Lithospheric studies based on array analysis of P-coda and microseisms
Tectonophysics, 140 (1):
103--113 (Aug 15, 1987)DOI: 10.1016/0040-1951(87)90143-0
Abstract
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.