Harvard University routinely estimates the faulting parameters and seismic moments of large earthquakes. They provide the information on these earthquakes in a freely distributed catalog (an ascii file). The CMT solutions are also published four times each year in Physics of Planetary Interiors (you should cite papers from the journal if you use the catalog). These quarterly reports also often contain information on changes in the procedure such as incorporation of new earth models, etc.
The catalog begins with earthquakes from 1977 and is complete to a moment magnitude (Mw) near 5.3-5.5 (See figure 9.30 in Lay and Wallace, 1996). For shallow earthquakes, the depth uncertainty can be on the order of 15 km, since they use only the longer period waves. If they need to fix the depth, they use a value of 15.00 km. The duration in the CMT is scaled with the moment, so the durations are not directly dependent on the observations of any one event.
Executing cmtsearch
We have a simple program called "cmtsearch" to search the CMT catalog. The executable is ~ammon/bin/cmtsearch. The paths to the catalog are "hard-wired" into the source code.
Here is an example of a single execution of the program.The input from the user is in blue.
mantle.eas.slu.edu:52% ~ammon/bin/cmtsearch Output to a file (f) or to the screen (s) f Enter the name of the output file: g.out ******************************************************************** Perform a geographic Search? (1-> yes, 0 -> no) 1 Search a geographic BOX (B) or about a REFERENCE POINT (P) ? b Enter the BOX Bounds; WESN -96,-88,12,18 Search by date? (1-> yes, 0 -> no) 0 Search by depth? (1-> yes, 0 -> no) 0 Search by event size? (1-> yes, 0 -> no) 0 Enter (1) for short output format (2) for long format. 2 ******************************************************************** Writing to file: g.out Examined: 13520 records, accepted: 196.
The catalog is ascii, so searchers can take a minute or so. Two output formats are available, a long and short version. The short version is a simple text listing of the information about the quake. The long version has a p-wave focal mechanism listed for each event. Here's a sample of the long format (the first entry in the file produced in the example above, "g.out"):
*** *** *** B040277B CHIAPAS, MEXICO 04/02/77 20:39:06.2 MLI Location: 16.880N -92.790E Depth: 249 km mb: 5.3 Ms: 0 HRV Location: 16.560N -93.120E Depth: 210 km Moment: 2.340e+17 Nm (Mw = 5.5) epsilon: 0.082 Best Double Couple Moment Tensor Strike Dip Rake Radial South East Plane 1: 93 48 152 1.130 -0.060 -1.280 Plane 2: 203 69 46 -1.260 -1.560 0.130 Used 14 body wavetrains (5 sta) & 0 surface wavetrains (0 sta) Standard Errors: tshift: 0.800 lat: 0.070 lon: 0.070 depth: 4.300 Mrr Mss Mee Mrs Mre Mse 0.120 0.220 0.210 0.130 0.150 0.190 -------- ---------##### ----------######## ---------########### --------############ --------############## #------##############- ###-###############- ###############----- ######------------ ###----------- --------
Generating a plot file for GMT. The text output of the search is fine for such tasks as preparing a data request, but more often you will want to plot the information. To strip the event locations, strike, dip, rake, and magnitude information from the cmtsearch output file, you can use the following nawk script.
{
if($2 == "Location:")
{
sub(/N/ , " " )
sub(/E D/ , " D" )
lat = $3;
lon = $4;
if(lon < 0) lon += 360;
getline;
getline;
gsub(/\)/ , " " )
mw = $6;
getline;
getline;
getline;
s = $3;
d = $4;
r = $5;
print lon, lat, s, d, r, mw / 10;
}
}
Note that the size of the mechanism is one tenth of the magnitude. If you wanted a bigger difference be tween the size of small and large events, cube the magnitude and divide by 2000. Assuming that you have saved the script in the file "cmt.nawk". Run the script using the command (note that I use nawk, not awk):
mantle% nawk -f cmt.nawk g.out
The result is:
-92.790 16.880 93 48 152 0.55 -91.680 13.950 259 33 31 0.55 -90.690 13.680 271 35 61 0.56 -91.380 14.250 105 22 -132 0.63 -92.200 13.920 286 20 92 0.55 -91.450 14.370 86 34 -148 0.62 -91.500 14.270 101 33 -132 0.61 -91.050 13.880 299 23 86 0.63 -89.640 13.150 324 32 113 0.61 -88.260 12.990 291 31 -120 0.56 -93.540 16.940 218 25 -2 0.59 -91.530 14.290 81 40 -153 0.61 -92.250 13.520 138 41 -91 0.53 -93.600 15.680 114 29 -102 0.58 -94.610 17.000 190 47 -40 0.69 -91.020 14.030 344 41 -53 0.56 . . .
Dziewonski, A. M., T. A. Chou and J. H. Woodhouse (1981). Determination of earthquake source parameters from waveform data for studies of global and regional seismicity, J. Geophys. Res., 86, 2825-2852.
Dziewonski, A. M., J. E. Franzen and J. H. Woodhouse (1984). Centroid-moment tensor solutions for January-March, 1984, Phys. Earth Planet. Int., 34, 209-219.
Dziewonski, A. M. and J. H. Woodhouse (1983). An experiment in systematic study of global seismicity: Centroid-moment tensor solutions for 201 moderate and large earthquakes of 1981, J. Geophys. Res., 88, 3247-3271.
Dziewonski, A. M., G. Ekström, J. E. Franzen and J. H. Woodhouse (1987). Centroid-moment tensor solutions for January-March 1986, Phys. Earth Planet. Int., 45, 1-10.
Dziewonski, A. M., G. Ekström and M. P. Salganik (1992). Centroid-moment tensor solutions for July-September 1991, Phys. Earth Planet. Int., 72, 1-11.