ALBERT

Description: 〈span〉〈div〉Abstract〈/div〉We estimated the site amplification of regional high‐frequency 〈span〉Lg〈/span〉 seismic phases by a reverse two‐station (RTS) method using seismic events (Mw 4–6) recorded by Earthscope’s Transportable Array from 2010 to 2013. We compare regional site amplification estimates (horizontal and vertical) from the RTS technique with horizontal‐to‐vertical spectral ratio (HVSR) estimates derived from ambient noise and earthquake records. We compare the RTS results with (1) shallow shear‐wave velocity estimates from near‐surface (horizontal/vertical) ratios of the local body‐wave (initial 〈span〉P〈/span〉‐wave) method, and (2) high topography, basins, and sediment thicknesses. Our RTS results show a strong positive correlation between regional site amplification and basins such as the Michigan basin, the Illinois basin, and the Mississippi embayment. In the case of the Illinois and Michigan basins, the higher the frequency, the higher the horizontal and vertical amplification. Waves passing through the Appalachian and Ozark plateaus are deamplified on both vertical and horizontal ground components; however, the variation in amplification with frequency is larger for horizontal motion than vertical motion. In some regions, such as the western edge of the Appalachian basin and southern Illinois basin, vertical amplification decreases with frequency but horizontal amplification is essentially invariant with respect to frequency. Topography and sediment thickness are likely to affect amplification and both factors likely frequency dependent. There is a negative correlation between the RTS‐measured amplification and shallow shear‐wave velocity, whereas HVSR shows a negative correlation only for low frequencies 〈2.0  Hz. We conclude that regional ground‐motion amplification is clearly a function of more than one variable. In general, it appears that both regional topography (i.e., long‐wavelength topography) and deeper subsurface seismic structures (basins and sediments) have a large impact on site amplification.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We estimated the site amplification of regional high‐frequency 〈span〉Lg〈/span〉 seismic phases by a reverse two‐station (RTS) method using seismic events (Mw 4–6) recorded by Earthscope’s Transportable Array from 2010 to 2013. We compare regional site amplification estimates (horizontal and vertical) from the RTS technique with horizontal‐to‐vertical spectral ratio (HVSR) estimates derived from ambient noise and earthquake records. We compare the RTS results with (1) shallow shear‐wave velocity estimates from near‐surface (horizontal/vertical) ratios of the local body‐wave (initial 〈span〉P〈/span〉‐wave) method, and (2) high topography, basins, and sediment thicknesses. Our RTS results show a strong positive correlation between regional site amplification and basins such as the Michigan basin, the Illinois basin, and the Mississippi embayment. In the case of the Illinois and Michigan basins, the higher the frequency, the higher the horizontal and vertical amplification. Waves passing through the Appalachian and Ozark plateaus are deamplified on both vertical and horizontal ground components; however, the variation in amplification with frequency is larger for horizontal motion than vertical motion. In some regions, such as the western edge of the Appalachian basin and southern Illinois basin, vertical amplification decreases with frequency but horizontal amplification is essentially invariant with respect to frequency. Topography and sediment thickness are likely to affect amplification and both factors likely frequency dependent. There is a negative correlation between the RTS‐measured amplification and shallow shear‐wave velocity, whereas HVSR shows a negative correlation only for low frequencies 〈2.0  Hz. We conclude that regional ground‐motion amplification is clearly a function of more than one variable. In general, it appears that both regional topography (i.e., long‐wavelength topography) and deeper subsurface seismic structures (basins and sediments) have a large impact on site amplification.〈/span〉

Description: Broadband seismic data from the regional seismic network operated by the China Earthquake Administration and 32 temporary seismic stations are used to image the crustal velocity structure in the northeast Tibetan plateau. Empirical Rayleigh- and Love-wave Green’s functions are obtained from interstation cross correlation of continuous seismic records. Group velocity dispersion curves for Rayleigh and Love waves between 10 and 50 s are obtained using the multiple-filter analysis method with phase-matched processing. The group velocity variations of Rayleigh and Love waves overall correlate well with the major geologic structures and tectonic units in the study region. Shear-wave velocity structures were then inverted from Rayleigh- and Love-wave dispersion maps. The results show that the Songpan–Ganzi terrane is associated with a low velocity at depth greater than 20 km. The northern Qilian orogen, with higher elevation and thicker crust compared to the southern Qilian orogen, is also dominated by low velocity at depth greater than ~25 km. However, there is no clear evidence of the low-velocity mid-to-lower crust beneath the southern Qilian orogen as the crustal flow model predicts. The low-velocity zone (LVZ) beneath the northern Qilian orogen may suggest that the crustal thickening and surface uplift of the northern Qilian orogen are related to the LVZ, and the LVZ may be considered as an intracrustal response to bear the ongoing deformation in the northern Qilian orogen. Online Material: Figures of crustal topography, number of group velocity measurements, checkerboard tests for NETS stations, and 1D velocity models.

Description: Reliable moment magnitude estimates for seismic events in the Middle East region can be difficult to obtain due to the uneven distribution of stations, the complex tectonic structure, and regions of high attenuation. In this study, we take advantage of the many new broadband seismic stations that have become available through improved national networks and numerous temporary deployments. We make coda envelope-amplitude measurements for 2247 events recorded by 68 stations over 13 narrow frequency bands ranging between 0.03 and 8 Hz. The absolute scaling of these spectra was calculated based on independent waveform modeling solutions of the moment magnitudes for a subset of these events to avoid circularity. Using our 1D path calibrations, we determined coda-based magnitudes for a majority of the events. We obtain fairly good agreement with waveform-modeled seismic moments for the larger events ( M w 〉4.5) at low frequencies (〈0.7 Hz). As expected, the coda-derived source spectra become increasingly scattered at higher frequencies (〉0.7 Hz) because of unaccounted 2D path effects, as well as mixing of both Sn coda and Lg coda, which have different attenuation behavior. This scatter leads to increased variance in the magnitudes estimated for smaller events in which low-frequency amplitudes are below the noise levels and the higher frequencies are the only signals available. We quantify the expected variance in coda envelope amplitudes as a function of frequency using interstation scatter as our metric. The net results of this study provide thousands of new 1D coda magnitude estimates for events in the broad region, as well as the necessary initial starting model for use in a new related 2D coda study ( Pasyanos et al. , 2016 ). Online Material: Table of site terms and moment magnitudes.