Significant Bulk Attenuation in the Tonga-Lau Mantle Wedge

Date: 4/25/2019

Time: 01:45 PM

Room: Cascade II

Seismic attenuation measures energy loss during seismic wave propagation and quantifies relaxation of rocks’ elastic moduli. Abundant seismic studies have observed shear attenuation, but few have quantified bulk attenuation (bulk modulus relaxation), and these studies controversially locate finite bulk attenuation in different parts of the Earth, from asthenosphere to inner core. Here we present the first conclusive evidence localizing bulk attenuation to a specific region of the uppermost mantle. By analyzing amplitude spectral decay of P and S waves from Tonga earthquakes recorded at local seismic stations, we observe unusually low Q_P/Q_S ratios (< 1.5) in the Tonga-Lau mantle wedge. Analysis also shows that the data are best fit by the assumption of a weak frequency dependence with 𝛼 ≈ 0.3, suggesting that the observed attenuation anomalies are mostly intrinsic (anelastic) rather than elastic scattering. All seismic rays with significant path-average bulk attenuation (Q_κ^–1 > 0.01 or Q_κ < 100) are confined immediately beneath the Lau back-arc spreading centers west of the Tonga Trench. Tomography results show that the highest bulk attenuation (Q_κ^–1 ≈ 0.03 or Q_κ ≈ 33) is about 70% of the in-situ shear attenuation. The observed high bulk attenuation anomalies coincide with an inferred zone of partial melting, invoking mechanisms involving mantle melts.

Presenting Author: Songqiao Wei

Authors