Temporal characteristics of some aftershock sequences in Bulgaria

We apply statistical analysis to study the temporal distribution of aftershocks in aftershock sequences of five earthquakes which occurred in Bulgaria. We use the maximum likelihood method to estimate the parameters of the modified Omori formula for aftershock sequences which is directly based on a time series. We find that: the maximum likelihood estimates of the parameter p show a regional variation, with lower values of the decay rate in North Bulgaria; the modified Omori formula provides an appropriate representation of temporal variation of the aftershock activity in North Bulgaria; the aftershock sequences in South Bulgaria are best modeled by the combination of an ordinary aftershock sequence with secondary aftershock activity. A plot of the cumulative number of events versus the frequency-linearized time t clearly demonstrates a transition from aftershock to foreshock activity prior to the second 1986 Strazhitsa (North Bulgaria) earthquake.

Microearthquake activity following the Parkfield, California earthquake of June, 1966

abstract A high sensitivity microearthquake recording station was established 10 km from the epicenter of the magnitude 5.5 Parkfield earthquake of June 28, 1966. Beginning 43 hours after the main shock, an hourly average of 22 microaftershocks was recorded for a period of 13 days. Events with magnitudes roughly equivalent to a Richter magnitude of −1.5 were recorded. The amplitude distribution suggests that there was a smaller percentage of small shocks in the Parkfield microaftershock series than has commonly been reported for Japanese and other California aftershock series. b values between 0.8 and 0.9 are commonly reported while the average b value for the Parkfield microaftershock series was 0.59. The distribution of S-P times for the microaftershocks is consistent with the epicentral area defined in other studies as a strip approximately 5 km wide astride a 35 km long trace of the San Andreas fault; however, some evidence suggests that the microaftershock activity extends beyond the zone defined by the larger aftershocks. The spatial distribution of microearthquake activity is shown to be strongly non-uniform within the aftershock zone. The microaftershocks, in general, did not cluster in time about the larger aftershocks (M > 2.0). Of 24 aftershocks with M greater than or equal to 2.0, only one event gave strong evidence of triggering a secondary aftershock series. Assuming that secondary foreshock and/or aftershock series imply the creation or reactivation of a secondary fault, one is led to the conclusion that secondary faulting was a rare occurrence in the Parkfield aftershock zone.