ON THE SLIP DISTRIBUTION OF THE TOHOKU EARTHQUAKE OF MARCH 11, 2011, JAPAN

The slip distribution during the Tohoku earthquake of March 11, 2011 (M_W=9.0), has been a subject of debate for many authors. Existing slip distributions all agree on the location of the shallow asperity which caused presumably the largest slip during the earthquake. This is claimed to be associated with the low-frequent part of the seismic energy which fit reasonably well with the extensive GPS data as well as the tsunami wave. However, there is a continuing discussion on whether the co-seismic slip extended down-dip rupturing also the smaller asperities at depths close to the termination of the contact zone.

Our slip inversion fit well with the shallow large asperity, which continue down-dip to intermediate depths through a neck-like structure. Additionally, we see clearly the role played by the foreshock on the 9^th of March, where a smaller asperity at intermediate depths ruptured causing static stress increase in the hypocentral area of the mainshock. This was probably the triggering mechanism for the March 11 mega-thrust event. Apart from this, the mega-thrust co-seismic slip probably did not cause any significant slip at intermediate depths. This is also confirmed by both the distribution of the aftershocks, as well as the location of the previous large earthquakes (7+) in this region.

An independent set of data provides important clues on the location of the most significant asperities along the subduction-interface where there is presumably strong coupling. The trench parallel gravity and topography anomalies have previously been used for mapping the location of asperities. Recently, use of trench parallel Bouguer anomaly (TPBA), was used to enhance that picture, delineating the location of strongly coupled areas more precisely. Using this methodology we find that there is a perfect match between the positive TPBA and the location of the shallow up-dip asperity as obtained by several authors. However, the smaller down-dip asperities do not seem to be affected by the co-seismic slip yet. The importance of the TPBA should be investigated further because it may provide independent evidence on the location of the asperities along the subduction zones prior to the occurrence of the mega-thrust events and hence will enable us to better estimate the likelihood and the extent of these infrequent but disastrous earthquakes.