Circular 78—An instrumental study of New Mexico earthquakes

By A. R. Sanford, 1965, 12 pp., 4 tables, 3 figs. Supplemet to circulars 102, 126, 171.

Data from seismic stations in New Mexico and surrounding states were used to establish locations and magnitudes of shocks that occurred within the state from January 1, 1962 to June 30, 1964. The number and strengths of shocks detected during this period indicated a seismicity for New Mexico about one fiftieth as great as southern California. The strongest shocks (M>3.5) recorded during the study originated in the northeastern part of the state, a region from which only one felt shock has been reported in more than 60 years. The majority of the located shocks were confined to the western half of the state and followed a geographical distribution in general agreement with the historical reports of earthquakes in the state.
A prediction of seismic activity in west-central New Mexico was made on the basis of the numbers and magnitudes of shocks detected from that region during the 2½ years. The predicted activity was much lower than the known seismicity from historical records. This result illustrates the possible dangers involved in making long-range predictions of seismicity based on short periods of observations.

This circular reports the strengths and the locations of instrumentally detected earthquakes that occurred within New Mexico from January 1, 1962 to June 30, 1964. Special emphasis is placed on shocks recorded from west-central NM, a region which has had fairly high seismicity in the past.

Prior to 1960, nearly all information on the strengths and distribution of earthquakes in NM was determined without the aid of instruments. The strength and location of a shock were determined from what people feel, do, and observe during an earthquake and also from the degree of damage to structures. Unfortunately, these measures of earthquake intensity are dependent on the type of ground as well as proximity to the shock. Under equivalent conditions, the ground motion on loosely consolidated alluvial materials common to many valleys in New Mexico could be ten times larger than the motion on competent rocks outcropping in the mountains. Thus, an earthquake in a mountainous region of New Mexico could be felt more intensely and do more damage in bordering valley areas than in the immediate vicinity of the shock. In this instance, the degree and location of maximum intensity of ground motion would not give a true indication of the strength and location of the shock.
Another difficulty with noninstrumental studies of earthquakes is the effect of population density. In regions where numbers of people are concentrated, reports of shocks are likely to be high and vice versa. Since vast areas of New Mexico are lightly populated, noninstrumental studies could have missed some small but significant areas of seismic activity.