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

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

Title: Seismicity on Conjugate Faults in Ölfus, South Iceland: Case Study of the 1998 Hjalli‐Ölfus Earthquake
Author: Mozhikunnath Parameswaran, Revathy   orcid.org/0000-0001-7094-8436
Þorbjarnardóttir, Bergþóra Sólveig
Stefánsson, Ragnar
Bjarnason, Ingi Þorleifur   orcid.org/0000-0001-5716-7053
Date: 2020-08
Language: English
University/Institute: Háskóli Íslands
University of Iceland
Háskólinn á Akureyri
University of Akureyri
School: Verkfræði- og náttúruvísindasvið (HÍ)
School of Engineering and Natural Sciences (UI)
Viðskipta- og raunvísindasvið (HA)
School of Business and Science (UA)
Department: Jarðvísindastofnun (HÍ)
Institute of Earth Sciences (UI)
Series: Journal of Geophysical Research: Solid Earth;125(8)
ISSN: 2169-9356
2169-9313 (eISSN)
DOI: 10.1029/2019JB019203
Subject: South Iceland Seismic Zone; Hjalli‐Ölfus; Earthquake relocations; Conjugate faulting; Seismotectonics; Jarðskjálftavirkni; Jarðskorpuhreyfingar; Jarðskjálftarannsóknir
URI: https://hdl.handle.net/20.500.11815/2045

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Parameswaran, R. M., Thorbjarnardóttir, B. S., Stefánsson, R., & Bjarnason, I. T. (2020). Seismicity on conjugate faults in Ölfus, South Iceland: Case study of the 1998 Hjalli‐Ölfus earthquake. Journal of Geophysical Research: Solid Earth, 125, e2019JB019203. https://doi.org/10.1029/2019JB019203


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.


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