Calibration Studies at TXAR

Abstract

Calibration studies at TXAR (Lajitas, Texas) used a modified version of the correlation method described by Cansi et al. (1993) in order to estimate azimuth and horizontal phase velocity of 144 events for which USGS m b values were available. Modifications to the correlation method include the Fourier interpolation of the data by a factor of 8 to obtain a virtual sample rate of 320/sec, use of an L1 norm (least absolute deviation) to obtain estimates of the azimuth and phase velocity, and a moving window display to indicate those portions of the waveform that show strongest correlation across the arra)~ Corrected phase velocities normally associated with Pn (less than 8.6 km/s) are generally seen for events at epicentral distance as far as 2,000 km. Upper mantle refracted first arrivals (P) with corrected phase velocities greater than 8.6 km/s are generally observed for epicentral distances beyond about 1,600 km. Phase identification is essential in order to select a suitable magnitude scale.Based on the 144 well-located events by USGS and using the Denny et al. (1987) formula, the most reliable magnitude estimates are as follows:1. For horizontal phase velocity less than 8.6 km/sec: mb(D) = logA + 2.4 (log D) -3.95 + C, with C= +0.3 2. For horizontal phase velocity greater than 8.6 km/sec: mb(D ) = logA + 2.4 (log D) -3.95 + C, with C= -0.50The M-discontinuity beneath TXAR was determined to the first order to strike along an azimuth of 109 degrees (NW-SE) and dip 11 degrees to the northeast. This result is consistent with the tectonic setting for the area.

Installation and Preliminary Results From a Small Aperture Seismic Array in Tripoli, Greece

“…Comparison of TRISAR location results to those of the National Observatory of Athens presents some differences that can be decreased by calibrating the array (Myers et al 2002, Tibuleac & Herrin 1997 as well as calculating travel-time, azimuth and slowness corrections for areas where abundant data are available (Ram & Yadav 1984, Schweitzer 2001b.…”

Advances in Mixed Signal Processing for Regional and Teleseismic Arrays

“…Tribuleac and Herrin (1997) use the sum of the absolute errors, robustified by dropping out some the largest ones.…”

“…The delays are functionally dependent on velocity and azimuth if the signals are propagating plane waves, and this is the assumption that is usually made. Methods that are commonly in use for analyzing such data, when a single signal is assumed to be present, can be roughly categorized as (a) beam-forming and plotting the power as a function of slowness, which can be converted to estimators of velocity and azimuth, (b) Capon's estimator (see Capon, 1969, Capon andGoodman, 1970), (c) beam-forming converted to an F-statistic by dividing by an estimator of the noise power (see Melton and Bailey, 1957, Blandford, 1970, Shumway, 1970, 1971, 1983, 2000, (d) Multiple Signal Characteristic (MUSIC) (Schmidt, 1979, Stoica andNehorai, 1989), and (e) cross correlation (Tribuleac and Herrin, 1997). Several algorithms such as the sequential F-detector proposed here and the multiple signal characteristic (MUSIC) algorithm are available that offer promise for handling array data with low signal-to-noise ratios and contamination from interfering signals.…”

Calibrating the Chiang Mai Seismic Array (CMAR) for Improved Event Location

“…Unlike the Bondar et al (1999) corrections, in our study an analyst estimates residuals for each event at epicentral distances greater than 2°. Such corrections are currently applied at the seismic arrays TXAR (Lajitas, TX), NVAR (Mina, NV), and ILAR (Eilson, AK) using a timedomain calibration methodology (Tibuleac and Herrin 1997, 2001a, 2001bLindquist et al 2005). Using this methodology we calibrate the Chiang Mai 19-element seismic array (18.42 N, 98.95 E) to reduce bias in bearing estimation and event location at regional and teleseismic distances.…”

“…In this study, we compare estimated horizontal velocity and back azimuth using two methods: 1) a time-domain cross-correlation method (Tibuleac and Herrin 1997) named ARPAC (Automatic Routine Package for Array Calibration version 1.0) and adapted for semi-automatic processing by Lindquist et al (2005); and 2) an enhanced frequency-wavenumber ( f-k) method called MUSIC (MUltiple Signal Classification, Schmidt 1979). To our knowledge, this type of comparison has not been attempted before.…”