THE JUNE 17 EARTHQUAKE

According to the Icelandic Meteorological Office (IMO) the origin time of the June 17 earthquake was 15:40:40.94 GMT, the hypocenter at 63.97$^{\circ}$N, 20.37$^{\circ}$W, and a hypocentral depth of 6.3 km. The aftershocks indicate an 11-12 km long rupture extending from the surface to 10 km depth. Assuming that the upper 1 km of the rupture does not contain considerable energy to be released in the earthquake the fault width is taken to be 9 km (Figure 2).

 

Figure 2: Aftershocks of the June 17 earthquake within 3 km of the fault plane are shown along with a tentative slip model. The frame at the top shows aftershocks during the first 32 hours. The second frame shows the aftershocks until December 10, 2000, and a rupture model to fit observations from local volumetric strainmeters, with variable right-lateral slip on a 12 km long fault (Stefánsson et al. 2000; Alan Linde and Kristján Ágústsson personal communication).

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The aftershocks indicate that the fault strikes N7$^{\circ}$E and dips 86$^{\circ}$ towards the east. Surface fissures showing right-lateral motion on an underlying fault coincide with and are found along the entire fault (Einarsson et al. 2000). Our model for the earthquake mechanism agrees very well with the USGS Rapid Moment Tensor Solution which has a nodal plane striking N4$^{\circ}$E and dip of 84$^{\circ}$ to the east. The moment calculated by USGS is $6.0\ast10^{18}$ Nm, assuming a best-fitting double-couple solution. The preliminary magnitudes estimated by the National Earthquake Information Center (NEIC) in USA were mb=5.7 and Ms=6.6. Preliminary modeling of local measurements by volumetric strainmeters, assuming rigidity 36 N/m², indicate variable slip as shown in Figure 2, and a moment of $4.8\ast10^{18}$ Nm (Alan Linde and Kristján Ágústsson 2000, personal communication; Stefánsson et al. 2000). Assuming uniform slip along the fault the rupture dimension suggested by aftershocks, and the moment estimated by USGS, an average right-lateral displacement of 1.5 m is inferred. Two minutes after the main shock an earthquake of local magnitude 5 (mb) occurred 6 km to the southwest of the main shock epicenter, not along a continuation of the main shock fault, but to the west of it. Studying the spatial distribution of the aftershocks (i.e. aftershocks within a couple of kilometers from the fault) the following observations are noticable (Figure 2). The observations suggest that the earthquake initiated in the center of the rupture which appears to have extended down to 10 km. This is slightly deeper, but comparable to the depth of the brittle/ductile boundary inferred from earlier microearthquake studies: around 8 km in this part of the SISZ (Stefánsson et al. 1993; Tryggvason et al. 2002). A few early aftershocks occurred at 12 km depth, well into what usually is considered the ductile zone. This suggests that ductility depends on strain rate which of course is expected to be high immediately after and near a large earthquake. The concentration of aftershocks at the southern and northern end of the rupture, reflect high stresses where the fault movement stopped. A similar effect is observed at the bottom of the fault, where ductile motion is expected following the earthquake. A concentration of aftershocks close to the hypocenter of the main shock is also noticeable. Further conclusions about these peculiarities require a more thorough investigation of a large amount of aftershocks of magnitudes down to zero, including joint hypocenter location by cross correlation of similar signals and, based on it, reevaluation of the microearthquake mechanisms.