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Local Earthquake Tomography in the Tjörnes Fracture Zone (North Iceland)

Local earthquake tomography has been carried out in the Tjörnes Fracture Zone. This transform region connects the Mid‐Atlantic Ridge to the Northern Volcanic Zone in Iceland in a mostly offshore area. The challenge to record seismic information in this area was the motivation for the North ICeland Experiment (NICE). Fourteen ocean‐bottom seismometers and eleven on‐land stations were installed in the project and operated simultaneously with the permanent Icelandic seismic network (SIL) during summer 2004. Data from the experiment were used to estimate P‐ and S‐wave crustal velocities. Also, the gravity anomaly was derived for comparison with the tomographic results. Upper‐crustal velocities are found to be relatively low in the offshore region. In particular, low velocities are mapped along the Húsavík‐Flatey Fault, where a more confined negative gravity anomaly and a sedimentary basin are found. Low velocities are also mapped along the Grímsey Oblique Rift and in a zone connecting these two main lineaments. The northern half of the aseismic Grímsey Shoal appears as a fast anomaly. Furthermore, localized high‐velocity anomalies are found beneath northern Tröllaskagi and Flateyjarskagi Peninsulas, where bedrock dates from Upper and Middle Miocene (10–15 Ma). Regions of low Vp/Vs ratios are mapped at depth along the main lineaments. Low velocities along the lineaments are interpreted as due to fracturing extending into the middle crust, while high velocities in the upper crust beneath Tertiary formations are associated with relic volcanoes. Low Vp/Vs variations along the lineaments are interpreted as due to the presence of supercritical fluids.