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

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

Title: Evolution of Stresses Over Conjugate Faults in Hjalli‐Ölfus, South Iceland
Author: Mozhikunnath Parameswaran, Revathy   orcid.org/0000-0001-7094-8436
Bjarnason, Ingi Þorleifur   orcid.org/0000-0001-5716-7053
Þorbjarnardóttir, Bergþóra Sólveig
Date: 2023-03-14
Language: English
Scope: e2022JB026201
University/Institute: Háskóli Íslands
University of Iceland
School: Verkfræði- og náttúruvísindasvið (HÍ)
School of Engineering and Natural Sciences (UI)
Department: Jarðvísindastofnun (HÍ)
Institute of Earth Sciences (UI)
Series: Journal of Geophysical Research: Solid Earth;128(3)
ISSN: 2169-9313
2169-9356 (eISSN)
DOI: 10.1029/2022JB026201
Subject: Earth and Planetary Sciences (miscellaneous); Geophysics; Jarðskjálftarannsóknir; Suðurland; Misgengi (jarðfræði); Spennubreytar; Reykjanes; Ölfus; Hengill; Stress rotation; Conjugate faults; South Iceland Seismic Zone; Hjalli-Ölfus; Focal mechanism; Stress-drop
URI: https://hdl.handle.net/20.500.11815/4128

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Parameswaran, R. M., Bjarnason, I. T., & Thorbjarnardóttir, B.S. (2023). Evolution of stresses over conjugate faults in Hjalli-Ölfus, South Iceland. Journal of Geophysical Research: Solid Earth, 128, e2022JB026201. https://doi.org/10.1029/2022JB026201


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.


Plain Language Summary Iceland hosts a predominantly rifting plate boundary that is offset by two ∼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-Ölfus, differs from the rest of the SISZ as it seems to host earthquakes in an ∼east-northeast direction, similar to its western, rifting neighbor, namely, the Reykjanes Peninsula. The activity in Hjalli-Ölfus 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-Ölfus segment. Here, we study earthquakes in Hjalli-Ölfus 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-Ölfus, and that this seismicity is similar to that of the Reykjanes Peninsula.


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