Network based evaluation of the infrasonic source location at sakurajima volcano, japan

An important step in advancing the science and application of volcano infrasound is improved source location and characterization. Semblance is a commonly used technique for determining both seismic and infrasound source locations at volcanoes. The method uses a forward grid search in which each unique sensor pair is time-shifted for each grid node, and then cross-correlated to determine which time-shift and respective node provides the best fit. This work evaluated the infrasonic source location at Sakurajima Volcano, Japan using the semblance method on data collected in July 2013. The activity at Sakurajima Volcano consists of strombolian to vulcanian style explosions. Five sensors were deployed in a network configuration around Sakurajima with vent to sensor distances ranging from 2.3 to 6.2 km and a maximum vertical relief across the network of 356 m. All sensors were deployed at elevations below that of the active vent. We investigated semblance in 2D and 3D to assess the necessity of considering 3D sensor and vent locations. Semblance-derived infrasonic source locations showed a clear offset of ~420 m from the actual vent. To determine the cause of the source location offset we evaluated semblance in multiple frequency bands, as diffraction around topography should be more severe at high frequencies. We also incorporated influences on sound speed such as diurnal temperature variations and wind speed, and tracked changes in the travel time associated with peak semblance over the dataset period. The source location offset is likely the result of significant local topography as the volcanic edifice lies between the vent and the sensor for two stations. This is evidenced by differences in frequency content across the network and consistent travel-time differences. Semblance can also be used to track and characterize volcanic activity, as periods of high semblance correspond to the eruption of ash and gas. Further details of this work can be found in McKee et al. [2014] or the poster below, which was presented at Cities on Volcanoes 8, Yogyakarta, Indonesia.