Thursday, February 21, 2008

Biggest EQ in the eastern Basin & Range in 25 years...

It's only a Mw 6.0. That means it's an order of magnitude smaller than the Borah Peak (Idaho) earthquake of 1983 (M 6.9) and the Hebgen Lake (Montana) earthquake of 1959 (M 7.3).

But, still, this morning there was a M 6 earthquake just east of Wells, Nevada. From the maps, it looks like it could be on a range-bounding fault on the next range east of the East Humboldt Range.

If there's a surface rupture, it should be a great place to visit - just south of I-80, on an alluvial fan, not much vegetation to obscure the geology.

The topography of the Basin and Range is controlled by normal faults: they have dropped the basins and lifted the ranges, and made the entire state of Nevada look, from space, like a washboard. But current deformation seems to be most intense near the California-Nevada border, where there is both strike-slip movement (part of the greater Pacific-North America plate boundary) and east-west extension. On the eastern side of the Basin and Range, there have been major earthquakes north of the Snake River Plain/Yellowstone area. The Wasatch Fault/Hurricane Fault, on the eastern edge of the Basin and Range, is considered potentially hazardous, but less so than the faults on the western side.

This earthquake is a couple hours drive from Salt Lake City, about 250 km from the Wasatch Fault. I'll be curious to see what the people who have been doing GPS monitoring of the Basin and Range have to say. Did they have continuous GPS stations in the area before the earthquake occurred? And did they see anything - anything - that could have told them that the fault was getting ready to go?

Edit: The USGS has added to its summary page. They think the earthquake occurred on a west-dipping fault, associated with the fault that bounds the Pequop Mountains (the range east of the earthquake). (The focal mechanism "beach ball" diagram has two possible solutions: one east-dipping fault, and one west-dipping fault.) That would make the rupture even more interesting to visit - it's very close to the east side of the hills between the Pequop and East Humboldt ranges. I want to go there and see for myself which side went up.

Edit 2: There's quite a bit of damage in Wells. Links from Salt Lake Tribune and KUTV News (with slideshow and video). There were lots of brick facades in downtown Wells - the damage looks pretty dramatic.


Maria said...

I'll be you (or anyone else) a beer that there were no GPS precursors.

Maria said...

Er, bet, I mean. Bet. I'm sure being you would be fine but I just can't handle the metaphysics.

Silver Fox said...

From your comment at Geotripper:

"I found the GPS study that I had heard discussed at GSA: Science 1999. The survey was done along US 50 rather than along I-80, but there's a step in the GPS data near the longitude of today's earthquake."

I'm not used to reading this kind of data, would the step you are talking about be just east of the Schell Creeek Range? Or between the Egan Range and Schell Creek Range? (The latter being very near Ely, NV.)

I think this is all very exciting. I was in Reno when this happened, didn't feel it (or notice it), and I will be spending some time working near the area.

Thanks for posting all the links and info.

Chuck said...

Is Wells built on an alluvial fan?

Kim said...

Maria - you're probably right. (And I think the GPS study was a campaign GPS, not continuous, and it was eight or more years ago, so it probably wasn't monitored anyway.)

Silver Fox - if I can find time, I'll try to write more about the GPS study, and how to read the diagrams in it, and what on earth it means.

Chuck - Wells is just off the image to the west. It's at the end of the East Humboldt range, and it's on one of the drainages that feeds... is that the Humboldt River? It's one of the internally drained systems of the Basin & Range. So I don't think it's technically an alluvial fan. (The USGS places the EQ ten miles to the east of Wells, though the location wasn't very good - the aftershocks are scattered all over the place, and their location quality is listed as "poor.")

Anonymous said...

There are several EarthScope (PBO) continuous GPS receivers (and seismometers) in the area. If you have google maps you can see them on the EarthScope website. The red ones are continuous and I think the blue ones were just deployed though that is just a guess.

Annoyingly there is no scale on this map.