Mode of strain release of three graben‐associated earthquakes

The nature of graben‐associated earthquakes has been investigated through a study of three earthquake sequences using various seismological, geophysical and geological data. The three earthquake sequences are the Hsingtai, China earthquakes (maximum magnitude Ms = 7.2) of March, 1966, the Nevada earthquakes (maximum M = 7.1) of 1954, and the New Madrid earthquakes (maximum Ms = 8.8) of 1811–1812. These earthquake sequences represent a unique mode of strain release away from plate margins and have bearings on earthquake hazard evaluation. A number of similarities have been observed during these earthquake sequences. For instance, all the three sequences are characterized by large multiple shocks or swarms, which last from several weeks to several months. Among the large multiple shocks in each sequence, the largest one usually is not the first one. Migrations of epicenters were observed during the Hsingtai and the New Madrid earthquakes. Significant amount of strike slip motion was associated or inferred with all these three sequences, in particular for the Hsingtai earthquakes. The swarm of multiple shock activities is probably due to induced instability or strong coupling and interactions among a series of adjacent faults in a heterogeneous graben. The strike slip motion can be explained by a change or a rotation of stress orientation after the formation of a graben. The graben events are similar to the earthquake swarms associated with spreading oceanic ridges but with significant differences. Available data indicate that the recurrence intervals for the three graben earthquake sequences studied are of the order of thousands of years.