| | AbstractLocal event
recordings are complex in the sense that relevant P and S
phases vary in an unpredictable manner even between closely spaced
stations; thus, manual analysis of such records is still commonplace.
Our approach to solving this long-standing problem of observational
seismology is to bandpass filter (3 to 6 Hz) to ensure good
signal-to-noise ratio SNR and then form envelopes to ensure simple
signals across a seismograph network. The physical basis is that
Pg and Lg are crustal wave-guide phases reflecting
P- and S-energy propagation. Extensive tests on envelope
analysis of local records from different areas found that arrival times
of the maxima of Pg and Lg envelopes increase very
consistently with distance even in different tectonic regimes, typical
velocities being 6.1 and 3.5 km/sec, respectively. These arrival-time
parameters are easy to extract in a semi-automatic manner and are highly
suitable for local epicenter determinations. Extensive tests on locating
mining explosions were conducted, and on average, the ``envelope''
location errors relative to ``true'' locations were similar to those in
bulletins that are based on conventional phase pickings. Occasionally,
the Pg/Pn envelope may be very weak but can be replaced by
the easily pickable (non-envelope) Pn phase. Additional
advantages with envelope locations are transportability (not overly
sensitive to details of crustal structure), and that envelope amplitudes
can be directly converted to ground motion and magnitudes. For modern
stations, envelopes can be formed in situ with low sampling rates
of 1 to 2 Hz, thus greatly reducing transmission costs.Return to Table of
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