Comparison of Continuously Recorded Seismic Wavefields in Tectonic and Volcanic Environments Based on the Network Covariance Matrix

Extended and sufficiently dense seismic networks capture spatiotemporal properties of the continuously recorded wavefields and can be used to compute the level of their coherence at different frequencies via the analysis of the network covariance matrix, which has been successfully used to study volcanic seismicity. Here, we present an application of the covariance matrix method in a subduction zone environment. We show that most coherent signals identified through the covariance matrix analysis are related to regional earthquakes with the wavefield properties affected by the scattering, which depends on the source location. Tectonic tremors, on the other hand, are not characterized by a high level of coherence. We compare real data results with a set of synthetic tests aimed at mimicking the properties of seismic sources and the main features of wave propagation. We conclude that highly coherent volcanic tremor wavefields could be produced in two ways: by a spatially localized group of monochromatic seismic sources or by a single source located in a highly heterogeneous medium. In both cases, the stability of the source position is a necessary condition to reproduce the observations in volcanoes. On the other hand, the low coherence of tectonic tremor wavefields can be explained by a spatially extended distribution of sources, in agreement with large portions of the subduction interface being nearly simultaneously involved in the episodes of slow deformation. In addition to earthquakes, weak and nearly continuous seismic signals known as "seismic tremors" are observed in the vicinity of active volcanoes and large seismic faults. They are related to slow processes that lead to accumulation of energy released during catastrophic events such as large eruptions and earthquakes. The exact mechanisms generating seismic tremors remain elusive. Possible physical processes include variations of the fluid pressure, slow deformation and rupturing of rocks. In order to better understand possible analogies and differences between seismic tremors occurring in the volcanic and the tectonic environments, we compare observations from the Kyuchevskoy group of volcanos in Kamchatka, Russia and from the Nankai subduction zone in Japan. We apply a mathematical formalism called "network-based covariance matrix analysis" that is based on correlating signals recorded by a network of seismographs. Our results show significant differences between the distribution of sources of volcanic and tectonic tremors. For the former, the sources are found to be very localized in space and stable in time while the latter are generated by a spatially broad and variable in time distribution of sources, in agreement with large portions of the subduction interface being nearly simultaneously involved in the episodes of slow earthquakes. We compare deep volcanic and tectonic tremors continuously recorded by regional-scale seismic networksVolcanic tremor wavefields are characterized by high level of spatial coherence across the networkLow level of coherence of tectonic tremor wavefields can be explained by a spatially extended distribution of their sources