Monitoring velocity variations in the crust using earthquake doublets: An application to the Calaveras Fault, California

Abstract

We present a technique that greatly improves the precision in measuring temporal variations of crustal velocities using an earthquake doublet, or pair of microearthquakes that have nearly identical waveforms and the same hypocenter and magnitude but occur on different dates. We compute differences in arrival times between seismograms recorded at the same station in the freqency domain by cross correlation of short windows of signal. A moving-window analysis of the entire seismograms, including the coda, gives g(t), the difference in arrival times versus running time along the seismogram. The time resolution of the method is an order of magnitude better than the digitization interval. The g(t) technique is illustrated with a pair of microearthquakes, M = 1.7 and 2.0, that occurred before and after the Coyote Lake, California, earthquake (M = 5.9) of August 6, 1979, and on the same segment of the Calaveras fault that ruptured during the earthquake. The coda wave arrivals for some stations are progressively delayed for the second earthquake in the doublet, so that its seismogram appears as a stretched version of the earlier event. We interpret this systematic variation in 6(t) along the coda as a change in the average S velocity in the upper crust in the time interval between the two doublets. S wave velocities appear to have decreased by 0.2% in an oblong region 5-10 km in radius at the south end of the aftershock zone.

Permanent tremor of Masaya Volcano, Nicaragua: Wave field analysis and source location

“…In this case, the coherency is equal to one. In a real case, the time delay is computed by using a weighted linear adjustment of the phase of the cross-spectrum [Poupinet et al, 1984;Frdchet, 1985] In order to test the method and our program for determining the spatial correlation coefficients and to determine the optimal window widths for the computations, we used a real seismic signal for which we modified the phase to simulate its propagation in a semicircular array. The spectrum of the reference and the propagated signal are the same, the phase difference is cot.…”

“…During the first campaign, in September 1992, we established a semicircular (R) array (r=120 m) labeled "R_array A", at a distance of 1.1 km from the Santiago crater We estimated the delays by applying a cross-spectral method, commonly used for seismic multiplet studies [Poupinet et al, 1984;Got et al, 1994]. If two signals S 1 and S 2 differ only by a scale factor K and a time delay r, S2(t) -KSl(t-r), it is easy to show that the phase of their cross spectrum is rco, where co is the angular frequency.…”

MIIC: Monitoring and Imaging Based on Interferometric Concepts

“…The concept of detecting temporal changes with interferometry of scattered waves in seismic coda was presented by Poupinet et al (1984) under the term Moving Window Cross that will be developed in the project we expect progress that is strongly required in the fields of building-ground investigation (seismic risk, Eurocode 7), subsurface monitoring (mining safety and subsurface storage) and geo-hazard evaluation (volcano and landslide monitoring).…”

Revision of earthquake hypocentre locations in global bulletin data sets using source-specific station terms

“…The use of station corrections along with differential arrival times is the basic concept behind most of the relative location techniques (e.g. Douglas 1967;Jordan & Sverdrup 1981;Pavlis & Booker 1983;Poupinet et al 1984;Got et al 1994;Richards-Dinger & Shearer 2000;Waldhauser & Ellsworth 2000;Myers et al 2007). When the travel-time-prediction errors are dominated by the biasing effects of lateral velocity heterogeneities, the relative location techniques can be highly effective for events within a localized region, provided the local source region is small compared to the heterogeneity.…”