Reno Quakes: an active deep fault zone

jlhredshift · #1 (**permalink**) 29-April-2008, 01:24 PM

These are deep epicenters usually associated with slab subduction.

List of quakes
Topographic maps

both from USGS.

jlhredshift · #2 (**permalink**) 29-April-2008, 04:34 PM

More details on the fault zone west of Reno. (Henry and Perkins; Geology 2001)

Quote:

ABSTRACT
Relative and absolute elevations of the Sierra Nevada and adjacent Basin and Range province, timing of their differentiation, and location, amount, and timing of strike-slip movement between them are controversial. The provincial boundary near Reno developed in two stages. (1) At ca. 12 Ma, the [approximately] 700 km2 Verdi-Boca sedimentary basin formed across what was to become the boundary, probably as a result of a small-magnitude but regional extensional episode that affected much of the western Basin and Range. (2) At 3 Ma, the basin was complexly faulted and folded during a larger magnitude extensional episode that established the modern Sierran structural and topographic boundary in this area. The boundary is really a transition zone with a western edge along the Donner Pass, California, fault zone, which is farther west than previously placed. Both episodes appear to have resulted from east-west extension only, which suggests that northwest motion of the Sierra Nevada relative to the Basin and Range shown by geodetic data began after 3 Ma or was taken up farther east.

They also say:

Quote:

The Sierra Nevada–Basin and Range boundary near Reno should be considered a left-stepping transition zone, the boundary stepping westward from the Genoa fault to the Donner Pass fault zone.

Edit to add At 12:15 EDT they had a mag 2.9.

Nick Theodorakis · #3 (**permalink**) 29-April-2008, 05:00 PM

Quote:

Originally Posted by jlhredshift

These are deep epicenters usually associated with slab subduction.
...

[grammar nitpick]I thought epicenter meant the location on the earth's surface that was above the focus of the quake.[/grammar nitpick].

Nick

jlhredshift · #4 (**permalink**) 29-April-2008, 05:06 PM

Quote:

Originally Posted by Nick Theodorakis

[grammar nitpick]I thought epicenter meant the location on the earth's surface that was above the focus of the quake.[/grammar nitpick].

Nick

True, nitpick accepted. I also would not recommend standing at the epicenters right now, just to be on the safe side, movement is occurring!

jlhredshift · #5 (**permalink**) 29-April-2008, 08:40 PM

I took the USGS data and plotted a chart in Excel.

jlhredshift · #6 (**permalink**) 29-April-2008, 09:32 PM

From Google Earth the ridge above the active zone.

jlhredshift · #7 (**permalink**) 30-April-2008, 12:34 PM

Link to map and data pertaining to the 5.2 Willow Creek quake.

jlhredshift · #8 (**permalink**) 06-May-2008, 02:24 PM

From LA Times

Quote:

An unusually intense swarm of earthquakes -- more than 1,000 over the last two months -- has struck beneath a small suburb of Reno, leaving residents shaken and scientists puzzling over the cause.... Normally, in an earthquake swarm, the main shock hits and then is followed by a series of ever smaller quakes. But this swarm "has been growing with time,
which is actually quite unusual," said geologist Brian Wernicke of Caltech. "I can't think of any other example where that has happened."

Earthquake swarms also usually occur much deeper, at least 5 miles below the surface and often much deeper than that. "It's very, very rare to get earthquakes of any kind in the upper kilometer," USC's Dolan said.

Reno, Kilauea, Chile, the Earth is very busy.

geonuc · #9 (**permalink**) 06-May-2008, 04:00 PM

Your OP addressed the unusual nature of the Reno area fault (deep hypocenter). Now the LA Times is reporting on a swarm of shallow quakes.

As you know, the Basin & Range is an extensional province, caused, I believe, from the effects of the shallow subduction of the Farallon plate. Extensional block faulting tends to run from shallow to moderately deep, does it not?

What's your take on this?

jlhredshift · #10 (**permalink**) 07-May-2008, 03:11 AM

Quote:

Originally Posted by geonuc

Your OP addressed the unusual nature of the Reno area fault (deep hypocenter). Now the LA Times is reporting on a swarm of shallow quakes.

As you know, the Basin & Range is an extensional province, caused, I believe, from the effects of the shallow subduction of the Farallon plate. Extensional block faulting tends to run from shallow to moderately deep, does it not?

What's your take on this?

