Evaluating changes of the Bárdarbunga caldera using repeating earthquakes
Abstract
The natural hazard monitoring in Iceland relies heavily on seismic monitoring. With an automated system for detecting earthquakes, locating and evaluating their focal mechanisms, 500 earthquakes are recorded weekly with magnitudes down to -0.5. During the Bárdarbunga volcanic unrest in 2014-2015 the seismicity intensified and up to thousands of earthquakes were recorded daily. The unrest was accompanied by caldera collapse, a rare event that has not been monitored in such detail before, providing a unique opportunity for better understanding the volcanic structure and processes. The 8x11 km caldera gradually subsided, triggering thousands of events with 80 earthquakes between M5-M5.8. A subsidence bowl up to 65 m deep was formed, while about 1.8 km3 of magma drained laterally along a subterranean path, forming flood basalt 47 km northeast of the volcano. The caldera collapse and magma outflow gradually declined until the eruption ended some 6 months later (27 February 2015). The seismicity continued to decline, both in the far end of the dyke as well as within the caldera for a few months. However, half a year later (in September 2015) seismicity within the caldera started to increase again and has been rather constant since, with tens of earthquakes recorded on the caldera rim every week and biggest events reaching magnitude 4.4. Here we present a seismic waveform correlation analysis where we look for similar repeating waveforms of the large caldera dataset. The analysis reveals a dramatic change occurring between February and May 2015. By allowing for anticorrelation we find that the earthquake's polarity reverses sign completely. The timing coincides with the ending of the caldera collapse and the eruption. Our results suggest that caldera fault movements were reversed soon after the eruption ended in spring 2015 when we also observe outwards movement of GPS stations around the caldera, indicating re-inflation of the magma chamber half a year before any seismicity increase was detected. These data and their interpretation are helpful to improve our understanding of the current status of the volcano and, eventually, to perform a more accurate and reliable hazard assessment.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.S22C..07J
- Keywords:
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- 7219 Seismic monitoring and test-ban treaty verification;
- SEISMOLOGY;
- 7230 Seismicity and tectonics;
- SEISMOLOGY;
- 7290 Computational seismology;
- SEISMOLOGY;
- 7294 Seismic instruments and networks;
- SEISMOLOGY