ALBERT

Description: Since the end of 2013, the region around the two volcanoes Masaya and Momotombo, which includes the Nicaraguan capital Managua, has shown an unusually high seismic and volcanic activity. In December 2015, the Momotombo volcano erupted after 110 years of quiescence. Since mid-December 2015, the Masaya volcano has also shown gradually increasing activity, including the formation of a lava lake in its main crater. By adding 30 broadband stations, we had temporarily (December 2016–March 2017)densified the permanent Nicaraguan seismic network around these volcanoes to study the local seismicity and image the subsurface structure. During the observation period, we observed an overall low level of seismicity. Recorded events around Momotombo likely consist of aftershocks of the M5.5 earthquake that struck this area on September, 26th, 2016. At Masaya, we did not observe volcano-tectonic events. Using the continuous waveform recordings, we perform a 3D ambient seismic noise tomography that reveals a first image of the subsurface velocity structure below the Masaya and Momotombo volcanoes. While Momotombo shows a typical elongated low shear-wave velocity anomaly that reaches depths of about 8 km, Masaya does not show indications of a deep plumbing system. At Masaya, we have indications of a shallow (0–3 km)magmatic chamber, offset to the west and not directly below the active Santiago vent, where the crater lake is located At greater depth (3–8 km)a low velocity anomaly towards the northeast coincides in location with a modelled positive gravity anomaly and could indicate the presence of a former intrusive body. With this study we want to trigger further interest in the diverse tectonic and volcanic features of Nicaragua. Future, long-term seismic imaging and monitoring projects are of critical interest for the estimation of seismic and volcanic risks in Managua and the surroundings.