# Earthworm Modules:
Pick_ew Overview

(last revised 26 May 2006)

This is the Earthworm picker. Pick_ew gets real-time data in the form of demultiplexed Earthworm waveform messages (TYPE_TRACEBUF messages). The program picks each channel independently, and produces pick messages, which are sent to an Earthworm transport ring. Pick messages are produced within two seconds of the pick time. Pick_ew also measures the length of the waveform coda following a pick and coda-length messages are also sent to a transport ring. Coda lengths are used by the "eqproc" program to assign a coda magnitude to an event. Pick_ew makes two types of coda measurements: The picker reports a "normal" coda if the amplitude decays to "cutoff" in less than 144 seconds after the P-wave arrival. If the coda does not decay to "cutoff" within 144 seconds, the picker times out, reporting the observed amplitude and a "truncated" coda duration of 144 seconds. For traces with high pre-event noise levels, the picker reports a negative or "noisy" coda duration. In addition to the duration, the picker reports the average absolute amplitudes of up to six pre-determined 2-second trace windows. Pick_ew does not start searching for new picks until the coda calculation is complete.

The pick is released by pick_ew within 3 sec (or whatever the MinCodaTerm value is, most default to 3 seconds), but it doesn't go back to looking for new picks until it reaches the coda termination point. Pick_ew cannot/will not produce any new picks between the pick time and the coda termination time for the given channel that is in coda determination mode.
If the coda termination is set to a very small value, that means the measured coda duration will be longer, and if a new event happens to occur to boost the signal again during this time, pick_ew assumes the energy belongs to the original pick and the coda gets extended. The longest that pick_ew will sit waiting for the signal to reach the coda termination value is 144 sec, after which the picker releases the coda message with 6 time/amplitude pairs and leaves it to eqcoda to extrapolate the duration time.

If the data has time gaps, pick_ew can be set up, using configuration parameters, to either interpolate through the gaps or restart itself. For time gaps up to several samples long, it is probably best to interpolate. For longer gaps, it is better to restart. The drawback to interpolating is that the data is somewhat corrupted. The drawback to restarting is that the picker will not start picking again for about one second after a restart.

Picks times are calculated using an algorithm developed by Rex Allen, in the late 1970's and early 1980's. The algorithm works well for short-period data (frequencies > 1 Hz) and has been tested extensively using data from the Northern California Seismic Network. The program tends to miss picks from teleseismic and volcanic events. S waves are occasionally picked, but most of the picks are first-arrival P waves. If the program is used in regions other than Northern California, pick parameters may need to be modified, or another algorithm may be needed. For more information about the picking algorithm used by pick_ew, see the following articles:

Allen, R.V., Automatic Earthquake Recognition and Timing From Single Traces, Bull. Seism. Soc. Am. 68, Oct 1978, pp. 1521-1532.

Allen, R.V., Automatic Phase Pickers: Their Present Use and Future Prospects, Bull. Seism. Soc. Am. 72, Oct 1982, pp. S225-S242.

In addition, Jim Pechmann of the University of Utah has run extensive tests of the earthworm picker. He has written up some suggested picker parameter changes for use with analog and broadband data streams.

Module Index | Pick_ew Commands