Seismic imaging of the shallow crust beneath the Krafla central volcano, NE Iceland

We studied the seismic velocity structure beneath the Krafla central volcano, NE Iceland, by performing 3-D tomographic inversions of 1453 earthquakes recorded by a temporary local seismic network between 2009 and 2012. The seismicity is concentrated primarily around the Leirhnjukur geothermal field near the center of the Krafla caldera. To obtain robust velocity models, we incorporated active seismic data from previous surveys. The Krafla central volcano has a relatively complex velocity structure with higher P wave velocities (V-p) underneath regions of higher topographic relief and two distinct low-V-p anomalies beneath the Leirhnjukur geothermal field. The latter match well with two attenuating bodies inferred from S wave shadows during the Krafla rifting episode of 1974-1985. Within the Leirhnjukur geothermalreservoir, we resolved a shallow (-0.5 to 0.5km below sea level; bsl) region with low-V-p/V-s values and a deeper (0.5-1.5km bsl) high-V-p/V-s zone. We interpret the difference in the velocity ratios of the two zones to be caused by higher rock porosities and crack densities in the shallow region and lower porosities and crack densities in the deeper region. A strong low-V-p/V-s anomaly underlies these zones, where a superheated steam zone within felsic rock overlies rhyolitic melt.