A program of seismological research was initiated at New Mexico Tech in the late 1950’s. Our review presents a chronological history from 1957 through 1999 that documents the milestones and accomplishments of the program. Two major long-term and on-going projects during the over 40 years of research were (1) the physical characteristics of a highly unusual mid-crustal magma body in the central Rio Grande rift beneath Socorro, New Mexico and (2) the seismicity of New Mexico. Our review includes a section on the Socorro Magma Body that presents observations which best constrain its depth, extent, internal structure, relation to seismicity and other geophysical properties. A section on the regional seismicity studies (1) describes development of catalogs based on instrumental data (1962-1998) and pre-instrumental data (1869-1961), (2) presents and discusses the geographic distribution of earthquakes in New Mexico and (3) presents and discusses a seismic hazard map for the region.

Introduction

The chronology of the research on the Socorro Magma Body is given in the first section of this institutional review. Presented below are the observations which are believed to best constrain the physical characteristics of this highly unusual geologic structure.

Evidence for Magma

The direct evidence for magma comes from analyses of amplitudes and spectra of reflected phases; PzP, SzP and SzS on microearthquake seismograms and PzP on COCORP crustal reflection profiles. Emphasized here are results of the New Mexico Tech studies of the microearthquake reflection phases. Data used in this research were from seismograms of small events, magnitudes generally less than zero, where reflected phases were not obscured by the codas of the direct P and S phases.

Amplitude versus Offset-AVO. From the very earliest observations of the microearthquake reflections (e.g. Sanford and Long, 1965), the amplitudes of these phases were found difficult to explain by single discontinuities across which the velocity increased. With the accumulation of more data it became possible to compare observed and theoretical ratios of SzP/SzS for offsets ranging from 5û to 30û (Sanford et al., 1973; Sanford et al., 1977; and Rinehart et al., 1979). The misfit for the case of the solid to solid Conrad discontinuity in the RGR (Toppozada and Sanford, 1976) was very large. Single discontinuities separating rigid and full or partial melts provided the best but not perfect match between observed and theoretical ratos.

Phase Changes and Waveform Characteristics. Digital microearthquake seismograms of a 1977 swarm 16 km southwest of Socorro were used to determine phase and waveform characteristics of PzP and SzS phases (Ake and Sanford, 1988). SzS and PzP undergo 180º phase reversals upon reflection, an indication that the first layer of any reflection producing structure is non-rigid. High correlation coefficients between S and SzS phases indicate little if any S wave energy penetrates beyond the first discontinuity, an indication that the underlying material lacks rigidity and is most likely magma from geologic considerations.

Internal Structure

The durations of PzP microearthquake reflections on digital seismograms range from 0.27 to 0.32 seconds which is nearly the same time range for mid-crustal reflections observed on COCORP profiles (Brocher, 1981). These durations place limits on the thickness of the magma body over much of its geographic extent. Ake and Sanford (1988) employed forward modeling of PzP phase to support the contention that the magma body is thin yet has resolvable differences in internal structure over distances as short as one-half kilometer. The preferred model for the SMB 16 km southwest of Socorro is a layer of full melt ~70 m thick underlain by layer of crystalline mush ~60 m thick.

Depth

The most reliable depth to the SMB, 18.75 ±0.28 (1 s.d.) km, was obtained from a simultaneous inversion for the one-dimensional velocity structure to and depth of the mid-crustal structure (Hartse et al., 1992). Earlier depth estimates from 17.8 km to 19.2 km were based on less constrained models of the average velocity structure to the SMB (Sanford and Long, 1965; Sanford et al., 1973; and Rinehart and Sanford, 1981).

Attitude

Examination of arrival time residuals for reflections PzP, SzP and SzS shows an even distribution of positive and negative residuals over the entire extent of the magma body for all reflected phases (Hartse et al., 1992; Balch et al., 1997). These observations indicate that the upper surface of the SMB is flat and horizontal.

Geographic Extent

A map of the geographic extent of the magma body was prepared using 697 PzP, 2169 SzP, and 2589 SzS reflections observed on seismograms recorded from 1975 to 1995 (Balch et al., 1997). The data indicate an area somewhat greater than 3400 km². Increases in the reported lateral extent of the magma body from 1200 km² (Sanford et al., 1977) to the present 3400 km² are the result of larger data sets both in number and wider distribution of stations in the Socorro area.

