| 33°45'N to 34°35'N | (87 km) |
| 106°30'W to 107°10'W | (62 km) |
| Magnitude Range | Number of Earthquakes |
| 2.0 - 2.9 | 147 |
| 3.0 - 3.9 | 27 |
| 4.0 - 4.9 | 10 |
| Total | 184 |
The great majority of earthquake activity in the Socorro area cannot be related to Quaternary and Pliocene faults (Machette, 1982), i.e., rupture is occurring on numerous "blind" faults similar to the "blind" thrusts responsible for damaging earthquakes in the Los Angeles basin. In the absence of data on hidden faults throughout the region, I have assumed in the analysis below that earthquake activity is equally probable throughout the 5500 square km area.
An analysis of numbers and strengths of earthquakes indicates a 0.40 probability that a magnitude 6.4 earthquake will occur somewhere on a known or hidden fault in the Socorro region in a 50 year period. However, the probability that this earthquake will occur within 25 km of a population center like Socorro is 0.16. Twenty-five kilometers is the approximate distance within which intensity VII or greater effects would take place during a magnitude 6.4 earthquake (Toppozada, 1975). The magnitude of ground shaking at intensity VII is sufficient to (1) produce considerable damage in poorly built or badly designed building (e.g. the NMT gymnasium), (2) break weak chimneys at the roof line, (3) dislodge bricks, stones (and perhaps clay roof tiles), (4) overturn furniture, e.g. bookcases, and (5) knock most loose objects from shelves, e.g. chemicals in laboratories.
The smallest earthquake that will produce intensity VII effects is magnitude 5.3 (Toppozada, 1975). Extrapolation of the statistics on Socorro area earthquakes indicates that there are five events between magnitudes 5.3 and 6.4 that have equal likelihood of occurring in a 50 year period. The probability that one of these five events will be close enough to a population center to produce intensity VII or greater affects is 0.25 compared to 0.16 for the magnitude 6.4 earthquake considered alone. The probability that one of these five shocks will produce intensity VIII or greater effects at a population center is 0.03. Intensity VIII ground motion produces all the effects of intensity VII, but stronger. In addition, the ground shaking is sufficient to lead to collapse of badly designed structures and partial collapse of masonry structures that are not designed or reinforced against horizontal forces.
References
Hartse, H.E., A.R. Sanford, and J.S. Knapp, Incorporating Socorro magma body reflections into the earthquake location process, Bull. Seism. Soc. Am.,
82, 2511-2532, 1992.
Larsen,
S., R. Reilinger, and L. Brown, Evidence for ongoing crustal deformation
related to magmatic activity near Socorro, New Mexico, J. Geophys.
Res., 91, 6283-6292, 1986.
Machette, M.N., Quaternary and Pliocene faults in the La Jencia and southern part of the Albuquerque-Belen basins, New Mexico: Evidence of fault history from fault morphology and Quaternary geology, New Mex. Geol. Soc. 33rd Annual Field Conference Guidebook, 161-169, 1982.
Sanford, A.R., L.H. Jaksha and D.J. Cash, Seismicity of the Rio Grande rift in New Mexico, in Slemmons, D.B., Engdahl, E.R., Zoback, M.D., Blackwell, D.D. eds., Neotectonics of North America: Boulder, Colorado, Geol. Soc. Amer., 229-255, 1991.
Toppozada, T.R., Earthquake magnitude as a function of intensity data in California and Western Nevada, Bull. Seismol. Soc. Amer., 65, 1223-1238, 1975.