My Thesis Area is Misbehaving: 6.0 Earthquake Near Walker, California

 

That was exciting. I live in the Central Valley of California. The valley is famously boring for a number of reasons, and one of the good ones is because we rarely feel earthquakes here. But that wasn't the case today. I was at my computer station and the desk started vibrating and I had to look and see if my wife was shaking it. She wasn't and she was not looking happy. 

It took a while for the USGS to untangle the many wave signatures in the seismic network (there was a brief report of a 4.8 quake just 20 miles from us), but as things have settled out, it looks like the quake took place smack dab in the middle of my masters thesis area in the eastern Sierra Nevada at the small town of Walker, California. The most recent estimate of the magnitude is 5.9 (upgraded to 6.0). The quake has the signature of a normal fault, the kind of faulting to be expected in a crustal region that is being stretched apart. 

Walker is the village in the picture above at the south end of the Antelope Valley, which exists because of faulting. The valley has sunk as the mountains on the right side of the photograph rose along the fault indicated by the solid black line. The epicenter of the quake would be just out of the picture on the lower right side (the picture is looking south).

Antelope Valley sits astride the boundary of the Sierra Nevada and Basin and Range provinces, where the solid block of the Sierra is being sliced up into a series of fault-bounded grabens. The picture below shows the upper (southern) end of the valley from Monitor Pass, with the Sweetwater Mountains in the distance. 

The next picture is from the hill behind Walker looking north. The break in slope on the left is where one would look for evidence of recent earthquakes, but slopewash has covered the fault terraces (scarps) in most places except for the streams and alluvial fans that cross the fault trace. That was what I was searching for when I was doing my masters thesis many years ago in this valley. I was very pleased when I found some.

The person who did the original mapping in the 1950's was working primarily on the rock exposures, and wasn't really looking for recently active faults. By the 1980's a number of people were looking a lot harder, trying to determine the seismic hazard for the region. Fresh alluvial fans provide a possibility of dating the occurrence and size of the last earthquake to cause ground rupture in an area.

The picture below is the Mill Creek fan, at the extreme south end of Antelope Valley. Under normal circumstances, an alluvial fan should be a smooth, gently sloping surface. Here at Mill Creek, the surface steps down to the left, forming a terrace. Immediately after the earthquake this terrace may have been essentially vertical (examples of scarps are shown on this post - Slinkard Valley lies immediately west of Antelope, and the post has a nice cut-away showing the arrangement of the fault blocks).

Scarps like these show that the last major earthquake took place in the recent geologic past, very likely less than 10,000 years ago, and maybe as recently as 3,000 years ago . The length of the fault and the size of the scarps are characteristic of quakes in the range of magnitude 6.5-7.0. A magnitude 5.8 event, the Double Springs Flat earthquake, shook the extreme north end of the Antelope Valley fault system in 1994. Today's quake one-upped that event, but do not be surprised if the magnitude is revised upward or downward (NOTE: the quake was revised upward to 6.0). It takes awhile to fully analyze the seismometer records. It is not inconceivable that some small cracks may appear along some of these older scarps.

I'm listening to reports of a rockfall off the cliffs above Meadowcliff Lodge. That would be very close to the epicenter.

I will revise this post as more information comes in. 

Postscript: I finally got to my office at Modesto Junior College to download the seismogram of the quake, and here it is. The shaking was off-scale for nearly two minutes.

The second shows a compressed version of the quake, along with some of the larger aftershocks.

The Other California: Dammed if we do dam, dammed if we don't dam!

I thank my father for the title, which derives from a college term paper he wrote many years ago, although today's post is about a different place, Topaz Lake, on the border of California and Nevada just east of the Sierra Nevada province. It lies at the north end of Antelope Valley, which I explored in the previous post in my series on the "Other California". I also thought of calling this post "A River Runs Through It (but not exactly)".

Antelope Valley is a half graben, a valley formed as movements dropped the valley along a curving fault surface, forcing the West Walker River to flow along the mountain front. The fault system becomes complicated at the north end of the valley and some low peaks, the Gray Hills, interrupt the otherwise flat topography of the valley floor.

Prior to 1921, the basin between the Gray Hills and the Sierra was occupied by a dry lake bed called Alkali Lake (or Rickey Lake). The West Walker River was blocked from flowing into the lake by the alluvial fan that had built out from Slinkard Creek (at the bottom of the GoogleEarth photo). Early ranchers in the region, especially the owners of the vast Rickey Ranch, wanted better control of the water for irrigation purposes, and devised a scheme to divert the West Walker into Alkali Lake for storage. Their plans precipitated a water war with other ranchers, especially California cattle baron Henry Miller downstream and the litigation eventually reached the Supreme Court. The Rickey Ranch lost, but ironically the other ranches decided he had a great idea, and a levee was built to divert the river into the dry lake bed. A tunnel was built at a low point in the Gray Hills to control the water level. So, Topaz Lake is a reservoir without a dam! The reservoir has about 65,000 acre feet of "dead water" and about 59,000 acre feet of usable storage space. The outlet tunnel is at the low point in the hills on the far side of the lake in the picture below.

