Alan Kafka
Weston Observatory
Department of Geology and Geophysics
Boston College
This page explains the complexities of demonstrating the existence of a “seismic zone” when many seismic stations are located in the vicinity of the hypothesized zone, as discussed in my blog Faults and Earthquakes in the Greater New York City Area: Reflections at the Intersection of Science, Media, and the Public.
The figure below shows the earthquake catalog for the greater New York City area published by Sykes et al. (2008 ) for various lower-magnitude cutoffs and periods of time. As was discussed by Kafka et al. (1985), arguments regarding relationships between seismicity and geological features need to be evaluated by choosing a lower-magnitude cutoff that is high enough that the pattern of seismicity under discussion is not affected by any uneven distribution of the seismic stations (see below).
Click on this figure to enlarge it.
The map in the upper left shows the entire Sykes et al. (2008 ) catalog and the Ramapo Fault (labeled RF, and shown by the thick black line). The part of the RF that goes through the Indian Point nuclear power plant is highlighted in green. Based on the entire catalog shown in the upper left map, one might argue that there is compelling evidence for the the existence of a “Ramapo Seismic Zone” (RSZ) that runs through the Indian Point site. However, as can be seen in that figure, and was pointed out by Kafka et. al. (1985), the largest known earthquakes occured elsewhere. Furthermore, if (as was done by Kafka et. al., 1985), we plot only the larger earthquakes so that the pattern of seismicity is less biased by station distribution (as shown in the figure), the existence of an RSZ is far less clear, and the fact that the largest earthquakes occured far from the RF (and far from Indian Point) is evident.
Earthquakes and Related Matters 