The spatially-averaged coherency (SPAC) method of processing microtremor noise observations for estimation of Vs profiles requires a circular or triangular array symmetry in order to allow spatial (azimuthal) averaging of inter-station coherencies over a constant station separation. Common processing methods allow for station separations to vary by typically ±10% in the azimuthal averaging before degradation of the SPAC spectrum is excessive. In this paper we develop a new wavenumber-normalised SPAC method (krSPAC). The traditional analysis of SPAC data involves performing averaging of sets of coherency versus frequency spectra and then fitting to a model SPAC spectrum. In our new approach we interpolate each spectrum to coherency versus kr, where k and r are wavenumber and station separation respectively, and r may be significantly different for each pair of stations. The averaging and interpolation changes with each iteration of velocity models, since k is a function of frequency and phase velocity and thus is updated each iteration. The method proves robust and is compared with alternative methodologies using asymmetric arrays in the Santa Clara Valley CA, Pleasanton CA, and Seattle WA, where station spacings are irregular and vary from 300m to 2000m.
The US National Seismic Hazard Model (NSHM) was updated in 2023 for all 50 states using new science on seismicity, fault ruptures, ground motions, and probabilistic techniques to produce a standard of practice for public policy and other engineering applications (defined for return periods greater than ∼475 or less than ∼10,000 years). Changes in 2023 time-independent seismic hazard (both increases and decreases compared to previous NSHMs) are substantial because the new model considers more data and updated earthquake rupture forecasts and ground-motion components. In developing the 2023 model, we tried to apply best available or applicable science based on advice of co-authors, more than 50 reviewers, and hundreds of hazard scientists and end-users, who attended public workshops and provided technical inputs. The hazard assessment incorporates new catalogs, declustering algorithms, gridded seismicity models, magnitude-scaling equations, fault-based structural and deformation models, multi-fault earthquake rupture forecast models, semi-empirical and simulation-based ground-motion models, and site amplification models conditioned on shear-wave velocities of the upper 30 m of soil and deeper sedimentary basin structures. Seismic hazard calculations yield hazard curves at hundreds of thousands of sites, ground-motion maps, uniform-hazard response spectra, and disaggregations developed for pseudo-spectral accelerations at 21 oscillator periods and two peak parameters, Modified Mercalli Intensity, and 8 site classes required by building codes and other public policy applications. Tests show the new model is consistent with past ShakeMap intensity observations. Sensitivity and uncertainty assessments ensure resulting ground motions are compatible with known hazard information and highlight the range and causes of variability in ground motions. We produce several impact products including building seismic design criteria, intensity maps, planning scenarios, and engineering risk assessments showing the potential physical and social impacts. These applications provide a basis for assessing, planning, and mitigating the effects of future earthquakes.
Spatial-autocorrelation (SPAC) microtremor-array data acquired at 14 sites in Salt Lake Valley, Utah, characterize S-wave velocities to depths as great as 300 m. Three data sets acquired at each site were analyzed simultaneously using equilateral triangular arrays with sensors deployed at 33.3-m, 100-m, and 300-m separation. Of the 14 sites, eight were within 1.2 km of active-source (vibroseis) body-and surface-wave acquisition sites, and two were within 0.7 km of boreholes logged for S-wave velocity to at least 50-m depth. A comparison to these existing active-source and borehole models indicates that these SPAC results typically differ by less than 10% on average to 100-m depth. At a majority of the investigation sites, SPAC modeling results can be interpreted confidently to more than 150-m depth. Linear ground-motion amplification spectra derived from these profiles of versus depth suggest amplification factors of more than three can occur at frequencies in the band of 0.5 to 4 Hz from the base of unconsolidated sediments in the upper 300 m.
A three-dimensional finite-difference simulation of a moderate-sized (M 6.5) thrust-faulting earthquake on the Seattle fault demonstrates the effects of the Seattle Basin on strong ground motion in the Puget lowland. The model area includes the cities of Seattle, Bremerton and Bellevue. We use a recently developed detailed 3D-velocity model of the Seattle Basin in these simulations. The model extended to 20-km depth and assumed rupture on a finite fault with random slip distribution. Preliminary results from simulations of frequencies 0.5 Hz and lower suggest amplification can occur at the surface of the Seattle Basin by the trapping of energy in the Quaternary sediments. Surface waves generated within the basin appear to contribute to amplification throughout the modeled region. Several factors apparently contribute to large ground motions in downtown Seattle: (1) radiation pattern and directivity from the rupture; (2) amplification and energy trapping within the Quaternary sediments; and (3) basin geometry and variation in depth of both Quaternary and Tertiary sediments
The Sevier Desert detachment (or “reflection,” SDR), which underlies the Sevier Desert basin along the eastern margin of the USA Basin and Range, is commonly cited as a “type example” of a low-angle normal fault (LANF) in continental crust. We present the results of reanalyzing the SDR on the COCORP (Consortium for Continental Reflection Profiling) deep seismic profile crossing the Sevier Desert basin (Utah Line 1). We employ a strategy of showing how shallow crustal velocity models, derived from first-break analysis of the shot records, may be used to reduce the effect of lateral velocity variations on imaging the SDR. Our results imply that the irregularities and discontinuities along the SDR are likely caused by overlying lateral velocity variations. The reprocessed versions of the section reveal a smoother, simpler, and more continuous SDR, lacking most of the apparent large offsets and structural variations on the currently available version of the profile. Seismic attribute and structural analyses of the reflection indicate significant variations along the profile, which are likely related in part to the field acquisition, but may also suggest lateral variations in the physical origin of the SDR. Such lateral variations may be consistent with previous studies that challenge the LANF interpretation of the SDR on the basis of attributing different origins to different parts of the reflector; however, a smoother and more continuous SDR points to a tectonically uniform origin and is thus interpreted to be more consistent with a LANF explanation.
Shallow seismic reflection prospecting has been carried out in order to investigate the faults that bound to the southwest and northeast the Quaternary Upper Tiber Basin (Northern Apennines, Italy). On the northeastern margin of the basin a ∼ 1 km long reflection seismic profile images a fault segment and the associated up to 100 meters thick sediment wedge. Across the southwestern margin a 0.5 km-long seismic profile images a 50-55°-dipping extensional fault, that projects to the scarp at the base of the range-front, and against which a 100 m thick syn-tectonic sediment wedge has formed. The integration of surface and sub-surface data allows to estimate at least 190 meters of vertical displacement along the fault and a slip rate around 0.25 m/kyr. Southwestern fault might also be interpreted as the main splay structure of regional Alto Tiberina extensional fault. At last, the 1917 Monterchi earthquake (Imax=X, Boschi et alii, 2000) is correlable with an activation of the southwestern fault, and thus suggesting the seismogenic character of this latter.
