Earth Institute News
posted 08/23/00 12:OO P.M. EST
Magnitude 2.5 Earthquake
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.
| STATION |
|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|
|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|
Recordings from the Lamont seismic network
can be observed near real-time by visiting the website:
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