Tomography of Seismic P-Waves from Earthquakes and Explosions —Part Ⅱ: Morphology of Faulting of the 2004 Sumatra-Andaman Earthquake from Spectral Seismograms

The paper is concerned with the analysis of broadband seismograms of the Sumatra-Andaman earthquake of 26 December 2004. The purpose of the analysis is to recognize the morphology of the faulting process in terms of the time of occurrence and of the frequency content of subevents. The analysis is based on spectral seismograms of P-waves, obtained from the corresponding broadband recordings. It is found that 15 major subevents with maximum radiation intensity at frequencies lower than about 1 Hz occurred during the time window prior to the arrival of the major S-wave. In the same time window however hundreds of subevents took place with maximum radiation intensity at higher frequencies, producing a quasi-continuous "humming" at infrasonic frequencies observable at distances of the order of 8 000 km. In view of the strong absorption of the high frequency waves during the propagation along the ray path, it may be concluded that the "humming" reflects a substantial portion of the seismic energy released during the earthquake.