V. G. Martynov

Aftershock Detection Thresholds as a Function of Time: Results from the anza Seismic Network following the 31 October 2001 ML 5.1 Anza, California, Earthquake

We examine aftershock detectability thresholds for events in the initial part of the 31 October 2001, M L 5.1 sequence in southern California. This sequence occurred directly below the broadband anza seismic network, which recorded continuous waveform data at 13 azimuthally well-distributed stations within the study region (seven had epicentral distances < 20 km). Of the 608 aftershocks (0 < M L < ∼2.8) in the initial 2 hr of this sequence, the first five aftershocks recorded were only identifiable at stations within 30 km after applying a high-pass filter. Using a cluster (radius ≤ 1.1 km) of 200 representative aftershocks, we track the maximum seismogram amplitude versus earthquake magnitude. This relationship helps us quantify the visibility of aftershocks within the mainshock coda and assess our detection capabilities. We estimate that detectable aftershocks within the mainshock coda include (1) those over magnitude ∼3 that are within 15 km of the network centroid that occur 12 sec or more into the sequence, and (2) those over magnitude ∼2 that are within 30 km of the centroid of the network that occur 60 sec or more into the sequence. We find a lack of large aftershocks in this sequence. The largest aftershock ( M L ∼2.8) is substantially smaller than the mainshock ( M L 5.1). We suggest this relatively large-magnitude differential is dictated by a combination of factors that includes complexity of the San Jacinto fault system and the lack of large earthquakes in the region in the past ∼20 years. Online material: Quicktime movies juxtaposing a 3.2 aftershock in the coda of a 5.1 mainshock.

Quantifying the remote triggering capabilities of large earthquakes using data from the ANZA Seismic Network catalog (southern California)

Received 24 August 2006; revised 22 June 2007; accepted 13 August 2007; published 3 November 2007. [1] Various studies have examined remote earthquake triggering in geothermal areas, but few studies have investigated triggering in nongeothermal areas. We search the ANZA (southern California) network catalog for evidence of remote triggering. Using three statistical tests (binomial, Kolmogorov-Smirnov, and Wilcoxon rank sum), we determine the significance of the rates and timing of earthquakes in southern California following large teleseismic events. To validate our statistical tests, we identify 20 local main shocks (ML � 3.1) with obvious aftershock sequences and 22 local main shocks (ML � 3.0) that lack obvious aftershock sequences. Our statistical tests quantify the ability of these local main shocks to trigger aftershocks. Assuming that the same triggering characteristic (i.e., a particular seismic wave amplitude, perhaps in a specific frequency band) is evident for both local and remote main shocks, we apply the same tests to 60 remote main shocks (mb � 7.0) and assess the ability of these events to trigger seismicity in southern California. We find no obvious signature of remote triggering. We find minimal differences between the spectral amplitudes and maximum ground velocities of the local triggering and nontriggering earthquakes. Similar analysis of a select few of our remote earthquakes shows that the related ground motion regularly exceeds that of local earthquakes both at low frequencies and in maximum velocity. This evidence weakly suggests that triggering requires larger amplitudes at high frequencies and that a maximum ground velocity alone is not the primary factor in remote triggering. Our results are complex, suggesting that a triggering threshold, if it exists, may depend on several factors.

Directional Variations in Travel-Time Residuals of Teleseismic P Waves in the Crust and Mantle beneath Northern Tien Shan

We study the directional variation in travel-time residuals using 13,820 P-wave arrivals from 1,998 teleseismic events (15 D 98, 4.1 mb 7.3) recorded in 1991-1997 by the Kyrgyz Digital Seismic Network (KNET). Based on a modified version of the iasp91 model that accounts for the Kyrgyz crustal thickness beneath KNET, we convert P-wave travel times to travel-time residuals dt. The de- pendence of dt on backazimuth is modeled as one-, two-, and four-lobed variations in a horizontal plane (Backus, 1965). A least-squares fit of the azimuthal variation of dt indicates that the crust in the northern Tien Shan is about 11-15 km thicker than it is in the Kazakh Shield and the Chu Depression. From nine KNET stations, the one-lobe model estimates that the slowest P-wave travel-time direction is 5.0 4.8 (almost directly north) and the magnitude of variation is 1.71 0.13 sec. This result is consistent with an upwelling lower mantle plume. For the two-lobe model, the slowest P-wave travel- time directions (anisotropy term) are 89.7 and 269.7 4.7 (i.e., trending east- west). We find P-wave velocity anisotropy of 2.0%-2.9% associated with a layer with a thickness of 440 km at the top of the lower mantle. The fast direction of the P-wave travel-time (north-south) azimuthal anisotropy at the top of the lower mantle is (1) parallel to the absolute motion of the India plate and (2) close to the direction of the upwelling hot mantle flow. The last result suggests that the azimuthal aniso- tropy of the travel-time residuals is due to the shape-preferred orientation of middle- mantle material that results from plume intrusion. Shear-wave splitting studies (Mak- eyeva et al., 1992; Wolfe and Vernon, 1998) estimated the fast polarization direction to be parallel to the strike of the geological structures of the northern Tien Shan (71 29). Thus, the fast polarization direction determined from these shear-wave splitting studies using KNET data contradicts (differs by 90) the fast travel-time direction (0.3 and 179.7 4.7) we determine here using P-wave travel-time residuals using KNET data. This suggests that the azimuthal anisotropy determined from P-wave travel-time variations and from shear-wave splitting in SKS and SKKS have different sources.