Seismicity Along a Segment of a Prominent ENE Trending Topographic Lineament in New
Mexico and West Texas*
by
Open-File Report 81
December, 1995
Geophysical Research Center
New Mexico Institute of Mining and Technology
Socorro, New Mexico
Seismicity
of New Mexico
New Mexico Tech, in collaboration
with Los Alamos National Laboratory, the U.S. Geological Survey, University
of Texas - El Paso, and Texas Tech University, has been using instrumental data
to map earthquake activity in
New Mexico since 1962. The seismicity for the period 1962 through October 1995
for duration magnitude 1.3 or greater shocks is shown in Figure
1 . Most of the
seismograph stations during this 33.8 year period were located along or very
near the Rio Grande rift, a major extensional structure extending northward from
the Mexico border at 32šN through central New Mexico into Colorado to at least
39šN. Despite the concentration
of instruments along the rift, this structure cannot be identified on the basis
of seismicity. At the 1.3 magnitude cutoff the earthquake activity appears
to be diffusely distributed over the entire state; the major exception is the
tight cluster of shocks in the Rio Grande rift centered near Socorro at 34.2šN
and 106.8šW. The relatively high level of activity near Socorro, which we call
the Socorro Seismic Anomaly (Sanford et al., 1995a), is believed to be the result
of crustal extension over an
inflating mid-crustal magma body. The magma body is ~ 150 m thick, ~ 19 km deep
and has a minimum lateral extent of ~ 2000 square km (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 ~ 2 mm/year (Larsen
et al., 1986).
In September 1995, we generated a list of New Mexico
earthquakes with duration magnitudes of 3.0 or greater (Sanford et al., 1995b)
for use by the U.S. Geological
Survey in preparing new seismic hazard maps for the United States. Unexpectedly,
the seismicity map at the magnitude 3.0 cutoff did reveal a prominent trend.
Figure 2 is a map of MD >= 3.0
events for New Mexico and bordering areas. Extending ENE through the Great Plains
of eastern New Mexico into the Panhandle of Texas is a clearly defined zone
of seismicity. Approximately 35 percent of the earthquakes located outside the
Socorro Seismic Anomaly occur
in this ~ 70 km wide, ~ 330 km long region. The ~ N70E orientation of the zone
is unusual because it is at a high angle to the generally north-trending basins
and bordering mountains of the Rio Grande rift. Using a Monte Carlo technique,
we have determined that the ENE trending zone of seismicity through eastern
New Mexico is not accidental.
ENE Trending Topographic Lineament
On their digital shaded relief map of the conterminous
United States, Thelin and
Pike (1991) called attention to a prominent topographic lineation that extends
from southwestern 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 Rio Grande rift, and the course
of the Canadian River in easternmost New Mexico and through the Texas Panhandle
(Figure 3 ).
The zone
of seismicity in eastern New
Mexico straddles the topographic lineation (Figure
4 ) which has an orientation of ~ N70E from just east of the
Arizona - New Mexico border to the Texas - Oklahoma border. The spatial coincidence
of seismicity and the topographic lineation suggests that the latter is
a physiographic expression of a major fracture zone which we have designated the
Socorro Fracture Zone (SFZ) in the discussion that follows.
Geologic
Observations in Support
of the SFZ
The postulated fracture zone as defined
by the topographic lineation intersects the Rio Grande rift ~ 80 km south of Albuquerque
at a place where the morphology of the rift undergoes a major change
(Figure 5 ). North for a distance
of ~ 500 km, the rift is primarily composed of four north-trending elongate
basins in a right-stepping echelon pattern. To the south the rift widens rather
abruptly into a series of adjacent
north-trending tilted basins and horst-blocks which collectively would appear
to require more extension than occurs in the single basin to the north. Estimates
of basin extension immediately north of the intersection are on the order
of 30% whereas just to the south it is believed to be ~ 50% (Chapin and Cather,
1994). The large change in extension in a short distance appears to require
a deeply penetrating detachment to accommodate the large differential movements.
