Earth Institute News posted 08/23/00 12:OO P.M. EST Magnitude 2.5 Earthquake Centered North of New York City Recorded at Lamont Columbia University, New York City  This is a seismogram of the earthquake of August 22, 2000 recorded at the Lamont-Doherty Earth Observatory in Palisades NY (PAL on map below). Time proceeds from the left to the right with first quiet before the earthquake; then the arrival of the p-wave followed by an increased amplitude as the s-wave arrives. These two waves have come from the quake epicenter near Carmel, NY to Lamont through the earth starting at the same moment but traveling at different speeds; thus they arrive offset in time. An earthquake is a slip on a fault which generates a shearing motion that produces the s-waves (secondary or shear waves). The earthquake also pushes and pulls in different directions generating the p-waves (primary or compressional waves). The p-waves arrive faster because rock is stronger under compression in comparison to under shearing motion. Like the difference in time between lightning and thunder, the distance from the event can be inferred from the difference in arrival times, in the case of an earthquake, arrivals of the p-waves and s-waves. For the August 22, 2000 earthquake, the time difference between their arrivals at Palisades, NY is approximately 7 seconds. This multiplied times a factor of 8 kilometers per second gives a distance of approximately 56 kilometers to the epicenter in Carmel, NY. On this sensitive seismograph, the shaking lasted for almost a minute, but a person would not likely have felt the earthquake at Palisades.

This seismogram is from a station at Mount Airy, NY (MANY on map below) which sends a radio signal to Palisades and is recorded at the Lamont observatory. The station is closer to the quake, thus it shows an earlier time for the arrival of the p-wave and a larger amplitude. The s-wave is not visible because the instrument went off scale with the p-wave.

STATION CODE	LAT (°N)	LONG (°W)	ELEV (m)	OPERATION (year/mo/da)	STATION NAME	COOPERATING INSTITUTION
ARNY	41.3032	74.1145	430	1993/12/16	Arden House, NY	LDEO
BRNJ	41.3032	74.1145	430	1993/12/16	Basking Ridge, NJ	LDEO
CRNY	41.3118	73.5482	293	1981/12/01	Cross River, NY	LDEO
GPD*	41.0177	74.4608	360	1976/08/01	Green Pond, NJ	LDEO
HKSI	40.5967	74.1220	107	1992/04/16-1992/12/01	Staten Island, NY	LDEO
MANY	41.2220	73.8686	133	1993/12/08	Mount Airy, NY	LDEO
PAL*	41.0042	73.9092	91	1949/01/01	Palisades, NY	LDEO
TBR	41.1417	74.2222	261	1975/08/01	Table Rock, NY	LDEO
WCC	41.0585	73.7918	100	1987/06/01	Westchester Comm. Coll., NY	LDEO

* three-component stations

Recordings from the Lamont seismic network can be observed near real-time by visiting the website: http://www.ldeo.columbia.edu/LCSN/lcsn.html.
 

^ back to top
< back to story