Circular 78An 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.
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