HR: 11:35h
AN: S31G-06    [PDF]
TI: How Stress Transfer Between Volcanic and Seismic Zones can Suppress or Trigger Earthquakes
AU: Brenner, S
EM: Sonja.Brenner@geo.uib.no
AF: Department of Earth Science, University of Bergen, Allegaten 41, Bergen, N-5007 Norway
AU: * Gudmundsson, A
EM: Agust.Gudmundsson@gwdg.de
AF: Department of Structural Geology and Geodynamics, University of Gottingen, Goldschmidtstrasse 3, Gottingen, D-37077 Germany
AB: In the South Iceland Seismic Zone (SISZ), major destructive earthquakes occur at intervals of 80-100 years, the largest events reaching M7.1. Although the zone is E-W trending, with an overall sinistral movement, most of the earthquakes are associated with NNE and ENE-trending strike-slip faults. Numerical models indicate that when the SISZ is loaded to failure, stresses are transferred to, and concentrate at, its ends: tensile in the northeast and southwest quadrants and compressive in the northwest and southeast quadrants. These model results agree with observations made when the SISZ was loaded to failure in June 2000, resulting in two M6.6 earthquakes. For several years prior to the June 2000 earthquakes, compression, uplift and intense earthquake swarms (some events exceeding M4) occurred in two active stratovolcanoes, located in the quadrants of compressive stresses. Most of this earthquake activity came to an end following the June 2000 earthquakes. Also, in the decades prior to the June 2000 earthquakes, the stratovolcano Hekla, located in one of the quadrants of tensile stresses, has erupted unusually frequently. These stratovolcanoes (with shallow magma chambers) act as "soft inclusions" to which stresses from the SISZ are transferred; they may be used as precursors to large earthquakes in the SISZ. Stress is also transferred from the volcanic zones to the seismic zones. For example, stress transfer from the overpressured 27-km-long feeder dike of the Laki 1783 eruption to the nearby SISZ triggered the 1784 earthquake sequence, the largest one in the SISZ. By contrast, dike injection in 1976 in the North Volcanic Zone during the Krafla Fires resulted in suppression of earthquake activity in the Husavik-Flatey Fault (HFF), an active transform fault partly exposed on land, for nearly two decades. For regional dikes such as these, the magmatic overpressure may reach tens of mega-pascals; part of this pressure is transferred as compressive stress to the nearby seismic zones, the SISZ and the HFF. The models presented here indicate that stress transfer from a dike injected in a volcanic zone either triggers or suppresses earthquake activity in a nearby seismic zone, depending on the location of the dike in relation to that zone. When the dike is so located that the dike-generated horizontal displacement is opposite to the normal displacement across the adjacent seismic zone, earthquakes are suppressed; when the dike-generated displacement is consistent with the normal displacement across the adjacent seismic zone, earthquakes are triggered.
DE: 7209 Earthquake dynamics and mechanics
DE: 7223 Seismic hazard assessment and prediction
DE: 8010 Fractures and faults
DE: 8020 Mechanics
DE: 8123 Dynamics, seismotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting