Preface

The earthquake catalogs, maps and discussion we present here are the outcome of 40 years of seismological research at New Mexico Tech and elsewhere in New Mexico and West Texas. Research at New Mexico Tech was started in 1958 when I learned from publications, primarily by Stuart A. Northrop and H.F. Reid, that based on felt earthquakes, the Socorro area had a history of strong earthquake swarms and was probably the most active region in New Mexico. In a short 3-day period of instrumental recording in the fall of 1959, 49 microearthquakes were detected, 40 within 20 km of Socorro. This was a strong indication that research based on seismograph recordings would be productive. At the end of 1960 we had a suite of instruments operating in an abandoned mine of the foot of Socorro Mountain. Through the years, New Mexico Tech seismograph stations have increased to 16, the number currently in operation.

Although our research program has emphasized earthquake activity within the Socorro area, beginning in 1962 we were able to locate earthquakes throughout New Mexico and bordering areas using data from New Mexico Tech instruments in conjunction with stations throughout the region. This report would not have been possible without seismograms or readings from (1) stations in operation during the VELA Uniform program, (2) USGS stations at Albuquerque and elsewhere in the region, (3) a seismograph network operated for a number of years by Los Alamos National Laboratory and (4) university stations at University of Texas at El Paso, University of Texas at Austin and Texas Tech University. Individuals who made particularly significant contributions by providing station readings and earthquake locations and magnitudes were Ken Olsen, Dan Cash, and Leigh House at Los Alamos National Laboratory and Diane Doser and G. Randy Keller at the University of Texas at El Paso.

This report would also not have been possible without the contributions of a long line of talented undergraduate and graduate research assistants at New Mexico Tech. They helped maintain instruments in the field and in the recording observatory. They also interpreted the seismograms, often completing special studies as well as routine determinations of the strengths and locations of earthquakes throughout the region.

An important aspect of the data presented here is that it establishes average rates of activity. For the Socorro area (~5000 km²), the expected numbers of earthquakes on average is (1) 5.8/year, moment magnitude 2.0 or greater and (2) 1.0/year, moment magnitude 3.0 or greater. For the remainder of New Mexico and bordering areas (700,000 km²) the average rates are (1) 19.1/year, moment magnitude 2.0 or greater and (2) 4.3/year, moment magnitude 3.0 or greater. As will be shown, earthquakes of magnitude 1.3 and greater occurred throughout New Mexico and bordering areas from 1962-1998, however, with a few exceptions, the level of activity has been low compared to regions like California.

Allan R. Sanford

Introduction

Starting in July 1960 and continuing through 1998, New Mexico Tech (NMT) generated files on all earthquakes they were able to locate in New Mexico and bordering areas. These files provided a starting point for generating the comprehensive and consistent catalogs presented here and were ultimately the source of 84 percent or more of the earthquakes in these listings. The event locations in the NMT files utilized phase readings from many organizations. For the period 1962 through 1972, the principal contributing organizations were the U.S. Geological Survey (USGS), the Air Force Office of Scientific Research (stations and observations in operation during the VELA Uniform Program), and universities (Texas Tech University (TTU) and University of Texas at El Paso (UTEP)). For the period 1973 through 1982, the principal contributing organizations were the USGS, universities (UTEP and TTU) and Los Alamos National Laboratory (LANL). From 1983 through 1998, the principal contributing organizations were LANL, the USGS, and UTEP.

Generation of the New Mexico Tech Catalog

A initial problem with the earthquakes on file at NMT was that different procedures were used to locate and assign magnitudes during different periods from 1960 through 1998. Therefore our first step was to relocate and reassign magnitudes for all events in the NMT files using the SEISMOS location program (Hartse, 1991) and a New Mexico duration magnitude scale (Newton et al., 1976; Ake et al., 1983). Eliminated from the NMT data were all known explosions from open-pit copper mines in southwestern New Mexico and southeastern Arizona, open-pit coal mines in northwestern New Mexico and northeastern Arizona, and miscellaneous explosions from quarry operations and road construction.

