Seismic Velocity and Attenuation Structure of the East Rift Zone and South Flank of Kilauea Volcano, Hawaii

Abstract

It has been proposed that a deep magma body beneath the east rift zone (ERZ) of Kilauea volcano, Hawaii, must be present in order to explain the observed deformation of the south flank. From November 1999 to June 2000, 29 IRIS-PASSCAL three-component seismographs were operated across Kilauea's ERZ and south flank. Using local earthquakes recorded by these stations, supplemented with data from the Hawaiian Volcano Observatory stations, we investigated the deep structure of the ERZ and south flank through two approaches: seismic velocity tomography and seismic attenuation tomography. Results along two north-northwest-south-southeast profiles are examined. The western profile shows a low-velocity region beneath and south of the Hilina Pali, and earthquakes at depth are sandwiched between a high V p /V s , high Q zone and a low V p /V s , low Q zone. The velocity and attenuation differences are interpreted to reflect the contrast in material properties between the overlying volcanic pile and the underlying fluid-rich ocean sediments and fractured, altered ocean crust beneath the south flank thrust fault. The eastern profile shows an anomalous feature with low V p , low V p /V s , high Q p , and low Q s at about 7 km depth beneath the ERZ. The low V p /V s ratio is inconsistent with the presence of partial melt. Instead, this anomaly is attributed to a trapped reservoir of CO 2. Our results do not support the existence of a sizable molten or partially molten body at depth beneath the ERZ. If the deep magma body exists, it must be relatively small. Based on synthetic resolution tests, an estimated limit on the maximum allowable crosssectional size of the deep magma body is about 2 by 4 km.

Three‐dimensional seismic velocity structure of Mauna Loa and Kilauea volcanoes in Hawaii from local seismic tomography

“…One of the main differences between this study and previous ones is the improved V p model resolution at deeper layers owing to the inclusion of events with large epicentral distances and application of the composite event method. Our V p model is well resolved to 20 km depth near the volcano calderas and their rift zones ( Figure 6) and generally agrees with the previous models [Ryall and Bennett, 1968;Hill, 1969;Ellsworth and Koyanagi, 1977;Zucca and Hill, 1980;Thurber, 1984;Rowan and Clayton, 1993;Okubo et al, 1997;Dawson et al, 1999;Haslinger et al, 2001;Hansen et al, 2004;Monteiller et al, 2005;Park et al, 2007;Got et al, 2008;Park et al, 2009;Syracuse et al, 2010]. The common features observed in these studies and also in our model are the high-velocity anomalies in the upper 9 km depth beneath Kilauea caldera and its rift zones, indicative of magma cumulates.…”

“…Poland et al [2013] suggested that the increased supply at Pu'u 'O'o-Kupaianaha during 2003-2007 must have been driven by increased flux of magma from the mantle, which is supported by increased CO 2 emissions. The low-V p and low-V p ∕V s anomalies observed in our model are consistent with the existence of CO 2 and were also observed by Hansen et al [2004]. The resolved area in the lower ERZ has one high-and one low-V p ∕V s body next to each other.…”

“…In the southeast flank of Kilauea, low-V p (<6.5 km/s) and V p ∕V s (<1.65) anomalies are prominent from 0 to 15 km depth. Hansen et al [2004] attributed an anomalous body of low V p and low V p ∕V s at 7 km depth in the ERZ to a trapped CO 2 reservoir. The difference in our study is the large depth range that this anomaly spans.…”

“…Refraction studies initially helped to resolve crustal structure [e.g., Ryall and Bennett, 1968;Hill, 1969]. The first three-dimensional (3-D) seismic velocity model for Kilauea was obtained by inverting teleseismic data [Ellsworth and Koyanagi, 1977] and was followed by numerous local tomographic studies [e.g., Thurber, 1984;Rowan and Clayton, 1993;Okubo et al, 1997;Dawson et al, 1999;Haslinger et al, 2001;Hansen et al, 2004;Monteiller et al, 2005;Park et al, 2007;Got et al, 2008;Park et al, 2009;Syracuse et al, 2010]. The majority of these studies focus on the compressional (P) wave velocity structure near Kilauea volcano.…”

“…Due to the smaller number of S picks in the seismic catalogs, only a few studies of V p ∕V s models have been conducted along with V p models. There are two V p ∕V s models available so far, one for Kilauea caldera [Dawson et al, 1999] and another for its east rift zone [Hansen et al, 2004].…”