SUMMARY
Applies an automatic event picking algorithm.
SYNTAX
APK {param v {param v} ... },{VALIDATION ON|OFF}
INPUT
param v: Define a new value for one of the pick parameters. param: C1|C2|C3|C4|C5|C6|C7|C8|D5|D8|D9|I3|I4|I6. These parameters are defined below. VALIDATION ON: Turn validation phase on. VALIDATION OFF: Turn validation phase off.
DEFAULT VALUES
APK C1 0.985 C2 3.0 C3 0.6 C4 0.03 C5 5.0 C6 0.0039 C7 100. C8 -0.1
D5 2. D8 3. D9 1. I3 3 I4 40 I6 3 VALIDATION ON
DESCRIPTION
The algorithm used in this automatic picker was originally obtained from the USGS in Menlo Park and is based upon work by Rex Allen (see reference below.) The detection of a pick is based upon abrupt changes in the ratio of a short term and long term running average of the signal. Once detected, the pick is subjected to an optional validation phase which attempts to distinguish a true event from cultural noise. Once validated, the pick is further evaluated to determine other characteristics of the event. Currently this is limited to its duration. Other features such as maximum amplitude, period, and decay rate may be added as required. Most of the parameters in this command need never be changed. They are available if the user wishes to fine tune the algorithm. Most of these parameters have the same meaning here as they do in the referenced article.
1. C1 is the constant used in the recursive high pass filter that is applied to remove any D.C. bias.
2. C2 is the constant used to vary the weight assigned to the amplitude and first difference in the characteristic function.
3. C3 is the timing constant, used to compute the short term average of the characteristic function.
4. C4 is the timing costant used to compute the long term average of the characteristic function.
5. C5 is the constant used to compute the threshold reference level. A potential event is declared when the short term average becomes larger than C5 times the long term average.
6. C6 is the timing constant used to compute the running mean absolute value of the filtered data.
7. A station is assumed to be dead when the absolute value of the characteristic function is greater than C7.
8. C8 is used to determine the signal termination level. The signal is terminated when its absolute value falls below this level for D8 seconds. There are currently two different algorithms in use so C8 has two different interpretations. If C8 is positive, then the termination level is C8 times the running mean absolute value of the signal just before the event was declared. This method is useful if the background level at a station is large. If C8 is negative, then the termination level is the absolute value of C8. This will give more consistent terminations from station to station if the noise level is well below this termination level.
9. D5 is the minimum duration in seconds for an event to be declared valid.
10. D9 is the duration in seconds used to initialize the long term average of the characteristic function.
11. I3, I4, and I6 are integer constants used during the validation phase and should not be changed.
HEADER CHANGES
The time of the pick is stored into A; the quality and sense of motion is stored into KA; the end of the event is stored into F.
ERROR MESSAGES
- 1301: No data files read in.
- 1306: Illegal operation on unevenly spaced file
- 1307: Illegal operation on spectral file
WARNING MESSAGES
- 1910: No valid pick found for the following file(s):
SEE COMMANDS
Rex V. Allen, Automatic Earthquake Recognition and Timing from Single Traces, BSSA, Vol. 68, No. 5, Oct. 1978.
LATEST REVISION
May 15, 1987 (Version 10.2)