A PETROLOGIC DATABASE FOR THE USGS NATIONAL CRUSTAL MODEL

Seismic hazard forecasts use ground motion models that rely on some representation of the shear-wave seismic velocity structure beneath a site of interest. This has typically been the time-averaged shear-wave velocity for the upper thirty meters of the crust, V_S30, and more recently includes the depths to 1.0 and 2.5 km/s, termed Z_1.0 and Z_2.5, which account for long-period site response. An upcoming improvement to the National Seismic Hazard Map involves making use of these Z parameters by extracting Zvalues from the USGS National Crustal Model (NCM), which is presently under development. The NCM is composed of geophysical profiles defined by a 3-D geologic model with petrologic and mineral physics databases, and Biot-Gassmann theory calibrated with measured seismic velocity and density data. The geologic model is derived from a modified version of the National Geologic Map of the United States and maps of the depth to bedrock and basement estimated using well, seismic, and gravity data.

We present the methodology behind the development of the petrologic database on which the NCM will rely. There are 177 geologic units within the National Geologic Map, which fall into one of five categories: igneous rocks, sedimentary rocks, metamorphic rocks, unconsolidated sediments, and other—nonspecific units with regard to these categories and geologic nomenclature of the categories. Well-established nomenclature was used to assign characteristic mineral assemblages to each unit (specific rock or sediment type). For the nonspecific geologic units, interpretations were made to best approximate mineral assemblages.

These assemblages were used to calculate shear-wave velocity profiles and V_S30 for each unit. Because velocity varies with density and shear modulus and these parameters are dependent on porosity, velocities for each unit were calculated for a range of porosities from 0 to 70 percent. More than 2,900 V_S30 measurements were previously compiled by the USGS for individual sites throughout the United States, but V_S30 measurements were found to be present in only 52 of the 177 geologic units. The average measured V_S30 value for each geologic unit represented by the measured data was calculated and then compared to V_S30 calculations to estimate the appropriate porosity of the measured geologic units.