The Columbia Earth Institute

  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.

Map showing short-period seismographic stations sending data to Lamont seismographers at Palisades, NY. The blue star is the approximate location of the August 22, 2000 earthquake near Carmel, NY.

ARNY 41.3032 74.1145 430 1993/12/16 Arden House, NYLDEO
BRNJ 41.3032 74.1145 430 1993/12/16 Basking Ridge, NJLDEO
CRNY 41.3118 73.5482 293 1981/12/01 Cross River, NYLDEO
GPD* 41.0177 74.4608 360 1976/08/01 Green Pond, NJLDEO
HKSI 40.5967 74.1220 107 1992/04/16-1992/12/01 Staten Island, NYLDEO
MANY 41.2220 73.8686 133 1993/12/08 Mount Airy, NYLDEO
PAL* 41.0042 73.9092 91 1949/01/01 Palisades, NYLDEO
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:

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