Class Exercise – Field Methods in Active Tectonics
Mapping and Interpreting the 2010 Mw 7.2 Baja Earthquake
· Learn to navigate and work in Google Earth Pro
· Import data – Baja California Earthquake and Eastern California Shear Zone
· Create maps of fault ruptures and alluvial deposits
· Annotate maps with observations in GE Pro
We first need to get access to Google Earth. The GE Pro version also allows data to be imported, and exported to excel, quickly measure features (line lengths, etc) and easily create topographic profiles. It also allows you to make movies such as flyovers along faults. In short, GE simulates programs such as MapInfo and ArcInfo that while more comprehensive, have a longer learning curve.
For the class exercise we will download, map and interpret a complex fault zone defined by a lidar dataset flown shortly after the Magnitude 7.2 Sierra Cucapah-El Mayor Earthquake in northern Baja California. Follow the set of instructions below to begin the exercise.
Obtain access to Google Earth. Sign up to get started on Google Earth see http://www.google.com/earth/index.html for how to gain access. If you want, you can get the Google Earth Pro version, which is free for students. It has the advantage of higher resolution output files.
The application will launch and a window that welcomes you to Google Earth will open. Select the 'Auto login' check box in order to avoid this prompt in the future, then click "Log In". You should see a spinning globe in the main viewer window.
Welcome to Google Earth! Google Earth Help Center:
http://earth.google.com/tour/ Google Earth Tutorials: http://earth.google.com/userguide/v5/tutorials/ Google Earth User Guide:
http://earth.google.com/support Google Earth Product Tour:
http://earth.google.com/userguide/v5/ Google Earth Movie Making Guide:
The tutorial and user guide are particularly useful, the others less so.
Spend some time cruising around GE Pro. Look for areas of interest (you can zoom there directly by typing the location) and check out the resolution of the images and other goodies. Some of the existing layers are useful, many are not. For starters go to the Benson Earth Sciences Building. I’ve had issues with a slow refresh rate at home, so it’s helpful to work where you have fast access. As you move around to points of interest it is very helpful to learn a few key commands, rather than use the direction finder at the upper right side of the screen (which is tedious). I can help show these to mac users, PC’s will need to get these from the tutorial. On a mac, command N pressed simultaneously returns the scene to north as up on the page. Command U brings the scene back to nadir, or looking directly downward. Control, mouse held down zooms and rotates, while shift, mouse held down moves the scene in 3D (i.e. obliquely)
Now let’s upload the data you want to use for the exercise. All the NCALM and Earthscope Lidar is readily available online and shaded relief maps have been created for all the existing fault lines from gridded data. Downloading the raw point cloud data takes forever. Consider for a minute the amount of data involved in lidar datasets. The Cucapah data strip is about 3 x 120 square kilometers. The raw data is about 0.5m or more in resolution. So that’s 360 km2 x 1,000,000 m2/km2 x 4 (0.5 pixels/m2). Roughly 1.5 billion data points minimum. Each of the areas you will map in the Eastern California Shear Zone is comparable in size.
Note that we’ve created colored shaded relief maps for all the areas we will visit or you will map on the Death Valley field trip. These will be available as .kml files that you will need to obtain (with a memory stick, the files are huge). But in the meantime, you can use the .kmz files I emailed to you to access the imagery
45-Degree Sun Angle (1st return).kmz
This is a shaded relief map available for the region. It creates a scene where illumination is at a low angle from a northeast (45 azimuth) direction
Scarps_2010.kmz is the file with the 2010 ruptures for your interest.
Put the resulting file into My Places in GE (everything is shown on the panel on the left side of the GE window). Or you can just drag and drop in a mac.
Now for the fun part
Review the dataset to see if you have the right file. Locate the 2010 earthquake rupture relative to California and Baja. It’s located at the northern end of the Gulf of California, the modern Pacific – North America plate boundary at this latitude. Answer the questions posed below. This exercise will form ~20% of your grade in the course.
To get a sense of the pattern of faulting in the 2010 earthquake, upload the file scarps_2010.KMZ (emailed to you). This is a map of the surface ruptures that formed in the Magnitude 7.2 El Mayor-Cucapah earthquake in April 2010. This map was the result of months of field mapping using Lidar, low elevation aerial photos and surface mapping by a team of experienced earthquake geologists. Led by John Fletcher and his postdoc Orlando at the Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California, it is state of the art, nowhere has an earthquake rupture been mapped in this much detail and accuracy. The lidar was flown specifically for this work. I will also send you a recently published paper from this mapping that I urge you to read as preparation for the field trip
Look at the 45o (azimuth) grey scale shaded relief map. Why was this image created at this orientation in the first place? (Shaded relief maps are made by illuminating topography from a specific perspective, including the direction relative to north the sun is located and the angle between it and the horizon). What does this tell you about the sense of vertical displacement across each of the ruptures? Would you expect strike slip faults to show up as well on the imagery? Which fault scarps are easier to spot on the 2010 rupture, the ones facing NE, or the ones facing SW? Why is this?
Given that the scarps that displaced alluvium form nearly vertical faces, how would the inclination of the illumination determine what you would see on the shaded relief map? By this, I want you to relate the strike and sense of dip slip on a scarp to both the azimuth and inclination of the illumination on the shaded relief scene. Draw a 3D block diagram to illustrate the above. Does the width of the shaded area always correspond to the width of the scarp? What would have to happen to have the width of the scarp wider on the image than it’s true width (i.e. fault heave) on the ground?
