P-wave velocity in Precambrian basement (Pg) and P-wave time terms in and around Albuquerque Basin, New Mexico Lawrence H. Jaksha, New Mexico Institute of Mining and Technology (Adjunct faculty, retired), Socorro, New Mexico 87801 Abstract Geology T = a + b + D/V (1) Where: A time-term method of analyzing seismic The Albuquerque Basin is one of a number T is the shot to station travel time. refraction data was used to process 62 of northerly elongate crustal features that a is the delay time associated with time-distance pairs observed on a 13 station in aggregate make up the Rio Grande the receiver. seismic network from 10 shot points in and depression as identified by Bryan (1938) and around Albuquerque Basin, New Mexico. b is the delay time associated with The time terms derived from the analysis described as the Rio Grande rift by Kelley the shot. range from 1.50 seconds at the Albuquerque (1952). The basin measures approximately D/V is the time the signal spends tra- volcanoes down to -.047 seconds at the 160 km long by 50 km in wide, and the surface versing the high-speed refractor Albuquerque Seismological Laboratory. In of the basin floor is roughly 1.6 km above sea (distance divided by velocity). a general way the time terms correlate with level. Parts of the basin probably began to the geologic foundation beneath the site: form during the Laramide, but most of the Delay time is defined by Sheriff (1973) as Large values are calculated for stations and features seen today are the result of Miocene the additional time taken for a wave to follow shots within the basin, intermediate values and younger faulting and uplift. The basin is a trajectory to and along a buried marker over on consolidated sedimentary rocks and linked through bedrock constrictions to the Tertiary intrusions, and the smallest on Pre- that which would have been taken to follow cambrian outcrops. The P-wave (Pg) velocity Española Basin on the north and to the San the same marker considered hypothetically to in bedrock was found to be 6.0 ± .02 km/s. Marcial Basin on the south. The uplifted east- have been at the ground surface. It is synony- This falls approximately midway between ern margin, fault blocks rotated to the east, mous with “time term” in this study. higher velocities reported to the east and consists of the Sandia–Manzano–Los Pinos An examination of equation (1) suggests west of Albuquerque Basin and lower veloc- Mountains. The less clearly defined western that if the coordinates of the shots and stations ities reported to the north and south. These margin is composed of Ladron Peak, Lucero are known, D can be calculated, and that if the numbers will be valuable in future efforts to uplift, and the Puerco fault belt. Deep hydro- shot origin time is known, T can be estimat- accurately locate shallow nearby earthquakes carbon test wells, seismic reflection profiles, ed from the arrival times. Figure 1 shows the and for studies seeking to unravel seismic and gravity/magnetic surveys in the basin complexities deeper in the crust. arrangement of explosions and stations used reveal a highly faulted and irregular basin in this study. There are 13 station time terms, bottom. These studies tally closely with the 10 shot time terms, and 1 velocity [the a, b, Introduction geologic fabric seen directly in the uplifted and V in equation (1)] to be determined. If margins (Kelley 1977). The basin seems trun- The U.S. Geological Survey in cooperation every station recorded every shot, 130 equa- cated at its northern end by the Miocene–late tions like (1) could be written, and there are with Los Alamos National Laboratory Pleistocene Jemez Mountains and the Valles (LANL) and New Mexico Institute of Mining only 24 unknowns so that the system of equa- caldera. Olsen et al. (1986) reported on a tions is over determined and has a solution. and Technology (NMT) operated a 13 station seismic experiment for the Jemez Mountains seismic network around and within the Multiple linear regression (Draper and Smith that analyzed 290 ray paths and found the 1966) was used to estimate the unknowns, Albuquerque Basin for a 66-month period P-wave velocity in bedrock to be 5.86 ± .01 between 1976 and 1981 with the goal of map- their associated standard errors, and the stan- km/s. The southernmost part of the Albu- dard deviation of the solution (Murdock and ping contemporary seismicity. In the course querque Basin is underlain by the northern of this earthquake monitoring, explosions Jaksha 1980). Berry and West (1966) discussed edge of the Socorro Magma Body described the errors associated with the time-term meth- originating from nearby mining operations by Sanford et