Shinzaburo Ozawa on the Deadly "Megathrust" Pacific Earthquake of 2011

November 2012
Shinzaburo Ozawa Geospatial Information Authority of Japan Tsukuba, Japan

A recent analysis of citation figures from InCites (drawing on data from Thomson Reuters Web of Knowledge) identified a 2011 report entitled “Coseismic and postseismic slip of the 2011 magnitude-9 Tohoku-Oki earthquake” (S. Ozawa, et al., Nature, 475(7356): 373, 2011) as a New Hot Paper in the field of Geosciences. Hot Papers, among reports indexed by Thomson Reuters within the last two years, are distinguished by being cited at a rate notably above reports of comparable age and type published in the same journal. To date, this paper has been cited 58 times in the Web of Science.

First author Shinzaburo Ozawa is affiliated with the Geospatial Information Authority of Japan, Tsukuba.

Below, Ozawa answers a few questions about this New Hot Paper and the lessons learned from the March 2011 earthquake and subsequent tsunami that killed upwards of 20,000 people.

SW: In addition to the magnitude of the earthquake and the worldwide attention it received, are there particular reasons why this paper is being highly cited so soon after publication?

Figure (a) coseismic displacements.The 2011 Tohoku Earthquake changed the conventional view in seismological circles that no moment magnitude-9-class (Mw-9) earthquakes would occur in the subduction zone in northeast Japan. In understanding the Mw-9 Tohoku earthquake, crustal deformation data is indispensable. In this regard, our paper provides coseismic and early postseismic displacements from the earthquake detected by the Global Positioning System network operated by the Geospatial Information Authority of Japan, as well as coseismic and postseimsic slip models that explain the displacements. These data and models will provide strong constraints in understanding how this megaquake occurred. The above-mentioned points seem to be the reason why our paper was highly cited immediately after its publication.

SW: Has this event added substantially to our knowledge of earthquakes (and if so, how?), or is it too soon to draw such conclusions?

The 2011 Tohoku Earthquake changed the view that magnitude-9-class earthquakes occur in only a few areas such as Chile, Alaska, Kamchatka, and Sumatra. Thus, it seems that even in an area where no great subduction earthquakes have been observed, there is a possibility of massive earthquakes. Furthermore, the assumed model of the earthquake cycle off the Tohoku region, northeast Japan, has to be modified substantially since it includes only magnitude-7-class or smaller than magnitude-8-class earthquakes at the largest. The 2011 Tohoku Earthquake hints at the existence of a super earthquake cycle, in which magnitude-7-class and magnitude-9-class earthquakes occur repeatedly.

SW: Would you summarize the significance of your paper in layman's terms?

Our coseismic model showed that a very large area stretching 400 km along the Japan Trench, northeast Japan, slipped during the 2011 earthquake to a great extent. The postseimic model shows that the afterslip area on the Pacific plate interface reached a deep portion of the subducting Pacific plate on the basis of highly accurate crustal deformation data. These points and crustal deformation data provided us with leading pictures of the earthquake immediately after the earthquake.

SW: How did you become involved in earthquake research?

Our institute operates the Global Positioning System network in Japan. The detected ground displacements are reported in several meetings regularly held for the examination of earthquake activities under the auspices of the central government. Since crustal deformation data is indispensable to earthquake research, I have been involved in earthquake research since my entry to the Geospatial Information Authority of Japan.

It is now clear that, contrary to the conventional belief, magnitude-9-class subduction earthquakes in Japan are possible.

SW: Where do you see your research leading in the future?

After the 2011 Tohoku Earthquake, postseismic deformation started; this has been continuing until now. The mechanisms of this postseismic deformation are thought to be an afterslip on a plate interface, a viscous relaxation induced by the earthquake, and a change in the property known as poroelasticity around the rupture area. There is no detailed report on the above-mentioned processes after magnitude-9-class earthquakes because of the scarcity of observations. In this regard, the 2011 Tohoku Earthquake is the best case to understand the processes after a great earthquake, since the continuous GPS network in Japan is one of the densest in the world.

SW: Given the scale of the destruction caused by this event, have there been additional social or political implications to your research?

It is now clear that, contrary to the conventional belief, magnitude-9-class subduction earthquakes in Japan are possible. Hence, the central government is now reevaluating the risk of subduction earthquakes. We were asked by several semi-governmental companies to provide our coseismic model. Since triangulation stations of the Geospatial Information Authority and other governmental institutes moved to a large extent on a large scale at the time of the 2011 earthquake, we are providing displacement data computed using our model to surveyors so that they can estimate how to survey triangulation stations whose locations are used in public-certified surveys. Since the Pacific coastal area in northeast Japan subsided by approximately several tens of centimeter from the 2011 Tohoku Earthquake, we are monitoring how the altitude of these areas changes after the earthquake, which is important for people who live in the Pacific coastal area damaged by the earthquake.

Download Figure (a)

Shinzaburo Ozawa
Geospatial Information Authority of Japan
Tsukuba, Japan

earthquakes, 2011 Tohoku Earthquake, Japan tsunami, Shinzaburo Ozawa, Geospatial Information Authority of Japan

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