Task 1: Continuous monitoring of shear-wave splitting

The basic remit of the project is to respond to any observed changes in shear-wave splitting and analyze data for hazard assessment. The bulk of the activity has been in this task as changes in shear-wave splitting are now observed routinely before larger earthquakes and before volcanic eruptions and subsurface movements of magma.

Shear-wave splitting is sensitive to changes in stress. In anisotropic media, the two shear-waves split into two orthogonally polarized waves that propagate with different velocities. Examination of the seismograms can identify the polarization direction of the first (or faster) shear-wave and the time delay between the two arrivals (Figure 7). Time-delays are particularly sensitive to changes in azimuthal anisotropy induced by stress-induced changes to microcrack distributions (Crampin 1999).

Shear-wave data from earthquakes provided by IMOR.DG are identified and analyzed routinely. Seismic stations in Iceland are shown in Figure 8 with all earthquakes with M$\ge$2 in the period January 1997 to December 1999. The stations with sufficient shear-wave arrivals for analysis of temporal changes, KRI, BJA, and SAU are marked with red triangles. Stations throughout Iceland with more than 10 polarization measurements during the four years of the PRENLAB projects, 1996-1999, are shown in Figure 9 (after Volti and Crampin 2000). The alignment of polarizations show average directions of the maximum horizontal stress SW-NE.

As in previous years, the stations with sufficient polarization and time-delay data are BJA, SAU and KRI, situated along the South Iceland seismic zone (SISZ) (Figure 8) and marginally GRI on the island of Grímsey, north of Iceland. During 1998, analysis was concentrated on station BJA, which had the best quality shear-wave arrivals. However, activity near BJA has declined during 1999 and 2000, whereas activity near KRI and SAU has increased (Volti and Crampin 2000).

 

Figure 7: Example of shear-wave splitting at station KRI. The upper diagram shows the three recorded components. On the bottom the rotated N-S and E-W components show the time-delay between the two shear-wave arrivals. (Event occurred at 03:02:40:6, April 9, 2000, at 5.7 km depth, 4.5 km from KRI).

 

Figure: Map of Iceland showing all earthquakes with $M\geq 2$, during the period March 1998 - June 2000. The small green triangles show the majority of SIL stations, whereas the stations KRI, BJA and SAU (from left to right) are shown with large red triangles.

 

Figure: Map of Iceland, showing the seismic network. Shaded areas are ice fields. Open circles mark the active volcanoes of, from north to south, Bárðarbunga, Grímsvötn, Hekla and Katla. The roundels are equal-area polar plots of polarizations of the faster split shear-wave arrivals in the shear-wave window (out to $45^\circ$) and rose diagrams, during 1996-1999. Roundels are shown only for those stations where there are more than 10 arrivals within the shear-wave window. The named stations without roundels (ASM, KRO, MID) have sufficient arrivals but the polarizations are severly disturbed by local rifting and/or local topography (after Volti and Crampin 2000).