Tsunami Source Reconstruction by Topex/Poseidon Data

Abstract

The data of the Topex-Poseidon mission (altimetry, files MGDR), recorded on the Sea, bay or strait shows essential deflection of the Sea surface from geo-id. The amplitude of deflection is near to 1 m, the correlation radia is near to 50 km. Most deflections are viewed as the same one for different cycles of the mission, if we examine the same point of the Earth. This means that the main part of anomaly do not depend on time.

The variation of the surface’s deflection on distance shows the peculiarities on the continental border. The several maximums and minimums are located on the boundary between the continent and ocean. The deep minima are observed on the variations recorded for each track which cross the trench. The tracks 34, 212, 136, 60 cross the trench between Pacific and Asia. If we join together the sequential minima (the sequential from North to the South) we receive the line of ravine on the surface of geo-id.

This line is the especial line from geophysical point of view. The epicenters of the most earthquakes are placed near the line, the principal axis of tsunami sources is directly along the line.

We interpret the deflections as a horizontal gravity anomaly with amplitude 10⁻⁵ g.

The great earthquake (October 4, 1994, Shikotan, M = 8.3 and December 28, Sanriku, M = 7.6) creates the most notable changing of gravity deflections. The variation has an amplitude of several percent (10⁻⁶ g). The perturbation is disposed near the area of aftershocks’ epicenters. The perturbation arrived after the main shock of the Earthquake in the case of earthquake December 4, 1994. The perturbations grow during the 6 months before the earthquake December 28, 1994 and shrink after the main shock.

Analogous phenomena are observed after the earthquake on December 26, 2004 (Sumatra).The epicenter of the earthquake is placed in the ravine that spreads from the point 3.37 N. 94.4 E to the point 12.7 N 92.4 E. The depth of ravine is 1.4 m. After earthquake the ravine disappears.

If we regard the geo-id’s perturbation as a result of gravity variation, then we can estimate the parameters of tsunami sources. If we use the piston model of tsunami source, then the parameters of plunger can be estimated. The location of plunger looks like the location of the aftershock’s area. The height of plunger can be estimated by amplitude of gravity variation, if we use the potential theory. It is 58 m for the 1994 earthquake and it is the 150 m for the December 26, 2004 tsunami.