Circular 143—Seismicity of proposed radioactive waste disposal site in southeastern New Mexico

By A. R. Sanford and T. R. Toppozada, 1974, 15 pp., 4 tables, 8 figs., 2 appendices.

An area encompassed within a 300-km radius from the proposed site was studied for seismic risk during expected lifetime of the facility. Seismicity was based on instrumental data, reports of felt shocks, and geological evidence. This study was undertaken to determine the seismicity at the proposed radioactive waste disposal site in southeastern New Mexico. The site is centered at about 42 km east of Carlsbad. Algermissen's seismic risk map of the U.S. indicates that the region around the site has a relatively low Zone 1 seismicity classification. The maximum expected seismic intensities from local or distant shocks in a Zone 1 region is V-VI. On an earlier seismic risk map, Richter places the Carlsbad region within a seismic zone where the probable maximum intensity is expected to be VIII.

Both of these risk maps are based on essentially the same data. The differences are due to varying interpretations. Most of these data are non-instrumental, simply reports of felt or damaging shocks. Our evaluation of the seismic risk of the region is based not only on the non-instrumental data, but on a substantial amount of instrumental data not available to Richter or Algermissen. In addition, they did not attempt to incorporate geologic evidence of recent crustal movements into their estimates of seismicity. This type of data, accounting for geologic features, especially fault scarps offsetting Quaternary geomorphic surfaces, is essential to accurate estimates of seismic risk over the planned lifetime of the disposal facility.

Seismicity was determined for the area within a 300-km radius from the proposed nuclear waste disposal site in southeastern New Mexico. The primary data used to establish seismic risk were: reports of felt shocks prior to 1961; instrumental epicenters and magnitudes from 1961 through 1972; and lengths, displacements, and ages of fault scarps cutting Quaternary geomorphic surfaces. The principal results of this study were: (1) earthquakes exceeding local magnitude 3.5 have not occurred within 40 km of the site in the past 12 years; probably not in the past 50 years, (2) on the average of once every 50,000 years major earthquakes are possible within 115 km of the site, but these events will produce accelerations of only about 0.07 g at the site; and (3) some evidence indicates that earthquakes located on the Central Basin Platform, 80-100 km southeast of the site, could be related to water injection for secondary recovery of oil.