HR: 11:35h
AN: S52B-06    [Abstracts]
TI: Investigations of Icequake Source Physics - Assessing the Link Between Englacial Fracturing and Glacial Floods.
AU: * Walter, F
EM: walter@vaw.baug.ethz.ch
AF: VAW-Glaciology, Gloriastrasse 37/39, Zurich, 8092, Switzerland
AU: Clinton, J
EM: j.clinton@sed.ethz.ch
AF: Swiss Seismological Service, ETH-Hoenggerberg / HPP P 6.3, Zurich, 8093, Switzerland
AU: Deichmann, N
EM: n.deichmann@sed.ethz.ch
AF: Swiss Seismological Service, ETH-Hoenggerberg / HPP P 6.3, Zurich, 8093, Switzerland
AU: Funk, M
EM: funk@vaw.baug.ethz.ch
AF: VAW-Glaciology, Gloriastrasse 37/39, Zurich, 8092, Switzerland
AB: We study the seismic activity recorded on the Gornergletscher, Switzerland, during annual glacial flooding sequences. Historically, these 'jokulhaup' events have caused considerable damage downstream in the Zermatt Valley. As part of a wider monitoring effort of the glacier, seismic networks have been deployed each spring during 4 recent lake drainages of Gornersee, a glacier-dammed lake at the confluence of the glacier's two main tributaries. We observed several thousand seismic events within the glacier ice ('icequakes') each day. Most occur near the surface and are associated with crevassing. However, a small fraction of events can be reliably located at intermediate depths and near the glacier bed. In particular, we are interested in investigating the relationship between the sudden subglacial passage of large volumes of water and the observed deep icequakes. The temporal occurrence of these basal events indicates a relationship to the lake drainage, and we suggest they may be caused by large fluctuations in subglacial water pressure. However, information about source type, strength and fault orientation are critical to a more complete understanding of these deep icequakes. We estimate the moment tensor of these deep events using a full waveform inversion based on the method of Dreger (1994), with and without constraint on the isotropic component. We modify the inversion scheme, usually applied to regional seismicity, to the scale of the glacier and the high frequency of the observed signals by scaling distances and frequencies to match simple 1-D Green's functions derived at longer periods. The results of initial icequake inversions exhibit good waveforms fits: we match both deep event reflectivity synthetics as well as observed first motion focal mechanisms for clearly double-couple surface events. This shows that moment tensor estimations can be tailored to the glacial environment and can elucidate the icequake source physics. If the fracture processes of the deep icequakes can be associated with the flooding sequence both in terms of their temporal occurrence and their source mechanism, icequake monitoring can potentially be used to provide a key early warning to the impending flood.
DE: 0776 Glaciology (1621, 1827, 1863)
DE: 7215 Earthquake source observations (1240)
SC: Seismology [S]
MN: 2007 Fall Meeting