Apparent Q for upper crustal rocks of the Central Rio Grande Rift

Abstract

Spectra from 130 digitally recorded microearthquakes in the magnitude range −0.9 to 0.3 were used to compute apparent seismic quality factors (Q) for upper crustal rocks within 45 km of Socorro, New Mexico. In this study, apparent Q was defined as a measure of seismic wave attenuation due to intrinsic absorption and scattering and was computed over the frequency range 0–50 Hz (most of the recorded energy was in the 3–30 Hz band). It was found that apparent Q increases with event distance for the eight stations used in this study. This increase was successfully modeled with a varying thickness, low Q, low‐velocity layer lying above a relatively high Q high‐velocity half‐space. It was found that (1) near‐surface low Q regions lying directly beneath the recording sites vary from 0.3 to 2.0 km in thickness and exhibit Qp and Qs values less than 50. (2) The average Qs for the upper crust in the central Rio Grande rift, excluding the near‐surface low Q region, is 535 (95% confidence interval 374–938). (3) The observed Qp/Qs ratio for events recorded at eight stations varied from 0.34 to 1.39. For earthquakes recorded at three stations, the Qp/ Qs ratio decreased with increasing event distance. For two other stations, no systematic increase or decrease in Qp/Qs with distance was evident, and for the remaining three stations, events were too closely clustered to draw any conclusions. These changes in the Qp/Qs ratio with increasing event distance may reflect varying degrees of fluid saturation in upper crustal rocks. (4) At least three anomalously low Qs regions exist within 20 km southwest of Socorro. These abnormally low Q regions may reflect small magmatic intrusions or high frature densities at depth and may lie in close proximity to microearthquake swarms at depths of 7.2–10.3 km.