EARTHQUAKES AND THEIR ASSOCIATED GROUND SHAKING HAZARDS IN THE RIO GRANDE RIFT, CENTRAL NEW MEXICO

The majority of New Mexico's population is located within the central Rio Grande rift, including the cities of Albuquerque and Santa Fe. Although no earthquakes larger than M 6 have occurred in the past 150 years and possibly no damaging events in the past 400 years, paleoseismic investigations indicate that prehistoric surface-faulting events of M > 6.5 occurred on many faults throughout the rift. To portray the earthquake ground shaking hazards in the central portion of the rift and to raise awareness among the state's citizens to this hazard, we have developed a set of scenario and probabilistic microzonation maps. The scenario maps are for a M 7 earthquake on the Rincon-Sandia faults which are adjacent to, and dip beneath Albuquerque. To calculate the ground shaking hazard, we characterized 57 Quaternary faults located within the rift in terms of their probability of activity, rupture behavior, geometry, segmentation, maximum magnitude, recurrence models, and slip rate. Most faults show dominantly normal slip. Preferred Mmax values range from M 6.1 to 7.4 and preferred slip rates from 0.01 to 0.12 mm/yr. To account for the hazard from background earthquakes (M < 6.5), events occcurring on faults with no surface expression, the historical seismicity record was used to spatially map the resulting hazard. The background seismicity was also considered to be uniformly distributed throughout the rift to allow for the possibility of "random" occurrence. Overall, there is a moderate level of seismic hazard in the central Rio Grande rift compared to other regions in the western U.S. The most perplexing issue in evaluating seismic hazards in the Rio Grande rift, as has been noted by others, is the difference between the predicted frequency of large-magnitude earthquakes based on the historical record versus the paleoseismic record. Either the historical seismicity is reflecting a temporary period of quiescence in the rift or the current rate of extensional deformation across the rift is not as high as indicated by long-term estimates of the fault slip rates.