Abstract: In studies of remote dynamic triggering, the common practice is to be reactionary, in that the impetus is to select a large earthquake and then systematically search for an increase in small earthquakes at remote distances. This approach requires specifying a duration (e.g., 5, 12, 24 hrs) used to search for seismicity rate changes. Rarely is the null hypothesis, that the rate change is merely a random occurrence, fully tested. Here, we take a different approach and simply count the number of events (above the magnitude of completeness) in a moving window of a given duration (e.g., 5, 12, 24 hrs), creating a catalog of count values for each sub-window over the full duration of a catalog. As expected the count increases substantially when there are local mainshock/aftershock sequences; however other rate increases can be identified as well. To identify times when the counts are significantly high (probabilities >= 96%), we use five times the standard deviation of the mean following Chebyshev’s Inequality Theorem. Applying this method to regional earthquake catalogs, we successfully find statistically high counts following the 2002 M7.9 Denali, Alaska, earthquake in Yellowstone and Utah indicative of the already known remote triggering. In addition, we find evidence of potential remote triggering in Montana following the 2002 M7.6 Papua Indonesia earthquake, and in Yellowstone following the 1985 M8.0 Michoacan, Mexico, earthquake. There can be a strong trade off between the length of the moving window and the robustness of the results. If the window becomes too small, an unrealistically large number of times may be flagged. These results indicate that we can use a simple earthquake count within temporal windows to identify potential remote triggering.
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Abstract: Demand on the development of non-invasive measurement methods for shallow S-wave velocity structure is increasing. Active and passive surface wave method will play important role in such measurements. Passive surface wave method or microtremor array measurements particularly receive large attention since the method can penetrate several hundreds to several kilometers easily. Applicability of microtremor array measurements to complex velocity structures with horizontal velocity change is the one of the issues to be figured out to apply the method to site investigations. We performed numerical simulation of microtremor array measurements at 2D and 3D structures using 3D finite-difference method to evaluate the effect complex structures on the analysis of microtremor array measurements. Two-layer models were used in the simulation. S-wave velocities of two layers are 200 and 400 m/s respectively. Depth to the boundary ranges 20 to 40 m. Receivers are deployed in a 100 x 100 m square. The size of the model is 550 x 550 x 150 m cube. Sources are randomly distributed outside of receiver array to simulate ambient noise field. Cell size is 1 m, time step is 0.25 ms and data length is 65 s. A 3D viscoelastic finite-difference method with 4th order velocity stress staggered grid scheme was used to calculate seismic wave field. Ten records are calculated with different source distribution. Ambient noise data were processed by common mid-point spatial auto correlation (CMPSPAC). SPACs were calculated all possible pairs and SPACs whose CMP belonged to the same bin were grouped. A dispersion curve was calculated for each bin and an 1D inversion was applied to each dispersion curve with horizontal constraint. The 1D velocity profiles were interpolated to a 3D velocity model. Obtained velocity were models generally consistent with true models and the simulation shows the applicability of the microtremor array measurements to complex velocity structures.
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Abstract: The concept of the ‘reference phase’ is widely used in seismic tomography and imaging. Usually, a differential traveltime between the prominent reference phase and another seismic phase is used with the hope to eliminate the influence of unknown velocity structure on their common travel paths. It is well known that Earth has inhomogeneities at all spatial scales and small-scale heterogeneities may violate this concept. In this work, we will investigate the validity of the ‘reference phase’ concept in the presence of small-scale heterogeneities whose length scales are below the spatial resolution of deterministic approaches in seismic inversion. We will first invert for the small-scale heterogeneity spectrum using the coherence function methods using amplitude and traveltime fluctuations of the transmitted wave. The inverted spectrum provides a quantitative statistical description of the small-scale heterogeneities. Then, we will produce realizations of the statistical random models. We use full-wave finite difference method to compute the wave propagation in the random medium. Finally, we evaluate the reference phase concept.