At this stage of the study, a decision was made to divide the NMT data into two catalogs of earthquakes with duration magnitudes of 1.3 or greater; one for a 5000 km² region surrounding Socorro that we have designated the Socorro Seismic Anomaly (SSA), the other for remainder of New Mexico and bordering areas (RNM). The SSA occupies 0.7% of the total study area but accounts for ~29% of the earthquakes in our most comprehensive catalog (M_d >= 1.3). The division was made not only because of the seismological importance of the SSA but also because the procedures followed were somewhat different for the SSA and the RNM.

The principal characteristics of this initial NMT catalog were:

1. An increase in the number of earthquakes with time because of an increase in the number of stations with a more favorable geographic distribution for location of events in New Mexico and bordering areas.

2. A bias towards earthquakes with locations near Socorro because of the geographic distribution of seismograph stations operated by New Mexico Tech.

Because of the temporal and spatial distribution of seismograph stations (see Appendix V), the catalogs generated for magnitude 1.3 or greater shocks were temporally and spatially biased. We realized that these problems could be removed by determining the lowest magnitude at which the data is complete throughout the study area. However before testing for completeness, we wanted to have as complete a catalog as possible at the magnitude 1.3 or greater level. This meant searching for missing events using catalogs from other organizations.

Catalogs of Other Organizations

The only organization cataloging earthquakes in New Mexico and bordering areas for the entire 1962-1998 period was the USGS. Their listings were particularly helpful in providing missing events at the outer margins of the study area that occurred from 1962-1980. From 1981-1998, all earthquakes cataloged by the USGS were also located by New Mexico Tech.

The NMT catalogs were cross-checked with LANL catalogs for the period 1 December 1972 through 31 December 1981. These catalogs filled gaps in the NMT catalog, particularly in the north-central area of New Mexico. The LANL catalogs were biased towards that area because of the distribution of stations in their network.

The NMT catalogs were cross-checked with UTEP catalogs for the period 1 December 1976 through 30 April 1978. These catalogs provided missing events in southeastern New Mexico and bordering West Texas at a time when NMT was not adequately monitoring that area.

In the process of cross-checking catalogs, we identified a substantial number of NMT cataloged earthquakes that were also located by the USGS, LANL, and UTEP. On the basis of these co-located events, we were able to establish magnitude adjustments to the missing events added to the New Mexico Tech catalog from the USGS, LANL, and UTEP listings (see Appendix IV).

Procedures

Earthquake Magnitudes

A major effort was made to have all magnitudes in our catalogs based on or tied to a New Mexico duration magnitude scale (Newton et al., 1976; Ake et al., 1983). For determining magnitudes the relation

, (1)

was used, where t_d is the duration in seconds. This relation was first developed by Dan Cash at LANL (Newton et al., 1976) for earthquakes in northern New Mexico. Later an essentially identical relation was derived at NMT (Ake et al., 1983). The basic data for the NMT duration magnitude equation were 64 Wood-Anderson seismograph recordings of earthquakes in New Mexico and bordering areas. Local magnitudes calculated from the amplitudes on the Wood-Anderson seismograms (Richter, 1958) were linearly related to the logarithm of t_d measured on seismograms from the Albuquerque (ALQ) station of the World Wide Seismograph Network (WWSN). Hanks and Kanamori (1979) have demonstrated that local magnitude is equivalent to moment magnitude.

All duration magnitudes calculated by NMT were based on t_d measurements on ALQ-WWSN seismograms or on seismograms produced by instruments with similar responses and magnifications. For a small fraction of events in the NMT catalog, duration magnitudes could not be calculated but magnitudes from the USGS, LANL, UTEP or ASL were available. To obtain consistency in magnitudes, we established magnitude adjustments based on differences in magnitudes for events co-located by NMT and the USGS, LANL, UTEP and the ASL (see Appendix IV for detailed listings). Table 1 summarizes the adjustments. The hierarchy adopted when magnitudes were available from two or more organizations was NMT, USGS, LANL, ASL, and UTEP.

Earthquake Epicenters

Most locations of earthquakes within the SSA were obtained from the inverse method program SEISMOS (Hartse, 1991) designed to incorporate crustal reflections into the location process. A relatively complex and tightly constrained crustal structure obtained from inversion of reflection data was used with the SEISMOS program in the SSA (Hartse et al., 1992).

