Case 25 application of the concentration parameter of seismoactive faults to Southern California

In this paper we evaluate the present state of the seismic regime in Southern California using the concentration parameter of seismogenic faults (K~/., SOI~OLEV and ZAVYALOV, 1981). The purpose of this work is to identify potential sites for large earthquakes during the next five or ten years. The data for this study derived from the California Institute of Technology's catalog of southern California earthquakes, and spanned the period between 1932 to June 1982. We examined events as small as ML --> 1.8 but used a magnitude cutoff at ML = 3.3 for a detailed analysis. The size of the target earthquakes (M~t) was chosen as 5.3 and 5.8. The algorithm for calculating Ksf used here was improved over the algorithm described by SOBOLEV and ZAVYALOV (1981) in that it considered the seismic history of each elementary seismoactive volume. The dimensions of the elementary seismoactive volumes were 50 km x 50 km and 20 km deep. We found that the mean value of K,u within 6 months prior to the target events was 6.1 _+ 2.0 for target events with M L _> 5.3 and 5.4 _+ 1.8 for targets with M L >_ 5.8. Seventy-three percent of the targets with M L _> 5.8 occurred in areas where Ksf was less than 6.1. The variance of the time between the appearance of areas with low Ksf values and the following main shocks was quite large (from a few months to ten years) so this parameter cannot be used here for accurate predictions of occurrence time. Regions where the value of Ksz was below 6.1 at the end of our data set (June, 1982) are proposed as the sites of target earthquakes during the next five to ten years. The most dangerous area is the area east of San Bernardino where Ksf values are presently between 2.9 and 3.7 and where there has been no earthquake with M c >_ 5.3 since 1948. Region of Study and Data T h e r e g i o n s t u d i e d is b o u n d e d b y 32 ~ to 3 8 ~ a n d 115 ~ to 121~ T h i s r e g i o n was d i v i d e d i n t o 50 b y 50 k m a r e a s w h i c h o v e r l a p p e d e a c h o t h e r b y 25 k m fo r t he c a l c u l a t i o n o f Ksz. T h e se i smic i ty d a t a o r i g i n a t e d f r o m the c a t a l o g p r e p a r e d b y t he C a l i f o r n i a I n s t i t u t e o f T e c h n o l o g y fo r t he p e r i o d b e t w e e n 1932 a n d J u n e 1982. T h i s c a t a l o g i n c l u d e s a b o u t 57 ,000 e v e n t s w i t h M L >_ 1.0. T h e c o m p l e t e n e s s o f r e p o r t i n g Institute of the Physics of the Earth, Moscow, USSR. 2 Cooperative Institute for Research in Environmental Sciences, University of Colorado, NOAA, Boulder, CO 80309. 130 Case 25 Pure appl. geophys. in this catalog varies strongly both in space and time, which results in large variations in the number of smaller events reported. Analysis of the detection history of this catalog was accomplished using the technique described by HABERMANN (1983), using the z test for a difference between two means. We found that if the entire region is considered, the reporting is consistent at the ML > 3.7 level since 1967 and at the ML > 3.3 level since 1974. With a magnitude cutoff at 3.3 and a depth cutoff at 20 km, the final data set included 6579 events between 1932 and June 1982. Techniques for Calculating Ksf Distribution The use of the concentration factor of seismogenic faulting for earthquake prediction is based on the kinetic theory of the strength of solid materials (ZnuRI(OV and NARSULAEV, 1953; ZHURKOV et al., 1977). According to this theory the failure of solid materials is a long-term process which depends on the properties of the material, the stress, and the temperature. The ultimate failure of a solid material is connected to the propagation of a major crack which is preceded by a period of creation and accumulation of smaller cracks in some volume of material. The ultimate failure occurs when these cracks reach a critical concentration. The critical concentration of cracks may be defined as: