Circular 120—Seismicity of the Rio Grande rift in New Mexico

By A. R. Sanford, A. J. Budding, J. P. Hoffman, O. S. Alptekin, C. A. Rush, and T. R. Toppozada, 1972, 19 pp., 7 tables, 8 figs., 2 sheets.

Text is written for the non-specialist as well as the seismologist. Engineers who must consider seismic risk in the design of structures, and officials responsible for public safety, will find this study informative. The Rio Grande rift zone is the most probable area of NM to have substantial seismic activity. The principal data used in establishing the seismicity of this region were:

(1) reports of strong earthquakes before 1960, (2) instrumental studies of moderate shocks after 1960, (3) analyses of microearthquakes recorded at Albuquerque, Socorro, and Las Cruces, and (4) analysis of fault scarps offsetting the Quaternary surfaces in the Socorro area. Historical reports indicate a moderately high seismic risk in a zone from Albuquerque to Socorro. In this region, particularly near Socorro, the largest shock in a 100-year period is likely to be magnitude 6. Instrumental data on earthquakes since 1960 show that activity is highest near Socorro and Las Cruces. However, estimated seismic risk from these data is low, with a maximum magnitude shock of about 5 each 100 years over the entire extent of the rift zone. Analyses of microearthquakes also lead to low estimates of seismic risk in the rift zone. Historical reports, spanning a century, are probably more reliable indicators of seismic risk than the relatively short-term instrumental data spanning only about a decade. The historical data indicate probability of a magnitude 6 earthquake each 100 years. Both categories of data indicate seismic risk is not uniform and its greatest in the following regions:

(1) Socorro–Bernado, (2) Albuquerque–Belen, and (3) El Paso–Las Cruces. In the region of highest seismic risk, Socorro-Bernado, little direct correlation exists between the distribution of microearthquakes and faults. Analyses of data for faults offsetting recent geomorphic surfaces indicates seismic activity has been occurring in the Socorro-Bernado region for thousands of years. Estimates of seismicity based on the fault scarps cover a fairly wide range because of the uncertainty in the age of scarps. However, the most reasonable estimates for the age yield siemicity values comparable to those calculated from earthquake data. The authors have attempted to establish the degree of seismic risk along the Rio Grande valley in New Mexico. The results of the study are necessarily preliminary because the data used for estimates of seismic risk are quite limited, particularly instrumental studies of earthquakes and geologic investigations relevant to the seismicity problem. Undoubtedly as more of these types of information become available, some of the conclusions reached in this paper will need to be modified.