Earthquake Swarm near Brawley, CA

This afternoon, in the course of a few hours, dozens of earthquakes shook the area around Brawley, CA along the southeastern side of the Salton Sea.  The largest of these quakes had a moment magnitude of 5.5 and a Mercalli Intensity at the epicenter of VII.  Below is a map and a partial list of the larger quakes, from the USGS.

On this map, the two darkest red traces, are the traces of the San Andreas fault.  If you were to walk along either strand, toward the other, you would have to make a big step to the right, right at  Brawley.

Because the San Andreas fault is a right-lateral strike slip fault, the western portion of this map is moving to the northwest and the eastern portion of this map is moving to the southeast, relative to each other.  This means that the crust at this stepover is tending to have a big hole ripped in it.  Since we cannot rip big holes in the crust, the crust instead stretches to fill in the gap.  In the upper crust it does this by forming normal faults.  The predicted orientation of these normal faults is about 120 degrees away from the trace of the San Andreas fault.  Interestingly, the orientation of the swarm at Brawley is more nearly perpendicular to the San Andreas trend, as is the trend of topographic features in the area, suggesting this is a long-lived fault orientation.  Update (8/27/12) The centroid moment tensor solution http://earthquake.usgs.gov/earthquakes/eqarchives/gcmt/neic_c000c7i2_gcmt.php shows that the largest of these quakes moved with slightly oblique strike-slip motion on either a right-lateral fault paralleling the San Andreas trend or a left-lateral fault perpendicular to it. 

In this area, the Salton Sea owes its existence to these normal faults and to the stretching of the crust.  The stretching occurring at this stepover is producing a depression, the Salton Sink, which filled with water when a 1905 flood overwhelmed a newly-constructed irrigation canal and diverted water from the Colorado River into the depression.  The diverted waters rapidly eroded the soft soils in the area, creating new river channels.  The flooding of the Salton Sink went on for two years before finally being stopped, leaving the large puddle of water we now call the Salton Sea.

Earthquake swarms are distinctive in that they have several to many quakes of similar magnitude along a fault system, as opposed to the more common pattern of a single large mainshock followed by many smaller aftershocks.  There are many reasons that have been suggested, including effects of fluid or magmatic pressures.  There are no active volcanoes at Brawley, so we can rule out volcanic mechanisms, but it is possible that some sort of hydrothermal or other fluid flow played a role. 

Massive Earthquake Hits Seismic "Gap" in Chile

Last night a M 8.8 earthquake struck offshore of Chile. Below is the helicorder image for the Da Vinci Science Center's seismometer, as of 9:15 AM EST, 2/27/10.


The live update of this seismometer can be found at http://www.davinci-center.org.

A detailed description of this earthquake can be found at http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/us2010tfan.php

This earthquake happened on the plate boundary between the Nazca plate and the South American plate, where the Nazca plate subducts eastward underneath the South American plate. This is the same plate boundary that gives rise to the volcanoes that form the core of the Andes Mountains. The portion of the plate boundary that slipped in this earthquake was between the portion that slipped in a M 9.5 quake in 1960 (the largest earthquake ever recorded by modern instruments), and the portion of the fault that produced a M 8.5 earthquake in 1922. Both of those earthquakes produced tsunamis that affected Hawaii and other areas of the Pacific rim. For more information of these tsunamis and others, see http://www.tsunami.org/index.html.

Miocene to Recent sediments of the northern Apennines

The materials we were studying in the Northern Apennines were very young sediments (geologically speaking, at least, where 1 million years is not much time at all). Below is a series of photos and descriptions that attempt to lay out the sequence of these rocks in the way they would be seen in a geologic column, youngest on top.


At the top of the section are the very recent Quaternary terraces, Qt8 and Qt9. These are 50-year old floodplain deposits that have been incised by downcutting related to gravel mining that began shortly after WWII.

Each of the surfaces separating these units in an unconformity (time passed without deposition of sediment). The lower boundary of Qt8 in this picture is an excellent example of an angular unconformity.