Position in the Crust

The SMB has a depth, 18.75 km, which coincides closely with the position of the Conrad mid-crustal discontinuity, 18.6 km, determined from a seismic refraction profile through the central RGR (Toppozada and Sanford, 1976). Sanford has suggested that basaltic magma rises to the level of the Conrad discontinuity where it encounters two conditions which impede its upward migration (Sanford and Einarsson, 1982). First it encounters a lower density crustal rock which reduces the buoyancy of the rising magma. Second, the crustal models by Hartse et al. (1992) and Singer (1989) indicate a decrease in velocity below the base of the seismogenic zone at ~10 km. The lower velocity and absence of earthquakes suggests a plastic crustal layer between the Conrad and the seismogenic zone that acts as a seal to further upward migration of the basaltic magma.

Seismicity

The Socorro Seismic Anomaly (SSA) is a region of unusually high earthquake activity in the central Rio Grande rift. This seismogenic province occupies ~5000 km², is centered over the SMB, and is surrounded by a clear aseismic halo. Sanford et al. (1995) have suggested that this distribution of seismicity can be explained by stresses arising from inflation of the SMB.

Uplift

Surface uplift determined from releveling of elevation benchmarks is centered on the SMB (Larsen et al., 1986). Maximum uplift near the center of the SMB averaged ~2.0 mm/year from 1911 through 1980 which is in good agreement with uplift rates over the past 20,000 years based on geomorphic evidence (Ouchi, 1983).

Teleseismic Structure

Sheetz and Schlue (1992) used receiver functions to look at the underside of the magma body; their modeling suggested both the top and bottom of the body present sharp velocity contrasts with surrounding material, with thickness between 25m ? 200m and shear wave speeds of 0.4 km/s ? 0.6 km/s.

In 1996, Schlue, Aster, and Meyer used arrivals from deep events from the Tonga-Fiji region, again to illuminate the underside of the SMB. This study noted evidence for a possible magma conduit near the western edge of the SMB that extends about 6 km into the lower crust.

Introduction

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 an 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. For this reason, the interval before 1962 is restricted to felt earthquakes with intensities of ground motion indicating magnitudes of 4.5 or greater. The earliest reported earthquake equaling or exceeding this magnitude threshold occurred near or at Socorro in 1869.

Seismicity from Instrumental Recording: 1962 through 1998

In the period from 1962 through 1998, several organizations used instruments to locate and determine strengths of earthquakes in New Mexico and bordering areas; notably New Mexico Tech (NMT), Los Alamos National Laboratory (LANL), U.S. Geological Survey (USGS), and the University of Texas at El Paso (UTEP). The periods of operation, number and sensitivity of instruments, and procedures for locating and assigning magnitudes were highly variable amongst these organizations during the 37-year period. For this reason, Sanford et al. (1995) and Sanford et al. (1997) undertook the project of collating data from all organizations into a comprehensive and consistent earthquake catalog for New Mexico and bordering areas. A major effort was made to have all magnitudes in the catalog based on or tied to a New Mexico duration magnitude scale (Newton et al., 1976; Ake et al., 1983). For determining magnitudes we used the relation

Md = 2.79 log Td - 3.63,

where Td 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) for earthquakes in central and southern New Mexico. The duration magnitude scales of both organizations are tied to the local magnitude scale which Hanks and Kanamori (1979) have demonstrated is equivalent to the moment magnitude.

The location program selected for the catalog was SEISMOS (Hartse, 1991). Originally developed to obtain hypocenters for earthquakes within or near a local network, it was modified to also locate regional earthquakes. SEISMOS like other inverse method location programs, e.g. HYPO71 (Lee and Lahr, 1975) and HYPOINVERSE (Klein, 1978), fails to obtain reasonable locations for regional events detected by small aperture networks. For the 37 years of New Mexico instrumental recording, this was a frequent occurrence. Lin (1994) and Lin and Sanford (1998) 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. Generation of the NMT catalog required relocation of nearly all of the earthquakes using SEISMOS modified to include the fuzzy-logic algorithm.

We have determined that the most accurate locations for regional earthquakes in New Mexico are obtained by using Pg and Sg arrivals only and a simple half-space crustal model with a velocity of 6.15 km/sec and a Poisson's ratio of 0.25. For earthquakes near Socorro, a more complex crustal structure is used in order to incorporate reflections into the location process.

The NMT catalog contains epicenters and magnitudes for over 2000 earthquakes with moment magnitudes greater or equal to 1.3; 89.1% from NMT, 6.8% from LANL, 3.5% from USGS, and 0.6% from UTEP. Tests indicate that a low cut-off magnitude of 2.0 assures completeness of the earthquake data throughout the study area since 1962. Therefore, presented here in Figure 1 are only those New Mexico earthquakes from 1962 through 1998 whose moment magnitudes are greater or equal to 2.0. Their distribution by magnitude is 581 greater or equal to magnitude 2.0, 117 greater or equal to magnitude 3.0, and 18 greater or equal to magnitude 4.0. A second map (Figure 2) shows the distribution of the earthquakes with moment magnitudes of 3.0 or greater.

