INTRODUCTION

In June 2000 two earthquakes with magnitude 6.6 (Ms) occurred in the central part of the South Iceland seismic zone (SISZ). They were immediately followed by seismic activity along zones of approximately 100 km length. This activity occurred after 88 years of relative quiescence in the 70 km long EW transform zone in SW-Iceland (Figure 1). Earthquakes in this region have, according to historical information, at some occations caused collapse of the majority of houses in areas encompassing 1000 km²and pose a significant threat to inhabitants of this area, a relatively densely populated farming region. Despite intensive surface fissuring caused by the earthquakes and measured accelerations reaching 0.8 g, the earthquakes in 2000 caused no serious injuries and no structural collapse. The relatively minor destruction led to some optimism regarding the safety of living in the area. Many of the ideas about the nature of strain release in the area have been confirmed. As far as the epicenter of the first earthquake is concerned, hazard assessments or long-term predictions were confirmed, and in hindsight precursors have been observed. Useful short-term warning, including the right epicenter and size of the second earthquake, was issued beforehand. Preliminary observations which were made of the earthquakes as well as of associated preseismic and postseismic processes, will be described in this paper. Among significant observational systems recording the events is the SIL-system. SIL stands for South Iceland Lowland, i.e. the test area for the SIL-project described in Stefánsson et al. 1993. The SIL-system is especially aimed at retrieving real-time information carried by microearthquakes (Böðvarsson et al 1999). Other significant observational systems include strong motion instruments, continuous GPS measurements, borehole strainmeters and hydrological observations in boreholes. Earlier survey-mode GPS measurements were repeated after the earthquakes, and detailed analysis of extensive surface fissures was carried out. InSAR images were collected to observe coseismic evidence and to compare with preseismic and postseismic deformation. We estimate that no more than one fourth of the available moment in the zone was released in the two earthquakes that occurred in 2000 and even larger earthquakes may occur in the zone during the next few decades.

 

Figure 1: The southwestern part of Iceland. Iceland as a whole is shown in the upper right corner. Dotted yellow lines denote the western volcanic zone (WVZ) and the presently more active eastern volcanic zone (EVZ). The South Iceland seismic zone (SISZ) is indicated as well as its prolongation in the Reykjanes peninsula (RP). The direction of the relative plate motion is shown by arrows. The faults of the two large earthquakes that ruptured on June 17 and 21 are indicated by 17 and 21 respectively. The epicenters of small shocks following the large earthquakes (red dots) show the extent of the seismically activated area.

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