Kinematic Modeling and Complete Moment Tensor Analysis of the Anomalous, Vertical CLVD Bardarbunga, Iceland, Event

Kinematic Modeling and Complete Moment Tensor Analysis of the Anomalous, Vertical CLVD Bardarbunga, Iceland, Event Full-waveform moment tensor inversion for a half-cone rupture scenario. The observed waveforms are shown by solid lines while the synthetic data are shown by dashed lines. The resulting mechanism is dominated by a vertically oriented strong compensated linear vector dipole. The caldera in finite source modeling is assumed to be a part of a conical surface, dipping at 45 degrees outward, and the rupture is assumed to take place along a segment between 3 and 5 km of depth. A finite source is described with a number of point sources and the rupture is simulated by initiating increment displacements at multiuple locations on the caldera walls.

Using a complete moment tensor inversion method and the seismic data from the HOTSPOT PASSCAL experiment acquired from the IRIS data center, we investigated the September 29, 1996, volcanic event of Mw = 5.6 originated beneath the Badarbunga caldera in Iceland. The corresponding moment tensor is characterized by a significant non-double-couple component (NDC) previously reported in the Harvard centroid moment tensor catalog (CMT) and confirmed by analysis of long-period and intermediate surface wave data. The deviatoric inversion performed by using Iceland HOTSPOT Project stations, yields a NDC solution with a 67% vertically oriented compensated linear vector dipole (CLVD) component, while the full moment tensor solution shows a similar, 66% CLVD component, 32% of double-couple component (DC) and a small volumetric contraction (ISO) of 2%. Statistical tests confirm that CLVD is a stable component of the moment tensor, while ISO is statistically insignificant. Using an elastic finite difference code with a large number of equidistantly distributed point sources, we simulated various rupture scenarios on the walls of a conical surface of the Bardarbunga caldera in order to compare them with the observations. Suites of seismograms for each independent run were produced at locations corresponding to HOTSPOT stations. We then inverted these synthetic data to investigate what portion of the original source information can be recovered by the moment tensor inversion (Figure 1). We were able to identify physical characteristics of a rupture scenario that produces synthetics resembling the observed data to a quite high level of detail. For example, we obtained the best results for the ruptures extending along one-half perimeter of the caldera, while one-quarter or full-length perimeter ruptures were unlikely scenarios. We found that the rupture velocity, which took place at Bardarbunga, could have been a super-shear one, and we hypothesize that it could have been triggered by a compressional wave field that spread throughout the volume of the caldera.


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