Properties of aftershock sequences in southern California

Abstract

The temporal behavior of 39 aftershock sequences in southem California, 1933California, -1988, was modeled by the modified Omori relation. Minimum magnitudes for completeness of each sequence catalog were determined, and the maximum likelihood estimates of the parameters K, p, and c, with the standard errors on each, were determined by the Ogata algorithm. The b value of each sequence was also calculated. Many of the active faults in the region, both strike slip and thrust, were sampled. The p values were graded in terms of the size of the standard error relative to the p value itself. Most of the sequences were modeled well by the Omori relation. Many of the sequences had p values close to the mean of the whole data set, 1.11 + 0.25, but values significantly different from the mean, as low as 0.7 and as high as 1.8, exist. No correlation of p with either the b value of the sequence or the mainshock magnitude was found. The results suggest a direct correlation of p values is with surface heat flow, with high values in the Salton Trough (high heat flow) and one low value in the San Bernardino Mountains and on the edge of the Ventura Basin (both low heat flow). The large fraction of the sequences with p values near the mean are at locations where the heat flow is near the regional mean, 74 mW/m 2. If the hypothesis that aftershock decay rate is controlled by temperature at depth is valid, the effects of other factors such as heterogeneity of the fault zone properties are superimposed on the background rate determined by temperature. Surface heat flow is taken as an indicator of crustal temperature at hypocentral depths, but the effects on heat flow of convective heat transport and variations in near-surface thermal conductivity invalidate any simple association of local variations in heat flow with details of the subsurface temperature distribution. The interpretation is that higher temperatures in the aftershock source volume caused shortened stress relaxation times in the fault zone materials, leading to a faster decay rate (higher p value). In 1967, Mogi proposed such a relation on the basis of Japanese data. Seismic velocity distributions published for southern California generally support the hypothesis, with low velocities (higher temperatures)corresponding to high p values, and vice-versa. mates of the three parameters K, p, and c in the relation n(t)=K(t +c) -p , where n(t) is the number of events per unit time at time t, were calculated following Ogata's [1983] approach for fitting the cumulative number of aftershocks as a function of time after the mainshock. The program also provides estimates of the standard errors of the values of the three parameters. Because the emphasis in this study is on the factors controlling the rate at which activity decays, only the p values are presented and discussed. K is dependent on the total number of events in the sequence and c on the rate of activity in the earliest part of the sequence.A few of the sequences are not described well by the Omori relation, as evidence...

Source parameters and stress release of seismic sequences occurred in the Friuli-Venezia Giulia region (Northeastern Italy) and in Western Slovenia

“…where ) (t n is the number of events per unit t ; k is related to the total number of earthquakes, c depends on the rate of activity in the earliest part of the sequence and p is related to the decay of aftershocks (Kisslinger and Jones, 1991). The results are summarized in Table 2.…”

A fractal model of earthquake occurrence: Theory, simulations and comparisons with the aftershock data

“…The fact that the Omori exponent p is not universally unity is a very well documented fact and some workers have reported variability in p from 0.5 to 2.5 [27]. But for seismically active zones p is generally larger than but close to unity.…”

Spatial variability of seismicity parameters in aftershock zones

“…Of these three parameters, p is the most important. Between different aftershock sequences, the value of p was found to vary within California between 0.7 and 1.8 [Kisslinger and Jones, 1991]. Kisslinger and Jones hypothesize a relationship between the decay rate of aftershocks measured by p, and the crustal heat flow in the source volume.…”

“…(t+c) p where R(t) is the rate of occurrence of aftershocks, and k, c, and p are constants [Kisslinger and Jones, 1991]. Of these three parameters, p is the most important.…”