Foreshocks and multiple shocks of large earthquakes

Details Foreshocks and multiple shocks of large earthquakes Foreshocks and multiple shocks of large earthquakes

Aug 1975

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1975pepi...10..355f&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 10, Issue 4, p. 355-368.

Physics

3

Scientific paper

The rupture process of the Tokachi-Oki earthquake (May 16, 1968) and that of the Kurile Islands earthquake (August 11, 1969) were investigated primarily on the basis of the long-period seismograms of WWSSN. These two large earthquakes are of the similar magnitude (M = 7.8-7.9) and are only about 460 km apart from each other. The detailed analyses of the long-period P waves and the multiple surface waves of G2, G3, and G4 revealed the following features. (1)The fracture at the earlier stages of the Tokachi-Oki event occurred in the form of multiple-shock activity. The largest two shocks were generated 57 km S and 93 km WSW of the initial hypocentre at 32 and 45 sec after the initial break respectively. (2)The main faulting of the Tokachi-Oki event commenced about 80 km S of the initial epicentre at 40-45 sec after the initial break and propagated in the northerly to northwesterly directions, the rupture velocity being about 4 km/sec. (3)No multiple event nature is seen for the Kurile event. Its initial break represents the commencement of the main-fault propagation. This earthquake was, however, preceded by an intense foreshock activity. The above features indicate that the multiple-shock activity of the Tokachi-Oki event and the foreshock activity of the Kurile event are the same type of precursory phenomena for the main phase of fault propagation. They are understood as the process for the nucleation of cracks at weak spots along the fault plane and their subsequent coalescence to grow larger cracks. This is the self-accelerated process resulting in the climax which marks the initiation of the main-fault propagation. We suggest that the rupture process of other large earthquakes can be described in a very similar way.

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