Temporal Seismic Velocity Changes Associated with the Mw 6.1, 2008 Ölfus Earthquake Doublet, South Iceland, Using Ambient Noise

Our study presents temporal seismic velocity changes (dv/v) associated with the May 2008 Ölfus earthquakes by computing the cross-correlations of ambient noise. The 2008 Ölfus doublet (M5.8 and M5.9, with a composite magnitude of M6.1) occurred in the South Iceland Seismic Zone, which is a highly active transform zone that accommodates plate motion with major earthquake sequences (e.g., 1896, 1912, 2000). We investigate co-seismic and post-seismic response of the crust in the epicentral area, to the 2008 Ölfus doublet. For our analysis, we used three-component continuous data from three stations of the SIL national seismic network operated by the Icelandic Meteorological Office. Using the MSNoise software package (http://www.msnoise.org), we calculated single station ambient noise cross-correlations and utilized the stretching approach to quantify relative seismic velocity variations. We found the highest co-seismic velocity decrease (<1 percent) in the high-frequency band (1-3 Hz) at a seismic station located 10 km from the rupture zone. The co-seismic dv/v drop is also observed at stations 35 km away from the earthquake epicenter, though the amplitude of the variation is less, at 0.5 percent. We identify three months of post-seismic period in both the high-frequency and low-frequency calculations, indicating the recovery process at different crustal depths after the mainshocks. We compare our dv/v time series to continuous GPS observations, local seismicity, and volumetric stress changes. Our analysis suggests that the velocity changes are mainly controlled by shaking-induced damage. Our findings provide considerable insights into the time-dependent seismic velocity changes caused by the 2008 Ölfus events. This work is supported by the IS-NOISE project (https://is-noise.earth/) and the Icelandic Research Fund, Rannis (https://www.rannis.is/).