Strongest Earthquakes in New Mexico-1869 through 1998

The historical record of earthquakes in New Mexico extends back to the middle of the 19th century. However, prior to 1962, the strengths of nearly all earthquakes were expressed in terms of the maximum intensity, Io, 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:

Md = 0.5 + 2/3 Io.

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 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 l and a map of their locations is presented in Figure 3

Geographic Distribution of New Mexico Earthquakes

The most striking feature of the seismicity in Figure 1 is the tight cluster of earthquake activity in the Rio Grande valley near Socorro. This Socorro Seismic Anomaly (SSA) occupies only 1.6% of the total area of the state but accounts for 37% of the earthquakes of magnitude 2.0 or greater in Figure 1 and 47% of the earthquakes of magnitude 4.5 or greater in Figure 3. 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.

In Figure 1 the pattern of seismicity outside the SSA is diffuse and well-defined seismic trends are not apparent. However, on the map of magnitude 3.0 or greater shocks (Figure 2), an interesting alignment of shocks does appear. Extending east-northeast from the SSA into the Great Plains of eastern New Mexico 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, 1998). The ~85 km wide track of this feature is defined by a lineation of many features such as rivers, elongate depressions, faults, and probably the contemporaneous seismicity in Figures 1 and 2. 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 in Figure 2 were randomly distributed over the state nearly 1000 times without reproducing the major east-northeast band of seismicity; a band that occupies 9 % of the total area of the state but produces 22 % of the earthquakes 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 on along an east-northeast track to beyond the northeast corner of the state (Figure 1). 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).

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, 1998) are associated with the RGR 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.

Probabilistic Seismic Hazard in New Mexico

A probabilistic seismic hazard map (Reiter, 1990) based on the 37 years of instrumental data is presented in Figure 4. Hazard is given in terms of maximum horizontal ground acceleration (expressed as a fraction of gravitational acceleration-g) with a 10% probability of exceedance in 50 years, an appropriate time scale for most structures in New Mexico. The map indicates seismic hazard throughout New Mexico is moderate to low with maximum horizontal ground motion ranging from a high of 0.18g within the SSA to near zero for much of the state (Lin et al., 1997; Lin, 1999). A horizontal ground acceleration of 0.2g can do considerable damage, for example, chimneys broken at roof lines, but not major destruction.

Geologic mapping has revealed faults in New Mexico that have produced major earthquakes in the Quaternary, particularly along the RGR (Machette, 1998). However, the recurrence interval between these magnitude 7 or greater shocks is many thousands of years, and therefore, they have no effect on a 50-year, 10% probability of exceedance seismic hazard map (Lin and Sanford, 1998).

The complete catalog for the 1962-1998 period was the initial raw data for the probabilistic seismic hazard analysis. The final data set was obtained after tests for completeness and removal of dependent events (Lin et al., 1997). Tests indicated that a cut-off magnitude of 2.0 assured completeness of data throughout New Mexico with a substantial margin of safety. Dependent events were identified and removed using moving time and space windows of 7 days and 4 km for the SSA, and 7 days and 25 km for the events in the remainder of the state. For the hazard analysis, we considered only two source zones, the SSA and the remainder of New Mexico (RNM).

For modeling recurrence relations for the two source zones, a truncated exponential recurrence model was adopted. A Poisson distribution with upper and lower bound magnitudes of 6.5 and 2.0 was assumed. In estimating the slope b, the uncertainty in the measurement of magnitudes was taken into account (Bender, 1983) and produced a combined value of 0.692 for the two source zones.

The region was divided into 20 X 20 km2 blocks, the size of the blocks set to accommodate the maximum horizontal epicenter error for most of the earthquakes in the data set. Each block had its own recurrence relationship and during the hazard analysis interacted with neighboring blocks. The cumulative number of events in the recurrence model for each block was a combination of 75% of the events that occurred in the block plus 25% background seismicity. The latter was based on the average earthquake activity in 20 X 20 km2 blocks throughout the state. For each block, probabilities of occurrence were calculated for ground accelerations ranging from 0.01g to 0.36g at 0.05g intervals. The value of the maximum horizontal ground acceleration was interpolated directly from the curve. For Figure 4, 1480 probability-ground acceleration curves were evaluated.

Acknowledgments

We would like to acknowledge the contributions to our earthquake catalog by Los Alamos National Laboratory (Ken Olsen, Dan Cash, and Leigh House), the U.S. Geological Survey, and the University of Texas at El Paso (Diane Doser and G. Randy Keller).

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