Calibration Studies at TXAR

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 array. 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: For horizontal phase velocity less than 8.6 km/sec: m b ( D ) = log A + 2.4 (log D ) - 3.95 + C , with C = +0.3 For horizontal phase velocity greater than 8.6 km/sec: m b ( D ) = log A + 2.4 (log D ) - 3.95 + C , with C = −0.50 The 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.