Westward from the Rio
Grande rift along the track of the topographic lineation for at least 160 km
are mapped normal faults with ENE and NE strikes (Figure
6 ). Faulting at high angles to the orientation of
the rift is relatively rare and generally occurs in association with proposed accommodation
zones (Chapin and Cather, 1994). One of these, the ~ 70 km long Socorro
accommodation zone (Chapin, 1989) is located ~ 30 km south of the center
of the SFZ and may define its
southern margin. Chapin's Socorro accommodation zone lies on the eastern end of
an earlier proposed Morenci lineament (Chapin et al., 1978) which to the west
deviates southwestward from the SFZ.
Additional Geophysical
Observations in Support of the SFZ
Socorro Magma Body
Although the boundaries of the Socorro Magma
Body (SMB) are not tightly constrained, it appears that its north-south dimensions
fall closely within the boundaries
of the SFZ (Figure 7 ).
Maximum surface uplift associated with the SMB occurs very close to the intersection
of the center lines of the SFZ and the Rio Grande rift. Recently, Schlue
et al. (1995) report evidence for feeder dikes immediately below the SMB based
on anomalous teleseismic converted phases. This region of possible conduits
into the mid-crustal magma body is located on its western edge ~ 10 km north
of the center of the SFZ.
Felt
Earthquakes Prior to 1961
We have
compiled a list of felt earthquakes prior to 1961 for the region 31š to 38šN and
101š to 111šW (Sanford, 1995). In order to minimize bias toward populated regions,
the list was restricted to felt events with maximum intensities of VI (Modified
Mercalli) or greater and/or felt areas greater than 100,000 km. The list
contains 39 events in the 132 year interval from 1830 through 1961 of which 11
were associated with the Socorro
Seismic Anomaly (Figure 8 ).
Considering the uncertainty in the epicenters of felt earthquakes, eight of
the remaining 28 felt earthquakes could have occurred along the Socorro Fracture
Zone. Of particular interest are the five earthquakes in the Texas Panhandle
on the eastern end of the SFZ which have felt areas up to 520,000 km (Shurbet,
1969; Coffman et al., 1982) which suggests magnitudes to ~ 5.2 (Reiter, 1990).
Summary of
Observations
€ On their digital shaded relief map of the
conterminous U. S., Thelin and Pike (1991) have called attention to a prominent
topographic lineament that extends ENE from southwestern Arizona to the eastern
border of the Texas Panhandle (Figure 3
).
€ A map of epicenters of earthquakes with magnitudes greater
or equal to 3.0 for the period 1962 through October 1995 (Figure 2
) reveals a prominent ENE band of seismicity that straddles the track of
the topographic lineament through the Great Plains of eastern New Mexico and the
Texas Panhandle (Figure 4 ).
€ A
southwestward extension of the zone of seismicity along the track of
the topographic lineament intersects the Rio Grande rift where its morphology
undergoes a major change (Figure 5 )
and where an active sill-like
magma body has been intruded at a crustal depth of ~ 19 km (Figure 7 ).
€ WSW from the Rio
Grande rift along the track of the topographic lineation for a distance of at least
160 km are mapped faults with ENE and NE strikes; in marked contrast the dominantly
NS structural grain of the rift (Figure
6 ).
Conclusions
€ The
concurrence of seismicity with
a topographic lineation suggests that the latter is a physiographic expression
of a major fracture zone (Figure 4
).
€ Additional evidence in support of a fracture zone along the topographic
lineament is the existence of a major mid-crustal magma body at the
intersection of the lineament and the Rio Grande rift (Figure
7 ). At the same location, the marked change in
rift morphology (Figure 5 ) appears to require a deeply penetrating
detachment to accommodate large differential movements expected with the
change in structural style.
€ Although there has been little seismicity
in the past 34 years southwest along the topographic lineament from the Rio Grande
rift, this region has prominent structural basins aligned with the lineament
(Figure 6 ). The orientation
of these basins relative to the
general NS structural grain of the rift would appear to support the existence
of a major ENE trending fracture zone.
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* Poster presented at the annual meeting of the AGU in San Francisco, CA, December 1995.