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Example seismograms from earthquake in Newcomb, NY
April 20, 2000 08:46:54.0 - Magnitude 3.8
On April 20, 2000 a magnitude 3.8 earthquake occurred near Newcomb, NY. The quake produced little damage but was felt throughout eastern New York and into southern New England. The exact mechanics and reason for the quake are under debate but it is consistent with other activity in the region.
The epicenter was a few kilometers from the magnitude 5.1 Goodnow earthquake of 1983 (Oct. 7).
The image below was created using the zoom feature of WebSeis. The recording is from the ACC station. The epicenter was just over 100 km from the station. The most obvious result of this is that the different phases in the signal have not had a chance to spread out in time. The entire event arrives at the station and has largely died out within 10 seconds (note: the scale at bottom is in fractions of a minute beginning at the time of the earthquake). Several features are evident in this recording. As usual, the first phase to arrive is the compressional (P) wave. While the arrival is clear on all components at just after 0.2 minutes. Though this phase is typically strongest on the vertical Z component, this is no the case for this event. It is of similar strength on all records. The S wave dominates the record beginning 0.4 seconds. One might be tempted to label this as a rayleigh wave. Note however that it is weakest on the vertical component - not characteristic of a rayleigh wave.
The distance to the event can be calculated from the arrival times. The P wave arrives after roughly 13 seconds.
0.23 minutes = 13.8 seconds
velocity x time = distance
8km/s x 13.8s = 110 km
Of course, to use this method, you must already know the time the earthquake occurred, in which case the location is likely known as well. Another way to get this distance is to use the S minus P time. The P and S waves separate with distance from the earthquake. This separation is a useful tool for calculating distance. The S-P time in seconds times 12 will give an approximate distance to the event
S arrival time: 0.38 minutes = 22.8 seconds
P arrival time: 0.23 minutes = 13.8 seconds
S-P time in seconds = 22.8-13.8 = 9.0 s
(S-P time) x 12 = distance
9.0 x 12 = 108 km
