ONGOING RESEARCH

1. Source characterization of long-period and very-long-period seismicity during the 2009 eruption of Redoubt Volcano Matthew M. Haney1, Bernard A. Chouet2, Phillip B. Dawson2, and John A. Power1 1Alaska Volcano Observatory, U.S. Geological Survey, Anchorage, AK and 2U.S. Geological Survey, Menlo Park, CA ABSTRACT The 2009 eruption of Redoubt produced several very-long-period (VLP) signals associated with explosions, with periods between 10 and 40 s. A network of 5 broadband seismometers situated between 3-5 km from the crater recorded the VLP signals. We invert for the source location and mechanism of an explosion at Redoubt volcano using waveform methods applied to the broadband recordings. Inversions are carried out assuming the volcanic source can be modeled as a point source, with mechanisms described by a) a set of 3 orthogonal forces, b) a moment tensor consisting of force couples, and c) both forces and moment tensor components. We find that the source of the VLP seismic waves during the explosion is well-described by either a combined moment/force source located northeast of the crater and at an elevation of 1.6 km ASL or a moment source at an elevation of 400 m southeast of the crater. Although the moment/force source provides a better fit to the data, we find that owing to the limited coverage of the broadband stations at Redoubt the moment-only source is the more robust and reliable solution. F E B A C D G H East-west cross-section of Redoubt vertically exaggerated by a factor of 2, with the locations of the 5 summit broadbands projected into the east-west vertical plane. Note that station RD02 (green square) is on the northern flank of the edifice and so appears to be inside of the volcano. The crater is shown as a black triangle and the VLP particle motions show a more or less radial pattern away from a point situated below and to the east of the crater. Also plotted are a solid and open circle showing the locations of the best fit sources using force and moment (solid) and moment-only (open). The lateral extents of the two volumes over which grid search was performed are shown by solid and dashed rectangles. Map of the 5 summit broadband sensors plotted along with exaggerated particle motions. Stations RDW and RD02 had inoperative north-south components, and are not represented in this plot. The crater is shown as a black triangle and the VLP particle motions show a more or less radial pattern away from the crater. Also plotted are a solid and open circle showing the epicentral location of the best fit sources using force and moment (solid) and moment-only (open). The lateral extents of the two volumes over which the grid search for best fit source location was performed are shown by solid and dashed squares. Slices from the error volume for the inversion for 6 moments and 3 forces (panels A-D). The minimum error solution is shown as a white circle. (Panel E) Moment tensor eigenvectors viewed from the south for the force and moment solution. (Panel F) The same eigenvectors as in Panel E, except viewed from the east. (Panel G) Moment tensor eigenvectors viewed from the south for the moment-only solution. (Panel H) The same eigenvectors as in Panel G, except viewed from the east. The color scheme in panels E-H is as follows: major eigenvector (blue), intermediate eigenvector (green), and minor eigenvector (red). DISCUSSION Several lines of evidence cast doubt on the force and moment solution, although it provides a better fit to the data. These include a) the depth of the solution coinciding with the average elevation of the stations, b) the lateral component of the force vector aligning with the average horizontal particle motions, and c) the vertical component of the force vector being out-of-phase with the horizontal components. Redoubt From the principal axes of the moment-only solution, we can estimate the volume and pressure changes at the VLP source. To do so, we find the projection of the measured principal axes [0.54:0.83:2.00] onto the principal axes of 3 orthogonal cracks (matrix on right). The left panel shows a map of the 4 broadband seismometers temporarily deployed at Redoubt Volcano from March through June 2009. One broadband (RDWB) was in operation on the Redoubt edifice within the permanent network, bringing the total number of broadband sensors to 5. Short period stations DFR and RDE, which are discussed below, are located on the northern side of the Drift River valley. Short period station REF is co-located with RD01. The right panel shows the instrument-corrected VLP waveforms from station RDWB for event #12 (see event listing below). Based on these considerations, we find that the VLP signal can be explained using only two cracks, a dominant dike (DV = 19500 m3 and DP = 7 MPa) and a subdominant sill (DV =5100 m3 and DP = 1.8 MPa) which act out-of-phase with each other. Waveform fits from the source mechanism inversion for 6 moments and 3 forces (left) and 6 moments only (right). For both panels, the three columns are, from the left to right, the vertical, north-south, and east-west components, respectively. Each row corresponds to one of the five summit broadbands, ordered from top to bottom as RD01, RD02, RD03, RDWB, and RDW. CONCLUSIONS 1. Very-long-period seismic waves were observed during the 2009 Redoubt eruption 2. Particle motions point to a source region to the east of the crater 3. Using waveform inversion, we find a source at an elevation of 0.4 km ASL 4. The limited number of stations causes there to be a spurious solution 5. Our study is important for interpretation when there are limited stations A B C D ONGOING RESEARCH Building on the results for the VLP signals, we plan to apply similar waveform methods to repeating long-period (LP) events associated with swarms during the Redoubt eruption. The individual LP events at Redoubt are characterized by a dominant period of approximately 0.5 s. Although the LP and VLP sources do not coincide in time or space, we seek a more complete view of the Redoubt magmatic system through a joint interpretation of the two types of eruptive seismicity. The source time functions from the source mechanism inversion for 6 moments and 3 forces (panels A and B) and 6 moments only (panel C). For panels A and C, from top to bottom, the 6 moments are sorted as Mxx, Myy, Mzz, Mxy, Myz, and Mzx. The force components in panel B are sorted from top to bottom as Fx, Fy, and Fz. Vertical axes are in SI units, either Nm for moment components or N for the forces. Panel D is a table of residual errors for the free inversions and the corresponding Akaike Information Criterion (AIC). Explosive events 1-9 (panels on the left) and 10-18 (panels on the right) for the 2009 Redoubt eruption. Shown for each event are 5 data streams consisting of (top-to-bottom): high-gain pressure sensor DFR, low pass filtered (< 0.05 Hz) broadband RDWBz, spectrogram of summit station REFz, unfiltered broadband RDWBz, and short-period station RDE. For each event, 23 minutes of data are plotted. Lahars are evident in the extended tails of RDE relative to the unfiltered RDWBz for events #5, #6, and #8. Compact VLP waveforms exist for events #2, #5, #8, #10-#14, and #16-#17. Note spectral gliding in events #11-#14. Acknowledgements: This work has benefited from the comments of 3 anonymous reviewers. We thank Helena Buurman and Cyrus Read from AVO for bravely deploying the temporary broadbands at Redoubt in the days before the explosive phase of the eruption began.