Triggered Seismicity after the June 17, Mw=6.5 Earthquake in the South Iceland Seismic Zone: The first five minutes.
Abstract
The Mw=6.5, June 17 earthquake occurred in the South Iceland Seismic Zone (SISZ), which is densely covered with short-period seismic instruments. However due to the event's size, most stations within 80 km epicentral distance were saturated. Consequently event detection and mechanism determination during the first few minutes after the event have proven difficult. The problem was compounded by two additional events of \bar M5, dynamically triggered by shear waves from the original event, as they swept along the Reykjanes Peninsula at an apparent velocity of 2.5 km/s. These two triggered events, at 64 and 77 km distance, respectively, in turn saturated stations within 20 km of their own epicenters. Two additional \bar M5 events occurred within the next five minutes; one just west of the main event after two minutes, the other after four minutes at 86 km distance on Reykjanes Peninsula. Of these four \bar M5 events, the first two were not detected teleseismically. To demonstrate this triggering sequence, seismic data and results from the first five minutes will be presented. The first triggered \bar M5 event occurred on a section of a known fault near Hvalhnúkur, 26 seconds after the main event. It initiated at 9 km depth and its mechanism is near-vertical, right-lateral, strike-slip striking just east of North. The second \bar M5 event triggered only 4 seconds after the first one and farther west, at approximately 4 km depth near the eastern shore of lake Kleifarvatn. Due to the short time interval, its waveforms were obscured at all nearby stations by the waves from the first triggered event, leaving the event undetected for some time, until a detailed study of the first few minutes was called for by observations of surface effects on Reykjanes (Clifton et al., 2003) and preliminary modeling of InSAR images from SW Iceland, which indicated large displacements near lake Kleifarvatn (Pagli et al., 2003). The mechanism of this event is virtually unconstrained by seismic data. Following the Kleifarvatn event, several small events (\bar M3) were detected at the southwest corner of the lake, leading up to the fourth \bar M5 event located at shallow depth under Núpshlídarháls, which is in the same general area. This earthquake was not dynamically triggered by any of the three preceding large earthquakes, but its proximity to Kleifarvatn (7 km) suggests it, and the preceding smaller events, could have been triggered by the Kleifarvatn event. The mechanisms of the third and fourth \bar M5 events are rather well constrained by the seismic data, revealing steeply dipping, right-lateral strike-slip faulting, striking just east of North.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA....11251V
- Keywords:
-
- Southwest Iceland;
- dynamically triggered;
- earthquakes;
- Reykjanes Peninsula