Variation of apparent velocity of teleseismicPwaves across the Large-Aperture Seismic Array, Montana

Apparent-velocity variations across the large-aperture seismic array (Lasa) have been studied by using P waves from six teleseisms at different distances and azimuths around Lasa. Data from 13 instruments per subarray were used in the analysis. Average apparent velocities between subarrays, taken in pairs, were measured by using arrival-time data. Apparent velocities at subarrays were estimated by a cross-spectral method involving the whole signal shape. For the 2 events in the southeast direction, there is a large zone of low apparent velocity to the northwest of A0 subarray. For the events in the other azimuths, regions of low and high apparent velocities are clearly contourable across Lasa. It appears that a 30% variation in apparent velocity exists across Lasa for events from all azimuths. The apparent-velocity measurements at subarrays, in general, follow the pattern shown by the measurements based on arrival-time data. However, there are some significant differences, especially at the subarrays in E and F rings. The apparent-velocity measurements indicate that structural complexities over and above those seen in the average crustal models proposed so far exist under Lasa.