Sendai earthquake in Japan - Magnitude Mw 9
2011 March 11
Short Updates - 2011 April 8, June 3
- Preprint of accepted paper on the ionospheric airglow response to the tsunami (Makela, Lognoné, Hébert, GRL, 2011, in press).
- Several shallow aftershocks occurred in the upper plate close to Fukushima (larger one on April 11 with Mw 7.0). Focal mechanisms correspond to normal or strike-slip faulting.
- On April 7, a Mw 7.1 earthquake occurred near the coastline about 60km east of Sendai. This ~40 km deep earthquake, with a thrust mechanism, is a large aftershock of the Mw9 march 11 earthquake. It probably broke the lower limit of fault ruptured by the march 11 event.
- Source models derived from geodetic and seismologic data show that the march 11 earthquake broke the whole subduction interface from to trench to about 40 km depth, and ~500 km from N to S. Slip is very strong, perhaps reaching 30m, in an area located between the epicenter and the trench. See for instance the GPS preliminary model by A.Socquet (tectonique IPGP), the seismologic-geodetic model by Shengji Wei (Caltech) and Anthony Sladen (Geoazur-CNRS), and the model using near field accelerometric data by the Meteorological Research Institute and the Japan Meteorological Agency. See also SCIENCE online articles published on may 19.
plus d'informations ici**
A large magnitude earthquake has occurred on March 11, 2011 at 5h46am UTC, offshore the island of Honshu, in the northern part of Japan. The moment magnitude Mw is estimated between 8.9 and 9.1. This earthquake ranks among the largest earthquakes ever recorded worldwide and is the largest event known for Japan. The epicenter is located about 400 km to the North-East of Tokyo, the capital city of Japan, and 160km offshore from the city of Sendai. This earthquake is followed by many aftershocks, some with magnitude as large as 7. This seismic activity will last for several weeks, with a possible occurrence of events larger than magnitude 7. During the two days preceding the March 11, 2011 earthquake, an unusual seismic activity had been recorded in the epicentral area, including an earthquake of magnitude 7.2.
The March 11th event has ruptured a 500 km long patch of the subduction interface that extend under Japan, accommodating the relative motion between the Pacific plate, to the West, and the Okhotsk micro-plate, to the East. The rupture has propagated southward in direction of the city of Tokyo and the co-seismic slip along the fault is probably larger than 10m. This earthquake caused a destructive tsunami, which has hit the Japanese coast very quickly and has also propagated eastward across the Pacific Ocean. Runup heights reached values of 10m along Honshu east coast. As all mega-earthquakes, this event has slightly modified the Earth rotation axis and the length of the day (both non detectable by humans). Coseismic displacement at the surface, measured by GPS, reached 4m along east Honshu coast facing the epicenter. It marks the elastic rebound of the upper plate, and vanishes towards the north and south.
The 2011 march 11th earthquake moment magnitude (Mw) has been estimated to 8.9 (USGS), 9.0 (GEOSCOPE-IPGP; Japan Meteorological Agency; Earthquake Research Institute Univ. Tokyo), 9.1 (Global CMT project). Both the focal mechanism, a thrust on a very gently west dipping (10-15°) fault plane, the shallow depth and the location of the hypocenter, imply that the earthquake occurred on the subduction zone that dips west under northern Japan. It is a megathrust earthquake comparable to the Sumatra 2004 and Chile 2010 events.The area of aftershocks is ~500 km long and ~150km wide. It indicates the probable extension of the ruptured part of the fault (see map). The size of the rupture is in agreement with the 3 minutes duration of the rupture. Preliminary source models derived from seismic waves inversion suggest that the co-seismic slip is highly localized on one or two asperities, with slip values possibly as large as 25m. [see another version of the seismotectonic map with 5 days aftershocks]
The dense Japanese network of GPS stations provides a spectacular view of the co-seismic strain field at the surface (see data reported by GEO supersite). Horizontal and vertical displacements respectively locally reach values of 4m and -70cm along the eastern coast of Honshu. Such displacements correspond to the elastic rebound of the upper plate located above the subduction zone during the earthquake. The large displacement is mostly restricted to the area of the main rupture and it decreases rapidly to the North and the South. Hence, the displacement is not affecting the entire Japan as a rigid body, as one can sometime read. Zones affected by large co-seismic subsidence remain partly inundated. Further modeling of the co-seismic strain field measured by GPS will provide a accurate description of the earthquake source, including its geometry and the amount of slip on the fault.
Japan is located at the western hedge of the Pacific plate. This area is part of a 5000 km long subduction zone that bounds the Pacific plate, from the Aleutian islands to the North to the Marianna trench to the South. The Island of Honshu is located at the junction of four tectonic plates, the Pacific plate to the East, the Eurasiatic plate to the West, the Okhotsk micro-plate to the North (also named the Honshu micro-plate, attached either to the Eurasian plate or to the North American plate, depending of geodynamic models) and the Philippines plate to the South. The convergence rate, offshore Honshu, between the Okhotsk micro-plate and the Pacific plate is about 8.5 cm/yr.
The seismicity of Japan is distributed, to the first order, between the subduction along the Nankai Trough (earthquakes of Tonankai, 1944, M8, earthquake Nankai, 1946, M8.1), the trenches of Japan and Izu-Bonin, and intraplate crustal faults, mostly strike-slip, located south-west of the island of Honshu (earthquake of Kobé, 1995, Mw7.4). In addition, the connecting area between these 3 systems, in central Honshu, generates complex deformation, including large earthquakes like the earthquake of Kanto, 1923, M7.9, which has devastated Tokyo and killed more than 150000 peoples. The Western limit of the island of Honshu, which corresponds to the western limit of the Okhotsk plate, is also responsible for diffuse seismicity. The impact of the March 11th, 2011 earthquakes on the possible activation of the various fault systems in Japan will need to be further studied.
Page by Robin Lacassin (Tectonics team - IPGP) with N.Feuillet, E.Jacques, Y.Klinger, P.Lognoné, C.Satriano, N.Shapiro, GEOSCOPE and E.Mitard.
Institut de Physique du Globe de Paris - Mise à jour 05/2013
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