Epicenters for the earthquakes everywhere in the state and bordering areas (RNM) except the SSA were obtained from the SEISMOS program modified to locate regional earthquakes. SEISMOS like other inverse method location programs, e.g. HYPO71 (Lee and Lahr, 1972) and HYPOELLIPSE (Klein, 1978), fails to obtain reliable locations for regional events whose epicenters may be far outside small aperture networks. For the 37 years of New Mexico instrumental recording, this was a frequent occurrence. Lin (1994) and Lin and Sanford (2001) solved this problem by developing a fuzzy logic algorithm that obtains a highly reliable initial estimate of the epicenter for input into the SEISMOS program. An important step in the generation of the NMT catalogs was relocation of nearly all of the earthquakes using SEISMOS modified to include the fuzzy logic algorithm. The research for relocation using a single location program was to obtain a more uniform set of epicenters. Also, locations in many cases were improved by combining readings from two or more organizations.

The velocity model used with SEISMOS to locate earthquakes outside the SSA was a simple half-space with a velocity of 6.15 km/sec and a Poisson’s ratio of 0.25. Because of this crustal model, only Pg and Sg arrival times were used in the location process. With this procedure, reasonable epicenters for the largest number of earthquake were possible. Many tests on the locations of known mining operations using this procedure indicated that for a significant fraction of New Mexico earthquakes, it may produce the most accurate locations.

For 10.9 percent of the events in our most comprehensive catalog (M >= 1.3), it was necessary to use locations obtained by the USGS (3.5%), LANL (6.8%), and UTEP (0.6%). We do not know the procedures used by these organizations to obtain earthquake locations.

Accuracy of Epicenters

We define accuracy as how close the locations calculated by SEISMOS are to the true epicenters. A primary factor influencing accuracy is how close the adopted crustal velocity model matches the true velocity structure. For the Socorro Seismic Anomaly (SSA), the match is close and the recording distances for most events short. For the remainder of New Mexico and bordering areas (RNM) the adopted half-space crustal model is an average for the entire study area. Some epicenters will not be accurate because all paths to recording stations for a given event are through crust which has a velocity different from the average. Recording paths for many events are long which can increase the error arising from crustal velocities different than the adopted model.

Other factors influencing accuracy of the epicenters are:

1. Number of stations.

2. Geographic distribution of stations.

3. Quality of P and S phase arrivals.

4. Number of paired P and S phase arrivals.

Quantifying the effect of these factors on the accuracy of epicenters is difficult and impossible for about 23% of the events in the catalog (M_d >= 1.3) because one or more of the four factors is not known. However the following generalizations about accuracy of locations in the catalogs can be made:

1. Accuracy improves with time because of an increase in number of stations with geographic distributions favorable for location of earthquakes in the region (see Appendix V for tables and figures showing temporal changes in numbers and distributions of seismograph stations).

2. Accuracy improves with an increase in the magnitude of an event.

3. Accuracy is highest for events located near seismograph networks operated by NMT (west central NM and southeast NM), LANL (north central NM) and UTEP (southeast NM and adjacent areas of West Texas).

The Catalogs

The first catalog (Appendix I) is for earthquakes with moment magnitudes of 1.3 or greater for the period 1962-1995. Separate listings are given for the SSA and the RNM. Percentage contributions by organizations to the 2000 locations in these listings are:

SSA (N = 572): NMT (99.6%), LANL (0.4%).

RNM (N = 1428): NMT (89.1%), USGS (3.5%), LANL (6.8%), UTEP (0.6%).

A map of epicenters for earthquakes in Appendix I is presented in Figure 1. Because of the distribution of seismograph stations (Appendix V) and the low cut-off magnitude, the seismicity for this data set is biased towards the western two-thirds of New Mexico.

Tests on a catalog covering a slightly smaller region than shown in Figure 1 indicated that the earthquake data were only complete over that area at magnitude of 2.0 or greater (Lin, 1999). For our second catalog (Appendix II), the same cut-off was adopted although we realized that there was a chance that we may have missed some magnitude 2.0 or so earthquakes at the outer limits of the study area. The time period for this second catalog is extended to 37 years (1962-1998). In Appendix II, separate listings are presented for the SSA and the RNM. Percentage contributions by organizations to the 923 locations in these listings are:

SSA (N = 215): NMT (99.5%), LANL (0.5%).

RNM (N = 708): NMT (86.5%), USGS (8.2%), LANL (4.2%), UTEP (1.1%).

