Location and mechanism of very long period tremor during the 2008 eruption of Okmok Volcano from interstation arrival times

摘要

We describe continuous, very long period (VLP) tremor that occurred during the 2008 eruption of Okmok Volcano, Alaska. Due to its low frequency content in band from the 0.2--0.4 Hz, the wave field of the VLP tremor is relatively free of path effects. From continuous recordings of the VLP tremor on 2 three-component broadband and 3 single-component short-period instruments, we devise a method to locate the epicenter of the tremor based on interstation arrival times computed with cross correlation. We find the epicenter since the vertical and radial components of the VLP tremor wave field are dominated by Rayleigh waves and the time shifts are related to lateral propagation. Over the 4 h period studied, this procedure yields a location NNW of Cone D, close to the new cone built over the course of the eruption. Similar analysis using the transverse horizontal components from the 2 three-component broadband instruments yields strong constraints on the source mechanism of the VLP tremor. We observe an anomalous interstation arrival time due to the existence of a nodal plane in the Love wave radiation pattern. The orientation of a compensated linear vector dipole (CLVD) source estimated from the transverse components closely aligns with the regional maximum horizontal stress direction. The depth of the CLVD source is constrained by matching the vertical components to the Rayleigh wave radiation pattern at all five stations. We find the VLP tremor source depth to be 2 km BSL, positioned between the magma chamber at Okmok (>3 km BSL) and the surface.