ALBERT

Description: Changes in the frequency of moderate-size events before and after the 28 June 1992 Landers earthquake are investigated, and their implications are discussed in the context of Coulomb failure stress (CFS) evolution since 1812 in southern California. We systematically considered circular regions and equal-area annuli centered on the epicenter of the Landers earthquake. Frequency-magnitude relationships for two 10-year periods before and two 5-year periods around the Landers event are compared. Only events with magnitude, M〉 or =4.0 are included; aftershocks are removed. For the larger circular regions with radii of 140 to 160 km, the rate and slope of the frequency-magnitude distribution for moderate-size events just before the mainshock appear to be anomalous compared to those for either the preceding or subsequent periods. For areas closer to the 1992 epicenter, however, the number of events is few, and the differences in the distributions are less obvious. When we examined the seismic activity in annuli of equal area, however, the largest changes occurred about 150 km from the epicenter of the mainshock, not closer as would be expected for a precursor to the Landers event. We also derive an index value to better quantify differences in the frequency of occurrence of moderate-size events as a function of time. The index value and the frequency-magnitude distribution show similar spatial dependence. Since 1812 a large region near Landers has moved closer to failure in terms of changes in CFS for faults of San Andreas type. These changes, however, are dominated by coseismic changes associated with the 1812 and 1857 earthquakes and by tectonic stress buildup related to the San Andreas fault, not by stress buildup associated with the Landers faults themselves, which are characterized by very slow long-term displacements. Hence, the most pronounced changes in the frequency of moderate-size earthquakes before 1992 do not appear to be related to stress buildup to the Landers sequence itself. They, along with the Landers sequence, may be indicative of a broad region that is approaching a high stress state prior to an eventual future great earthquake. The failure to find a pronounced increase in moderate-size shocks close in to Landers is in accord with the idea that such increases on a timescale of years to decades are associated with the regional buildup of stress to large earthquake along faults of high (not low) long-term slip rates.

Description: Cross-correlation (CC) determined relative time delays, or related differential travel times, between pairs of seismic events at the same station are often used as input data to improve earthquake relocation results. Researchers generally select those time delays with associated CC coefficients larger than a chosen threshold. When two similar time series are contaminated by correlated noise sources, the relative time delay between them calculated with the CC technique is sometimes not reliable. Noise sources at a station for different events can be partially correlated or just randomly correlated. In this work, we use the bispectrum (BS) method, which works in the third-order spectral domain, to check the reliability of the CC determined time delay. We calculate two time delays with the BS method, one using the band-pass-filtered waveforms and the other with the raw data, and use them to verify (select or reject) the CC estimate computed with the filtered waveforms. We apply this technique to obtain bispectrum-verified CC differential times for 822 New Zealand earthquakes in the Wellington area. Our work demonstrates that the CC time delays verified with the BS method provide improved (smaller root mean square residual and more clustered) earthquake relocation results compared to those selected with the standard threshold criterion.

Description: We examine 16 moderate-size earthquakes that have occurred since 1990 in the northeastern United States and southeastern Canada. Source parameters, such as focal mechanism, focal depth, and seismic moment, are determined using a regional waveform inversion technique in which the best-fitting double-couple mechanism is obtained through a grid search over strike, dip, and rake angles. Most of the 16 events and 10 previous ones with well-determined focal mechanisms show horizontal compression with near-horizontal P axes striking northeasterly, which is consistent with the maximum horizontal stress orientation for midplate North America. Four shocks along the St. Lawrence River, however, have P axes trending south-east (100 degrees -130 degrees ), which is not compatible with the regional stress field and requires a local stress perturbation close to the St. Lawrence River. We examine several generating mechanisms for this localized stress variation, including that a "rift pillow" structure may exist in the lower crust below the St. Lawrence River. All of the 11 shocks in the United States occurred at shallow depths from 2 to 8 km; the 15 Canadian events are systematically deeper, with foci between 5 and 28 km. This geographic variation in focal depth is well correlated with the difference in surface heat flow values between the older Grenville and younger Appalachian provinces in the study area. We also examine apparent discrepancies between M (sub w) and M (sub b) (Lg) magnitude for 17 events. Five earthquakes of very similar M (sub w) show variations in m (sub b) (Lg) as large as 0.6, which implies that for seismic events with similar seismic moment in the study area, high-frequency seismic radiation behavior differs considerably.