3D multi-step travel time tomography: Imaging the local, deep velocity structure of Rabaul volcano, Papua New Guinea

In this paper, we put forward an alternative approach to combine the travel times from regional events with local earthquakes (including explosions) to reconstruct the deep velocity structure, but still on a local crustal scale. A synthetic example is first used to explain the multi-step seismic tomographic procedure before progressing to recover the 3D velocity structure beneath Rabaul volcano, Papua New Guinea, down to a depth 20 km. The main velocity structures at shallow depth range are similar to those reported previously [Finlayson, D.M, Gudmunsson, O., Itikarai, I., Nishimura, S., Shimamura, H., 2003. Rabaul volcano, Papua New Guinea: Seismic tomographic imaging of an active caldera. J. Volc. Geotherm. Res. 124, 153–171] based on both wide-angle seismic profiling using explosion data and 3D tomography using only local earthquake information (including the explosions). But we have found the bottom boundary of the interpreted shallow magma chamber (under Rabaul caldera complex) to be 4 km depth, and not 6 km depth as previously reported. The other difference in this shallow depth range is that an additional low velocity zone was delineated beneath Tavui—another caldera near Rabaul volcano. In the medium depth range, we obtained a similar V-shaped low velocity zone under Rabaul volcano to that found by Finlayson et al. (Finlayson, D.M, Gudmunsson, O., Itikarai, I., Nishimura, S., Shimamura, H., 2003. Rabaul volcano, Papua New Guinea: Seismic tomographic imaging of an active caldera. J. Volc. Geotherm. Res. 124, 153–171), but this extends to 12 km depth in our study compared to the previous determination of 9 km depth. In the deeper range, below 10 km, the dominant seismic structure is a cylindrical-shaped low velocity zone in the depth range (from 12 to18 km) beneath Rabaul volcano. This deep, low velocity zone appears to be linked to the shallow interpreted magma chamber by a low velocity funnel feature, which suggests the possible existence of deep magma storage.