New Mexico Geological Society Annual Spring Meeting — Abstracts

The majority of New Mexico's population is located along the Rio Grande River corridor that extends from Belen in the south to Espanola in the north and which includes the cities of Albuquerque and Santa Fe. The corridor is situated within the Rio Grande rift, a tectonically, volcanically, and I seismically active feature in the western U.S. Although no historical earthquake larger than M 6 has occurred, paleoseismic investigations indicate that prehistoric surface-faulting earthquakes of M 6 ½ and greater occurred on many faults throughout the rift.

In an effort to portray the ground shaking hazards in the Albuquerque-Belen-Santa Fe corridor, we have developed a set of deterministic earthquake scenario and probabilistic microzonation maps. These GIS-based maps display color-contoured ground motion values in terms of peak horizontal acceleration and horizontal spectral accelerations at 0.2 and 1.0 sec periods. The maps depict surficial ground shaking and thus incorporate the site response effects of soils and near-surface unconsolidated sediments. The probabilistic maps are for the two return periods of building code relevance, 500 and 2,500 years. The scenario maps are for a M 7 earthquake on the Sandia and Rincon faults which are located adjacent to Albuquerque and dip beneath the city.

We included a total of 55 late-Quaternary faults in the probabilistic hazard analysis. These faults were characterized in terms of their rupture behavior, geometry, segmentation, M_max and slip rates. Best estimate Mmax values ranged from M 6.1 to 7.4 and slip rates from 0.01 to about 0.15 mm/yr. An areal source zone and Gaussian smoothing of the historical seismicity were also included in the probablistic hazard analysis to account for the hazard from background earthquakes (M ≤ 6½).

Amplification factors were calculated to incorporate site response into the hazard maps. Amplification factors were developed using only simple generic geologic site categories (e.g., hard rock, stiff soil, etc.) because very little subsurface geologic and geotechnical data are available. Factors were calculated using a numerical ground motion modeling approach coupled with an equivalent-linear methodology. This modeling approach and empirical attenuation relations appropriate for extensional tectonic regimes were used to compute the scenario and probabilistic ground motions.