Look at the original GE satellite image. Identify alluvial fans in the region. Why are some alluvial fans larger than others (assuming that rainfall and rock type is the same across the region)?
Given that sedimentation rate is important in earthquake geology (the faster the sedimentation rate, the more likely a given surface rupture or deposit will be buried), how would you expect sedimentation rates to vary in the region (i.e. where are the areas located where sedimentation rate is higher or lower). Show this by adding pins to the landscape (appropriately labeled as higher or lower) and then by saving an oblique scene of the location (and then including in your report). Why would an earthquake geologist worry about the rate of sedimentation along an active fault they were going to measure displacement along an old earthquake scarp? What parts of the fault would be the best place to measure vertical displacement, at the head of fans, or at their edges?
Now for the hard part.
Create a map of the faults in the region shown on the file I have emailed to you (Laguna Salada Map Area to students.jpg). The map should be shaped like a strip centered over the contact between alluvial deposits along the sharp linear rangefront and the crystalline rocks that make up the high part of the range. It should extend into the lake deposits (marked by the “bathtub ring” beach ridges that mark old lake shorelines) and southward to the exposures of red sediments offset across a large releasing bend.
Faults should be mapped using the path tool (see header). Keep the line weight at 1.0, and colored red (you can change this with the get info command). You will eventually create a bunch of kmz files that when turned on will define the fault map. The final map (that you turn in for a grade) will be a screen shot of the area, oriented similarly as the strip.
Basically you are looking to map scarps that mark faults that rupture the ground surface. The most obvious fault is marked by the basement/alluvium contact in the northern part of the region. This part of the fault ruptured in a large earthquake in 1892 AD. Other scarps offset alluvium and cross alluvial fans.
Some things to consider.
The age of surfaces on alluvial fans vary, and can be related to: 1) switching of active channels (a depositional process called avulsion related to buildup of sediment and movement of the channel into lower areas) and/or 2) offset of the ground surface that affects local base level. Remember that a decrease in base level (such as from a fault rupture that faces towards the basin) causes incision on the higher side of the fault. The converse is true where scarps face upstream, sediment builds up behind them (an increase in local base level). This typically causes streams to be deflected laterally and the scarp to become undercut and eroded.
Alluvial fans in arid regions usually consist of surfaces of markedly different age. Related to the time of abandonment by active stream channels, fan surfaces undergo geomorphic processes that include:
Weathering (mechanical and chemical) weathering of rocks and boulders left on surfaces. This is often accelerated by the presence of salt where fault systems bound playas with evaporites (halite and gypsum) in their centers.
Development of surface patina. Rocks in arid deserts often have a dark coating comprised of iron and aluminum sesquioxides deposited on clasts that increase in intensity with age. Called desert varnish, this causes fan surfaces to darken with age. This will be apparent in the study area in Baja, as well as the regions we will visit on the field trip. Note that rocks that are relatively more silica rich (quartzite is a good example) form better patina than clasts made of limestone (which often won’t form a patina at all).
Desert pavement. Older fan surfaces weathered in very arid regions (i.e. 50,000 years or so) can be characterized by clasts that fit together into a close pattern, almost as if they were arranged (by man) that way. Related to wind deflation (removal of fines) and/or shrink-swell processes in underlying clayey argillic soil, these correspond to fans with dark, almost black surfaces with very well developed rock varnish.
Surface roughness Alluvial fan surface become smoother with time (it’s easier to walk along older surfaces with desert pavement). In contrast, younger surfaces have bar and swale topography that is considerably rougher (you will see what I mean when we walk across young fans made of coarse deposits in northern Death Valley, north of Redwall Fan). This is particularly apparent at the scale of lidar ~1m for the data we will use.
All these characteristics (darker colors, surface roughness) allow one to determine a relative chronology of deposits preserved along an active fault system. Given that fault scarps are continually modified with time (buried or incised), older surfaces will preserve a longer record of movement on a particular fault strand. Conversely, younger deposits preserve a shorter record of earthquakes. So displacement on a particular scarp should be greater as a function of the age of the deposit/surface it cuts. If a deposit is younger than the last surface rupture, a scarp will not be present. This can be a challenge where scarps end for structural reasons (i.e. the ends of a fault segment).
Once you have mapped the fault scarps in the region, you will need to identify and map (in general) different fan surfaces. Use the polygon tool to mark the edges of fan surfaces from different depocenters with green lines to distinguish them from fault surfaces.
In modern studies, various geochronologic methods are used to directly date alluvial deposits, such as cosmogenic nuclides, optically stimulated luminescence and radiocarbon dating (see paper by Frankel at Red Wall fan).
Locating sites for defining a history of multiple paleo-earthquakes. The absolute crux to becoming a qualified earthquake geologist, or paleoseismologist is determining where to excavate trenches or date deposits. Please pick the two best sites for such an analysis by examining the lidar data carefully for a set of deposits that are progressively offset by a fault (typically terrace deposits in stream channels) in the study area. Hint: One site is on the range-bounding fault and another on one of the large scarps further out in the alluvium. Both sites sit in the southern half of the study area. This is not an easy question!