Seismic Attenuation from within Mauna Loa and Kīlauea: A Comparative Analysis

Summary This study presents the first comparative measurements of seismic attenuation between Mauna Loa and Kīlauea volcanos on Hawai’i Island. The focus is on key physical variables found within Kīlauea, and extending our knowledge of these from Kīlauea to Mauna Loa. The measurements of attenuation, elastic/anelastic moduli (µ rigidity and K bulk), T temperature, P pressure, basalt activation energy, are uniformly applied to these adjacent volcanos (34 km separation) for comparative analyses. While numerous seismic attenuation studies have been conducted at Kīlauea, Mauna Loa has remained unexamined in this context until now. We extend previous methodologies to measure both shear (Qµ) and bulk (QK) attenuation over propagation paths from both volcanic calderas to the Aloha Cabled Observatory (ACO), located 442-464 km away at 4728 m depth. Utilizing earthquake displacement source spectra from shallow (near sea level) events beneath both calderas, we derive frequency-dependent effective Q values across the 2-35 Hz frequency band. Our analytical approach employs the t* formulation (ratio of travel time to Q) to separate attenuation along path segments, allowing direct comparison between the two volcanic systems. Results reveal that Mauna Loa exhibits substantially higher attenuation (lower Q values) than Kīlauea for both bulk and shear waves. At 10 Hz, Qµ is approximately four times higher for Kīlauea (∼400) than Mauna Loa (∼115), while QK displays even greater contrast with Kīlauea (∼425) exceeding Mauna Loa (∼25) by a factor of 17. Both volcanoes demonstrate QK < Qµ across most frequencies, emphasizing the significance of bulk losses in volcanic environments. This contradicts traditional assumptions held, that bulk attenuation is negligible in Earth. The pronounced difference in attenuation between these adjacent volcanoes, which share the same hot spot origin, cannot be explained solely by temperature-pressure dependent activation energy models. While we calculated expected Q variations using established basalt activation energies (59-68 kJ/mole), the observed differences exceed predictions by an order of magnitude. This suggests additional mechanisms are at work, likely involving partial melting processes. Our findings indicate that the internal structure of Mauna Loa may contain a greater proportion of partial melt or different melt geometry than Kīlauea, significantly affecting seismic wave propagation. At higher frequencies (17-33 Hz), both volcanoes show evidence of comparable scattering effects. This research provides new insights into the internal composition and dynamics of Hawaiian volcanoes, demonstrating that despite their proximity and shared magmatic source, Mauna Loa and Kīlauea possess distinctly different attenuation characteristics that reflect fundamental differences in their internal structure and melt distribution. These findings enhance our understanding of volcanic processes and contribute to improved interpretation of seismic data in volcanic environments.