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Calibration of the U.S. Geological Survey National Crustal Model for Seismic Hazard Studies

Session: Advances in Upper Crustal Geophysical Characterization [Poster]

Type: Poster

Date: 4/21/2021

Presentation Time: 11:30 AM Pacific

Description: 

The U.S. Geological Survey National Crustal Model (NCM) is being developed to improve estimates of site response in seismic hazard assessments. Primary outputs of the NCM are continuous velocity and density profiles from the Earth’s surface to the mantle at each location on a 1-kilometer grid across the conterminous United States. Basic subsurface information is provided by the NCM geologic framework (NCMGF), thermal model, and petrologic and mineral physics database. In this study, velocities and densities within the NCM are calibrated through the development of a porosity model using Biot-Gassmann theory and over 2000 P- and/or S-wave velocity profiles (< 10 km deep) from across the conterminous United States and southwest Canada.

Sediment and rock porosities are derived from S-wave velocity and are found to depend on effective pressure, rock type, and age (in the case of sedimentary and extrusive volcanic deposits: SEVR). Porosity-effective pressure functions are then estimated for each rock type (and age for SEVR): unconsolidated sediments are found to have higher porosities than consolidated units, which have higher porosities than unweathered igneous units; young sedimentary units tend to have higher porosity than older sedimentary units; porosity decreases with increasing effective pressure; and porosities can decrease quickly through the weathered layer of intrusive rocks.

Upon comparison with the cvmh and cvms426m01 Los Angeles area velocity models and the USGS Bay Area velocity model, the NCM does a better job of reproducing observed S-wave velocities below 1 km/s, having less bias and spread. Approaching and above 1 km/s, the NCM tends to underpredict observed S-wave velocity. While several factors could contribute to this, the primary factor is probably bias in the NCMGF. For example, cases in which the NCM will predict lower velocities include places where the depth to bedrock and basement appear shallower in the measured velocity profiles than specified in the NCMGF.

Presenting Author: Oliver S. Boyd

Student Presenter: No

Authors