A map of epicenters for earthquakes in Appendix II is presented in Figure 2.

The third catalog (Appendix III) is for earthquakes with moment magnitudes of 3.0 or greater. As before separate listings are presented for the SSA and the RNM. Percentage contributions by organizations to the 194 locations in these listings are:

SSA (N = 36): NMT (100%).

RNM (N = 158): NMT (83.6%), USGS (16.4%), LANL (0%), UTEP (0%).

A map of epicenters for earthquakes in Appendix III is presented in Figure 3.

A final catalog (Table 2) is presented for the strongest earthquakes within the borders of New Mexico from 1869-1998. Incorporated into this listing are earthquakes documented by felt reports prior to 1962. The record of historical seismicity for New Mexico begins in the mid-nineteenth century. Although settlement by the Spanish began in 1598, little is known of earthquake activity in the state prior to its becoming a territory of the United States in 1848. No doubt, reports of earthquakes exist in Spanish and Mexican archives; such information, however, is difficult to extract, and to our knowledge no such attempt has been made. The earliest report after U. S. occupation is the description of a swarm of shocks in the Rio Grande valley at Socorro by a U. S. Army surgeon (Hammond, 1966). No shock in this 22 event swarm from 11 December 1849 through 8 February 1850 was felt at distances greater than 25 km, an indication that these earthquakes did not exceed magnitude 3.5. Similar sequences of shocks located away from population centers in the state could easily have gone unreported before the start of instrumental studies in 1962.

Prior to 1962, the strengths of nearly all earthquakes were expressed in terms of the maximum intensity, I_o, a quantity assigned on the basis of what people observe during an earthquake and damage to structures. The scale used for ranking earthquake intensity in United States is the Modified Mercalli-Revised 1931 (Richter, 1958). An empirical relation between maximum intensity and duration magnitude (equivalent to moment magnitude) has been derived by Sanford (1998) for New Mexico earthquakes:

M_d = 0.5 + 2/3 I_o. (2)

This equation was used to convert maximum observed intensities for shocks prior to 1962 into magnitudes in order to obtain a list of strongest earthquakes with the same measure of strength.

The difficulty with using maximum intensity reports as a measure of the strength of an earthquake is that it implies the existence of people and/or structures directly at the epicenter of the shock. Therefore the reliability of the reports is dependent on population density which was very low for nearly all of New Mexico for the period prior to 1962. To reduce but not eliminate any population density problems, we have restricted our list of strongest earthquakes for the period prior to 1962 to shocks with maximum reported intensities of VI or greater. This places a lower limit of about moment magnitude 4.5 on the list of strongest earthquakes. Data on the 30 earthquakes equaling or exceeding magnitude 4.5 from 1869 through 1998 are given in Table 2 and a map of their locations is presented in Figure 4.

Geographic Distribution of Earthquakes

The most striking feature of the seismicity in Figures 1 and 2 is the tight cluster of earthquake activity in the Rio Grande valley near Socorro. This Socorro Seismic Anomaly (SSA) occupies only 0.7% (~5000 km²) of the total area of the figures but accounts for 23% of the earthquakes of magnitude 2.0 or greater. The SSA 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 lateral extent of ~3400 km² (Ake and Sanford, 1988; Hartse et al., 1992; Balch et al., 1997). Level-line data indicate that the surface above the magma body is undergoing uplift at a maximum rate of ~1.8 mm/year (Larsen et al., 1986) presumably because of injection of new magma into the thin extensive mid-crustal chamber. Stresses in the upper crust generated by an inflating magma body at a depth of 19 km are expected to extend over a larger region than the lateral extent of the magma body (~3400 km²).

