Thursday, February 21, 2008

Nevada 6.0 Quake: USGS Numbers' Game in Seismology

According to the AP, a 6.0 magnitude quake struck today in a sparsely populated area of Nevada, 11 miles southeast of Wells near the Nevada-Utah line.
WELLS, Nev. - A strong earthquake shook rural northeastern Nevada Thursday, causing at least one building to collapse, authorities said. The magnitude of the quake, initially estimated at 6.3, was later revised to 6.0 by the U.S. Geological Survey National Earthquake Information Center. The quake, which struck at 6:16 a.m., was centered in a sparsely populated area 11 miles southeast of Wells near the Nevada-Utah line.

While the quake occurred in an unpopulated area in Nevada, what's generally not known is that over a period of the last 150 years, Nevada ranks in the top three states where large earthquakes have struck. On Oct. 3, 1915, a 7.6 earthquake hit Pleasant Valley near Winnemucca. In 1954, four quakes happened near Fallon, the smallest, a 6.6, the largest, a 7.2.

According to Seismo,
Magnitude 3 and 4 earthquakes are commonly felt, but rarely cause damage. Minor to moderate damage can accompany a magnitude 5 or 6 event, and major damage commonly occurs from earthquakes of magnitude 7 and greater. Although earthquakes don't occur at regular intervals, the average frequency of earthquakes of magnitude 6 and greater in Nevada has been about one every ten years, while earthquakes of magnitude 7 and greater average once every 27 years.
It's also a matter of location, location, location, in where a quake strikes. If the same 6.0 that struck this morning near Wells had hit Las Vegas the damage toll might have been far different.

Most deaths are not due to the movement of the ground but from structural collapse, flying debris such as metal and glass, furniture and appliances, fires, broken gas lines, and downed electrical lines.

For earthquakes, it's also not only a matter of size but also that of duration that counts.

According to the USGS, there is a correlation between "shaking" and "duration" that relates to magnitude.
The duration of fault rupture is related to both how long it takes for a spot on the fault to slip (which seems to be quite fast) and the time it takes rupture to proceed along a fault. You have to think of an earthquake as an area on a fault rather than just a point. It starts at a point and then the rupture propagates along the fault at around 2 kilometers or so per second. So the larger the area of the fault that ruptures, the longer the duration of the earthquake. And larger magnitude earthquakes have larger fault areas. So there is a general relationship between duration and magnitude.
And then there is "duration of shaking." This what one feels when in the midst of a quake, the ground suddenly shaking or "rolling" under one's feet or the building one is in, swaying or shaking, and how long it lasts. The duration of shaking is figured by a point on the ground and depends on how long the earthquake took to occur and how the waves move through the ground to that point.

There's also a matter of what type of ground the area has, does it contain soil, sand, bedrock or swamp? This too can make a difference.

On March 28, 1964, a 9.2 quake struck Prince William Sound and Anchorage, Alaska. This quake still ranks as the largest earthquake ever recorded. The quake lasted an incredible 4-5 minutes, resulting in avalanches and landslides, but incredibly, only nine people were killed.

The epicenter was located 75 miles east of Anchorage, the depth of the rupture was estimated at 14 miles beneath the Earth's crust. An estimated 90% of the deaths were due to tsunamis. Today the coastlines of Alaska are monitored 24 hours a day at the West Coast & Alaska Tsunami Warning Center located in Palmer.

USGS Seismic Hazard Map

A great site for earthquake info is the United States Geological Survey Site.

This map shows the liquefaction hazard in the communities of Alameda, Berkeley, Emeryville, Oakland, and Piedmont for a magnitude 7.1 earthquake on the Hayward fault. The map predicts the approximate percentage of each designated area that will liquefy and show surface manifestations of liquefaction such as sand boils and ground cracking. Liquefaction is a phenomenon that is caused by earthquake shaking. Wet sand can become liquid-like when strongly shaken. The liquefied sand may flow and the ground may crack and move causing damage to surface structures and underground utilities. The map depicts the hazard at a regional scale and should not be used for site-specific design and consideration. Subsurface conditions can vary abruptly and borings are required to address the hazard at a given location.

Magnitude 7.2 Blender

Liquefy? (Yikes!)

A more sobering map from the site is the Liquefaction Hazard and Shaking Amplification Maps of the Oakland, California area. In simpler terms "liquefaction" means the same effect as putting the city on the highest setting in a giant blender.

In summary, a 6.o quake does indeed, seem high but there also other factors such as duration, type of soil stability and population density and, if you live near the coastline, the dreaded tsunamis.

Earthquakes are truly frightening. There are no advance warning systems such as weather forecasters who can alert people to dangerous weather conditions. The first signs of a quake could be a rumbling noise under the ground, buildings beginning to sway, houses shaking, or the ground beginning to "roll" beneath your feet.

There are very few places to run and hide and it's over in a matter of seconds, or a truly frightening scenario, minutes, with, at times, unbelievable damage.


Source - WCATWC
Source - Seismo
Image - USGS
Image - Chemistry Land


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