Seismic bulk and shear attenuation along a transect from Kama‘ehuakanaloa volcano through Mauna Loa to the Aloha Cabled Observatory: Implications for the distribution of partial melt

Abstract

Bulk (QK) and shear (QS) attenuation are measured and modeled to ~ 50 km depth beneath Hawai‘i. High-frequency (> 50 Hz) earthquakes are routinely observed from the Aloha Cabled Observatory (ACO) along the azimuth to Mauna Loa, Pāhala, and Kama‘ehuakanaloa volcano. Bulk attenuation is consistently larger than shear attenuation beneath Hawai‘i at frequencies > 2 Hz. The Mauna Loa Summit shows the smallest Q values, and transects approaching the Summit from the southeast differ asymmetrically with those departing to the northwest from the Summit. Transect maps of Q are created from the measurements to present in plan view the distribution of QK and QS near the moho. Activation energy E* models of QS are tested both at Pāhala and Kama‘ehuakanaloa for experimentally determined olivine E* using the temperature derived from a Hawai‘i Hotspot geotherm and pressure. The QK arising from water-filled pores in vesicular basalts within the shallow oceanic crust are a hypothesized mechanism for bulk attenuation measured in the shallow crust near ACO and Wake Island. Below the shallow oceanic crust, partial melt presents a feasible bulk attenuation mechanism at volcanos. Fitting a thermodynamic equilibrium model for frequencies > 1 Hz to the QK measurements shows a very good match to the QK data, predicting partial melt fractions of 0.1–10%. Translating the Q maps into partial melt regions near Mauna Loa, Pāhala, and Kama‘ehuakanaloa volcano gives a first view of the observation, location, and distribution of partial melt along the ~ 100 km transect from southeast to northwest of Mauna Loa. Index terms [3050, 5144, 7280, 3619, 3909]