In Figures 1 and 2, the pattern of seismicity outside the SSA is diffuse and occurs in all physiographic provinces including the presumably tectonically stable Colorado Plateau and Great Plains provinces. Well-defined seismic trends are not apparent. However, on the map of magnitude 3.0 or greater shocks (Figure 3), an interesting alignment of shocks does appear. Extending east-northeast from the SSA into the Great Plains of eastern New Mexico and West Texas is a band of epicenters that straddles the trace of a prominent topographic lineation identified by Thelin and Pike (1991) on a digital shaded relief map they generated for the conterminous United States. The lineation, a possible fracture zone of Precambrian origin, extends 1400 km east-northeast from southwestern Arizona to the Texas Panhandle-Oklahoma border (Sanford and Lin, 1998b). The ~85 km wide track of this feature is defined by a lineation of many features such as rivers, elongate depressions, faults, and the historical seismicity in Figures 2 and 3. We have designated this feature the Socorro Fracture Zone. Using Monte Carlo techniques, we have tested the possibility that the alignment of epicenters overlying the topographic lineation is accidental. The earthquakes of magnitude 3.0 or greater outside the SSA were randomly distributed over the region nearly 1000 times without reproducing the band of seismicity extending east-northeast from the SSA; a band that occupies 7% of the total area of the region but produces 19% of the earthquakes of magnitude 3.0 or greater outside the SSA.

A large fraction of the earthquakes in northern New Mexico appear to be related to the Jemez lineament (Aldrich and Laughlin, 1984), a fracture zone that extends from southwest of Grants to Los Alamos and Espanola in the Rio Grande valley and then continues on along an east-northeast track to beyond the northeast corner of the state (Figure 3). The Jemez lineament is a 50 km to 80 km wide leaky fracture zone defined by many hundreds of magmatic eruptive centers, including the very large aseismic Jemez volcanic complex just west of Los Alamos (Sanford et al., 1991). Other areas (Figures 1 and 2) where there appears to be an association between volcanism and seismicity are (1) near the Albuquerque volcanoes, 35.1 ûN and 106.8 ûW (Jaksha et al., 1981); and (2) near the vents for the very recent Carrizozo flow, 33.9 N and 106.0 W (Sanford et al., 1991).

Perhaps the most unusual characteristic of earthquake activity from 1962-1998 is its failure to define the Rio Grande rift (RGR), a major continental rift extending north-south through the state from north of Taos to south of Las Cruces (Chapin, 1971 and 1979). The overwhelming majority of Quaternary faults in New Mexico (Machette et al., 1998) fall within the boundaries of the RGR shown in Figure 3, and yet earthquakes are absent or nearly so over much of its extent; for example, from just south of Socorro to just north of Las Cruces (Figures 1 and 2).

The 30 largest earthquakes for the period 1869-1998 (Figure 4) appear more concentrated along the rift than the weaker shocks (Figures 1 through 3). However, because of the very low population density over most of the region east and west of the rift, there is a good possibility that events with maximum intensities of VI to VII (MM 1931) may have gone unreported in the period prior to the start of instrumental recording in the early 1960’s. Note that 14 of the 30 strongest earthquakes in New Mexico (Table 1) had epicenters within the Socorro Seismic Anomaly. Five of these 14 earthquakes occurred during a prolonged earthquake swarm that began in July 1906 and extended into 1907 (Sanford et al., 1991). The two strongest earthquakes in New Mexico from the mid-1800’s to the end of the millennium took place during this swarm.

Off the southeastern corner of New Mexico in the Great Plains is an extensive cluster of earthquake activity. In this area, the spatial distribution of epicenters appears to correlate with locations of oil and gas fields, taking into account uncertainties in epicenters. Published studies (Sanford et al., 1981) and ongoing research strongly suggest that a large fraction of activity in southeastern New Mexico and adjacent areas of West Texas is induced by production, secondary recovery or waste injection within this petroleum and natural gas province.

Temporal Variation of Earthquake Activity

Presented in Figure 5 is the number of events versus time for the Socorro Seismic Anomaly (SSA) and the remainder of New Mexico and bordering areas (RNM). The earthquakes considered are magnitude 2.0 or greater and the area covered for the RNM is slightly smaller than in Figures 1 through 3. For the period 1962-1998, the seismicity of both source zones was temporally irregular at short and long intervals. For example, there were periods of up to two years without a magnitude 2.0 or greater shock within the SSA. The number of recorded events within the SSA for the 17 year period 1982-1998 was 147 which is significantly higher than the 68 shocks in the 20 year period 1962-1981. For the RNM, there were no very long term variations such as observed for the SSA, but intermediate term variations with periods of ~5 years were recorded. The temporal variations illustrate the danger of using short term histories of earthquake activity to estimate long-term hazard. The sharp peaks in the temporal seismicity for the two seismic source zones are the result of earthquake swarms and main-shock aftershock sequences.

