SINGAPORE: The recent earthquake that struck Osaka is a reminder to all of us that most cities in Japan are at risk of seismic hazard.
Japan sits on a complicated and tectonically-active zone, deeply affected by the interaction of four major tectonic plates.
Studies conducted by Japanese geologists in past few decades have revealed hundreds of active faults on the islands of Japan, including those in the vicinity of the Hanshin area where the recent earthquake occurred.
Many of these active faults are capable of producing earthquakes measuring 7.0 or greater in magnitude near Japan’s populated cities every few thousand years.
Although earthquakes produced by these active faults are not as powerful and do not occur as frequently as earthquakes generated in subduction zones, they could still bring about heavy damage because their shallow epicentres are much closer to populated cities.
ACTIVE UEMACHI FAULT
The recent earthquake may have brought the Uemachi fault one step closer to its next rupture. It is one of the most jeopardous active faults in the Hanshin area because it cuts right through the populated city of Osaka.
Even though this active fault is not associated with any known damaging earthquakes in Japan’s written history, recent studies using high-resolution geologic data have shown that the Uemachi fault has disturbed shallow sediment layers and produced a clear topographic feature on the surface.
This suggests that the Uemachi fault is active and capable of producing earthquakes. If the Uemachi fault should rupture, it could cause more than 1,000 causalities within the city, based on a scenario published by the Japanese government.
But exactly when the Uemachi fault could rupture is currently unknown. It could happen within the next decade, or next few centuries.
Several earthquakes in past decades have revealed that moderate to strong quakes could result in a change in the stress status in a shallow crust residing close to a ruptured fault, thereby increasing the probability of failures to the adjacent active faults in a geologically short time period (i.e. days to years) – meaning changes from one quake could provoke another major quake.
One of these so-called “stress trigger” examples is the 2016 Kumamoto earthquakes. An early shock (foreshock) of 6.2 magnitude triggered a magnitude-7.0 mainshock after about two days.
A preliminary simulation performed by Japanese scientist Professor Shinji Toda showed a possibility of the recent quake triggering another shallow earthquake in the area of Osaka.
His findings revealed that the recent earthquake may have increased the stress loading on a few adjacent active faults, in particular, the Urmachi fault that sits underneath the city of Osaka. Extra precautions should be taken by those residing in or visiting the Osaka-Kobe (Hanshin) area.
One relief is even though the quake could have transferred extra tectonic stress to the Urmachi fault, it does not mean that the fault will go on to generate an even more damaging earthquake within the Osaka area in the very near future.
The fault may remain locked for several more decades before producing another major earthquake.
NANKAI AREA STANDS A GREATER CHANCE OF A LARGE EARTHQUAKE
The possibility of a larger shallow earthquake to hit the greater Osaka area is not the only future seismic hazard Japan needs to worry about.
In fact, the subduction zone off western Japan could pose an even greater threat to this earthquake-prone country. The Nankai Trough, which has shown itself to be capable of producing mega-earthquakes about every one to two centuries, is close to its tipping point.
In the next few decades, the Nankai Trough could produce an earthquake with magnitude greater than 8.0 off Japan. The western Honshu Island and the Shikoku Island could experience strong ground motions and large tsunami waves.
The western Honshu Island’s economic activities and population density are much greater than that near the massive Tohoku earthquake and tsunami that led to the Fukushima disaster, so the estimated losses from this Great Nankai earthquake could well exceed 2011’s devastation.
Even though large, destructive earthquakes are hazards that countries like Japan cannot escape from, these issues need to be tackled head on.
Proper earthquake engineering, education and research, insurance, and prompt community response could significantly reduce economic losses and the loss of lives from such catastrophic events.
While the Japanese government has expended great effort to strengthen Japan's infrastructure to mitigate potential earthquake damages, residents in and visitors to these earthquake-prone areas are encouraged to take precautions and get acquainted with some simple self-protection measures.
They should also familiarise themselves with the locations of nearby shelters and evacuation routes to be taken during such disasters.
Early-warning systems provide a lead time of a few seconds to less than a minute depending on the earthquake’s epicentre.
It might not allow for city evacuation but it is sufficient for citizens to perform the “Drop, Cover, and Hold on” practice and move to a safe location like emergency relief shelters after the tremors have ceased.
Dr Wang Yu is a visiting scientist at the Earth Observatory of Singapore.