Tracking Multiple Tremor Sources Below Kilauea's Summit

Various tremor sources, shallow as well as deep, are commonly observed below the island of Hawaii. Eruption tremor is almost constantly and continually present since the onset of the Pu`u`O`o east rift zone eruption 20 years ago. Its source is found at shallow depth below the eruption crater. Indirectly related to the ongoing eruption, tremor is also recorded below the summit of Kilauea about 15 kilometers away from the eruption site. Time-frequency transforms (sgram) are automatically processed in real-time at the Hawaii Volcano Observatory via the Earthworm system for several seismic stations situated in Kilauea's summit area. This processing indicates the presence of marked spectral peaks, sometimes common to several stations. It also emphasizes changes in tremor spectral content and cyclic behaviors. During small eruption crises, changes in the signal relative amplitude at the different stations suggest the presence of several tremor sources. We locate the sources using seismic amplitude distributions corrected for the site effects. Amplitudes are calculated in several frequency bands and spatial amplitude distributions across the summit area are usually smooth and coherent. We search for the source of those distributions by approximating the decay of the amplitude as a function of the hypocentral distance. The application of our location method to tremor recorded between March and August 2001 indicates the presence of at least two shallow sources whose locations are persistent with time. In the case of a small crisis that occurred in April 2002, we distinguish two main sources of activity, with a first deeper one triggering the activation of a second shallower source. Those results suggest the possibility of using tremor for mapping fluid transport below Kilauea volcano.