The 2008 May 29 earthquake doublet in SW Iceland

Abstract

an earthquake doublet shook the southwestern part of Iceland. The first main shock originated beneath Mt Ingólfsfjall, located near the western margin of the South Iceland Seismic Zone (SISZ) approximately 40 km east of the capital Reykjavík. Immediate aftershock activity was recorded by the SIL seismic network, operated by the Icelandic Meteorological Office (IMO), with both N-S and E-W structures illuminated over a broad area. A continuous GPS (CGPS) network, also operated by the IMO, recorded coseismic offsets with up to 200 mm of horizontal motion at the closest stations. We estimate the coseismic surface deformation observed by campaign and continuous GPS and satellite radar data (InSAR). We invert the geodetic data to find the optimal geometry, location and slip on the main faults, accounting for variation in the elastic parameters of the crust with depth. Our models indicate that most of the slip occurred on two N-S structures spaced ∼5 km apart. From a joint inversion of GPS and InSAR data for variable slip models we find that most of the slip for the first (Ingólfsfjall) event was concentrated at 2-4 km depth with a maximum of 1.9 m, whereas the slip on the second (Kross) fault was located deeper, at 3-6 km depth with up to 1.4 m of motion. The models give similar geodetic moments for the two main events, equivalent to a moment magnitude of M w 5.8 and M w 5.9 for the first and second event, respectively. Our estimated composite moment therefore equals a M w 6.1 for the doublet, smaller than the M w 6.3 estimated from teleseismic data (e.g. NEIC and Harvard).The geodetic data support rupture on two main faults and analysis of high-rate (1 Hz) CGPS data suggests that slip on the second fault initiated within 3 s of the first main shock. Static Coulomb failure stress calculations indicate that the first event caused a stress increase in the area of the main asperity (i.e. at the location of the largest slip patch) on the second fault. However, we cannot rule out dynamic stress triggering due to the short time between the two main events. The 2008 May 29 earthquake doublet appears to be a continuation of the earthquake sequence that started in 2000 June, when two M w 6.5 events struck the eastern and central part of the South Iceland Seismic Zone, in the span of 81 hr. The 2000 June-2008 May sequence has released about half of the moment accumulated by plate motion since the previous earthquake sequence in [1896][1897][1898][1899][1900][1901][1902][1903][1904][1905][1906][1907][1908][1909][1910][1911][1912]. Therefore, continued earthquake activity with moderate size events rupturing N-S faults in the SISZ in the coming decades is likely.

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

“…The only significant change in the horizontal stress orientation related to the 2008 May earthquake doublet was observed in the S-cluster, which is the location of maximum slip based on models of coseismic surface deformation (Hreinsdóttir et al 2009;Decriem et al 2010). All values for S H were consistently found >30 • before and <30 • after the main rupture in the S-cluster.…”

“…The third cluster covers the E-W elongated zone west of the main shock region (EW-cluster). No significant slip was identified in the EW-cluster from geodetic studies (Decriem et al 2010). Aftershock activity on the eastern Ingólfsfjall fault was low compared to the Kross fault.…”

“…The activation of an east-west aligned segment west of the main shock region (Figs 1 and 2) is likely to have been statically triggered by an increase of Coulomb failure stress (Decriem et al 2010). Comparable induced stress changes on adjacent faults have been suggested for a number of earthquakes including the SISZ 2000 earthquakes (Árnadóttir et al 2003) as well as the 1992 Landers M W 7.3 earthquake (Harris & Simpson 1992).…”

“…The 2008 May events occurred during a span of about three seconds, on two adjacent faults 4-5 km apart, close to the Hengill triple junction (Hreinsdóttir et al 2009). Being first reported as a single event with a composite magnitude of M W 6.3 (Global CMT), detailed geodetic studies of the coseismic surface deformation revealed two main shocks with moment magnitudes ranging from M W 5.8 to M W 6 (Hreinsdóttir et al 2009;Decriem et al 2010).…”

“…2). Slip models determined from inversion of geodetic data indicate maximum slip during the main event in the northern part of the S-cluster (Hreinsdóttir et al 2009;Decriem et al 2010). The third cluster covers the E-W elongated zone west of the main shock region (EW-cluster).…”