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Spatial Statistics of Densely Measured Seismic-Velocity Variations

Session: Advances in Upper Crustal Geophysical Characterization I

Type: Oral

Date: 4/21/2021

Presentation Time: 10:30 AM Pacific

Description: 

Seismic velocity measurements show high degrees of spatial variability, even when taken at close spacing. P- and S-wave well logs have long shown substantial, apparently stochastic vertical variability in velocity values to a degree similar to the deterministic velocity variability related to the different formations logged. Transects of Vs30 measurements (the geotechnical time-averaged Vs from the surface to 30 m depth) show similar characteristics. Reno and Las Vegas, Nevada, have each hosted 15 km transects with 300 m spacing between refraction microtremor measurements. A 60 km transect with the same spacing crosses San Gabriel Valley and the Los Angeles Basin in Southern California. These transects show apparently stochastic and deterministic velocity variations. Both types of variation represent aleatory variability in the ground, well above the level of epistemic uncertainty of the measurements. Vs30 transect spatial spectra suggest fractal variations horizontally. Fractal dimensions D are 1.5 to 1.8. Deep ROSRINE well logs of Vs near the Los Angeles Basin transect show a very similar D, for vertical variations, of 1.7. Covering over 80% of Las Vegas Valley, Nevada, the Clark County Parcel Map (CCPM) incorporates over 10,000 refraction microtremor measurements of Vs30, generally spaced at 300 m. Horizontal Vs30 variations across the CCPM yield a fractal dimension of 1.7. Wave-propagation modeling to 1.0 Hz of earthquake scenarios in and around Las Vegas Valley, through a 3D velocity model with a geotechnical layer including the CCPM, produce similarly fractal-appearing maps of peak ground velocity (PGV) estimates. The fractal dimensions of the horizontal variations in PGV are lower, from 1.1 to 1.4. Wave propagation averages over and smooths the rapid lateral velocity variations to some extent. Even with the small 300 m spacing between measurements, the CCPM can only resolve near-surface wave-propagation effects at frequencies up to 0.5 Hz.

Presenting Author: John N. Louie

Student Presenter: No

Authors