Tsunami jepang

Long lasting threat of aftershocks"

 

 

52-year-old local resident Emiko Ohta stands in front of the debris of her destroyed home in Kuji, Iwate prefecture. (Source: Reuters)

The tsunami that hit Japan's eastern coast was caused by a massive earthquake measuring 9.0 on the Richter Scale. Markus Treml, a seismology expert specializing in earthquakes at Allianz Reinsurance, explains the situation.
What were the main factors contributing to the disaster on Japan's coast?

With a magnitude of 9.0 this earthquake is the strongest ever recorded in Japan and the fourth or fifth strongest recorded anywhere in the world. The fact that the epicenter of the quake was 130 km offshore meant fewer damages inland but also caused the tsunami that devastated cities and cost many lives.

Japan is situated in a high earthquake risk zone. There is a lot of seismic activity there, but for the most part it is rather weak and doesn't cause much damage. Interestingly, the northern part of the island of Honshu was by Japanese standards in a rather low earthquake risk zone.

What exactly happened, and couldn't it have been foreseen?

Historically speaking, earthquakes in Japan have often been major catastrophes – the tremors were frequently accompanied by tsunamis and large scale fires. Certainly Japan had anticipated the possibility that a large earthquake could hit Tokyo, but further southwest from the zone where this quake occurred, namely in the Nankai trough. There the Philippine Plate descends into the earth. In the area where this quake occurred, the Pacific Plate "subducts."

There was a high probability that segments of the Pacific Plate could shift (causing an earthquake, Ed.), but no one expected that the entire area east of Honshu would fracture.

When such large portions of tectonic plates are pushed under each other during an earthquake, strong tremors can be observed worldwide. The plates in this case moved more than 17 meters against each other. The area in which more than 10 meters of movement happened was about 300 km wide. This releases massive amounts of energy.

Markus Treml is a seismology expert specialized in earthquakes at Allianz Reinsurance. (Source: Allianz)

Tokyo also observed strong tremors, yet the damages there are not nearly as devastating as in the northeastern part of the country. Why is that?

Tokio is 375 km away from the epicenter. That doesn't mean much for an earthquake of this magnitude since the epicenter is simply the center of the tremor. The actual area of fracture in such a quake is very, very large. Still most of the fracture from this quake happened more than 200 km away from Tokyo, closer to the city of Sendai. There the damages – also from the tsunami – were much worse.

That Tokyo was affected – major fires were reported – is partly due to the relatively insecure bedrock under the region. This can strengthen earthquake tremors significantly.

In your opinion, is there still a possibility of further, stronger tremors in the region?

An earthquake of this magnitude is followed by an extended period of aftershocks. The aftershocks can sometimes be of significant strength even long after the original quake, as we saw in Christchurch in December. After a quake, tension is stored up in the tectonic region. In the worst case this could cause another earthquake in a neighboring earthquake-prone area, although it is questionable whether or not this danger is significantly higher than normal in such areas.
How well is Allianz prepared for these kind of natural catastrophes? What concrete actions have you and your team taken in the past days?

We use earthquake risk models to calculate the probability of the damages we insure against. We do this regularly and it is the basis of our risk management. Directly after an earthquake our job is to identify the affected insurance coverage and prepare data to assess damages. The basis for this is again models, however the tsunami and fires must be examined relative to their actual occurrence.