1978 Volume 26 Issue 3 Pages 253-274
Mechanism Studies fox five destructive earthquakes in the Izu peninsula, central Japan, since 1930 were made on the basis of an extensive set of P-wave first-motions, aftershocks, close-in long-period seismograms, leveling, triangulation and field data. Physical source parameters such as dislocation, source dimension and seismic stress were determined for the Northern Izu earthquake of 1930 (M=7.0), Southern Izu of 1934 (M=5.5), Izu-Oki of 1974 (M=6.9), Amagi of 1974 (M=4.9) and Kawazu of 1976 (M=5.4). All of these earthquakes represent a purely or nearly vertical strike-slip faulting at a very shallow depth. These faults constitute a system of conjugate faults produced by a single force system, e.g., north-south compression, with the exception of the northern fault of the 1930 Northern Izu earthquake which indicates NW-SE compression. The strikes of the faults coincide closely with a general trend of geologically active faults in this area. The stress drop varies from 10 to 70 bars, and larger shocks indicate larger stress drops. The dislocation velocities for the three small shocks are low, about 10 to 15cm/sec, in good agreement with their low stress drops.