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Displacement Above the Hypocenter of the 2011 Tohoku-Oki Earthquake

Science  17 Jun 2011:
Vol. 332, Issue 6036, pp. 1395
DOI: 10.1126/science.1207401

The Quake That Rocked Japan

The 11 March 2011 magnitude 9.0 Tohoku-Oki megathrust earthquake just off the Eastern coast of Japan was one of the largest earthquakes in recorded history. Japan's considerable investment in seismic and geodetic networks allowed for the collection of rapid and reliable data on the mechanics of the earthquake and the devastating tsunami that followed (see the Perspective by Heki). Sato et al. (p. 1395, published online 19 May) describe the huge displacements from ocean bottom transponders—previously placed directly above the earthquake's hypocenter—communicating with Global Positioning System (GPS) receivers aboard a ship. Simons et al. (p. 1421, published online 19 May) used land-based GPS receivers and tsunami gauge measurements to model the kinematics and extent of the earthquake, comparing it to past earthquakes in Japan and elsewhere. Finally, Ide et al. (p. 1426, published online 19 May) used finite-source imaging to model the evolution of the earthquake's rupture that revealed a strong depth dependence in both slip and seismic energy. These initial results provide fundamental insights into the behavior of rare, very large earthquakes that may aid in preparation and early warning efforts for future tsunamis following subduction zone earthquakes.

Abstract

The moment magnitude (Mw) = 9.0 2011 Tohoku-Oki mega-thrust earthquake occurred off the coast of northeastern Japan. Combining Global Positioning System (GPS) and acoustic data, we detected very large sea-floor movements associated with this event directly above the focal region. An area with more than 20 meters of horizontal displacement, that is, four times larger than those detected on land, stretches several tens of kilometers long along the trench; the largest amount reaches about 24 meters toward east-southeast just above the hypocenter. Furthermore, nearly 3 meters of vertical uplift occurred, contrary to observed terrestrial subsidence.

On 11 March 2011, a large interplate earthquake [moment magnitude (Mw) = 9.0] occurred at the plate boundary off Miyagi Prefecture, northeastern Japan. The focal region inferred from the distribution of aftershocks stretches about 500 km long and 200 km wide offshore (1).

Various studies have been under way to understand the mechanism of occurrence of this earthquake. For example, the Geospatial Information Authority of Japan (GSI) has reported co-seismic displacements on land, on the basis of the dense Global Positioning System (GPS) network (2). The largest displacement has been detected at the Oshika peninsula (Fig. 1), amounting to about 5 m toward east-southeast (ESE) and about 1 m downward. The GSI also estimated slip distribution on the plate boundary from the observed displacements, and the maximum slip was about 24 m near the hypocenter (2). Because the Oshika peninsula is located about 130 km away from the epicenter of the earthquake, it is preferable to measure crustal movements closer to the focal regions, that is, on the sea floor, to better constrain the focal mechanism of the event.

Fig. 1

Horizontal (A) and vertical (B) coseismic displacements at the sea-floor reference points, associated with the 2011 Tohoku-Oki earthquake. Red squares and a yellow star show locations of sea-floor reference points and the epicenter, respectively. The position reference is Shimosato (an open triangle).

In order to monitor crustal movements offshore, we have been carrying out sea-floor geodetic observations by using the GPS/acoustic combination technique (35) (fig. S1). Five sea-floor reference points were installed off the Tohoku region between 2000 and 2004 (Fig. 1) with campaign observations carried out three times a year on average.

The latest observations before the event were conducted in November 2010 at KAMS and KAMN and in February 2011 at MYGI, MYGW, and FUKU. After the event, we conducted observations at these sites for the period from 28 March to 5 April (6).

Comparison between before and after the event yielded coseismic displacements of 5 to 24 m toward ESE and –0.8 to 3 m upward (Fig. 1, table S1). In particular, at MYGI near the epicenter, we detected a huge coseismic displacement of about 24 m toward ESE and about 3 m upward. Observation errors after the event are somewhat large (up to 50 to 60 cm) compared with those in regular campaigns (up to several centimeters) (6). The observed displacements include any postseismic movements for about 20 days after the mainshock. They would also include coseismic displacements by foreshocks and aftershocks (1), some of which are large enough to affect these sites. However, a displacement caused by each of them is estimated to be a few tens of centimeters at most, and the total amount other than that of the coseismic signal by the mainshock is not larger than 1 m. Therefore, these data illustrate huge coseismic movements and its spatial variance by the mainshock just above the focal region.

The horizontal movement at MYGI is more than four times larger than that detected on land and almost equal to the maximum slip on the plate boundary inferred from terrestrial measurements (2). Additionally, the horizontal displacement at KAMS, located about 70 km northeast of the epicenter, is as large as that at MYGI. Therefore, it is reasonable to interpret that the area where coseismic displacement is greater than 20 m spans at least 70 km. These results suggest that slip on the plate boundary near the trench exceeded the 20- to 30-m level estimated as a maximum by the terrestrial data (2), because slip on the plate boundary should be much larger than displacement of the sea floor.

It is also evident that the up-down components of displacement at MYGI and MYGW show opposite polarity. Because the terrestrial data exhibit subsidence (2), the polar reversal of the vertical displacement from downward to upward expected from the upper plate rebound at the event occurred offshore. Thus, the hinge line corresponding to null displacement is located on the east side of MYGW.

With only five observation sites, we may not be able to constrain the detailed feature of focal mechanism, but we believe that the coseismic displacements obtained offshore in this study will provide far better constraints than only the terrestrial data in inferring a fault model for this event.

Supporting Online Material

www.sciencemag.org/cgi/content/full/science.1207401/DC1

Materials and Methods

Fig. S1

Table S1

References

References and Notes

  1. Materials and methods are available as supporting material on Science Online.
  2. Acknowledgments: We thank Y. Honkura for valuable comments and suggestions, O. L. Colombo for assistance with GPS software, and the GSI for providing us with the GPS data. The installation of MYGW was funded by the Ministry of Education, Culture, Sports, Science and Technology of Japan. Averaged position data for the five stations are available in the supporting online material.
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