High-amplitude seismic waves radiated from the bright spot of an earthquake

Abstract

Abstract In the source region of a large shallow earthquake, there is likely to be a small spot characterized by the production of high accelerations and a gap in the aftershocks. On the seismograms of most large shallow earthquakes, and on some seismograms of moderate main shocks, two types of P-waves may be detected: the relatively low-amplitude waves following phase P 1 and the predominant high-frequency waves following phase P 2 . The duration of the seismic waves following phase P 1 (the P 1 −P 2 time) increases proportionately with the earthquake magnitude. This fact strongly suggests that the preliminary rupture period is closely related to the radiation of the predominant high-frequency seismic waves. To explain the above observation, the shear strain waves radiated from a moving rupture are calculated. Because the seismic wave velocity is generally higher than the rupture propagation velocity, maximum shear strains by seismic waves usually precede the rupture front. If the total shear strain, which is equal to the static pre-earthquake shear strain plus dynamic shear strain excited by seismic waves, exceeds the critical shear strain (~ 10 −4 ), the secondary ruptures break out in the forward direction from the initial rupture. The rupture processes of large or moderate earthquakes occur in two stages: one is the initiation of rupture corresponding to phase P 1 , and the other is the development of secondary ruptures triggered by seismic shear strain waves. The latter process generates an earthquake bright spot which radiates high-amplitude seismic waves.