Lucile M. Jones — Page 2

USGS1985

Determination of elastic wave velocity and relative hypocenter locations using refracted waves. I. Methodology

An arrival time difference method utilizing refracted arrivals from earthquakes in a homogeneous, layered earth model has been developed for the simultaneous determination of near-source (in situ) velocity and relative locations of earthquakes. The method is particularly applicable when analyzing data from arrays in which most of the recording stations are far (i.e., several focal depths) from a group of events. This iterative scheme locates earthquakes relative to a master event and performs an inversion for in situ velocity using a generalized inverseleast squares estimation procedure. Direct arrivals, when available, may be included to stabilize the inversion and increase the accuracy of the event locations. We tested this scheme on artificial data contaminated by random and systematic arrival time errors, gaps in azimuthal coverage, and inaccuracies in the assumed velocity model. As usual, depth is the least well-resolved hypocenter coordinate, but this scheme yielded accurate locations of most events while converging to the correct velocity model.

Bulletin of the Seismological Society of America↗

USGS1985

Foreshocks and time-dependent earthquake hazard assessment in southern California

The probability that an earthquake in southern California (M ≧ 3.0) will be followed by an earthquake of larger magnitude within 5 days and 10 km (i.e., will be a foreshock) is 6 ± 0.5 per cent (1 S.D.), and is not significantly dependent on the magnitude of the possible foreshock between M = 3 and M = 5. The probability that an earthquake will be followed by an M ≧ 5.0 main shock, however, increases with magnitude of the foreshock from less than 1 per cent at M ≧ 3 to 6.5 ± 2.5 per cent (1 S.D.) at M ≧ 5. The main shock will most likely occur in the first hour after the foreshock, and the probability that a main shock will occur decreases with elapsed time from the occurrence of the possible foreshock by approximately the inverse of time. Thus, the occurrence of an earthquake of M ≧ 3.0 in southern California increases the earthquake hazard within a small space-time window several orders of magnitude above the normal background level.

California↗