Nearly all of the region covered in the compilation had a low population through most if not all of the 132 year period. To minimize bias towards settled regions, for example, the Rio Grande Valley, Table 1 and Figure 1 were restricted to earthquakes with maximum reported intensities of VI (MM) or greater and/or felt areas greater than 100,000 square km.

Twelve of the 39 earthquakes in Table 1 and Figure 1 occurred within the Socorro Seismic Anomaly (SSA) (Sanford et al., 1995a). The SSA is located in the central Rio Grande rift in the vicinity of Socorro, New Mexico. This region of unusually high earthquake activity 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 ~ 3400 square km (Ake and Sanford, 1988; Rinehart and Sanford, 1981; Hartse, 1991; Hartse et al., 1992; Balch et al., 1996). Level-line data indicate that the surface above the magma body is undergoing uplift at a maximum rate of ~ 2mm/year (Larsen et al., 1986).

On the basis of instrumental data accumulated from 1962 through 1995, the SSA occupies an area of ~ 5000 square km which is only 0.7% of the total area (~ 713,000 square km) from 31°N to 38°N and from 101°W to 111°W. However, 31% (12 of 39) of the events listed in Table 1 fall within the SSA. The data on felt earthquakes from 1830 through 1961 appear to indicate an even more remarkable concentration of seismic activity in the Socorro area than the instrumental data from 1962 through 1995 (Sanford et al., 1995a).

The Socorro Seismic Anomaly falls within a prominent ENE trending linear zone we have designated the Socorro Fracture Zone (SFZ) (Sanford et al., 1995c). Summarized below are observations in support of the SFZ:
€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.
€A map of the epicenters of earthquakes with magnitudes greater or equal to 3.0 for the period 1962 through October 1995 (Sanford et al., 1995b) 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.
€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 (Baldridge and Olsen, 1989) and where an active sill-like magma body has been intruded at a crustal depth of ~ 19 km (Ake and Sanford, 1988; Hartse et al., 1992).
€WSW from the Rio Grande rift along the tract 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 (Woodward et al., 1978). On Figure 1, six of the 27 shocks outside of the SSA fall within the boundaries of the SFZ. Considering the uncertainities in the boundaries of the fracture zone and the epicenters of the felt quakes, an additional nine events may be associated with the SFZ.

Of considerable interest is the cluster of five events in the Texas Panhandle on the eastern end of the proposed SFZ. These earthquakes had felt areas up to 520,000 square km (Shurbet, 1969; Coffman et al., 1982) which suggests magnitudes up to 5.2 (Reiter, 1990). Perhaps the SFZ provides an explanation for this unusual cluster of relatively strong quakes located within the tectonically stable Great Plains.

References

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Balch, R., H. Hartse, A. Sanford, and K. Lin (1996). A new map of the geographic extent of the Socorro midcrustal magma body, Bull. Seism. Soc. Am., submitted 1996.

Coffman, J.L., C.A. von Hake, and C.W. Stover (1982). Earthquake History of the United States, Publication 41-1, U. S. Department of Commerce, NOAA, U. S. Department of Interior, Geological Survey, 208 p.

DuBois, S.M., M.L. Sbar, and T.A. Nowak (1982). Arizona earthquakes, Arizona Bureau of Geology and Mineral Technology, Bulletin 193, 456p.

Earthquakes of Arizona (1939). Bull. Seism. Soc. Am., 29, 294.

Hartse, H.E. (1991). Simultaneous hypocenter and velocity model estimation using direct and reflected phases from microearthquakes recorded within the central Rio Grande rift, New Mexico, Ph. D. Dissertation , New Mexico Institute of Mining and Technology, 251p.

Hartse, H.E., A.R. Sanford, and J.S. Knapp (1992). Incorporating Socorro magma body reflections into the earthquake location process, Bull. Seism. Soc. Am., 82, 2511-2532.

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Northrop, S.A. and A.R. Sanford (1972). Earthquakes of northeastern New Mexico and Texas Panhandle, New Mex. Geol. Soc., Twenty-third Field Conf. Guidebook, 148-159.

Olsen, K.H. (1979). The seismicity of north central New Mexico with particular reference to the Cerrillos earthquake of May 28, 1918, New Mex. Geol. Soc., Thirtieth Field Conf. Guidebook, 65-75.

Reid, H.F. (1911). Remarkable earthquakes in central New Mexico in 1906 and 1907, Bull. Seism. Soc. Am., 1, 10-16.

Reiter, L. (1990). Earthquake Hazard Analysis: Issues and Insights, Columbia University Press, New York, 254p.

Rinehart, E.J. and A.R. Sanford (1981). Upper crustal structure of the Rio Grande rift from inversion of microearthquake S-wave reflections, Bull. Seism. Soc. Am., 71, 437-450.

Sanford, A.R. (1994). The July 1960 earthquake sequence in the central Rio Grande rift of New Mexico, New Mexico Institute of Mining and Technology, Geophysics Open- File Report 64, 16p.

Sanford, A.R., R.S. Balch and K.W. Lin (1995a). A seismic anomaly in the Rio Grande rift near Socorro, New Mexico, New Mexico Institute of Mining and Technology, Geophysics Open-File Report 78, 17 p.

Sanford, A.R., K.W. Lin, I.C. Tsai, and L.H. Jaksha (1995b). Preliminary listing and discussion of New Mexico earthquakes 1962-1994 with duration magnitudes of 3.0 or greater, New Mexico Institute of Mining and Technology, Geophysics Open-File Report 79, 11p.

Sanford, A.R., K.W. Lin, I.C. Tsai, and L.H. Jaksha (1995c). Seismicity along a segment of a prominent ENE trending topographic lineament in New Mexico and West Texas, New Mexico, New Mexico Institute of Mining and Technology, Geophysics Open-File Report 81, 16 p.

Shurbet, D.H. (1969). Increased seismicity in Texas, The Texas Journal of Science, 21, 37-41.

Taggart, J. and Baldwin B. (1982). Earthquake sequence of 1938-1939 in Mogollon Mountains, New Mexico, New Mexico Geology, 4, 49-52.

Thelin, G.P. and R.J. Pike (1991). Landforms of the conterminous United States - a digital shaded-reflect portrayal, Misc. Investigation Series Map I-2206, U.S.G.S., U.S. Dept. of Interior.

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