Investigation for Anomalous Focal Mechanisms: Bárdarbunga Volcano, Iceland Events

Using the complete moment tensor inversion method as described in Dreger at al. (2000), we investigate the September 29, 1996 Mw=5.6 volcanic event beneath Bárdarbunga caldera in Iceland. Green's were calculated using a suite of crustal models determined by combining teleseismic receiver functions and surface wave dispersion curves (Du and Foulger, 2001) to account for lateral heterogeneity. Long-period (20 to 50 sec) three-component waveform data from 6 Iceland Hotspot Project stations were inverted yielding a solution that is characterized by a 67% vertically-oriented CLVD. This solution is similar to that previously reported in the Harvard centroid moment tensor (CMT) catalog, and later confirmed by analysis of long-period and intermediate surface wave data (e.g. Nettles and Ekstrom, 1998). The full moment tensor inversion yielded the same CLVD (66%) with an insignificant amount of volumetric contraction. Sensitivity tests investigating the stability of the CLVD and isotropic components as a function of the numbers of stations used indicate that the CLVD component is very robust. The isotropic component is seen to decrease to a nominal level as more stations are used in the inversion. In addition, we investigated the level of fit afforded by pure double-couple, pure isotropic, and combined double-couple plus isotropic models. None of these provided a better fit than the vertically oriented CLVD. It is unlikely that smaller events in the sequence would resemble the NDC nature of the main event, and that the same physical mechanism would be responsible for them (e.g. faulting on an outward dipping cone-shaped ring fault beneath the caldera, as proposed by Nettles and Ekstrom (1998)). Unfortunately, due to poor signal to noise ratios the same waveform method cannot be applied to the smaller events. For these events the linear-programming method developed by Julian and Foulger (1996) to invert simultaneously polarities and amplitude ratios was used. Our analysis shows that there exist at least several events that have NDC solutions that are similar to the main event. Although our results cannot rule out the hypothesis of faulting on an outward dipping cone-shaped ring fault beneath the caldera, similar FM solutions found for the smaller events suggest that a different type of physical mechanism is responsible for Bárdarbunga, September 29, 1996 earthquake sequence.