Local Earthquake Tomography in Tjörnes Fracture Zone (North Iceland)

Abstract:

The Tjörnes Fracture Zone (TFZ) is a transform zone connecting the Kolbeinsey Ridge with North Iceland. The main transform motion takes place on the Húsavík-Flatey Fault (HFF). The zone contains two additional seismic lineaments: one named after Grímsey to the north of the HFF and another named after Dalvík to the south of the HFF. Most of the zone is offshore, which limits the historical information and geological observations. Between June and September of 2004, a temporary seismic network was operated offshore on the insular shelf of Iceland and onshore in North Iceland, during the project North ICeland Experiment (NICE). Data from this seismic array and data from the permanent Icelandic Network (SIL) have been used to constrain the sub-surface structure using local earthquake tomography. Comparison between results including and without NICE data shows the capability of this denser network to illuminate the upper-most crust. Although the microseismicity registered by the NICE network in a short period is dominated by low-magnitude, shallow events, SIL data recorded for more than ten years are useful to get information about the deeper crust. The results from the tomography reveal a transition from anomalously thick Icelandic crust to a more typical oceanic crust to the north of Grímsey. Major velocity anomalies are found along the Grímsey lineament and a low-velocity anomaly beneath Flatey Island along the HFF. The clustering of refined earthquake locations is enhanced. The seismicity is concentrated along the Grímsey Lineament, the Húsavík-Flatey Fault and a possible transversal lineament between them. Seismicity associated with the Grímsey lineament varies in depth down to 15 km.