Characterization of long period tremor bursts during the 2008-present summit eruption of Kilauea volcano, Hawaii (Invited)

We characterize the timing, amplitude, and source location of long period (peak frequency 0.5Hz) tremor bursts during the 2008-present summit eruption of Kilauea volcano, Hawaii. Seismic tremor levels at Kilauea’s summit, which began rising steadily in early November 2007, reached nearly five times the background level by early 2008. Matching the trend of increasing tremor, Kilauea summit SO2 emission rates rose above normal rates of less than 400 tonnes per day, peaking at the highest recorded (1950 tonnes per day) since regular measurements began in 1979. The steadily increasing volcanic activity culminated in a small explosion on March 19, 2008-the first explosion at the summit since 1924. Tremor levels at Kilauea’s summit subsequently reached a high in early April of approximately 7 times background levels. In mid-April, 2008, summit tremor began to exhibit episodic fluctuations in amplitude, and from April to early May was showing regularly timed bursts, with an average spacing of 2.5 minutes between tremor burst peaks. In mid-June, tremor burst amplitudes began increasing, and the spacing between tremor bursts also changed from 2-3 minutes to 6-7 minutes between bursts. At the beginning of July tremor burst amplitudes began decreasing before stopping altogether by mid-July. After July, 2008, episodic tremor occurrence was irregular, sometimes taking only a few hours to go from relatively steady tremor to strongly fluctuating amplitudes. This trend continued for the rest of 2008, with many intervals of episodic tremor initiating during the inflation phase of transient summit tilt events. Since December, 2008, episodic tremor has rarely occurred. Summit area seismic data was processed to characterize the timing, amplitudes, and locations of these tremor bursts. For tremor bursts with amplitudes greater than 2 times the local background, source locations were determined using a grid search methodology. A total of 7870 tremor bursts were located using this technique, and the results show a tight clustering of locations centered on the new Halemaumau vent. The depth distribution of computed source locations for April through July, 2008 indicated that nearly all of the tremor bursts occurred at 0-200 m depth. August tremor locations show a similar shallow (0-200 m depth) concentration, but with additional secondary groupings of sources at 300m and 420m depth. This trend of deepening source locations continued in September and October, 2008, with primary concentrations of events at 300m and 420m depth, and secondary groupings at 0-200m depth. These tremor burst locations are consistent with a tremor source region that is at the top of the magma column. This source location is also support by infrasound data and video observations which indicate that cyclic degassing observed at the lava surface occur in concert with episodic tremor bursts. The change in source depth may reflect the observed changes in vent geometry (deepening and widening) by collapses and explosions at the new Halemaumau vent.