Seismicity and focal mechanisms

The figures below show the two interfaces to plot either epicenters locations or draw focal mechanisms (the seismologists beach balls). To plot seismicity use either an ASCII file with data in lines organized as: lon, lat, depth, magnitude, year, month, day, [hour, minute, second]. As an alternative one may use a ISF formatted catalog (like the ones we can get from www.isc.ac.uk). Reading the ISF format is done with a MEX file. A “Posit file” reads the format used by PMEL to divulgate the hydrophone detected seismicity. Once you have loaded a seismicity file, there are many operations available through a right-click. Namely, play a seismic movie (a separate window controls the “movie” parameters), display several types of histograms and fit a Modified Omori Law.  This later as well as the Mc and b value estimate was taken from parts of code of the Zmap toolbox. A finer refinement is achieved when we select to “Draw a polygon” around an interest area. Then the same options are available but will apply only to the data contained inside the polygon. It contains also a tool to detect swarms. That is a very basic tool that uses a homemade criterion of space and time distances to detect the events connectivity.

To plot focal mechanisms one can use the formats shown in the list. In the Aki & Richards convention the data is ordered as: lon lat depth strike dip rake (in degrees) magnitude. Optionally add two more columns with the lon and lat at which to place the beach ball. The Harvards CMT convention uses: lon lat depth str1 dip1 rake1 str2 dip2 rake2 mantissa and exponent (of moment in N-m). Again, optionally add two more columns with the lon and lat at which to place the beach ball. WARNING, contrary to psmeca, the moment magnitude uses N-m (and not Dyn-cm). Also, it is not yet possible here to have a last string column containing the event label. However, the ISF formatted files uses the event date to build up an event label.

A big advantage of using the focal mechanism tool over using psmeca is that the user may easily displace the beach balls using the mouse. This is particularly useful when we have many events on top of each other. Also, the Save GMT script tool is able to detect beach balls and to make the appropriate calls to psmeca that reproduce the graphical display.

Copyright © <2010>, <Joaquim Luis>

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