Seismo Blog

Of Ghost Towns and Explosions

2016-12-28 00:00:00 -0800

Categories: CaliforniaSwarms

In these cold winter days, the ghost town of Bodie is even ghostlier than usual. Located east of the Sierra Nevada north of Mono Lake, the hamlet became a boomtown in the 1880's after a prospector named Waterman Body discovered gold in the barren hills of this remote area. During the middle of last night it got even spookier amongst the uninhabited, windswept buildings. Within one hour the town and its surroundings were rattled by three earthquakes with similar magnitudes of around 5.7. Their epicenters lay about ten miles northeast of Bodie just across the Nevada border. These significant shakers did not cause any damage in the ghost town, but were felt over thousands of square miles from Chico in the north all the way to Fresno and Visalia in the south. Because a quick glance at the map of active earthquake faults does not reveal anything in this area, speculations about the cause of these good size temblors and their dozens of aftershocks immediately popped-up in social media.

Seismogram of Hawthorn EQ Swarm recorded at the seismic station HELL

The three earthquakes of similar size (magnitude around 5.7), which shook Hawthorne Nevada last night, as recorded by the Berkeley Seismological Laboratory seismic station HELL in the Sierra Nevada near Kings Canyon National Park.

There is reason to speculate indeed. After all, the epicenters are very close to the Aurora Volcanic Field. The Aurora Crater, less than a mile away, is an impressive one mile wide crater completely surrounded by lava flows. The lavas of the Mud Springs volcano, located slightly to the east of the epicenter, flowed for more than four miles, leaving behind a crusted, barren landscape. Could it be that the latest earthquakes are associated with a reawakening of the Aurora Volcanics? Well, anything is possible within the interior of the Earth, but the last eruption in this field was about two million years ago and no signs of any magmatic activity have been observed recently.

And then there is the area to the northeast of the epicenter. Just south of Walker Lake, the US Army operates the largest ammunition depot in the world. An area of 230 square miles is dotted with almost 2,500 bunkers in which millions of pounds of conventional explosives are stored: from rifle bullets to mortars, artillery shells and much more. Even though the depot has been spared any major accidental explosion since it was established in 1930, there is enough fire power stored in the area to blow up all of Nevada several times over. Depending on the circumstances, obsolete ammunition is detonated in a controlled fashion several times a week. This detonation hot spot is marked on aeronautical charts as restricted area R-4811, helping pilots to avoid getting caught in the blasts. Could the earthquakes have actually been accidents on the Army Depot? Even though there is some secrecy involving the depot, news of such major explosions would have been leaked by now.

Seismogram of Hawthorn EQ Swarm recorded at the seismic station HELL

So, what caused the quakes then? The epicentral area is part of what is geologically known as the Basin and Range Province. This area, which stretches from the east of the Sierra Nevada all the way to Utah, is a zone where the Earth's crust is expanding. It is characterized by a sequence of long, deep valleys - the basins - and running parallel between them high mountain crests - the ranges. The crustal expansion is accompanied by a large number of earthquakes. During the five year period between 2010 and 2015 alone, the Nevada Seismological Lab at the University of Nevada in Reno registered more than 18,000 quakes of magnitude 1 and greater. Most of these events come in swarms and short sequences.

One of these swarms shook the area southwest of Hawthorne in the spring of 2011. At least nine of the more than 5,000 temblors recorded during that time had magnitudes of four and above. These quakes were located in the same epicentral area as the one which shook Bodie and its environs last night. All indications are that the lastest quakes are a new manifestation of the tectonic movement within the Basin and Range province. The earthquake sequence more than five years ago lasted about two months. At the end of April 2011 the Earth near Hawthorne quieted down again. Hence it is very likely that Bodie and the ammo bunkers will be shaken for a few more weeks, before every thing calms down again - except for the occasional detonations of obsolete Army ordnances. (hra134)


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Of Ghost Towns and Explosions

In these cold winter days, the ghost town of Bodie is even ghostlier than usual. Located east of the Sierra Nevada north of Mono Lake, the hamlet became a boomtown in the 1880's after a prospector named Waterman Body discovered gold in the barren hills of this remote area. During the middle of last night it got even spookier amongst the uninhabited, windswept buildings. Within one hour the town and its surroundings were rattled by three earthquakes with similar magnitudes of around 5.7. Their epicenters lay about ten miles northeast of Bodie just across the Nevada border. These significant shakers did not cause any damage in the ghost town, but were felt over thousands of square miles from Chico in the north all the way to Fresno and Visalia in the south. Because a quick glance at the map of active earthquake faults does not reveal anything in this area, speculations about the cause of these good size temblors and their dozens of aftershocks immediately popped-up in social media.

Temblors where Three Plates Meet

We have often said in this blog that California's most seismically active region lies around Cape Mendocino, halfway between San Francisco and the Oregon border. Thursday morning's off-shore temblor with a magnitude of 6.5 was a reminder of the seismic hazard in this region. People in all of Humboldt County and some places beyond were rudely awakened around 7 am by the swaying motion of the ground. Fortunately no serious damage or injuries were reported.

An Earthquake of Two Flavors

Not all earthquakes are created equal. Instead, there are certain distinct flavors of earthquakes. In temblors "California Style" the two flanks of an earthquake fault slide past each other horizontally with no or only very little vertical movement. This flavor is called "strike-slip". In other quakes, the movement of one of the blocks of a fault is mostly down - called "normal" faulting - or up, dubbed "thrust" faulting. And then there are earthquakes which combine some horizontal with some vertical movement. They are of a flavor called "oblique". But no matter what the flavor and no matter what the magnitude: The vast majority of earthquakes are of just one kind. When a quake starts as strike-slip, it stays the same flavor throughout the duration of the fault rupture. The same is true for the other flavors, i.e. what starts as a thrust ends as a thrust with no changes in the orientation of the fault movement in between.

The Missing Link

It has long been suspected, but scientists could not be sure. Are two of the most dangerous earthquake faults in Northern California, the Hayward Fault and the Rodgers Creek Fault, separate entities or are they connected in some way? The answer to this question lies literally in the mud. It is hidden in the invisible geology under the murky sediments of San Pablo Bay, the northern extension of the San Francisco Bay. San Pablo Bay, with its almost circular shape, separates the narrows between Point San Pablo and Point San Pedro just north of the Richmond Bridge from the Carquinez Strait, through which the Sacramento and the San Joaquin rivers bring their freshwater supplies from further east. Research just published in the journal Science Advances suggests very strongly that the missing link between these two faults has been found. The Hayward and the Rodgers Creek Faults have now to be considered as one more than 110 mile long fault.

Predicting Presidents and Not Earthquakes

Many years ago, when we at the Berkeley Seismology Lab decided to start publishing this Seismo Blog, we resolved not to do certain things: We were not going to trivialize earthquakes and the horrors they can cause as the most potent force of nature on our planet - and we were staying away from politics. And yet here we are, talking about politics less than a week after one of the most consequential upsets in American politics in decades. Well, the blogger will refrain from actually commenting on the results of the recent elections. But he can't bite his tongue any longer after having endured endless months of polls, predictions and forecasts, and after wasting countless hours in front of the TV listening to pundits, analysts and the infamous "talking heads." The bottom line is: All of them were absolutely wrong. In fact, never in modern times have their predictions about who will be our next president been worse.

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