PSN-L Email List Message

Subject: event detection
From: sean@...........
Date: Sat, 5 Aug 2000 14:45:21 -0500 (CDT)


I have been trying to finness event detectors for years, and of course
have never found one that can sort out earth-related from man-made
events, only "something" from "not something". 

The most common is the "LTA-STA" process that can be implemented either
by analog filters or digital means. Basically, a short increase of short
period average energy is detected above a running long term average.
The STA is long enough to avoid spikes, like from radio telemetry.
Some digital detectors manage to use running FFTs before determining
the mean amplitudes. The detector parameters must be "tuned" to the
local noise situation, ideally not missing any events nor filling
the event file with junk. Of course, local and teleseismic detectors
need different parameters. I don't know where some code examples could
be found, but I would snoop around some major university seismo sites.

And of course, all the methods involve a pre-event memory generally 
long enough to catch the P-wave of a local event that triggers on the
S-wave. In the '70s, I used multi-channel analog tape recorders to
record the multiplexed FM telemetry carriers on a tape loop to provide 
the delay; the loop output was written to the event tape (still telemetry 
carriers) when the LTA-STA detector triggered. The tape loop had to be
replaced daily, and the splice in it often made glitches in the data.
Obviously digital delays are easier to do, as long as time tagging is
preserved in the delayed or pre-event data.

The most competent detector I am familiar with is the one used in the
IRIS broadband stations. It was developed by Albuquerque, and is called
the MHH detector, or Murdock-Hutt-Halbert. It is quite elaborate, and
is set up or "built" by 11 parameters specified in the configuration file
for the station, with separate detectors for data of different sample
rates (derived by decimation and FIR filters from a single instrument).
Each MHH detector selects a particular IIR filter at its input.
It has proven quite robust in not missing events, and reports 
detection quality info with each detection along with the period and
amplitude of the event and the background average counts.

I believe that the code is available in C, and might be found 
on the ASL web site ( . The IRIS stations run OS-9 
in 68020/30 multitasking processors, so I don't know if it is PC 
compatible or if a stand-alone version is available. 


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Larry Cochrane <cochrane@..............>