New Mexico Geological Society Annual Spring Meeting — Abstracts

Spectra from over 400 digitally rec orded microearthquakes in the magnitude range -0.8 to 1.2 were used to compute apparent seismic quality factors (Q) for upper crustal rocks (0-19 km) within 45 km of Socorro. In this study apparent Q is a measure of seismic wave attenuation due to intrinsic absorption and scattering. It was found that:

(1) Relatively low Qp and Qs regions exist at depths of 7-10 km beneath Socorro Peak and the southwestern part of the La Jencia basin. These relatively low Q regions may result from small magmatic intrus ions or a high density of fluid-filled fractures at depth.

(2) Near-surface low Q regions have a profound effect on apparent Qp or Qs measured at the surface. Near-surface low Q regions vary from 0.3-2 km in thickness and exhibit Q values less than 50, and often less than 10.

(3) Qp/Qs varies from less than 0.5 to about 1.0. The observed Qp/Qs depends on the event distance.

(4) The average Qs for the upper crust in the central Rio Grande rift, excludin g the near-sur face low Q layer, is greater than 500.