Lithospheric studies based on array analysis of P-coda and microseisms

A simple coherence measure—the semblance coefficient—has been used to sample the lithosphere beneath the NORSAR array in terms of its ability to produce coherent seismic energy. The data set is comprised of two samples of teleseismic P-coda waves (south of Honshu and Hindu-Kush events) and two samples of microseismic noise data with duration of about one hour each.Concerning the constitution of the earth structure there are two possibilities for coherent energy to be present in noise-like wavefields. The first is tied to scattering effects and the second may be brought about by seismic emission effects. The emissive component represents the seismic response of the medium to variations in the background stress field which may arise from various sources: passing seismic waves (body or surface), free oscillations of the Earth, tides, an abnormal level of heat flow, or geodynamic processes in general.The presence of a well known source of seismic signal—Hunderfossen dam—in the vicinity of NORSAR area provided an opportunity to test the data processing algorithm. Results obtained for two independent microseismic data samples point to the same location in close vicinity to the dam.For P-coda data the signal energy from the upper mantle areas, as indicated by the maximum of semblance measure values at the depth around 116 km, reaches about 3.8% and 5% of the total P-coda energy for Honshu and Hindu-Kush events, respectively. For microseismic noise data this value is about 1% at a depth around 100 km. Comparison with the results of holographic studies supports the assumption that, in the case of microseismic data, the detected signal energy is of emissive nature, while for P-coda data we need more coda samples to make a sound judgement about the nature (scattering or emission) of the detected signals.