NEW OBSERVATIONAL INFORMATION ON THE PRECURSORY ACCELERATING AND DECELERATING STRAIN ENERGY RELEASE

Abstract

Recent reliable data are used to study the behavior of seismic activity before 46 strong shallow earthquakes (M ≥ 6.0), which correspond to five complete samples of mainshocks. These samples include 6 mainshocks (M = 6.0-7.1) that occurred in western Mediterranean since 1980, 17 mainshocks (M = 6.0-7.2) which occurred in the Aegean (Greece and surrounding area) since 1980, 5 mainshocks (M = 6.4-7.5) that occurred in Anatolia since 1980, 12 mainshocks (M = 6.0-7.3) that occurred in California since 1980 and 6 mainshocks (M = 7.0-8.3) that occurred in Japan since 1990. In all 46 cases, a similar precursory seismicity pattern is observed. Specifically, it is observed that accelerating Benioff strain (square root of seismic energy) release caused by preshocks occurs in a broad circular region (critical region), with a radius about eight times larger than the fault length of the mainshock, in agreement with results obtained by various research groups during the last two decades. However, in a much smaller circular region (seismogenic region), with a radius about four times the fault length, the corresponding preshock strain decelerates with the time to the mainshock. The time variation of the strain follows in both cases a power law but the exponent power is smaller than unit (m - = 0.3) in the case of the accelerating preshock strain and larger than unit (m - = 3.0) in the case of the decelerating preshock strain. Predictive properties of this "Decelerating In-Accelerating Out Strain" model are expressed by empirical relations. The possibility of using this model for intermediate-term earthquake prediction is discussed and the relative model uncertainties are estimated. © 2006 Elsevier B.V. All rights reserved.