Abstract
A sudden increase in the scale of seismicity has occurred as a long-term precursor to twelve major earthquakes in California and Northern Mexico. These include all earthquakes along the San Andreas system during 1960-2000 with magnitude M* 6.4. The full list is as follows: Colorado Delta, 1966, M 6.3; Borrego Mt., 1968, M 6.5; San Fernando, 1971, M 6.6; Brawley, 1979, M 6.4; Mexicali, 1980, M 6.1; Coalinga, 1983, M 6.7; Superstition Hills, 1987, M 6.6; Loma Prieta, 1989, M 7.0; Joshua Tree, 1992, M 6.1; Landers, 1992, M 7.3; Northridge, 1994, M 6.6; Hector Mine, 1999, M 7.1. Such a Precursory Scale Increase (s ) was inferred from the modelling of longterm seismogenesis as a three-stage faulting process against a background of self-organised criticality. The location, onset-time and level of s are predictive of the location, time and magnitude of the future earthquake. Precursory swarms, which occur widely in subduction regions, are a special form of s ; the more general form is here shown to occur frequently in a region of continental transform. Other seismicity precursors, including quiescence and foreshocks, contribute to or modulate the increased seismicity that characterises s . The area occupied by s is small compared with those occupied by the seismicity precursors known as AMR, M 8 and LURR. Further work is needed to formulate s as a testable hypothesis, and to carry out the appropriate forecasting tests.
Highlights
Anomalies of seismicity are intuitively the most plausible of all proposed earthquake precursors
Swarms are common in the shallow seismicity of subduction regions: systematic studies in New Zealand, Japan and Greece show that swarms make a large contribution to long-term preshock activity in subduction regions, where swarms are predictive of mainshocks (Evison and Rhoades, 2000)
Once the occurrence of a jump in the rate of seismicity indicates that seismogenesis has begun, it is the time of the jump, and the value of the new magnitude level, that allow the parameters of the major earthquake to be estimated, as will be explained below
Summary
Anomalies of seismicity are intuitively the most plausible of all proposed earthquake precursors. Swarms are common in the shallow seismicity of subduction regions: systematic studies in New Zealand, Japan and Greece show that swarms make a large contribution to long-term preshock activity in subduction regions, where swarms are predictive of mainshocks (Evison and Rhoades, 2000). This phenomenon is explained by modelling seismogenesis as a three-stage faulting process occurring against a background of self-organised criticality (Evison and Rhoades, 1998, 2001). The aim is to show that the scale increase is a seismogenic phenomenon, preparatory to formulating an hypothesis that can be tested by long-range forecasting
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