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Abstract: Long Valley in eastern California is sandwiched by the Sierra Nevada and the Basin and Range Province. There has been a stress perturbation in the vicinity of the caldera with respect to the regional stress field. Previous studies suggest that this perturbation is a result of the left-step offset in the Sierran range bounding normal faults instead of magma chamber inflation beneath Long Valley Caldera. In this study, we take advantage of the abundant seismic data and state-of-the-art finite-element numerical modeling to reinvestigate this local stress anomaly. We utilize the HASH program to compute focal mechanisms from P-wave first motion polarity observations for the relocated earthquakes between 1984 and 2014. The final ~42,000 good-quality focal solutions are then used to invert for the stress fields in 11 sub-areas by applying the SATSI algorithm. The orientations of the inverted minimum horizontal principal stresses (ShMIN) greatly agree with previous studies based on analyses of focal mechanisms, borehole breakouts, and fault offsets. The NE-SW oriented ShMIN in the resurgent dome and south moat of the caldera is in contrast to the dominating ~E-W orientation in the western Basin and Range province and Mammoth Mountain. In order to examine the source of this stress direction difference, we apply 3-D Finite Element Modeling with different parameters and combinations of tectonic and magmatic stress sources to search for the model that best fits the observed ShMIN orientations. Our preliminary results show that the local stress perturbation in the Long Valley area may be explained by the combination of a regional tectonic stress and an ellipsoidal magma source located next to the Mono Craters, indicating that the Mono Craters may play an important role in the local stress distribution in the area, although the current deformation is dominated by Long Valley Caldera.
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Abstract: The impacts of seismic shaking on urban basins have been in the news again this past year. Construction projects for tall buildings have been delayed out of concerns that current design standards may not sufficiently account for the shaking amplification that occurs in geologic basins. Building codes in Nevada pertaining to seismic hazard use the USGS National Seismic Hazard Mapping Program (NSHMP), which does not include site or basin amplification factors. The NGA-West2 ground motion prediction equation (GMPE) incorporates basin amplification factors homogeneously in one dimension, based on minimum depths to certain shear-velocity values (e.g., Z1.0, Z2.5) and on the geotechnical average velocity to 30 m depth (Vs30). We investigate whether such GMPEs may adequately predict amplifications recorded in the Reno-area urban basin of western Nevada. We are quantifying and comparing basin amplification factors recorded from a series of local and regional events in and around the Reno-area basin. The focus of our analysis lies in the variation of amplification factor with spatially distributed source locations relative to the Reno-area urban basin. Broadband records we are examining include the: 2008 Mogul sequence; 2015 M4.3 Thomas Creek; and three 2016 M~5.5 Nine Mile Ranch events. Initial investigation is into peak ground velocity (PGV) ratios of basin over bedrock stations; leading to including other measures of shaking intensity such as H/V spectra and duration. We have generated 3D physics-based SW4 synthetic seismograms for these events that partially account for basin effects at low frequencies of shaking (<1.0 Hz), and we can examine how well the synthetic PGV ratios predict the recorded ratios. Using the computational models, we can perform sensitivity testing on the model through varying Vs30, basin shear velocity profiles, and incorporating deep volcanic sub-basins.
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Abstract: We study the coherence of one-year continuous waveforms from the Piñon Flats Observatory Array (PY) and five dense linear arrays (JF, DW, SGB, RA, and BB from south to north) along the San Jacinto fault using the multitaper spectral analysis. The examined data include ambient noise and earthquake signals from local and teleseismic sources recorded by different types of seismometers at different locations. In the PY array, the coherence of noise data remains high in the 0.1-1.0 Hz band when the interstation distance is small (<65 m), and decreases quickly outside the dominant frequency band and with increasing station distances. The local earthquake signals exhibit high coherence up to 20 Hz, while the teleseismic waves have high coherences in the low frequency band (<0.1 Hz). The coherences show complex daily variations up to 12 Hz. The high coherence (>0.95) frequency bands in the two horizontal components exhibit seasonal variations. In addition, the coherences of horizontal components are higher in the 2-5 Hz band when compared to the vertical component, while the reverse is true in the low frequency range (<0.1 Hz). The coherence at PY also contains high anomalies in ~2-4 Hz lasting for ~16 days before the 2016 M5.2 Borrego Springs event, which might be related to anthropogenic effects. In the JF and DW arrays, coherence exhibits similar patterns in 0.1-1.0 Hz without significant daily or seasonal changes, while in the SGB and RA arrays there are no highly coherent noise signals in 0.1-1.0 Hz.
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