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Investigation of local very long period seismic signals registered at Mýrdalsjökull glacier, south Iceland
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
(English)Manuscript (Other academic)
Abstract [en]

Strong and repeating very long period (vlp) signals (10-30 sec) on and around the western Mýrdalsjökull ice cap, which covers the Katla volcano in South Iceland, are recorded simultaneously with long period (lp) events of magnitude >2. The signals are recorded on 6 broadband and 2 higher frequency three-component seismic stations. The vlp signals are linearly polarized and the horizontal polarization direction points to the same source as the lp events, which have recently been interpreted as being generated by ice falls over an escarpment in a steep outlet glacier. The vlp events are registered on several different instrument types, which implies that they must be due either to a generic problem with the instruments or that they are real signals. They do not appear to be consistent with P-, S- or surface wave behaviour.Similar signals elsewhere have been interpreted as the combined effect of static displacement and tilt caused by ground deformation due to magma movements within a volcano. The three-component data are therefore modelled as the combined effects of tilt and vertical translation caused by the ice movements. Simple deformation modelling shows that the observed vlp signals are inconsistent with the ice fall source in the relative magnitudes of the vertical and radial displacements and, in line with results from elsewhere, the magnitudes of the observed effects are large relative to the model response. Thus the origin of the vlp signals remains enigmatic.Our analysis suggests that it is unlikely that the signal is an instrumental artefact. Possible physical origins are discussed.

Keyword [en]
Mýrdalsjökull, Katla volcano, seismic tilt, vlp-signals, lp-signals, acceleration, static deformation
National Category
Geophysics Geophysics
Research subject
Geophysics Specialized In Seismology
Identifiers
URN: urn:nbn:se:uu:diva-101011OAI: oai:DiVA.org:uu-101011DiVA: diva2:211530
Available from: 2009-04-16 Created: 2009-04-16 Last updated: 2010-01-14
In thesis
1. Studies of Unusual Seismicity and Long Period Events at the Glacier Overlain Katla Volcano, Iceland
Open this publication in new window or tab >>Studies of Unusual Seismicity and Long Period Events at the Glacier Overlain Katla Volcano, Iceland
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Earthquake catalogues are usually dominated by diffusive behaviour consistent with the Omori law of aftershocks. This is investigated in terms of waiting times, i.e. the time between successive events in a time-sorted earthquake catalogue. The theoretical waiting time probability distribution for the Omori law is derived and shown to predict the numerically produced Omori aftershock sequence well. These results enhance our understanding of aftershock processes and demonstrate that previous waiting time interpretations were severely flawed.

Iceland earthquake catalogues are studied in terms of waiting times. Omori aftershock sequences are shown to predict most datasets well but there are some significant exceptions. One of these is data from the glacier covered Katla volcano in South Iceland, with few aftershocks. This dataset can be further split into two geographical groups: Several hundred volcano-tectonic earthquakes occurring within the caldera, reaching depths down to 15 km, and thousands of emergent low frequency earthquakes with a poorly defined shallow source in Goðabunga, in the western part of Katla. These events are investigated further.

The lp events at Goðabunga have been recorded for decades and show a clear seasonal and climate-related correlation where their number increases in the autumn as well as during warmer years. Many of them form groups with very with similar waveforms. New broad-band seismic data suggests that the lp events originate in a steep outlet glacier covering Katla. Here, ice movement leads to ice falls over the steep escarpment, and we now believe that the lp events are generated by large ice falls rather than being related to gas or magma movements within the volcano, and are not precursors to an eruption as previously suspected. This observation probably has major significance for hazard estimation at the many ice-covered volcanoes around the world.

We report near-field (vlp) signals simultaneous with the largest lp events. Our data is partly consistent in character with surface deformation (displacement and tilt) due to the ice movements. However, in line with results from elsewhere, the magnitudes of the observed effects are large relative to those from mathematical modelling. Our analysis suggests that the signal is not an instrumental artefact. Possible explanations are discussed.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2009. 85 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 636
National Category
Geophysics
Research subject
Geophysics Specialized In Seismology
Identifiers
urn:nbn:se:uu:diva-100771 (URN)978-91-554-7500-0 (ISBN)
Public defence
2009-05-08, Hambergssalen, Geocentrum, Villavägen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2009-04-16 Created: 2009-04-07 Last updated: 2009-04-27

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