A 23 hour tremor burst was recorded on July 8-9 th 2011 at the Katla subglacial volcano, one of the most active and hazardous volcanoes in Iceland. This was associated with deepening of cauldrons on the ice cap and a glacial flood that caused damage to infrastructure.Increased earthquake activity within the caldera started a few days before and lasted for months afterwards and new seismic activity started on the south flank. No visible eruption broke the ice and the question arose as to whether this episode relates to a minor subglacial eruption with the tremor being generated by volcanic processes, or by the flood. The tremor signal consisted of bursts with varying amplitude and duration. We have identified and described three different tremor phases, based on amplitude and frequency features. A tremor phase associated with the flood was recorded only at stations closest to the river that flooded, correlating in time with rising water level observed at gauging stations. Using back-projection of double cross-correlations, two other phases have been located near the active ice cauldrons and are interpreted to be caused by volcanic or hydrothermal processes.The greatly increased seismicity and evidence of rapid melting of the glacier may be explained by a minor sub-glacial eruption. It is also plausible that the tremor was generated by hydrothermal boiling and/or explosions with no magma involved. This may 2 have been induced by pressure drop triggered by the release of water when the glacial flood started. All interpretations require an increase of heat released by the volcano.
A double-correlation method is introduced to locate tremor sources based on stacks of complex, doubly-correlated tremor records of multiple triplets of seismographs back projected to hypothetical source locations in a geographic grid. Peaks in the resulting stack of moduli are inferred source locations. The stack of the moduli is a robust measure of energy radiated from a point source or point sources even when the velocity information is imprecise. Application to real data shows how double correlation focuses the source mapping compared to the common single correlation approach. Synthetic tests demonstrate the robustness of the method and its resolution limitations which are controlled by the station geometry, the finite frequency of the signal, the quality of the used velocity information and noise level.Both random noise and signal or noise correlated at time shifts that are inconsistent with the assumed velocity structure can be effectively suppressed. Assuming a surface-wave velocity, we can constrain the source location even if the surfacewave component does not dominate. The method can also in principle be used with body waves in three dimensions, although this requires more data and seismographs placed near the source for depth resolution.
We introduce a back‐projection method to locate tremor sources using products of cross‐correlation envelopes of time series between seismic stations. For a given subset of n stations, we calculate the (n − 1)th‐order product of cross‐correlation envelopes and we stack the back‐projected products over combinations of station subsets. We show that compared to existing correlation methods and for realistic signal and noise characteristics, this way of combining information can significantly reduce the effects of correlated (spurious or irrelevant signals) and uncorrelated noise. Each back‐projected product constitutes an individual localized estimate of the source locations, as opposed to a hyperbola for the existing correlation techniques, assuming a uniform velocity in two dimensions. We demonstrate the method with synthetic examples and a real‐data example from tremor at Katla Volcano, Iceland, in July 2011. Despite very complex near‐surface structure, including strong topography and thick ice cover, the method appears to produce robust estimates of tremor location.
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