Feature

How Does Nevada Rank?

Nevada is one of the most active states in terms of the numbers of earthquakes. To determine just how active, we consulted the catalog of earthquakes that has been compiled by the Advanced National Seismic System, and available through the Northern California Earthquake Data Center (http://www.ncedc.org/anss/catalog-search.html). The catalog contains data from 1898 to 2005. Earthquakes in the catalog with magnitude 5.0 or greater are shown in the following map. We supplemented this with the USGS catalog of significant earthquakes after 1769(http://neic.usgs.gov/neis/epic/epic.html)

Next, we counted all earthquakes more than magnitude 3.5, 5.0, 6.0, and 7.0 within the boundary of each state. For those states that have coastlines, we also counted earthquakes up to 100 kilometers off of the coast. The following table lists the top 10 states for numbers of earthquakes in each of these size ranges.

Without question, the most active state is Alaska, and California is second.

For potentially damaging earthquakes with magnitude 5.0 or larger, Nevada ranks in third place. Curiously, when one goes down to smaller magnitudes (3.5), Hawaii comes up higher, as noted by the U S Geological Survey (http://earthquake.usgs.gov/regional/states/top_states.php). Hawaiian earthquakes are mostly driven by the volcanic Hawaii hot spot. Large numbers of earthquakes in Hawaii occur on the south side of the main island and also offshore to the south, where the hot spot is slowly building a new island. Due to this concentrated volcanic source, the numbers and especially the density of small earthquakes are extremely impressive, giving this small state a very high ranking, and locally a very high hazard (Klein et al, 2001).

A better way to compare activity rates is to generate cumulative magnitude distribution curves showing numbers of events per year with magnitude that equals or exceeds the magnitude on the abscissa. Anderson and Miyata (2006) developed the distribution curves that are shown in figure 2.



In table 2, for each state we list the magnitude, M1, that occurs at an average rate of once per year as picked from the curves in figure 2. The physics of earthquakes does not guarantee that the distributions in figure 2 will obey the exponential Gutenberg-Richter model for the distribution, but that model is a satisfactory approximation to most of the curves. The Gutenberg-Richter relationship can be approximated as logN = -b (M - M1 ) . The b-values are all between about 0.9 and 1.0. As an aside, the meaning of the parameter M1 is in some ways more easily communicated than the productivity, a, when the equation is written in the usual form log N = a-bM .


The references:

Klein FW, Frankel AD, Mueller CS, Wesson RL, Okubo PG (2001). Seismic hazard in Hawaii: High rate of large earthquakes and probabilistic ground-motion maps, Bull. Seism. Soc. Am. 91 (3): 479-498.

Anderson, J. G. and Y. Miyata (2006). Ranking states by seismicity, Seismological Research Letters 77, 672-676.

The catalog used here goes back longer in time than the USGS ranking, and includes the active earthquake period associated with the 1954 Dixie Valley and Fairview Peak events, which are the most recent Nevada earthquakes with magnitude over 7.0.

Ranking states by numbers of earthquakes can be tricky, and results can be different depending on the time period and the magnitude threshold. But however you count it, Nevada has a lot of earthquakes. In the last century, within its borders there have been over 100 earthquakes strong enough to cause damage.

This research was carried out by John Anderson and Yui Miyata at the Nevada Seismological Laboratory.

Last updated 10/30/06