JMA
NIED Hi-Net
Active Fault Database of Japan, https://gbank.gsj.jp/activefault/

Toda S., H. Kaneda, S. Okada, D. Ishimura, and Z. Mildon, Slip-partitioned surface ruptures for the Mw 7.0 16 April 2016 Kumamoto, Japan, earthquake, Earth, Planets and Space, 68, 188, doi: 10.1186/s40623-016-0560-8

Coulomb 3.3 software, https://pubs.usgs.gov/of/2011/1060/

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