Earthquake in New Zealand - and “Beach Balls”
Saturday, September 4, 2010 10:03:11 AM
The earthquake however gave me an opportunity to try out my new piece of FREE software for my MAC, OSXGeoCalc, with which I can draw stereonets. Stereographic projections on a stereonet is a way of picturing three-dimensional features (spheres) on a two-dimensional plane. This technique is used within many different geological disciplines.
Within seismology the direction of slip and the orientation of the fault on which it occurs are three-dimensional. Their directions can be calculated from seismograms, and these so-called “focal mechanisms” can be displayed as what is popularly known as “beach balls”, which is in fact a stereographic projection.
Based on information from USGS (time of moment solution: 10/09/03 16:35:44.00 UTC) I made the following “beach ball”.
It looks slightly different from the “beach ball” at Higlyallochtonous, based on moment solution half a minute later (time: 10/09/03 16:36:14.29 UTC).
USGS has a very useful page on “beach balls” and focal mechanisms. Just looking at the “beach ball” (often seen on earthquake maps) you can distinguish between the different fault types (strike slip, normal, reverse). This was apparently strike-slip faulting.
PS: The earthquake may really have been three quakes which makes it a bit complex!