WAVE-FORM INVERSION OF VERY LONG-PERIOD IMPULSIVE SIGNALS ASSOCIATED WITH MAGMATIC INJECTION BENEATH KILAUEA VOLCANO, HAWAII

We use data from broadband seismometers deployed around the summit of Kilauea Volcano to quantify the mechanism associated with a transient in the flow of magma feeding the east rift eruption of the volcano. Th e transient is marked by rapid inflation of the Kilauea summit peaking at 22 mu rad 4.5 hours after the event onset, followed by slow deflat ion over a period of 3 days. Superimposed on the summit inflation is a series of sawtooth displacement pulses, each characterized by a sudde n drop in amplitude lasting 5-10 s followed by an exponential recovery lasting 1-3 min. The sawtooth waveforms display almost identical shap es, suggesting a process involving the repeated activation of a fixed source. The particle motion associated with each sawtooth is almost li near, and its major swing shows compressional motion at all stations. Analyses of semblance and particle motion are consistent with a point source located 1 km beneath the northeast edge of the Halemaumau pit c rater. To estimate the source mechanism, we apply a moment tensor inve rsion to the waveform data, assuming a point source embedded in a homo geneous half-space with compressional and shear wave velocities repres entative of the average medium properties at shallow depth under Kilau ea. Synthetic waveforms are constructed by a superposition of impulse responses for six moment tensor components and three single force comp onents. The origin times of individual impulses are distributed along the time axis at appropriately small, equal intervals, and their ampli tudes are determined by least squares. In this inversion, the source t ime functions of the six tensor and three force components are determi ned simultaneously. We confirm the accuracy of the inversion method th rough a series of numerical tests. The results from the inversion show that the waveform data are well explained by a pulsating transport me chanism operating on a subhorizontal crack linking the summit reservoi r to the east rift of Kilauea. The crack acts like a buffer in which a batch of fluid (magma and/or gas) accumulates over a period of 1-3 mi n before being rapidly injected into a larger reservoir (possibly the east rift) over a timescale of 5-10 s. The seismic moment and volume c hange associated with a typical batch of fluid are approximately 10(14 ) N m and 3000 m(3), respectively. Our results also point to the exist ence of a single force component with amplitude of 10(9) N, which may be explained as the drag force generated by the flow of viscous magma through a narrow constriction in the flow path. The total volume of ma gma associated with the 4.5-hour-long activation of the pulsating sour ce is roughly 500,000 m(3) in good agreement with the integrated volum e flow rate of magma estimated near the eruptive site.