Earthquakes, stress, and strain along an obliquely divergent plate boundary: Reykjanes Peninsula, southwest Iceland

Abstract

[1] We investigate the seismicity and the state of stress along the obliquely divergent Reykjanes Peninsula plate boundary and compare the directions of stress from inversion of earthquake focal mechanisms with the directions of strain rate from GPS data. The seismicity on the peninsula since early instrumental recordings in 1926 shows a systematic change from primarily earthquake swarms in the west to main shock-aftershock sequences in the east. The largest earthquakes on the Reykjanes Peninsula typically occur by right-lateral slip on N-S faults and reach magnitude 6 on the eastern part of the peninsula. During 1997-2006 most earthquakes on the Reykjanes Peninsula were located in two areas, Fagradalsfjall and Krísuvík on the central part of the peninsula, as recorded by the South Iceland Lowland (SIL) seismic network. The state of stress estimated by inversion of microearthquake focal mechanisms from the SIL catalogue is mainly oblique strike slip, with a tendency toward a normal stress state. Mapping the directions of the least compressive horizontal stress (S hmin ) shows an average S hmin direction of N(120±6)°E and a remarkable agreement with the directions of greatest extensional strain rate ( _ Hmax ) derived from GPS velocities during [2000][2001][2002][2003][2004][2005][2006]. The agreement between the directions of stress at depth and strain rate observed at the surface indicate that the earthquakes are primarily driven by plate motion.Citation: Keiding, M., B. Lund, and T. Á rnadóttir (2009), Earthquakes, stress, and strain along an obliquely divergent plate boundary: Reykjanes Peninsula, southwest Iceland,

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

“…A maximum horizontal stress direction of N30 • E ± 6 • observed for both the Reykjanes peninsula (Keiding et al 2009) and theÖlfus region (Lund & Slunga 1999), is consistent with our findings for the S-and EW-clusters. The average 54 ± 6 • found for the northern cluster are within the range of S H directions found for the whole SISZ (Stefánsson et al 1993).…”

“…Pre-main shock earthquakes in the N-cluster are located at shallow depths of around 5 km. This fits observations on the adjacent Reykjanes peninsula, where seismic activity occurs in comparable local normal and strike-slip faulting regimes (Einarsson 1991;Keiding et al 2009). In contrast, earthquakes in the S-and EW-clusters mainly occur between 4 and 8 km depth with almost pure strike-slip faults.…”

“…The strike directions of maximum and minimum stress axes, σ 1 and σ 3 , determined by the two different inversion algorithms differ by 4-5 • , and are within the uncertainties of each other. The minimum horizontal stress direction using the Lund & Slunga (1999) inversion method (N122 ± 8 • E) is comparable to the average minimum stress direction in theÖlfus area (N120 • E, Lund & Slunga 1999) and on the Reykjanes peninsula west ofÖlfus (N120 ± 6 • E), determined by Keiding et al (2009). The overall spreading direction across south Iceland predicted by the MORVEL plate motion model is ∼101 • (DeMets et al 2010), about 20 • off the minimum stress direction observed in the target region in this and previous studies in South Iceland.…”

Reliability of Coulomb stress changes inferred from correlated uncertainties of finite‐fault source models

“…These values are similar to those used in other studies on Iceland [e.g., Árnadóttir et al , 2003]. We then resolve the calculated stress changes onto hypothetical vertical North–south oriented right‐lateral strike‐slip receiver faults according to the prevailing orientation of faults and style of faulting on the Reykjanes region [ Árnadóttir et al , 2004; Keiding et al , 2009]. We limit the orientation of the receiver faults to one set of strike, dip and rake values as we intend to analyze only the influence of the source model uncertainties.…”

Correlation between tectonic stress regimes and methane seepage on the west-Svalbard margin

“…Here, we focus on the tectonic stress due to ridge push. We use the 108 approach by Keiding et al (2009) based on the analytical solutions derived by Okada (1985), to model the plate 109 motion and tectonic stress field due to spreading along the Molloy and Knipovich Ridges. Because the model only 110 incorporates plate spreading, it is likely that the actual stress field on the west-Svalbard margin differs to some 111 extent from the stress field predicted by our model.…”