37. Explosion filtering, EXFILTER

The program EXFILTER is used to identify probable explosions in a catalog of seismic events. Man-made seismic events like quarry blasts, mining explosions and other explosions show a certain distribution in time and space. Therefore the method of explosion identification here is based on normalizing the time of day distribution of seismic event occurrence as a function of area. The program works on the following principle: Areas where explosions occur are defined. If an event is located in one of these areas, with a magnitude below a given maximum magnitude, with a depth less than a given maximum depth, within a given time of day interval and within a given year interval, it is identified and marked as probable explosion. The areas are defined by polygons of any shape. For definition of the filter areas, a list of mine locations (with consideration of location accuracy), locations of explosions and locations of event clusters (they might be clearly related to mine locations, but others might indicate unknown explosion sites) can be used. The next step is to define the parameters for each area to get a normal time of day distribution. They can be determined following the steps:

1)	- get the time of day distribution of events (program CATSTAT)  - select a time window of probable explosions  - select events within time window of probable explosions
2)	- get the distribution of magnitudes of events within time window of probable explosions (program BVALUE)  - select the maximum magnitude
3)	- test parameters defined with program EXFILTER for the defined area and adjust the parameters if the time of day distribution is not normal.

For more details, see Ottemöller (1995).

The program uses a parameter file, EXFILTER.PAR which MUST be located in the DAT directory.

An example of the parameter file EXFILTER.PAR

--------------Parameter file for program EXFILTER-----------------
------------------------------------------------------------------
        This file must follow the following format rules: __________
1. Any number of comment lines
2. Any line with first character # defines the parameters of that area
3. Any line with first character * defines the parameters of exceptions within
   an already defined area. I.e. you can define an area around a volcano,
   and make an exception for that very small  area.
4. In the first line of each parameter set the parameters are:
    Maxdepth: Events with depth above this value are not explosions
    Maxmag  : Events with magnitude above this value are not explosions
    Lhour   : Lower limit of time interval for explosion time 
    Hhour   : Upper limit of time interval for explosion time
    Btime   : Lower limit of yearly interval
    Etime   : Upper limit of yearly interval
    N       : Number of latitude longitude pairs in polygon
5. From the second line the lat long pairs are given
------------------------------------------------------------------
 Area------------Maxdepth Maxmag   Lhour   Hhour   Btime   Etime       N         

# area 1           100.0     3.6      11      15  197001  200012      12
                   64.0    23.8    66.70   23.80   68.80   20.20   69.80    18.30
                   70.50   20.0    71.20   26.00   70.80   29.50   70.00    27.50
                   68.50   28.50   67.00   29.50   64.30   29.50   64.00    34.00

# area 2           100.0     3.8      15      20  197001  200012       4
                   67.90   17.80   68.80   20.20   66.70   23.80   66.50    21.00

# area 3           100.0     3.5       0       1  197001  200012       4
                   67.90   17.80   68.80   20.20   66.70   23.80   66.50    21.00

# area 4           100.0     4.0       2      17  197001  200012       4
                   68.50   28.50   68.50   36.00   67.00   36.00   67.00    29.50

The EXFILTER program searches for probable explosions using a catalog-file as input and marks events that might be explosions with 'P' as Event ID in the output file exfilter.out. Example of program run

<exfilter> 

NUMBER OF AREAS: 55 

 FILENAME... ? 
june.cat
 ************************************
 Number of probable explosions found: 90
 Output written in file: exfilter.out
 ************************************

Figure 37.1: Figures that show how the exfilter works for events in Scandinavia:
The top figure shows the filter areas used for Scandinavia. The bottom right figure shows the time of day distribution for a 10 year Scandinavian catalog before filtering (made with CATSTAT) and the figure bottom left shows the distribution after filtering.