Economics and Geosciences
United States Air Force Academy
Thermal Infrared Imagery/Differentiating geologic units part of Cutting Edge:GIS and Remote Sensing:Activities2
Sarah Robinson, U.S. Air Force Academy Summary A two part lab in which students a) manipulate thermal imagery to look at rock types and b) determine what wavelengths are appropriate for discriminating between rock ...
Remote Sensing and Imagery Analysis part of Cutting Edge:GIS and Remote Sensing:Courses
This is an introductory remote sensing and imagery analysis course.
Using geomorphology to determine tectonic slip at Wallace Creek part of Vignettes:Vignette Collection
The Carrizo Plain along the San Andreas fault in California is world-famous for its dramatic offset features. Sag ponds, linear ridges, beheaded channels and scarps define the landscape along the fault (Vedder and Wallace, 1970; Wallace, 1975; Sieh and Wallace, 1989; Arrowsmith and Zielke, 2009; Figures 1-4). These geomorphologic features allow for the identification of a right-lateral strike-slip fault. The most famous offset feature is the offset channel at Wallace Creek (Sieh and Jahns, 1984; Figure 1). Using offsets such as the one at Wallace Creek and others nearby, tectonic slip that has accumulated over different timescales can be determined. Knowledge about the release of tectonic slip in earthquakes informs our understanding of the earthquake process and how it relates to the long term relative motion of the plates (Figure 1), and it is a basic ingredient of earthquake hazard assessment. In order to understand how tectonic slip can be determined from offset features, several geologic and geomorphologic concepts must be considered. First, ground-rupturing earthquakes have formed the offsets like the one created at Wallace Creek. Second, these channels form as a result of runoff driven by strong precipitation events, which may or may not occur more often than earthquakes (e.g., Grant-Ludwig, et al., 2010). Channels are subsequently offset by ground-rupturing earthquake events. Thus, the geomorphologist can analyze channels which are offset to determine the timing of ground-rupturing earthquakes, as well as the amount of slip associated with these events. Subsequent geomorphic modifications by erosion and deposition can complicate the interpretation. Assuming that incision events occur frequently enough to record the tectonic (i.e., earthquake) history, the offsets can be measured to determine tectonic slip. If the channels can be dated accurately, slip rate may be calculated. If the earthquakes can be dated, we can attribute the amount of slip to individual earthquakes (usually the last) or to sequences of earthquakes. Slip released in an earthquake is an indicator of that earthquake's magnitude, and if we divide the slip per event by the slip rate, we calculate an average earthquake return time. The San Andreas has a long slip history. Many older channels have been offset more than once. The main channel at Wallace Creek has been offset 130 meters. However, just south of Wallace Creek, smaller offsets can be measured, some between 5 and 8 meters. Such high gradients in slip (100 m) are not expected. Therefore, Wallace Creek is the result of repeated earthquakes, offsetting the channel multiple times while remaining active. Researchers have determined via carbon dating of sediments deposited just before Wallace Creek cut down that the currently offset channel is 3700 years old (Sieh and Jahns, 1984). Knowing the total offset of Wallace Creek (130 m) and the age of the channel, an average rate of motion can be easily determined, giving a rate of 35 mm/yr. This is consistent with GPS and other geodetic data measuring the relative motions of monuments spanning this portion of the San Andreas Fault over the last few decades (Figure 5). The monuments are separated by tens of kilometers and measure the broad warping that loads up the fault and which is released abruptly during an earthquake. Thus, the strain accumulation (measured by geodesy) and the strain release (measured by the offset channel) along San Andreas Fault are similar at about 35 mm/yr at Wallace Creek. We do not see any motion immediately at the ground surface at Wallace Creek because the San Andreas fault is locked. As Figure 1 shows, the San Andreas Fault only accommodates about two-thirds of the total relative plate motion between the Pacific and North American Plates. The last ground-rupturing earthquake at Wallace Creek occurred in 1857 (Sieh, 1978). Thus, over 150 years later, the portion of the San Andreas Fault below Wallace Creek has accumulated over 5.25 meters of slip not yet released. Early investigations of the topography and stratigraphy along the San Andreas Fault near Wallace Creek determined that the offset in 1857 (and prior earthquakes) was 8-9 m (Wallace, 1975; Sieh, 1978; Sieh and Jahns, 1984; Liu-Zheng, et al., 2006), which combined with the 35 mm/yr slip rate gave an earthquake recurrence interval of about 250 years (Figure 6). However, looking more closely at high resolution topography and aerial photography around Wallace Creek (Figures 2, 3, and 4), Zielke, et al., 2010 found evidence that slip in the last earthquakes was 5-6 m or less and thus the return period for earthquakes is closer to 140 years or less and similar to what has been determined for the dates of recent earthquakes (Akciz, et al., 2009) and to the elapsed time since the 1857 earthquake.