The flood of 1997 keeps coming up in my recent posts. It was an extraordinary event in which the river peaked at almost double the size of all previous floods, at 12,000 cubic feet per second. This flood, and an earlier smaller event in 1986 revealed a serious weakness of the reservoir system. Because of channel blockages below the diversion, there was no way to stop floodwaters, and especially debris from flowing into the lake, causing the lake to rise far above normal levees, and flooding some residential areas. Modifications to the levee system are currently being made (the inlet canal is marked by the line of Cottonwood trees in the picture below).

Topaz Lake today is fishing destination, and if you are one of those really desperate gambling junkies, it has one of those casinos that sits almost on the state boundary with Nevada. It also has some nice views of the mountains, and now that you know what you are looking for, it is a wonderful place to see the results of some heavy duty fault action.

One more post to go on the West Walker River series. The next post will involve the end of the river in Nevada, but the repercussions extend into our fair state of California.

The Other California: The West Walker River and the Antelope Valley

The West Walker River, the latest entry in our "Other California" series, changes character many times on its journey from the headwaters in the High Sierra to the end of the river in the desert at Walker Lake in Nevada. It begins as a glacially carved valley, plunges into a deep river-cut gorge, and then it flows out into a wide open alluvial plain, the Antelope Valley. Unlike the erosion that is taking place upstream, this is a place where the river deposits material. In other words, Antelope Valley was not carved by a river at all. As can be seen in the GoogleEarth image below, the use of the river changes as well: upstream is protected watershed and recreation/wilderness country, but the Antelope Valley is given over to agricultural development.

Notice in the image above how asymmetrical the valley is: on the left (west) it has a sharp boundary with the mountain flank; on the right (east), the valley slopes gradually up into the foothills of the Sweetwater Mountains. It is the expected shape of a fault-formed valley, a graben (really a half-graben, since the major faults are on one side, the left). Antelope Valley sits astride the boundary of the Sierra Nevada and Basin and Range provinces, where the solid block of the Sierra is being sliced up into a series of fault-bounded grabens. The picture below shows the upper (southern) end of the valley from Monitor Pass, with the Sweetwater Mountains in the distance.

Upon hearing that this is a fault valley, most people might ask if the faults are still active. In a word, yes. Besides the youthful appearance of the mountain front (look at the sharp slope break in the photo below where the valley is sliding down relative to the mountain slope), the river has been forced to flow mostly on the west side of the valley, where the valley has dropped the most. The break in slope is where one would look for evidence of recent earthquakes, but slopewash has covered the fault terraces (scarps) in most places. Except for the streams and alluvial fans that cross the fault trace. That was what I was searching for when I was doing my masters thesis many years ago in this valley. I was very pleased when I found some.

The person who did the original mapping in the 1950's was working primarily on the rock exposures, and wasn't really looking for recently active faults. By the 1980's a number of people were looking a lot harder, trying to determine the seismic hazard for the region. Fresh alluvial fans provide a possibility of dating the occurrence and size of the last earthquake to cause ground rupture in an area.

The picture below is the Mill Creek fan, at the extreme south end of Antelope Valley. Under normal circumstances, an alluvial fan should be a smooth, gently sloping surface. Here at Mill Creek, the surface steps down to the left, forming a terrace. Immediately after the earthquake this terrace may have been essentially vertical, much like the scarps formed in April in Baja California (examples of scarps are shown on this post from last month - Slinkard Valley lies immediately west of Antelope, and the post has a nice cut-away showing the arrangement of the fault blocks).

Scarps like these show that the last major earthquake took place in the recent geologic past, very likely less than 10,000 years ago, and maybe as recently as 3,000 years ago (the latest incarnation of the state fault hazard map is in agreement). The length of the fault and the size of the scarps are characteristic of quakes in the range of magnitude 6.5-7.0 (USGS analysis is here). A magnitude 5.8 event, the Double Springs Flat earthquake, shook the extreme north end of the Antelope Valley fault system in 1994.

Earthquakes are not the only hazards associated with the Antelope Valley. I had a personal experience with the massive flood of 1997 that I chronicled in a post last year. The flood was twice the size of the previous record flood, and resulted in a new channel being formed. The Army Corps of Engineers put the river back after a few weeks. I have video of the event, which I hope to find and post as soon as I can.

If you visit the Antelope Valley in the fall, you may have a unique and increasingly rare experience. The very first time I saw the valley, in 1976, I was on my second geology field trip. We were in a hurry to get somewhere, and we rolled into the valley at a high rate of speed, only to be brought to a complete stop by a ... stampede. Well, ok, to us flatlanders it was a stampede. It was actually a very large herd of cattle being directed down onto the valley floor to spend the winter after fattening up in the adjacent high country. Oh, and I must say that the 100-year-old Cottonwood trees along the highway at Coleville and Topaz have a wonderful blazing yellow glow when the weather is turning cold and the leaves are turning.