EVIDENCE OF BULK ATTENUATION IN THE ASTHENOSPHERE FROM RECORDINGS OF THE BOLIVIA EARTHQUAKE

Radial modes excited by the 9 June 1994 Bolivia earthquake are analyzed to improve constraints on bulk attenuation in the Earth's interior. Using data recorded at 53 stations, quality factors for six radial modes are determined. The new observations are combined with previously published surface wave and normal mode attenuation measurements, and inverted for radial variations in shear and bulk attenuation, Q(m)u(-1)(r) and Q(k)appa(-1)(r>). The consequences of restricting finite Q(k)appa to specific regions of the Earth's interior are investigated using a layered model parameterization. The optimal model is characterized by: 1) no bulk attenuation in the core, 2) a ratio of bulk to shear dissipation in the asthenosphere (similar to 30%) consistent with the presence of partial melt, and 3) a ratio in the remainder of the mantle (similar to 1%) consistent with solid-state mechanisms of attenuation.