Epilocator is a stand-alone hypercard-based application that will locate the epicenter of an earthquake from arrival time data and calculate the magnitude. It also is a self-contained data base that allows the user to paste in additional pictures and text.
When you look at the new version of Epilocator and click the drop-down menu for choosing a phase arrival, you will see the following choices: P, Pg, SP, S, Sg, LLg. Larry has clarified to me how Epilocator works behind the scenes and when and why you should choose any of the above phases.
- P -- This is the first wave arrival. At distances less than 200 km, the P-wave follows a direct path from the hypocenter to the station. Beyond 200 km, the Pn phase, which is a P raypath that goes downward from the hypocenter to the Moho, travels along the Moho, then comes back up, will generally be the first arrival. Epilocator is designed to make this choice, depending on distance of the station, as you go through the iterations. You should choose "P" for the first arrival, except in the following case.
- Pg -- This is the designation for the direct raypath P wave. You would use Pg, instead of P if, quoting Larry, "In some situations you may know that the Pn arrives first but with small amplitude, hence yu pick the Pg arrival. In this case, you tell Epilocator that your pick is for Pg, even though the epicentral distance may be greater than the crossover distance (200 km). This situation does arise because Pn can be much smaller than Pg out to 300 or 400 km distance."
- SP -- This is S minus P; that is, the time difference between the P arrival and the S arrival (subtract the P time from the S time). This is very useful if a station does not have time lock.
- S and Sg -- See explanations for the P and Pg waves. Similar choices apply. We have found the S wave to be very useful to us with some of the small, local events (<3.0). In many cases the P amplitude is so small that we have had difficulty picking it out of the local background noise at a particular station (some stations are fairly noisy). However, the S wave is usually much higher amplitude than the P and we have become fairly confident in picking it.
- LLg -- These are the high amplitude (usually with periods of 0.5 to about 1.0 seconds) that we use for magnitude calculation. Larry has designed Epilocator to use these waves for arrival times. Pick the arrival time of the the largest wave on each seismogram. I did a location on a limestone-quarry blast in northwest Ohio using just Lg arrivals (that is mostly all you see with quarry blasts) and was able to get the epicenter to less than a kilometer from the known quarry that did the blasting. So it works!