Seismic attenuation at Rabaul volcano, Papua New Guinea

The attenuation structure around Rabaul volcano, New Britain, Papua New Guinea, is studied using broadband records from a pair of sites, inside and outside of the Rabaul caldera complex, using regional earthquakes. Estimates of attenuation for P and S waves between 0.5 and 5 Hz indicate that near-surface rocks within the caldera complex are significantly more attenuative than outside the caldera. The average strength of this anomaly is defined in terms of t*S∼0.2 s over and above the region outside the caldera (t*S is the time integral of the inverse quality factor of attenuation, in this case along the path of a shear wave). This attenuation anomaly appears to be strongest at depth and to the south of the Kaivuna site in the northern part of the caldera complex. The ratio of attenuation estimates for P and S waves is δt*S/δt*P≈2.7, indicating that intrinsic attenuation contributes significantly to the process of attenuation. No frequency dependence of the quality factor is resolved within this frequency band. Path-averaged lithospheric attenuation is estimated from S/P spectral ratios at both stations, yielding quality factors on average Qβ∼180, but possibly significantly lower around and below the volcano. Shear waves are observed along paths passing underneath the central southern caldera at depths greater than about 5 km, although they are strongly attenuated. This indicates that they have not passed through a large, extensively molten body.