Physical mechanisms for vertical‐CLVD earthquakes at active volcanoes

Abstract

[1] Many volcanic earthquakes large enough to be detected globally have anomalous focal mechanisms and frequency content. In a previous study, we examined the relationship between active volcanism and the occurrence of a specific type of shallow, non-double-couple earthquake. We identified 101 earthquakes with vertical compensated-linear-vector-dipole (vertical-CLVD) focal mechanisms that took place near active volcanoes between 1976 and 2009. The majority of these earthquakes, which have magnitudes 4.3 ≤ M W ≤ 5.8, are associated with documented episodes of volcanic unrest. Here we further characterize vertical-CLVD earthquakes and explore possible physical mechanisms. Through teleseismic body-wave analysis and examination of the frequency content of vertical-CLVD earthquakes, we demonstrate that these events have longer source durations than tectonic earthquakes of similar magnitude. We examine the covariance matrix for one of the best-recorded earthquakes and confirm that the isotropic and pure vertical-CLVD components of the moment tensor cannot be independently resolved using our long-period seismic data set. Allowing for this trade-off, we evaluate several physical mechanisms that may produce earthquakes with deviatoric vertical-CLVD moment tensors. We find that physical mechanisms related to fluid flow and volumetric changes are incompatible with seismological, geological, and geodetic observations of vertical-CLVD earthquakes. However, ring-faulting mechanisms explain many characteristics of vertical-CLVD earthquakes, including their seismic radiation patterns, source durations, association with volcanoes in specific geodynamic environments, and the timing of the earthquakes relative to volcanic activity.

Moment Tensor Inversion for Nuclear Explosions: What Can We Learn from the 6 January and 9 September 2016 Nuclear Tests, North Korea?

“…However, this decreases the resolution of certain MT components. Because of the poor resolution of the isotropic component (Kawakatsu, 1996;Dufumier and Rivera, 1997;Shuler et al, 2013), often only deviatoric MTs are considered. Such an approach is not adequate for explosion source.…”

“…Such an approach is not adequate for explosion source. Further ambiguities among MT configurations were discussed by Shuler et al (2013), concerning isotropic and horizontal tensile crack solutions of different signs, but also concerning pure deviatoric MTs, involving compensated linear vector dipoles (CLVDs) and double couples (DCs) with different orientations (Bukchin et al, 2010). Beside the uncertainties of MT components arising for certain types of observations (Kawakatsu, 1996;Dufurmier and Rivera, 1997) or in case of poor monitoring conditions (e.g., Domingues et al, 2013), further ambiguities in the MT interpretation may arise from the adopted MT decomposition.…”

Volcano Seismology: Detecting Unrest in Wiggly Lines

“…Synthetic modelling and moment tensor inversions of low frequency seismic wavefields are powerful tools for gaining information on the source mechanisms underlying volcanic earthquakes. Once instrument response and path effects have been accounted for, real data can be compared to synthetic models, and on the basis of a best-fit approach the obtained model parameters allow insights into the nature and geometry of the source (Chouet 1996;Shuler et al 2013). However, as the fundamental assumptions behind the commonly used moment tensor inversions are based on plane surface geometries which are believed to be too simple to explain the generation of low frequency events in a volcanic environment, the application to more complex seismic sources has so far been inconclusive.…”

Focal mechanisms and size distribution of earthquakes beneath the Krafla central volcano, NE Iceland

“…A source dominated by near-vertical single force or a vertical-CLVD mechanism might produce only dilatational first motions as well at the stations, but this presumes that there is a small region at the surface where we could have recorded compressional first motions. Physical sources for such mechanisms may include fluid movement or cone-shaped fault structures [Shuler et al, 2013]. Although we cannot rule out such an alternative explanation, we stick with the simple implosive source explanation.…”