Preliminary Listing and Discussion of New Mexico Earthquakes 1962-1994 with Duration Magnitudes of 3.0 and Greater
by
Open-File Report 79
September, 1995
Geophysical Research Center
New Mexico Institute of Mining and Technology
Socorro, New Mexico
Preface
At
New Mexico Tech we have started to compile a comprehensive and detailed catalog
of earthquakes in New Mexico and bordering states for the period 1962 through
1994. Completion of this catalog will require collaboration with several
organizations, primarily Los Alamos National Laboratory. We expect to finish
the project within two years.
The catalog will be based primarily on data
obtained from seismograph stations
operated by New Mexico Tech, Los Alamos National Laboratory (LANL), U.S.
Geological Survey (USGS), University of Texas at El Paso, and Texas Tech University.
The new catalog will be a revision and extension of the listing of earthquakes
we submitted for the seismicity maps published in conjunction with the
DNAG (Decade of North American Geology) volume entitled Neotectonics of North America
(Sanford et al., 1991). Our listing of events for that publication covered
the period 1 January 1962 through
30 September 1986 and only included events with magnitudes of 2.5 or greater.
Since the DNAG listing, we have developed a better procedure for locating earthquakes
that avoids the problem of local minima encountered frequently when
using relatively small aperture arrays to locate regional events outside the arrays
(Lin, 1994). Use of the new procedure will improve the overall reliability
of locations for shocks with magnitudes as low as 1.3. Another problem with our
DNAG listing was a non-uniform
procedure of calculating magnitudes. For about the first half of the ~25 year
period, magnitudes were based on amplitudes of ground displacements converted
to equivalent Wood-Anderson trace amplitudes. Determination of ground-motion from
analog recordings at a rate of 1 mm/second is difficult and consequently the
results uncertain. We are convinced now that more reliable measures of the magnitude
can be obtained using durations of ground motions as we did for the second
half of the DNAG listing.
At
New Mexico Tech, we have accumulated data on approximately 2200 earthquakes
in New Mexico and bordering areas, particularly West Texas, with duration
magnitudes of 1.3 or greater for the time period 1962-1994. At the low magnitudes,
the data set is geographically biased towards the positions of networks, most
of which have been in the Rio Grande rift. In order to avoid this bias, we
will be progressively lowering the threshold magnitudes to determine geographical
completeness of the data. Reported
herewith is our first cut which is for earthquakes of magnitude 3 or greater
within the borders of New Mexico. We believe the data is complete over all
of the state for the entire 33 year period, and because of the strength of these
events, the chance that any earthquake is badly off in location is small. However,
this New Mexico Tech list has not been compared with other lists generated
by the USGS and LANL to check for omissions or differences.
Allan R.
Sanford
September, 1995
INTRODUCTION
From a New Mexico Tech database
of ~2200 earthquakes, we have identified 111 earthquakes within the state
of New Mexico that have duration magnitudes of 3.0 or greater (Table 1). For determining
the magnitudes, we have used the relation
MD=2.79 log TD - 3.63
where TD is the duration in seconds. This relation was developed for earthquakes
in northern New Mexico by Dan Cash at Los Alamos National Laboratory (Newton,
1976). An analysis of earthquake
durations at station ALQ versus local magnitudes based on Wood-Anderson
recordings at Albuquerque and Socorro produced a duration magnitude relation for
central New Mexico which is close to the one for northern New Mexico (Ake et
al., 1983). The duration magnitudes listed in Table 1 are based on the LANL formula
above. For the time period 1962-1981, nearly all duration magnitudes are
based on TD's measured on the WWSSN station ALQ short-period vertical-component
seismograms. From 1982 through
1994, the magnitudes are based on durations measured on New Mexico Tech seismograms
which are produced by instruments with responses similar but not identical
to the ALQ WWSSN station. In general, events which appear in Table 1 and also
the DNAG catalog have somewhat higher magnitudes in Table 1. However, these may
change as we refine the duration magnitude scale, e.g., determine whether a
distance correction is required.
