We have compiled a list of the strongest earthquakes to occur in New Mexico for the interval 1860 through May, 1998. Earthquake magnitudes based on instrumental records are available for only about one-quarter of this ~ 140 year period. Prior to 1962, the strengths of earthquakes were expressed in terms of the maximum earthquake intensity Io, a quantity assigned on the basis of what people observe during an earthquake, damage to structures, etc. The scale used for ranking intensities in the United States is the Modified Mercalli-1931 (Richter, 1958). In order to generate a list of strongest earthquakes with the same measure of strength, maximum intensities for earthquakes prior to 1962 were converted to magnitude. For this purpose, we used the empirical relation derived by Sanford (1998) for New Mexico earthquakes,
M = 0.5 + 2/3 Io. (1)
The list of strongest earthquakes in New Mexico (Table 1) is based on data from two earlier New Mexico Tech Geophysics Open-File Reports. New Mexico earthquakes with maximum intensities of VI or greater from ~ 1860 through 1961 were extracted from a compilation of felt earthquakes over the region 31o to 38oN and 101 o to 111 o W (Sanford et al., 1995c). The maximum intensities for the 25 shocks prior to 1962 were converted to magnitudes using equation (1) above. On the basis of this equation, an event with a maximum intensity of VI has a magnitude of 4.5. This places a lower limit on magnitudes for the compilation of strongest earthquakes. For the period 1962 through May, 1998, five earthquakes with instrumentally determined magnitudes of 4.5 or greater were obtained from a listing of magnitude 3.0 or greater New Mexico shocks, 1962-1994 (Sanford et al., 1995b) and from data on file at New Mexico Tech.
Geographical Distribution. The last column of Table 1 indicates that 14 of the 30 strongest shocks were nearest the communities of Socorro and Bernardo. These earthquakes occurred in the central Rio Grande rift within the Socorro Seismic Anomaly (SSA) (Sanford et al., 1995a). The SSA is a well-defined cluster of seismic activity that occupies only 1.6 percent of the total area of the state but contributes nearly 50 percent of the events in our compilation of the strongest earthquakes for New Mexico.
Five of the 30 earthquakes in Table 1 occurred within the high population segment of the Rio Grande Valley from Belen to Santa Fe. The remaining shocks in Table 1 are scattered throughout the state.
Temporal Distribution. Thirty-six percent (11 of 30) of the earthquakes in Table 1 are from two swarms, one near Socorro in 1906, and the other near Glenwood in 1938-39. If we consider these swarms as single events, then the average interval between shocks of magnitude 4.5 or greater in New Mexico is 6 to 7 years. The number of earthquakes in Table 1 since the beginning of instrumental monitoring of the entire state in 1962 is five; close to the expected number for the ~36 year period. This observation suggests that the conversions of maximum intensities to magnitudes have not greatly over or under estimated the level of activity prior to 1962.
Richter, C.F. (1958) Elementary Seismology, W.H. Freeman and Co., San Francisco, 768 p.
Sanford, A.R., R.S. Balch, and K.W. Lin (1995a) A seismic anomaly in the Rio Grande rift near Socorro, New Mexico, Geophysics Open-File Report 78, New Mexico Tech, Socorro, NM 87801, 17 p.
Sanford, A.R., K.W. Lin, I.C. Tsai, and L.H. Jaksha (1995b) Preliminary listing of New Mexico earthquakes 1962-1994 with duration magnitudes of 3.0 or greater, Geophysics Open-File Report 79, New Mexico Tech, Socorro, NM 87801, 12 p.
Sanford, A.R., K.W. Lin, and I.C. Tsai (1995c) Felt earthquakes in the region 31 o N to 38 o N and 101 o W to 111 o W from 1830 through 1961 and their relation to the Socorro Seismic Anomaly and the Socorro Fracture Zone, Geophysics Open-File Report 80, New Mexico Tech, Socorro, NM 87801, 8 p.
Sanford, A.R. (1998) An empirical relation between magnitude and maximum intensity for New Mexico earthquakes, Geophysics Open-File Report 79, New Mexico Tech, Socorro, NM 87801, 12 p.