Evolution of Stresses Over Conjugate Faults in Hjalli‐Ölfus, South Iceland

Abstract

Hjalli-Ölfus is the westernmost segment of the east-west transform South Iceland Seismic Zone (SISZ), which is the eastward extension of the ∼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-Ölfus 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-Ölfus 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 ∼ENE or ∼WSW directions (75° ± 15° or 255° ± 15°). We identify 5 stages of stress evolution from January 1991 to December 1999 over which Hjalli-Ölfus experiences both spatial and temporal shifts in stress-states. The Hengill inflation likely loaded both the fissure zone and western Ölfus, culminating in the Mw 5.4 (Hengill) and Mw 5.1 (Hjalli-Ölfus) earthquakes. Following these events, the maximum compressive stress (SHmax) orientation near the location of the Mw 5.1 earthquake showed a ∼5°–7° counterclockwise swing, compared to SHmax before June 1998. The average SHmax (∼40° ± 1°) and minimum principal stress (sigma3 ∼ 130° ± 1°) are comparable to geological trends in the RP. We conclude that Hjalli-Ölfus 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.