The origin times and epicenters listed
in Table 1 from 20 September 1982
through 1994 were obtained from the program SEISMOS (Hartse, 1991) modified
to locate regional as well as local earthquakes. The principal modification was
the addition of a fuzzy-logic algorithm to obtain an initial estimate for the
epicenter(Lin, 1994). Prior to September 1982, several different programs and
procedures were used to obtain origin times and locations. However, the crustal
structure used for location of most regional earthquakes throughout the 33
year period was a half-space with
a velocity of 6.15 km/sec and a Poisson's ratio of 0.25. For the events which
occurred within small networks, e.g. the one surrounding Socorro, more complicated
crustal models were used. Major tasks ahead are to locate all earthquakes
with the program SEISMOS and to determine the best average crustal structure to
use for the New Mexico region.
Discussion
Figure
1 is a map of the 111 epicenters of earthquakes with duration magnitudes
greater or equal to 3.0 for
the time period 1962 through 1994. Figure 2 is the same as Figure 1 but with the
addition of the approximate boundaries of the Rio Grande rift (RGR) proposed
by Chapin (1971) in relation to other physiographic provinces in New Mexico. Geologic
characteristics indicate that the Colorado Plateau and Great Plains are
tectonically stable relative to the RGR. However, for the period 1962-1994,
earthquakes were as numerous and as strong on the Colorado Plateau and the Great
Plains as they were in the
RGR. On the basis of seismicity for the 33 year period, the RGR is not clearly
identified nor is the other major young geologic structure, the Jemez Lineament.
There are two areas of seismicity worth commenting on. The first is a
tight cluster of activity in the central Rio Grande which we call the Socorro
Seismic Anomaly (Sanford et al., 1995). The cluster which occupies ~5500 km2, or
approximately two percent of the total area of New Mexico, accounted for ~25
percent of the earthquake activity
greater or equal to magnitude 3.0 during the 1962 through 1994 period. At the
magnitude 4.0 or greater level, the cluster contributed 39 percent of the seismicity.
The relatively high level of activity near Socorro is believed
to be the result of crustal extension over an inflating mid-crustal magma body.
The magma body is ~150m thick, ~19km deep and has a minimum lateral extent of
~2000km2 (Ake and Sanford, 1988; Rinehart and Sanford, 1981; Hartse, 1991). Level-line
data indicate that the
surface above the magma body is undergoing uplift at a maximum rate of ~2mm/year
(Larson et al., 1986).
Extending ENE from the Socorro Seismic Anomaly
into the Great Plains is a clearly defined band of earthquakes that coincides
with a prominent topographic lineation identified by Thelin and Pike (1991) on
their digital shaded relief map for the conterminous U.S. The lineation extends
from south-central Arizona, through New Mexico and the Texas Panhandle to the
Oklahoma border. Included along
its path are sections of the Gila and Salt rivers in Arizona, ENE trending basins
west of the RGR in New Mexico (Baldridge and Olsen, 1989; Woodward et al.,
1978) and the course of the Canadian river in northeastern New Mexico and the
Texas Panhandle. Strong historical earthquakes prior to 1962 have occurred along
the Canadian river trend in West Texas (Shurbet, 1969).
The alignment
of topographic features, the seismicity along the same trend, and the existence
of a mid-crustal magma body
at the intersection of the lineation and the RGR suggest a major crustal flaw which
because of its orientation probably pre-dates Laramide and later structures
which have northerly orientations. Even accounting for errors in epicenters,
the seismicity indicates a broad zone of crustal deformation perhaps on the order
90 km. On a seismicity map with a lower cutoff magnitude of 2.2, the ENE trending
band of activity broadens to ~140 km (all the increase to the south), but
remains well defined. However,
as the threshold magnitude is lowered below 2.2, the ENE band becomes progressively
obscured by weaker events in the regions adjacent to it.
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