Question of the Day - 26 June 2017

Believe it or not, Nevada ranks third among the 50 states for frequency of earthquakes. Only California and Alaska have more seismic events than the Silver State.

Geologically speaking, Nevada is a fairly young piece of real estate. Nearly all the state’s 250 mountain ranges, in fact, continue to grow — one earthquake at a time.

Las Vegas, specifically, is underlain by a number of local fault lines. Most of these delineate the borders of the valley where it edges up against the surrounding mountains, though some run under the city itself. In addition, larger tectonic faults undercut Las Vegas. These are the ones about which geophysicists and seismologists worry most.

Some of the faults have the potential to produce strong earthquakes on rare occasions.

According to a 2003 seismic survey conducted by a UNLV geophysicist for the Department of Energy studying the Nevada Test Site and nuclear repository, Las Vegas will sustain a magnitude 6.5-7 earthquake someday. Of course, someday is highly variable, but to seismographers, "on rare occasions" means that the average time between the large earthquakes on any one of the faults is somewhere between 1,000 and 10,000 years.

Closer to the current calendar, the likelihood of an earthquake of that magnitude hitting Las Vegas is slim. Seismologists calculate a roughly 1-in-10 chance that a magnitude 6.0 earthquake — large enough to cause significant damage — will strike the valley in the next 50 years.

But it’s not just the fault lines under Las Vegas that could have an impact here. The worst shaking the valley experienced reportedly occurred in 1857, when a massive earthquake was sustained by the San Andreas Fault, topping out at an estimated 8 on the Richter Scale. An earthquake of that magnitude will inevitably be visited again upon the San Andreas Fault and when it is, even though its epicenter would be hundreds of miles away, it could create havoc in Las Vegas.

Experts project that it could kill hundreds, damage thousands of buildings, force tens of thousands of people into public shelters, and wreak economic havoc of up to $25 billion.

The last major earthquake experienced in southern Nevada occurred in May 2015, centered near Caliente, roughly 120 miles northeast of Las Vegas. Originally rated 5.4 on the Richter Scale, it was quickly downgraded to 4.8. The temblor could be felt around Las Vegas Valley and caused some minor damage to a bridge and freeway exit ramp. The United States Geological Survey (USGS) reported four significant aftershocks, with magnitudes between 2.5 and 3.8, an hour after the main shake.

In October 1999, a massive quake only 150 miles southwest of Vegas in the California Mojave registered 7.0 at the epicenter and 5.0 in Las Vegas. It was the largest temblor felt in Las Vegas Valley since the 7.3 quake in Landers, California, in 1992.

That earthquake was centered 220 miles southwest of Las Vegas. It was so powerful that it was felt from San Diego to Boise; Las Vegas residents experienced rolling and shaking.

A little closer to home, also in 1992, an earthquake registering 5.6 on the Richter Scale was centered at Little Skull Mountain near Yucca Mountain, about 90 miles from the city, where it was felt as a sizable quake event.

The USGS database shows that 368 earthquakes have been recorded within 30 miles of Las Vegas since 1931. The largest of those was 3.9-magnitude in 1995.

EarthquakeTrack.com reports that Las Vegas has experienced 27 earthquakes in the past 365 days, averaging between 1.5 and 3 on the Richter Scale.

And speaking of earthquakes, the results of the previous poll on the three choices for the membership are posted at this link.