Here we see Qt3 on top of Qt2. The top of Qt2 has a deeply weathered "fossil" soil or paleosol. The presence of carbonate nodules in that soil indicate a prolonged period of warm and dry climate. The lower gravel (Qt2) was deposited about 450,000 years ago and the soil developed about 400,000 years ago





Here is Qt2 on top of Qt1. Qt1 is the oldest Quaternary gravel stream terrace in the region. It is moderately tilted everywhere along the mountain front and is characterized by a reddish weathering soil at its top. This soil developed about 630,000 years ago







This is the upper mud in the AEI (Association Emilliano Inferiore - Lower Emillia Association). The AEI consists of alternating lake muds, representing glacial periods, gravels, representing glacial melting, and soils developed on top of the gravels, representing interglacial times. These sediment cycles occur due to 100,000 year cycles in the eccentricity of Earth's orbit. The reddish strata in the background are debris flow gravels that lie on top of this mud.





Here is another mud interval (yellow and grey material at the bottom of the photo), overlain by a thin fluvial gravel and then by debris flows. This is the second of three such cycles in AEI.
















Here is a fossil cypress(?) stump at the top of the lowermost AEI mud unit This shows the unit was deposited on land,or in very shallow water








This is the lowest unit in AEI. This mud contains terrestrial fossils and was deposited in a lake on land. The base of this mud is roughly 800,000 years old.

Somewhere in the AEI unit, should be the most recent magnetic reversal in Earth's history. At this time, the magnetic field went from a period of reversed polarity (Matuyama chron) to a period of normal polarity (Brunhes chron). The Brunhes-Matuyama reversal occurred 780,000 years ago.




These interbedded sands and gravels lie at the very top of the Sabbie Gialle.

This unit is approximately 1 million to 800,000 years old








The muds of the Argille Azzure are overlain by medium to thick bedded sands and gravels of the Sabbie D'Imola (Sand of Imola), locally known as the Sabbie Gialle (Yellow Sand)








In the upper part of the Argille Azzure, fossils are abundant. In particular, there are beds of very coarse calcarenite (calcite sands) made of shells and shell fragments. These beds range from 3.1 Ma (million years ago) to 1.8 Ma, corresponding to the Gelasiano and Piacenzian Ages of the middle Pliocene Epoch.







These grey muds of the Argille Azzure show distinct bedding. Much of the Argille Azzure is completely homogeneous mud with no evident bedding. The differences between the bedded and unbedded parts may reflect differences in the depositional environment, in the sediment supply, r in the degree of bioturbation (churning of the sediment by living creatures). The Argille Azzure was deposited between approximately 4.9 and 1 Ma (million years ago).



Cross-bedded sandstones of the upper Miocene Colobacci Formation. This formation is 5.6-5.3 Ma in age. There is an unconformity (time without deposition) between the Colombacci and the overlying Argille Azzure.










Beneath the Colombacci is a very unique rock unit, formed roughy 6 million years ago. These are the Messinian evaporites. This picture shows bedded gypsum that grew out of the evaporating waters of the Mediterranean Sea. Above these rocks lie layers of gysum crystal conglomerates (resedimented gypsum).












Here is a detailed view of the gypsum crystals that make up the Messinian evaporites in this section.

Day 3 in the field: we have measured and sampled more than 60 meters of sand and mud, sampling every .75 m. The rocks are Miocene and Pliocene and are barely lithified. They look like rocks, but you sink in to your ankles when you step in a wet spot. We've also seen a whole set of Quaternary gravel terraces, some of which are tilted by deformation.

More details on Thursday!

Getting ready for Italy

Right now, the geology on my mind is the geology of the northern flank of the Apennine Mountains in Italy. From April 30 to May 12, I will be joining a team of structural geologists, tectonicists, sedimentologists, and paleomagnetists to study Plio-Pleistocene sedimentation along the north flank of the tectonically-active Apennine Mountains, in and around the spa town of Salsomaggiore Terme (salty thermal springs). The project is evaluating possible links among tectonics, sedimentation, and climate in the development of mountain belts. I hope to blog from the field, although I may be limited by how often I have access to the Internet.

A lot of what we will be doing is detailed sampling of Plio-Pleistocene sediments in river banks, collecting mud that is just barely lithified, for paleomagnetic analysis to establish a detailed mangento-stratigraphy. Once a chronology can be established, then depositional rates can be determined and compared to the tectonic and climatic history of the area. It's a different kind of field work from anything I have ever done before and I am really looking forward to it.

My role, beyond providing labor in the field, is to use the work of the project to create classroom materials for earth science teachers. I will be looking for ways in which the data sets we generate can drive useful inquiry by students at a variety of levels. I hope to create materials that are specifically connected to existing inquiry-based earth science curricula (EarthComm and Investigating Earth Systems), but will be useful to teachers of any curriculum. If you have any comments about what would make such materials most useful, please share them.