Computation of the Observed Ellipticity


Program objectives

The computation of the observed ellipticity can be performed by using of the Multiple Filtering Technique (MFT). This technique is a very useful technique that allows the computation of a very good ellipticity curve, from the seismic signals recorded in a station with three components (up, north and east). This technique is described with detail by Corchete (2013). You can compute the observed ellipticity by using of this technique by means of the present program, as it will be described below.

Program description

The Ob_ellipticity program and its files, which are needed to run this application, are enclosed into a ZIP file named "Ob_ellipticity.zip". When you have got the ZIP file and you have uncompressed this file, you have four files named Ob_ellipticity.exe, Ob_ellipticity.dat, SZ.dat and SE.dat. The file Ob_ellipticity.exe contains a program (in FORTRAN code for PC) for the computation of the observed ellipticity. This computer program will be used to obtain the observed ellipticity, from the pre-processed traces contained in the files SZ.dat and SE.dat. The file named SZ.dat contains the vertical component of the seismic signal and the file named SE.dat contains the east component of the same signal. The observed ellipticity computed with this program will be contained in the file named ellipticity.dat. The program also generates a data file named SL.dat which contains the radial component of the seismic signal. Figure 1 shows the results of this program. All the program capabilities are controlled by parameters enclosed in the file named Ob_ellipticity.dat. The list of these parameters is as follows:

N, NU, TMIN, TMAX, NT, ALFA, BAZ, ICOMP

The description of all parameters is as follows:

N = Number of the records in the input files SZ.dat and SE.dat (maximum number = 16384).
NU = Number which value is given by the relationship N = 2**NU.
TMIN, TMAX, NT = Period range (in seconds) in which the computations will be performed. TMIN is the minimum and TMAX is the maximum of this interval. The maximum value for NT is 256.
ALFA = Parameter that controls the gaussian filter (Dziewonski et al., 1969).
BAZ = The angle (measured from North and clockwise) between the meridian passing through the station and the geodesic, which connects the epicentre and the station. This angle can be computed by DistAz program.
ICOMP = With value 1 selects the north component of the seismic signal and with other value the east component. If the value given is 1, the input file with the north component of the seismic signal will be contained in the file named SN.dat. If another value is given for ICOMP, the east component will be considered and the file named SE.dat will be used by the program.

Fig. 1 (a) Seismograms corresponding to the radial (L) and vertical (Z) components of the seismic signal, contained in the files SL.dat and SZ.dat, respectively. (b) Observed ellipticity curve obtained by this program and contained in the data file named ellipticity.dat. (c) Rayleigh-wave particle motion composed with the radial (L) and vertical (Z) components of the seismic signal.

References

Corchete V., 2013. A methodology for the analysis of the Rayleigh-wave ellipticity. Earth Sci. Res. J., 17, 127-133.