SAC Data Formats

SAC can read data in a variety of formats:

Use the "help" facility in SAC or refer to the online manual for detailed descriptions of these formats.

SAC Binary Format

This is the standard format for most seismic data.  A SAC binary file consists of a header of ascii data that defines the following binary seismic data.  The data may consist of equally incremented or irregularly spaced time series data, or complex spectral data.

SAC header:

The header portion of the file is very important to the datafile since it contains information on sampling interval, station location, event location, ground motion components, and all other parameters important in earthquake data processing.  These parameters are used by the various SAC commands to manipulate the data.

For example, here is a listing of the header variables for NS waveform data recorded at MLNT from the recent Shelby Co. microearthquake:
 
 
Commands/Results
 Explanation
SAC> r MLNT.EGN.NM.1.sac
SAC> lh
 

  FILE: MLNT.EGN.NM.1.sac
 -----------------------

        NPTS = 18100
           B = 0.0000000e+00
           E = 1.8098999e+02
      IFTYPE = TIME SERIES FILE
       LEVEN = TRUE
       DELTA = 9.9999998e-03
      DEPMIN = -4.1800000e+02
      DEPMAX = 6.6300000e+02
      DEPMEN = 1.0872928e+00
     OMARKER = 0
      KZDATE = JAN 31 (031), 2001
      KZTIME = 01:22:59.370
      IZTYPE = BEGIN TIME
       KINST = NM
       KSTNM = MLNT
       CMPAZ = 0.0000000e+00
      CMPINC = 9.0000000e+01
        STLA = 3.5334000e+01
        STLO = -8.9830002e+01
        EVLA = 3.5228001e+01
Waiting

        EVLO = -8.9998001e+01
        DIST = 1.9284590e+01
          AZ = 5.2375816e+01
         BAZ = 2.3247301e+02
       GCARC = 1.7342965e-01
      LOVROK = TRUE
SAC>

 Read the data file
list the header using "lh"
 

file name
 

number of points in the file
begin time
ending time
type of file
evenly sampled time series
time increment
minimum amplitude in file
maximum amplitude in file
mean amplitude of file
origin marker time
date (Julian day)
UT reference
reference time equivalence
generic name of instrument
station name
component azimuth relative to north
component "incidence angle" reletive to the vertical
station latitude
station longitude
event latitude
 

event longitude
station distance from event in km
station azimuth from event in degrees
event back azimuth from station in degrees
great circle arc length in degrees
parameter enabling the file to be modified
 
 


Some of the information in the SAC header is computed automatically once the appropriate header variables have been defined.  For example, "DIST", "AZ", "BAZ", and "GCARC" are computed once the station and event locations are defined by their geographical coordinates.

Other information is essential for particular SAC commands to work.  For example "CMPAZ" and "CMPINC" must be defined to use the "rotate" command to rotate horizontal seismic data into tangential and radial motions or to compute a particle motion plot using "ppm".

Once data are manuipulated by SAC, the result can be written to disk, in SAC binary format, using the "write" ("w") command.

SAC> r file1
SAC> hp bu co 1 n 2 p 1
SAC> w file2

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last modified 3/05/01