Summary and Conclusions

Catalogs and maps of the seismicity for New Mexico and bordering areas based on instrumental data are presented for the time period 1962 through 1998. An important consideration in the generation of the catalogs was to maximize uniformity in the magnitudes and epicenters by using a single magnitude scale (100% of the time) and a single location program (89% of the time) (Ake et al., 1983; Hartse, 1991). The largest catalog, which contains 2000 earthquakes of magnitude 1.3 or greater, is temporally and spatially biased because of an increase with time in the number of stations with a more favorable geographic distribution for location of events in the New Mexico region. Two additional catalogs have cutoff magnitudes of 2.0 and 3.0, and respectively, they are temporally and spatially almost complete to complete over the entire region for the entire 37 year interval. The final catalog presented is for the strongest earthquakes in New Mexico for the time period 1869-1998. This catalog utilizes felt reports of earthquakes having maximum intensities of VI or greater (MM 1931) before 1962, and is probably incomplete before that date.

The catalogs document a long known feature of the seismicity of the New Mexico region; a disproportionate fraction of the activity is occurring in the middle of the Rio Grande rift at Socorro. A 5000 km² area surrounding this community is contributing 23% of the earthquakes of magnitude 2.0 or greater. The enhanced seismicity over this small area is believed to be the result of crustal extension arising from inflation of a thin (~150 m), extensive (~3400 km²) magma body at a depth of ~19 km (Ake and Sanford, 1988; Hartse et al., 1992; Balch et al., 1997).

The catalogs also document the existence of earthquakes in all physiographic provinces of the region including those considered tectonically stable, such as the Colorado Plateau and the Great Plains, as well as the Rio Grande rift. A somewhat surprising observation is that the rift is not clearly defined by the seismicity although this province contains nearly all faults with evidence of Quaternary movement (Machette et al., 1998).

Perhaps the most important outcome of our compilations is that it establishes average rates of activity for the anomalous Socorro area (SSA) and for the remainder of New Mexico and bordering areas (RNM). For the SSA, the expected number of earthquakes of moment magnitude 2.0 or greater is 5.8 each year, and for moment magnitude 3.0 or greater it is 1.0 each year. For the RNM, the expected number of earthquakes of moment magnitude 2.0 or greater is 19.1 each year, and for moment magnitude 3.0 or greater it is 4.3 each year. The latter are modest rates of activity for such a large region and the resulting earthquake hazard is for the most part low. On the other hand, the rate of activity per square kilometer is relatively high for the SSA and consequently the resulting hazard is considerably higher but still quite moderate when compared with most areas of California.

Acknowledgements

The authors are grateful to Leigh House, Diane Doser and Noel Barstow for their many useful comments and suggestions on the text accompanying the catalogs of this publication.

References

Ake, J. P., A. R. Sanford, and S. J. Jarpe (1983). A magnitude scale for central New Mexico based on signal duration, New Mexico Institute of Mining and Technology Geophysics Open-File Report 45, Socorro, New Mexico, 26 pp.

Ake, J. P., and A. R. Sanford (1988). New evidence for existence and internal structure of a thin layer of magma at mid-crustal depths near Socorro, New Mexico, Bull. Seism. Soc. Am., 78, 1335-1359.

Aldrich, M. J., Jr., and A. W. Laughlin (1984). A model for the tectonic development of the southeastern Colorado Plateau boundary, J. Geophys. Res. 89, 10,207-10,218.

Balch, R. S., H. E. Hartse, A. R. Sanford, and K. W. Lin (1997). A new map of the geographic extent of the Socorro midcrustal magma body, Bull. Seism. Soc. Am., 87, 174-182.

Chapin, C. E. (1971). The Rio Grande rift: Part 1, Modifications and additions. New Mex. Geol. Soc., 22nd Annual Field Conference Guidebook, 191-201.

Chapin, C. E. (1979). Evolution of the Rio Grande rift: A summary, in Rio Grande Rift: Tectonics and Magmatism, R. E. Riecker (editor), American Geophysical Union, Washington, D.C., 1-5.

Hammond, J. F. (1966). A Surgeon's Report on Socorro, New Mexico, 1852, Stagecoach Press, Santa Fe, New Mexico, 47 pp.