I've read papers theorizing that the Farallon plate is a shallow angle, i.e. low angle, subduction. In the May issue of Geology Bulletin; GSA Wells and Hoisch suggest that the extension is due to anatexis, delamination, of the bottom of the overriding NA plate. The Reno area is at the extreme northern edge of the Sierra Nevada range. The Luning thrust belt is half way across Nevada to the east, i.e. the basin and range is to the east. There are former accreationary island arcs to the west, i.e. California. Eight kilometers deep is not even close to the MOHO, so in that sense, shallow. I am certainly not in a position to express any opinion as to what is going on beneath Mogul, Nevada. I do believe we need to watch it closely for any clues as what kind of "readjustment" is occurring there.

The Earth does seem active right now. Can one area affect another? Plate tectonics says that it should. On Human time scales, I do not know.

geonuc · #11 (**permalink**) 07-May-2008, 11:00 AM

I'd like to read that article, but I don't take the Bulletin anymore.

jlhredshift · #12 (**permalink**) 07-May-2008, 11:50 AM

Quote:

Originally Posted by geonuc

I'd like to read that article, but I don't take the Bulletin anymore.

I know the feeling. Link to abstract. After weighing the cost vs. benefits it was decided that belonging to the GSA would gain the most bang for the buck. With all the reading I do their publications filled any time I had left. The list of journals that I would like to subscribe/read is impossibly long. And this business of only remembering a percentage of what you read is very annoying!

Untangling the rocks in the western half of the U.S. will continue to provide employment to geologists for many years!

geonuc · #13 (**permalink**) 07-May-2008, 04:09 PM

Quote:

Originally Posted by jlhredshift

Untangling the rocks in the western half of the U.S. will continue to provide employment to geologists for many years!

And enjoyment for me.

I fondly remember my weeks of field class at the Poleta Folds in the White - Inyo Mtns.

Kaptain K · #14 (**permalink**) 07-May-2008, 10:29 PM

Quote:

Untangling the rocks in the western half of the U.S. will continue to provide employment to geologists for many years!

Make that lifetimes!

jlhredshift · #15 (**permalink**) 07-May-2008, 11:31 PM

Quote:

Originally Posted by geonuc

And enjoyment for me.

I fondly remember my weeks of field class at the Poleta Folds in the White - Inyo Mtns.

It is amazing when you see rock folded like it was a bed blanket.

I found this about a north-south fault system south southeast of Reno and the referenced paper.

The Discovery News story, pg2, states:

Quote:

As a strike-slip fault, however, it could be part of a growing set of north-south running faults in eastern California and western Nevada that some geologists suspect are gradually taking over the San Andreas' role as the major boundary between the North American and Pacific tectonic plates.

I can not state that this is part of the quakes in the Reno-Mogul area, but it points in that direction.

geonuc · #16 (**permalink**) 08-May-2008, 10:24 PM

That paper talks about something I've kept an eye on for a few years (just casually, of course - I'm not a geologist) - the eastward migration of the NA-Pacific transform margin. As I recall, a lot of geologists predict the major movement on the boundary will jump to the Owens Vally area or nearby.

The Reno activity may be confusing to those who want to nail it down precisely, but I can't imagine too many geologists being surprised there's activity in the region not manifesting as Basin & Range extension.

As they say, buy your Nevada beachfront property now while it's cheap.

jlhredshift · #17 (**permalink**) 08-May-2008, 11:44 PM

Quote:

Originally Posted by geonuc

And enjoyment for me.

I fondly remember my weeks of field class at the Poleta Folds in the White - Inyo Mtns.

I am interested in the historical perspective. At the time that you took the course what was the explanation and other details of cause of the folds?

geonuc · #18 (**permalink**) 09-May-2008, 09:54 AM

Quote:

Originally Posted by jlhredshift

I am interested in the historical perspective. At the time that you took the course what was the explanation and other details of cause of the folds?

The Poleta area is complex (which is why they send geology students there).

The rocks are Cambrian shallow-sea carbonate deposits, mostly. Structurally, they've been altered by the Antler, Sonoma, Cordilleran orogenies, as well as some more recent intrusive events. I believe the Antler (Late Devonian) was responsible for most of the folding, with the later events causing a lot of faulting and rotation.

I made a good geologic map of the area (my instructor thought so, anyway), but I can't share it. The details of Poleta are supposed to be kept secret.

BigDon · #19 (**permalink**) 09-May-2008, 10:17 AM

Is it secret so they can still use it as a learning tool for future students or is that where they re-hid the All Spark?