Seismicity on Conjugate Faults in Ölfus, South Iceland: Case Study of the 1998 Hjalli‐Ölfus Earthquake

• Published in: Journal of Geophysical Research: Solid Earth
• Main Authors: Mozhikunnath Parameswaran, Revathy, Þorbjarnardóttir, Bergþóra Sólveig, Stefánsson, Ragnar, Bjarnason, Ingi Þorleifur
• Other Authors: Jarðvísindastofnun (HÍ), Institute of Earth Sciences (UI), Verkfræði- og náttúruvísindasvið (HÍ), School of Engineering and Natural Sciences (UI), Viðskipta- og raunvísindasvið (HA), School of Business and Science (UA), Háskóli Íslands, University of Iceland, Háskólinn á Akureyri, University of Akureyri
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Geophysical Union (AGU) 2020
• Subjects: South Iceland Seismic Zone; Hjalli‐Ölfus; Earthquake relocations; Conjugate faulting; Seismotectonics; Jarðskjálftavirkni; Jarðskorpuhreyfingar; Jarðskjálftarannsóknir; Hengill; Hjalli; Iceland
• Online Access: https://hdl.handle.net/20.500.11815/2045; https://doi.org/10.1029/2019JB019203

Publisher's version (útgefin grein) The Ölfus seismic belt lies at the western end of the ~E‐W sinistral transform shear zone in South Iceland, called the South Iceland Seismic Zone (SISZ), where most seismicity and surface faulting show ~N‐S dextral slip. Unlike the rest of SISZ, seismicity in west Ölfus is predominantly along the ~ENE‐WSW direction. Throughout recorded history, Ölfus has shown an interactive behavior with the Hengill volcanic system that lies northwest of the zone. For instance, the 13 November 1998 Mw 5.1 earthquake in the Hjalli area (west Ölfus) and its ~ENE trending aftershock sequence were likely triggered by the 4 June 1998 Mw 5.4 Hengill earthquake sequence. These events point to an interplay between conjugate ~N‐S and ~ENE‐WSW faults in the region. Relative relocations of earthquakes in Hjalli‐Ölfus from July 1991 to December 1999 (Icelandic Meteorological Office, 2017) are chiefly limited to 4‐ to 8‐km depth along the ~ENE direction with a few distributed on smaller ~N‐S faults. The foreshocks of the November 1998 earthquake occurred on a ~N‐S fault until a day prior to the mainshock when they shifted to the ~ENE direction. The subsequent aftershocks are also mainly restricted to the ~ENE direction. We find that the Mw 5.1 (Global Centroid Moment Tensor moment = 5.43 × 10E16 N‐m) Hjalli‐Ölfus earthquake ruptured a near‐vertical ~ENE fault area of 24–40 km2 with left‐lateral average slip of 5–8 cm. Multiple relocations of the mainshock using various constraints indicate that the event likely occurred close to the junction of the conjugate ~ENE‐WSW and ~N‐S faults. This study was part of the project titled “4D seismic of the South Iceland Seismic Zone: Strong earthquake forecasting” (152432‐053) funded by the Icelandic Research Fund (IRF) (Rannís). The authors thank Felix Waldhauser for sharing a more recent version of the hypoDD routine, which was then modified for the purposes of this study, and for advice on fine‐tuning the analyses. The authors thank Haukur Jóhannesson, Kristján ...