Hanks, T. C., and H. Kanamori (1979). A moment magnitude scale, J. Geophys. Res., 84, 2348-2350.

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

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

Jaksha, L.H., J. Locke, and H.J. Gebhart (1981). Microearthquakes near the Albuquerque volcanoes, New Mexico, Geol. Soc. Am. Bull., 92, 31-36.

Klein, R. W. (1978). Hypocenter location program HYPOINVERSE Part I: Users guide to versions 1, 2, 3 and 4, U.S. Geol. Surv. Open-File Report 78-694.

Larsen, S., R. Reilinger, and L. Brown (1986). Evidence for ongoing crustal deformation related to magmatic activity near Socorro, New Mexico, J. Geophys. Res., 91, 6283-6292.

Lee, W. H. K. and J. C. Lahr (1975). HYPO71 (Revised): A computer program for determining hypocenter, magnitude, and first motion pattern of local earthquakes, U.S. Geol. Surv. Open-File Rept. 75-311.

Lin, K. W. (1994). Regional earthquake hypocenter location using a fuzzy logic algorithm enhanced SEISMOS program, New Mexico Institute of Mining and Technology Geophysics Open-File Report 74, Socorro, New Mexico, 82 pp.

Lin, K. W. (1999). Probabilistic seismic hazard in New Mexico and bordering areas, Ph.D. Dissertation, New Mexico Institute of Mining and Technology, Socorro, New Mexico, 195 pp.

Lin, K. W., and A. R. Sanford (2001). Improving locations of regional earthquakes using a modified G matrix and fuzzy logic, Bull. Seism. Soc. Am., in press.

Machette, M. N., S. F. Personius, K. I. Kelson, K. M. Haller, and R. L. Dart (1998). Map and data for Quaternary faults and folds in New Mexico, U.S. Geol. Surv., Open-File Report 98-521, 443 pp.

Newton, C. A., D. J. Cash, K. H. Olsen, and E.F. Homuth (1976). LASL seismic programs in the vicinity of Los Alamos, New Mexico, Los Alamos Scientific Laboratory Informal Report LA-6406-MS, 42 pp.

Richter, C. F. (1958). Elementary Seismology, W. H. Freeman, San Francisco, California, 768 pp.

Sanford, A. R., L. H. Jaksha, and D. J. Cash (1991). Seismicity of the Rio Grande rift in New Mexico, in Neotectonics of North America, D. B. Slemmons, E. R. Engdahl, M. D. Zoback, and D. D. Blackwell (Editors), Geological Society of America, 229-244.

Sanford, A. R. (1998). An empirical relation between magnitude and maximum intensity for New Mexico earthquakes, New Mexico Institute of Mining and Technology Geophysics Open-File Report 86, Socorro, New Mexico, 4 pp.

Sanford, A. R. and K. W. Lin (1998a). Strongest earthquakes in New Mexico: 1860 to 1998, New Mexico Institute of Mining and Technology Geophysics Open-File Report 87, Socorro, New Mexico, 4 pp.

Sanford, A. R. and K. W. Lin (1998b). Evidence for a 1400 km long Socorro Fracture Zone, New Mexico Institute of Mining and Technology Geophysics Open-File Report 89, Socorro, New Mexico, 18 pp.

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. Geol. Survey, U.S. Dept. of Interior.

Appendix I
Earthquake Catalogs for New Mexico and Bordering Areas: 1962 through 1995; Moment Magnitude >= 1.3

Two catalogs are presented, one for the Socorro Seismic Anomaly (SSA), and the other for the remainder of the state and bordering areas (RNM). For each event, the catalogs indicate (1) organizations which have calculated magnitudes and the magnitude selected, and (2) the organization whose location is used. The symbols used for the latter are:

nmt New Mexico Tech
lanl Los Alamos National Laboratory
utep University of Texas at El Paso
asl Albuquerque Seismological Laboratory ­ USGS Facility
usgs United States Geological Survey

Also in the magnitude columns is the symbol cln. This is the designation for a NMT station in SE New Mexico whose records were used to establish magnitudes for a large number of events from July 1974 through April 1979.

