HR: 17:45h
AN: V14B-08 INVITED     [Abstracts]
TI: Sub-surface Fault Mapping with Microearthquakes Reveals Volcano-Tectonic Interaction in the Hengill Region, Iceland
AU: * Vogfjord, K S
EM: vogfjord@vedur.is
AF: Icelandic Meteorological office Physics Department, Bustadavegi 9, Reykjavik, 150 Iceland
AU: Hjaltadottir, S
EM: slauga@vedur.is
AF: Icelandic Meteorological office Physics Department, Bustadavegi 9, Reykjavik, 150 Iceland
AU: Slunga, R
EM: slunga@foi.se
AF: Uppsala University Department of Geophysics, Villavagen 16, Uppsala, 752 36 Sweden
AB: During a six-year period between 1993 and 1998, a major volcano-tectonic episode occurred in the Hengill volcanic region in Iceland, at the intersection of the Western Volcanic Zone and the South Iceland Seismic Zone (SISZ). The episode was driven by an intrusion at 7 km depth inside the northwestern margin of the Graendalur volcanic system during 1993-1997, causing uplift of 2 cm/yr and inducing over 80 thousand earthquakes. The uplift was fairly uniform, but the seismic activity was episodic, and initially mostly confined near the center of uplift, at the northern margin of the SISZ, with only occasional small swarms at the SISZ's southern margin. Finally in June 1998 enough stress had accumulated north of the Seismic Zone to unlock the asperities within it, allowing rupture to propagate through to its southern margin along a segmented N-S fault, which is characteristic for faulting in the SISZ. This faulting generated an M~5 event and two M~4 events, which were accompanied by over six thousand microearthquakes during the following weeks. Five months later, in November, a second M~5 event struck at the southern margin of the SISZ, on an E-W striking fault and was followed by over five thousand microearthquakes during the remainder of November. After this last swarm the stress had finally propagated through the Seismic Zone and seismic activity died out, being induced again only briefly in 2000 by the two M6.5 earthquakes, occurring over 30 km farther east in the center of the SISZ. The faults activated during the Hengill episode are mapped by joint interpretation of the microearthquake distribution and focal mechanisms of individual earthquakes, which allows also determination of slip direction. Improved earthquake locations are obtained through cross-correlation of both P and S wave forms and subsequent inversion of the relative arrival times. Focal mechanisms are obtained by a combination of grid-search through all possible mechanisms and inversion of spectral amplitudes of P and S waves. The inter-weaving fault pattern emerging from this mapping reveals mostly near-vertical and steeply dipping faults, a few hundred meters to a few kilometers long. Fault-strikes in the region north of the SISZ vary greatly, but most common directions are are NE and NNE, with a few N-trending sections. Within the SISZ the dominant strike direction is near N-S and the faults are longer, extending nearly 7 km. A few ENE faults offset the N-S faults creating a right-stepping pattern. South of the Seismic Zone the fault pattern is quite opposite, consisting of one 9 km long, vertical ENE-striking fault, offset by a few short faults with a northerly strike. Slip directions are generally right lateral on N and NE striking faults and left lateral on E and ENE striking faults. Normal faulting is also observed on short fault segments north of the SISZ. Focal mechanisms of the larger events are usually in agreement with the fault planes defined by the microearthquake distribution.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7250 Transform faults
DE: 8123 Dynamics: seismotectonics
DE: 8419 Volcano monitoring (7280)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005