Crustal deformation from geodetic techniques: Earthquakes and plate movements in the South Iceland Seismic Zone

Abstract

The South Iceland Seismic Zone (SISZ) is one of the most seismically active area in Iceland. The last two major events occured in the central and western parts of the SISZ in June 2000 and May 2008, respectively. In this thesis, I use geodetic methods to estimate the deformation due to earthquakes and plate spreading and derive models of the crustal processes. In the first paper we derive a uniform-slip and a variable-slip fault models for the two 29 May 2008 earthquakes based on GPS and InSAR observations.We account for the variation in the elastic parameters of the crust with depth. Our models indicate that the slip for the first event (Ingolfsfjall) was concentrated at 2-4 km depth, with a maximum of 1.9 m, whereas the slip on the second fault (Kross) was located deeper, at 3-6 km depth with up to 1.4 m of motion. Static Coulomb failure stress calculations indicate that the first event caused a stress increase in the area of the main asperity on the second fault. We estimated a composite moment that equals a Mw 6.1 for the doublet. We also find that the June 2000 - May 2008 sequence only released about half of the moment accumulated by plate motion since the previous earthquake sequence in the SISZ during 1896-1912. In the second paper we document the transient following the June 2000 earthquakes, extending a previous study with GPS data from 2004 to 2008. We explore whether viscoelastic models based on Maxwell and/or standard linear solid (SLS) rheologies can reproduce the surface velocities observed during 2000-2008 and calculate the corresponding stress changes. Our preferred model consists of a ~15 km thick elastic crust overlying a SLS upper mantle with a viscosity of 1-2*10E18 and relaxation strength between 0.1-0.25. The viscoelastic transient promotes failure in the area of the May 2008 earthquake. However, the Coulomb failure stress changes due to the viscoelastic relaxation are very small (0.05 MPa) compared to the static stress changes from the June 2000 main shocks. Viscoelastic loading may therefore not be the main trigger for the May 2008 events. In the third paper we study the deformation following the May 2008 earthquakes using GPS and InSAR observations. We test poroelastic, viscoelastic and afterslip models in order to determine the dominent mechanism driving the transient observed. Our best model involves viscoelastic relaxation and shallow afterslip to explain both the far field and near field displacements. We also propose that an impermeable structure disrupts fluid flow and causes pore pressure changes that could be an explanation for a sharp EW lineament highlighted in some interferograms in an area where a large number of aftershocks occured, west of the two mainshocks.This PhD was funded by grants from the Icelandic Research Council for the projects "High-rate continuous GPS observations in Iceland" (Grant of Excellence, nr. 060243013), and Recent earthquake sequences in the South Iceland Seismic Zone" (project grant nr. 090237021)." GPS fieldwork was funded by the Icelandic Research Fund, as well as the Icelandic Research Council, and the Nordic Volcanological Center. During the June 2008 fieldwork additional GPS equipment was supplied by UNAVCO and assistance by the National Land Survey of Iceland, the Icelandic Road Authority (Vegagerdin), and the National Power Company (Landsvirkjun). New CGPS stations were funded by the Icelandic Research Fund, United State National Science Foundation (grants number EAR-0711446 and EAR-0711456 ), the University of Arizona, Pennsylvania State University