Appendix II
Earthquake Catalogs for New Mexico and Bordering Areas: 1962 through 1998; Moment Magnitude >= 2.0

Two catalogs are presented, one for the Socorro Seismic Anomaly (SSA), and the other for the remainder of the state and bordering areas (RNM). For each event, the catalogs indicate (1) organizations which have calculated magnitudes and the magnitude selected, and (2) the organization whose location is used. The symbols used for the latter are:

nmt New Mexico Tech
lanl Los Alamos National Laboratory
utep University of Texas at El Paso
asl Albuquerque Seismological Laboratory ­ USGS Facility
usgs United States Geological Survey

Also in the magnitude columns is the symbol cln. This is the designation for a NMT station in SE New Mexico whose records were used to establish magnitudes for a large number of events from July 1974 through April 1979.

Appendix III
Earthquake Catalogs for New Mexico and Bordering Areas: 1962 through 1998; Moment Magnitude >= 3.0

Two catalogs are presented, one for the Socorro Seismic Anomaly (SSA), and the other for the remainder of the state and bordering areas (RNM). For each event, the catalogs indicate (1) organizations which have calculated magnitudes and the magnitude selected, and (2) the organization whose location is used. The symbols used for the latter are:

nmt New Mexico Tech
lanl Los Alamos National Laboratory
utep University of Texas at El Paso
asl Albuquerque Seismological Laboratory ­ USGS Facility
usgs United States Geological Survey

Also in the magnitude columns is the symbol cln. This is the designation for a NMT station in SE New Mexico whose records were used to establish magnitudes for a large number of events from July 1974 through April 1979.

Appendix IV
Listings of Magnitude Differences Between New Mexico Tech and Contributing Institutes

For most events in the NMT catalogs, we were able to determine a duration magnitude (Newton et al., 1976; and Ake et al., 1983). For the remainder, we had to use magnitudes calculated by contributing institutions, adjusted by the average differences appearing in the tables of this appendix. Below are the institutions for which magnitude corrections were established.

usgs United States Geological Survey
lanl Los Alamos National Laboratory
utep University of Texas at El Paso
asl Albuquerque Seismological Laboratory ­ USGS Facility

A correction was also established for CLN, an NMT station in SE New Mexico whose records were used to establish magnitudes for a large number of events from July 1974 through April 1979.

Appendix V
Seismograph Station Locations

Four tables and three figures are presented. Table V-1 is a master list of the latitudes and longitudes of the 120 seismograph stations used to locate earthquakes in New Mexico and bordering areas from 1962 through 1998. Most of these stations were not in operation for the entire 37 year study. Tables V-2 through V-4 list stations used during three separate periods of the earthquake research; 1962 through 1972, 1973 through 1982, and 1983 through 1998. Figures V-1 through V-3 are maps of the seismograph stations for these three periods.

Figure 1. Seismicity of New Mexico and bordering areas; time period 1962-1995, moment magnitudes 1.3 or greater. A total of 2000 earthquakes are plotted on this map, 572 inside the Socorro Seismic Anomaly (SSA) (see gray area above and in Figures 2 and 3).

Figure 2. Seismicity of New Mexico and bordering areas; time period 1962-1998, moment magnitudes 2.0 or greater. A total of 923 earthquakes are plotted on this map, 215 inside the Socorro Seismic Anomaly (SSA) (see gray area above and in Figure 3).

Figure 3. Seismicity of New Mexico and bordering areas; time period 1962-1998, moment magnitudes 3.0 or greater. A total of 194 earthquakes are plotted on this map, 36 inside the Socorro Seismic Anomaly (SSA) (see gray area above). Thirty of the 158 events outside the SSA fall within the Socorro Fracture Zone. Also shown are the approximate boundaries of the Rio Grande rift and the Jemez lineament.

Figure 4. Strongest New Mexico earthquakes; time period 1869-1998, moment magnitudes 4.5 or greater. Number in parenthesis indicates multiple events at that location.

Figure 5. Temporal seismicity for the Socorro Seismic Anomaly (SSA) and the remainder of New Mexico and bordering areas (RNM); time period 1962-1998, moment magnitudes 2.0 or greater. A total of 215 events are plotted for the SSA (bottom) and 492 for the RNM (top). In this case the area covered by RNM is smaller than in Figures 1 through 3. The sharp peaks in the temporal seismicity for the two seismic source zones are the result of earthquake swarms and main-shock aftershock sequences.