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We use relative arrival times and locations for similar earthquake pairs that are found using a cross-correlation method to analyze the time dependence of P and S station terms in southern California from 1984 to 2002. We examine 494 similar event clusters recorded by Southern California Seismic Network (SCSN) stations and compute absolute arrival-...
Contexts in source publication
Context 1
... station-term estimates for these nine stations are plotted in Figure 5, and their apparent offset times are listed in Table 1. The offsets range from about 20 to 70 msec and occurred at distinctly different times, although the exact oc- currence time of each time offset is limited by our 3-month averaging interval. ...
Context 2
... timing difference may be due to slightly different passband or phase response in the filters. We list the times for these hardware changes in Table 1. For stations CIBAR, CIMWC, CIRVR, CISBB, and CIVPD, the offsets in the station corrections happened at the time of the equipment changes. ...
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Citations
We present an analytical solution for the evaluation of timing errors at seismological stations. The method makes use of differential P- and S-wave arrival time measurements demeaned over a network that recorded a set of densely located seismic events. In this configuration, one can assume coincident P and S ray paths between sources and receivers, and cancel out dependencies associated with absolute event origin times, event locations, and P- or S-wave velocities in the problem. Relative timing errors can be obtained by linear inversion, using only a limited amount of input data: differential P- and S-wave arrival times, and a local VP/VS ratio. By setting at least one reference station in the network, supposed to be devoid of any timing error, one can retrieve reliable timing errors for other stations. We validate the approach against synthetic and real data. We also analyze the sensitivity of results on errors in the input data. Although picking uncertainties do affect the variability of estimates, we also identified a significant bias when an incorrect VP/VS ratio is used. However, this bias can be reduced if one uses the optimal VP/VS value that minimizes the root mean square of travel-time residuals. Application of the method to a collection of manually picked arrival times for the 2002–2003 Tricastin, France, earthquake, swarm allowed us to identify nonstationary timing errors from tenth to tens of seconds during the monitoring campaign.
Very accurate timing of seismic recordings is critical for modern processing techniques. Clock synchronization among the instruments constituting an array is, however, difficult without direct communication between them. Synchronization to Global Positioning System (GPS) time is one option for on-land deployments, but not for underwater surveys as electromagnetic signals do not propagate efficiently in water. If clock drift is linear, time corrections for ocean-bottom seismometer (OBS) deployments can be estimated through GPS synchronization before and after the deployment, but this is not sufficient for many applications as the nonlinear component of the drift can reach tens to hundreds of milliseconds for long-duration experiments. We present two techniques to retrieve timing differences between simultaneous recordings at ocean-bottom instruments after deployment has ended. Both techniques are based on the analysis of the cross correlation of ambient seismic noise and are effective even if clock drift is nonlinear. The first, called time symmetry analysis, is easy to apply but requires a proper illumination so that the noise cross-correlation functions are symmetric in time. The second is based on the doublet analysis method and does not have this restriction. Advantages and drawbacks of both approaches are discussed. Application to two OBS data sets shows that both can achieve synchronization of recordings down to about five milliseconds (a few percent of the main period used).
Some studies of coda Q(-1) have found temporal changes that may be associated with earthquake activity, but these analyses are subject to biases due to differences in source locations and other nonstationary behavior in earthquake catalogs. These biases can be greatly reduced by using clusters of repeating earthquakes; studies using this approach have generally found no resolvable changes in coda Q(-1). We examine coda Q(-1) variations across southern California using 22 similar event clusters identified from a recent large-scale waveform cross-correlation project to improve earthquake locations. These clusters are found across the region and span the time period between 1981 and 2005. We apply the method of Beroza et al. (1995) to compute differential coda Q(-1) using waveforms from similar earthquake pairs and analyze the results to constrain any possible temporal variations. Results from individual event pairs show a great deal of scatter in differential coda Q(-1), but exhibit no clear temporal variations or changes associated with large earthquakes. Application of a median filter to smooth the results shows that any persistent large-scale changes in coda Q(-1) during this time period are less than about 30%.
