D21 Study of the history of stress distribution in the Hengill area – IMOR.DG

The catalog of roughly 45 thousand relocated earthquakes between 1997 and 2000 has been completed and the main faults that were active during this time have been mapped. The results, shown in Figure 1, reveal a multitude of interweaving steeply dipping faults. At the southern and northern margins of the South Iceland Seismic Zone (SISZ), clear E- to ENE-striking faults are visible. The northern fault generated an M>4 event in June 1998, the southern one generated a M>5 event in November 1998. In addition many NE-striking faults were active north of the center of intrusion (black circle). Within the SISZ (between 63.95 and 64.06 degrees latitude) the dominant fault directions are NNE. They were all active in June 1998, generating two M>4 earthquakes and one M~5.

The seismicity was induced by an intrusion at depth, causing measurable uplift of 2 cm/yr during 1993-1997. The center of uplift was determined to be at the location of the black circle (Feigl et al., 2000). The imprint of the radial stress field caused by the intrusion on the loading of the underlying left-lateral shear across the SISZ, increases shear stress on E- and ENE-striking faults in the NE and SW quadrants of the intrusion. The activated E- and ENE-striking faults, in fact are all in these two quadrants and all show left-lateral strike-slip motion. Hardly any faults are activated in the intrusion’s SE quadrant due to the resulting decrease in shear stress on the NS-striking asperities in the SISZ. The predominant strike direction of activated faults in the intrusion’s NW quadrant is NE, and motion is predominantly right-lateral strike slip with a smaller normal component. These faults also experienced increased loading by the intrusion.

The time history of rupture during this period is revealed in Figure 2, where events defining the main faults are colour coded according to time. Epicenter of events greater than M4 are marked with stars, and their mechanisms are also shown. In general these mechanisms agree very well with the fault strikes and dips. The figure shows that activity from 1997 to the end of May 1998 (purple and dark blue) was mainly north and west of the intrusion, with mainly short patches active farther south. In June 1998 seismicity (turquoise) started north of the intrusion, propagated westward, and finally ruptured south through the asperity in the SISZ causing the M5 earthquake, as well as one M4 event just before, and one just after. In November 1998 (pastel green), activity started at the southern margin of the SISZ, ruptured the ENE fault in an M5 event, followed the next day by an M4 event on a NS fault a few km farther east. In May 1999 (forest green) the faults north of the intrusion were reactivated in a swarm and finally, following the June 2000 earthquakes in the SISZ, activity on the faults in the intrusion’s NE quadrant was induced.

Fig. 1: Relatively located events (open purple circles) from 1997 through 2000. Events greater than M4 are displayed with stars (yellow for 1997, green for 1998) and mechanisms for the two M5 events are shown. Previously mapped surface faults are shown with thin black lines. The arching yellow line indicates the mapped fault scarp following the June 1998 earthquake swarm. The center of uplift is indicated by an open black circle. Seismic stations (kro and bja) are shown as purple triangles. The over-lapping green boxes outline the sections used to divide up the data for relocation analysis. The width of the South Iceland Seismic Zone is between 63.95 and 64.06 degrees latitude.

The volcano-tectonic episode in the Hengill region during 1993 and 1998, was caused by an intrusion at the northern margin of the South Iceland Seismic zone. The episode induced over 80 thousand earthquakes in the Hengill region, initially mostly confined north of the SISZ. However, in June 1998 the asperity in the SISZ broke in an M5 and two M4 earthquakes, and seismicity propagated through the zone to its southern margin. In November that same year, an ENE-striking fault at the southern margin slipped in a second M5 earthquake, followed by thousands of microearthquakes. This swarm terminated the transfer of stress caused by the intrusion, from the northern margin of the SISZ to its southern margin. A small swarm (over one thousand events) occurred in the NE quadrant in 1999, and seismic activity was also induced in the same location, following the June 2000 earthquakes in the SISZ.

The interaction of the radially symmetric stress field of the intrusion with the forces driving the E-striking left-lateral shearing in the SISZ, caused left-lateral slip on faults with easterly strikes in the intrusion’s NE and SW quadrants, while northerly striking faults were activated in the NW quadrant. No slip occurred in the SE quadrant.

Fig. 2: Mapped faults, active between 1997 and 2000, colour coded according to time. Events greater than M4 are marked with stars (yellow for 1997, green for 1998), and their mechanisms are shown. Previously mapped surface faults (black thin lines) and the one mapped in June 1998 (yellow thick line) are shown. The center of uplift is indicated by an open black circle. Seismic stations are shown as purple triangles.