Temporal stress changes associated with the 2008 May 29M_W 6 earthquake doublet in the western South Iceland Seismic Zone

Abstract

two magnitude M W ∼ 6 earthquakes occurred on two adjacent N-S faults in the western South Iceland Seismic Zone. The first main shock was followed approximately 3 s later by the rupture on a parallel fault, about 5 km to the west. An intense aftershock sequence was mostly confined to the western fault and an E-W aligned zone, extending west of the main shock region into the Reykjanes oblique rift. In this study, a total of 325 well-constrained focal mechanisms were obtained using data from the permanent Icelandic SIL seismic network and a temporary network promptly installed in the source region following the main shocks, which allowed a high-resolution stress inversion in short time intervals during the aftershock period. More than 800 additional focal mechanisms for the time period 2001-2009, obtained from the permanent SIL network, were analysed to study stress changes associated with the main shocks. Results reveal a coseismic counter-clockwise rotation of the maximum horizontal stress of 11 ± 10 • (95 per cent confidence level) in the main rupture region. From previous fault models obtained by inversion of geodetic data, we estimate a stress drop of about half of the background shear stress on the western fault. With a stress drop of 8-10 MPa, the pre-event shear stress is estimated to 16-20 MPa. The apparent weakness of the western fault may be caused by fault properties, pore fluid pressure and the vicinity of the fault to the western rift zone, but may also be due to the dynamic stress increase on the western fault by the rupture on the eastern fault. Further, a coseismic change of the stress regime-from normal faulting to strike-slip faulting-was observed at the northern end of the western fault. This change could be caused by stress heterogeneities, but may also be due to a southward shift in the location of the aftershocks as compared to prior events.

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

“…It has been argued that part of the latest stress buildup since the 1896 and 1912 events in the eastern part of SISZ manifested as the 1987 Mw 5.9 (centroid moment magnitude) earthquake that occurred at the junction of SISZ and the western flank of EVZ, ~10 km east of the 1912 earthquake (Bjarnason & Einarsson, 1991). In more recent times, two large sequences of earthquakes in the Years 2000 (17 June Mw 6.5 and 21 June Mw 6.4) (Árnadóttir et al, 2001; Stefánsson et al, 2000) and 2008 (29 May; ~Mw 6.3 doublet in a span of 3 s) (Brandsdóttir et al, 2010; Hensch et al, 2016; Hreinsdóttir et al, 2009) have occurred on SISZ (Figure 1d), all on ~N‐S faults (Clifton & Einarsson, 2005; Hjaltadóttir, 2009).…”

Stress Field Variation During the 2019 Ridgecrest Earthquake Sequence

“…Fortunately, we can obtain the principal stress direction and the stress ratio R by analyzing focal mechanisms. The spatiotemporal variation of the focal mechanisms can be used to study the state of stress release (Chiba et al, 2012; Hasegawa et al, 2011), the magnitude of the deviatoric stress (Hardebeck & Hauksson, 2001; Hasegawa et al, 2011; Hensch et al, 2016; Huang et al, 2011; Yoshida et al, 2016), the coseismic stress rotation (Hardebeck, 2012; Hauksson, 1994; Huang et al, 2011; Michael, 1987; Otsubo et al, 2013; Vavryčuk, 2015; Zhao et al, 1997), and the postseismic stress recovery (Hardebeck, 2012; Hauksson, 1994; Ickrath et al, 2014; Zhao et al, 1997).…”

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

“…Hensch et al. (2016) conducted stress inversions using aftershocks of the 2008 ∼M6 Ölfus earthquakes, east of the Hjalli‐Ölfus region, and found that SHmax orientations in Hjalli‐Ölfus showed a counterclockwise rotation (36° ± 8° to 35° ± 8°) over the first month, followed by clockwise rotation (35° ± 8° to 40° ± 6°). A more recent study conducted using GPS data from 2001 to 2015 in SISZ to understand the pre‐seismic strain anomalies before the 2008 ∼M6 earthquakes in Ölfus also points to a varying stress field in the region (Árnadóttir et al., 2018).…”