Using technology as an aid to the geomorphologist part of Vignettes:Vignette Collection
Geomorphology requires characterization of the earth's surface at sufficient high resolution in 3 dimensions to explicitly represent landforms. Measuring change requires repeat survey, thus adding the 4th dimension. One exciting powerful tool that many geomorphologists have begun to use in their research is LiDAR (Light Detection and Ranging). LiDAR technology measures the earth's surface by using a scanning laser along with inertial navigation (IMU) and GPS to measure the landscape at high resolution (decimeter accuracy). LiDAR can be used to create high-resolution imagery, reconstruct topography and allow geomorphologists to evaluate the landscape at a level beyond the capabilities of photography, satellite imagery, and topographic maps alone. Although it is not a replacement for being in the field and seeing things firsthand, it allows for an advanced level of analysis away from field sites as well as increasing efficiency and thoroughness in the field. An important capability associated with LiDAR data is processing to virtually "remove" vegetation and obstructions in order to see the bare earth underneath. This can allow for the identification of features such as faults or landslides otherwise invisible under the vegetative canopy. It can also help with field planning in order to know where to go to find various features. And, ecologists use the data to characterize the canopy structure and its relationship to the underlying topography. Collecting airborne LiDAR for field study is both expensive (several hundred dollars per square kilometer) and involved. In order to collect the data, a low-flying aircraft scans a laser at pulse rates of 10s to 100s of kilohertz. Laser returns are collected within the aircraft, cataloguing the timing of return, the scanner orientation (IMU), and position (using GPS). There are also terrestrial LiDAR (TLS) units that allow for LiDAR data to be collected for a small area from a ground-based vantage. Many of the large LiDAR datasets such as the "B4" project flown along the southern San Andreas Fault in California are available online for free. One of the more dynamic free LiDAR websites is www.opentopography.org where many of the western United States airborne LiDAR datasets are available to download. Once LiDAR data have been collected for an area of interest, geomorphologists can use the data products for research (See Figure 1). LiDAR data can be processed and made available in many different file formats, allowing for usage in computer programs like ArcGIS, Matlab, and the free program Google Earth. The applications of LiDAR for the geomorphologist are vast. LiDAR can be used to locate faults and measure offsets that could not be measured previously with satellite imagery alone (See Figure 2). The high resolution (<1m) of LiDAR topography allows for more detailed analysis at the sub-meter scale. Repeat LiDAR or comparison of LiDAR with aerial photography can be used to evaluate landscape change due to rivers and streams, landslides, coastal change, volcanic activity or earthquakes. LiDAR can be used to generate topographic maps, cross sections, geologic maps, and 3-D imagery. One interesting study involving LiDAR and its applications to geomorphology is the work of Dr. Ramon Arrowsmith and Dr. Olaf Zielke (Arizona State University) studying offsets along the San Andreas Fault in California. Recently active fault breaks along the San Andreas Fault were extensively mapped using the B4 project LiDAR data, and troughs, ridges, sags, and offsets channels along the zone were assessed confidently (Arrowsmith and Zielke, 2009). Using the LiDAR data in Matlab, offset channel profiles can be back-slipped and a best fit determined, allowing for total offsets to be analytically calculated. This process has traditionally been done in the field, but LiDAR allows for many of these offsets to be analyzed in a matter of minutes.