Paul, I am getting the impression that you did not see the NEIS info about the 20 April quake? VIS: The following is from the United States Geological Survey, National Earthquake Information Center: Preliminary hypocenter for earthquake of 2000 Apr 20, NEW YORK: latitude 44.0 degrees north, longitude 74.3 degrees west, origin time 08 46 54.0 utc, depth shallow, magnitude 3.7 mbLg. The earthquake was felt in eastern New York and at Montpelier, Vermont. There have been no reports of damage. This is located in the same general area as a magnitude 5.1 earthquake on October 7, 1983, that caused minor damage and was felt in 12 U.S. states and 2 Canadian provinces. Stations used: NCB P 084656.2 HNH P 084720.5 LBNH P 084724.7 BINY P 084731.3 LSCT P 084734.9 WES P 084737.1 PAL P 084742.0 SSPA P 084800.5 MCWV P 084823.8 This is their calculation of the origin time based on the P arrival time at these 9 stations. Any additional arrival time data should only refine the origin time estimate. A single seismogram can only provide the unique arrival time at the station site; the reading is used with data from other stations, and a minimum of three are needed to calculate a hypocenter location, which results in the origin time estimate. A VERY approximate method we use with the wall maps is to use the P and S times to guess the distance of a station to an event, letting each 1 second difference to represent 8 kilometers of travel time. This lets us draw three circles on the wall map around the reporting stations, so their intersection is the epicenter guess. An origin time can also be guessed at with a single station P and S, again using 8 seconds/S-P time to get a distance, then figuring the P travel time. SO if S-P is 10 seconds, the event is about 80 km from the station (this is only good out to about 200 km). At a velocity of 5 km/second, the P wave took 16 seconds to get to the station, so the origin time is the P time minus 16 seconds. THis VERY approximate estimating can be improved by averaging the available data. MUCH better results are obtained using hypocenter programs that use recursive algorithms to minimize the error ellipses. Regards, Sean-Thomas __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>