al. (1977). Singer (1989) studied were routinely recorded, along with special od and stated that the estimates should be the entire crust beneath a seismic network taken as indicators of “goodness of fit” but shots set off for scientific research by the Uni- centered on Socorro and from an analysis versity of New Mexico, the U.S. Department not as accurate fiducial limit calculations. of 265 ray paths reported that the P-wave The time-term method assumes a starting of Defense, and LANL. These data were used velocity in bedrock was best modeled as to estimate some of the seismic parameters model that includes: increasing from 5.71 km/s at the top of the 1. A high velocity layer that is constant of the shallow crust beneath the network. Precambrian section to just under 6.0 km/s This report gives an estimate for the P-wave beneath the study area. at approximately 10 km depth. This study 2. Dip on the refractor surface is small. velocity in the Precambrian basement (Pg) fills the gap between the Valles caldera to and the P-wave time terms (delay times) 3. Curvature on the refractor is small. the north and the contemporary magma 4. The velocity beneath the sites varies associated with each of the recording stations injection proposed to the south of the basin. and shot sites along with the standard errors only with depth. for the computed values. These numbers The method produces relative time terms are important for the accurate location of Method unless a recording station and an explosion earthquakes and for deducing structure at occupy the same site, in which case absolute The time-term method of analyzing seismic deeper levels in the crust. The delay times values are calculated. In this study a station refraction data was proposed by Scheideg- also are related simply to the thickness of at the Occidental Mineral Corporation mine ger and Willmore (1957) and has been used the Phanerozoic section above the basement, (OXM) recorded the first arrival from explo- successfully by many others. The method or, where the site is already located on base- sions at LANL shot points SP5 and SP6, and assumes that the travel time for the first ment, they are related mainly to the amount on the White Sands Missile Range (WSMR) a arriving refracted signal from an explosion of confining pressure in the rocks beneath station at the site of DICE THROW recorded can be partitioned into 3 terms: the explosion or recording station. the shot at DISTANT RUNNER 2. November 2013, Volume 35, Number 4 NEW MEXICO GEOLOGY 95 the permanent seismic stations along with TAOS their elevations are given in Table 2. A SAN RIO ARRIBA description of the instruments in the field JUAN (short period vertical sensors and radio SP4 telemetry) and at the recording site is given EUM by Jaksha et al. (1977). The seismic data were recorded on 16 mm film displayed at Valles 1 cm per second. Two analysts timed the Los Alamos caldera refracted P-wave arrivals independently, Nacimiento Mts. Nacimiento SP6 TSP and if the results varied by more than ± 100 ms the data were rejected. Shot to station Mc KINLEY SP5 distances were computed using DISTAZ, an unpublished computer program from COH SANTA CDN FE the U.S. Geological Survey, Menlo Park, OXM California. The data were then assembled SANDOVAL into matrix algebra format and processed San Pedro- using a TI-89 calculator manufactured by MTL Ortiz porphyry belt Texas Instruments Inc. VOL GNM Residuals for each ray path were calculat- Albuquerque SAN JM MIGUEL ed, and those with values greater than 200 TJQ ms, the estimate for our ability to read the BERNALILLO CIBOLA seismograms, were reviewed, rejected, or ABQ EST both. These data are shown in Figure 2 and CRF Pedernal presented in tabular form in the Appendix. MLM Basin e Hills Albuquerqued Distance constraints were used to look for n ra aberrations associated with arrivals from G TORRANCE o shots too close (direct arrivals) or too far (a VALENCIA Ri deeper refractor) from the stations. It was found that Pg appears as a first arrival in LAD the range 10–150 km in and around the Albuquerque Basin and that its velocity is LPM 6.0 ± .02 km/s. This result is in agreement with a refraction line partially in the basin Plateau Colorado analyzed by Olsen et al. (1979). WTX Estimates for the station and shot time Socorro terms are listed in Table 3. The standard Great Great Plains LINCOLN error for the estimate is given along with Rio Grande rift SOCORRO the number of observations associated with each site. The standard deviation for the entire solution is 103 ms. The estimate for DT our ability to read the arrival times given above is 200 ms, so the solution appears to MRC be well determined.