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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 Leirhnjúkur 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 (Vp) underneath regions of higher topographic relief and two distinct low‐Vp anomalies beneath the Leirhnjúkur 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 Leirhnjúkur geothermalreservoir, we resolved a shallow (−0.5 to 0.5 km below sea level; bsl) region with low‐Vp/Vs values and a deeper (0.5–1.5 km bsl) high‐Vp/Vs 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‐Vp/Vs anomaly underlies these zones, where a superheated steam zone within felsic rock overlies rhyolitic melt.

MTt is a Python module for Bayesian moment tensor source inversion of earthquake seismic data using polarities, amplitudes or amplitude ratios. It can solve for double couple or full moment tensor solutions, taking into account uncertainties in polarities, take-o angles of the rays from the source to the receiver, and amplitudes. It provides an easily accessible and extendable approach to earthquake source inversion which is particularly useful for local and regional events.