Evolution of Stresses Over Conjugate Faults in Hjalli-olfus, South Iceland

Hjalli-Olfus is the westernmost segment of the east-west transform South Iceland Seismic Zone (SISZ), which is the eastward extension of the similar to ENE-trending transtensional Reykjanes Peninsula (RP). Historically, the area has shown an interactive behavior with the Hengill volcanic system to the north and the central SISZ to the east. We analyzed the state of stress and faulting mechanisms in Hjalli-Olfus between July 1991 and December 1999, in connection with the Hengill inflation episode (Feigl et al., 2000, https://doi. org/10.1029/2000JB900209) and the 13th November 1998 Mw 5.1 Hjalli-Olfus earthquake. We find that this region predominantly hosted oblique-normal and left-lateral strike-slip events (4-10 km-depth), with most nodal planes oriented along similar to ENE or similar to WSW directions (75 degrees +/- 15 degrees or 255 degrees +/- 15 degrees). We identify 5 stages of stress evolution from January 1991 to December 1999 over which Hjalli-Olfus experiences both spatial and temporal shifts in stress-states. The Hengill inflation likely loaded both the fissure zone and western Olfus, culminating in the Mw 5.4 (Hengill) and Mw 5.1 (Hjalli-Olfus) earthquakes. Following these events, the maximum compressive stress (SHmax) orientation near the location of the Mw 5.1 earthquake showed a similar to 5 degrees-7 degrees counterclockwise swing, compared to SHmax before June 1998. The average SHmax (similar to 40 degrees +/- 1 degrees) and minimum principal s....tress (sigma(3) similar to 130 degrees +/- 1 degrees) are comparable to geological trends in the RP. We conclude that Hjalli-Olfus shows clockwise SHmax rotation upon loading, while a stress-drop reverses the rotation. We also posit that the region, especially the western end, behaves like the RP during interseismic periods. Plain Language Summary Iceland hosts a predominantly rifting plate boundary that is offset by two similar to east-west trending, horizontally sliding (transform-faulting) segments, one in the north and the other in the south. The southern segment, known as the South Iceland Seismic Zone (SISZ), is seismically productive and is flanked by diverging volcanic arms. The SISZ has hosted several moderate to large earthquakes on north-south faults that cut across it. However, the westernmost end of the SISZ, also known as Hjalli-Olfus, differs from the rest of the SISZ as it seems to host earthquakes in an similar to east-northeast direction, similar to its western, rifting neighbor, namely, the Reykjanes Peninsula. The activity in Hjalli-Olfus also seems to be responsive to volcanic/magmatic activity in the Hengill volcano to its north. This suggests the existence of multiple stress fields-volcanic, tectonic, or both-acting on the Hjalli-Olfus segment. Here, we study earthquakes in Hjalli-Olfus from January 1991 to December 1999, including a magnitude 5.1 earthquake in November 1998, to identify possible stress changes along the segment over time. Results indicate that magmatic deformation and seismic activity near the Hengill volcano directly influence the seismic productivity of Hjalli-Olfus, and that this seismicity is similar to that of the Reykjanes Peninsula.

Automated summary

This study analyzes the stress state and faulting mechanisms in the Hjalli-Olfus region of Iceland between July 1991 and December 1999. It finds that the region experiences left-lateral strike-slip events with nodal planes oriented in an ENE or WSW direction, possibly influenced by the Hengill volcanic system and the central SISZ.