In this study, we investigate the shallow shear wave velocity structure of the Los Angeles Basin in southern California, using ambient noise correlations between 5 dense arrays and 21 broadband stations from the Southern California Seismic Network (SCSN). We observe clear fundamental mode and first overtone Rayleigh waves in the frequency band 0.25–2.0 Hz, and obtain group velocity maps through tomography. We further derive a 3D shear wave velocity model, covering a large portion of the central LA Basin for the depths shallower than 3 km. We found that the small scale shallow velocity structure heterogeneities are better resolved compared with the SCEC Community velocity models. Our model captures the presence of the Newport‐Inglewood fault by a NW–SE trending high velocity belt. Our model provides more accurate constraints on local ground motion predictions with detailed mapping of structural heterogeneities.
We use broad-band stations of the ‘Los Angeles Syncline Seismic Interferometry Experiment’ (LASSIE) to perform a joint inversion of the Horizontal to Vertical spectral ratios (H/V) and multimode dispersion curves (phase and group velocity) for both Rayleigh and Love waves at each station of a dense line of sensors. The H/V of the autocorrelated signal at a seismic station is proportional to the ratio of the imaginary parts of the Green’s function. The presence of low-frequency peaks (∼0.2 Hz) in H/V allows us to constrain the structure of the basin with high confidence to a depth of 6 km. The velocity models we obtain are broadly consistent with the SCEC CVM-H community model and agree well with known geological features. Because our approach differs substantially from previous modelling of crustal velocities in southern California, this research validates both the utility of the diffuse field H/V measurements for deep structural characterization and the predictive value of the CVM-H community velocity model in the Los Angeles region. We also analyse a lower frequency peak (∼0.03 Hz) in H/V and suggest it could be the signature of the Moho. Finally, we show that the independent comparison of the H and V components with their corresponding theoretical counterparts gives information about the degree of diffusivity of the ambient seismic field.
more » « less- NSF-PAR ID:
- 10123854
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Geophysical Journal International
- Volume:
- 220
- Issue:
- 1
- ISSN:
- 0956-540X
- Page Range / eLocation ID:
- p. 415-427
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The proliferation of dense arrays promises to improve our ability to image geological structures at the scales necessary for accurate assessment of seismic hazard. However, combining the resulting local high‐resolution tomography with existing regional models presents an ongoing challenge. We developed a framework based on the level‐set method that infers where local data provide meaningful constraints beyond those found in regional models ‐ for example the Community Velocity Models (CVMs) of southern California. This technique defines a volume within which updates are made to a reference CVM, with the boundary of the volume being part of the inversion rather than explicitly defined. By penalizing the complexity of the boundary, a minimal update that sufficiently explains the data is achieved. To test this framework, we use data from the Community Seismic Network, a dense permanent urban deployment. We inverted Love wave dispersion and amplification data, from the Mw 6.4 and 7.1 2019 Ridgecrest earthquakes. We invert for an update to CVM‐S4.26 using the Tikhonov Ensemble Sampling scheme, a highly efficient derivative‐free approximate Bayesian method. We find the data are best explained by a deepening of the Los Angeles Basin with its deepest part south of downtown Los Angeles, along with a steeper northeastern basin wall. This result offers new progress toward the parsimonious incorporation of detailed local basin models within regional reference models utilizing an objective framework and highlights the importance of accurate basin models when accounting for the amplification of surface waves in the high‐rise building response band.
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SUMMARY The near-surface seismic structure (to a depth of about 1000 m), particularly the shear wave velocity (VS), can strongly affect the propagation of seismic waves and, therefore, must be accurately calibrated for ground motion simulations and seismic hazard assessment. The VS of the top (<300 m) crust is often well characterized from borehole studies, geotechnical measurements, and water and oil wells, while the velocities of the material deeper than about 1000 m are typically determined by tomography studies. However, in depth ranges lacking information on shallow lithological stratification, typically rock sites outside the sedimentary basins, the material parameters between these two regions are typically poorly characterized due to resolution limits of seismic tomography. When the alluded geological constraints are not available, models, such as the Southern California Earthquake Center (SCEC) Community Velocity Models (CVMs), default to regional tomographic estimates that do not resolve the uppermost VS values, and therefore deliver unrealistically high shallow VS estimates. The SCEC Unified Community Velocity Model (UCVM) software includes a method to incorporate the near-surface earth structure by applying a generic overlay based on measurements of time-averaged VS in top 30 m (VS30) to taper the upper part of the model to merge with tomography at a depth of 350 m, which can be applied to any of the velocity models accessible through UCVM. However, our 3-D simulations of the 2014 Mw 5.1 La Habra earthquake in the Los Angeles area using the CVM-S4.26.M01 model significantly underpredict low-frequency (<1 Hz) ground motions at sites where the material properties in the top 350 m are significantly modified by the generic overlay (‘taper’). On the other hand, extending the VS30-based taper of the shallow velocities down to a depth of about 1000 m improves the fit between our synthetics and seismic data at those sites, without compromising the fit at well-constrained sites. We explore various tapering depths, demonstrating increasing amplification as the tapering depth increases, and the model with 1000 m tapering depth yields overall favourable results. Effects of varying anelastic attenuation are small compared to effects of velocity tapering and do not significantly bias the estimated tapering depth. Although a uniform tapering depth is adopted in the models, we observe some spatial variabilities that may further improve our method.
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