Geologist's work yields insights into fault

LONG BEACH - Coping with the summer heat at the base of Southern California's San Jacinto Mountains, boiling mud at the Salton Sea, and chilly Arctic summers and wandering polar bears on Norway's Spitsbergen island are all part of Nate Onderdonk's work as an assistant professor in the Department of Geological Sciences at California State University, Long Beach (CSULB).An expert in tectonics and geomorphology - how landforms evolve over time - Onderdonk has been busy with a variety of field studies focusing on how faults are changing the landscape at home and abroad. One of his primary interests is the San Jacinto Fault Zone, which runs along the western side of the San Jacinto Mountains near Hemet and Moreno Valley, and north into the Cajon Pass. It's one of Southern California's most seismically active areas and poses a threat to the Inland Empire. He recently received an additional $65,516 grant from the U.S. Geological Survey's Earthquake Hazards Fund, the latest of several USGS grants over the past five years to study the fault's history. "That project is all about figuring out the earthquake hazard of the San Jacinto Fault and the pattern of prehistoric earthquakes-how often they occur and when was the last one-so we can guess at when the next big earthquake should be expected," he said. "I just finished writing a paper which will be submitted to the Bulletin of the Seismological Society of America. We've been able to figure out the timing of the last seven earthquakes and it's interesting because it looks like they occur about every 200 years on average, and the last one was a little over 200 years ago. The USGS grant money is to allow us to look even further back in time to see whether that's a consistent pattern." This month, he and colleagues from San Diego State and Cal State San Bernardino, along with several undergraduate and graduate students, will begin cutting a new series of trenches across the fault and will continue the work through next year. By examining layers of earth and traces of fault movements, they can learn more about the frequency of historic quakes. Onderdonk also is intrigued by the boiling mud volcanoes called gryphons, steaming mud pots and other geothermal features at the southern end of the Salton Sea. The San Andreas Fault runs through the area, allowing water to seep deep underground and become superheated. Power companies are taking advantage of the naturally occurring steam to run electricity generation plants. After Norwegian research colleagues introduced him to the locale, he thought the area was just a geological curiosity until he realized its value to his research agenda. "We've been working down there for nearly six years. One of the goals was to monitor the temperature of these things and see how they relate to local earthquakes. I also have been doing differential GPS, which is surveying and watching how they grow and erode. We're trying to understand the process that controls the growth and shape of these things." Their latest work appeared in the July 15 issue of the journal Geomorphology. Initial written reports describing the gryphons date from the early 1800s, but the tallest ones are only about six feet, he said. "We're trying to figure out where all this mud is going and why they're not getting any bigger. The area around them is actually dropping. You see little ring faults like a caldera volcano. As the mud comes up, it leaves a void below and the whole thing collapses back." The area also fascinates the public and visitors often stop by to ask what his group is doing. "Nobody comes to watch you study a fault that could devastate L.A. because it's not as easy to see and comprehend. It changed my view of things because we're always focused on, 'Why would I bother to do this unless it's beneficial to society or advances something?' One of my main goals in getting involved is to say, 'What if we can see temperature pulses right after an earthquake, or more interestingly, right before an earthquake? That might be able to give us a warning.'" "We were monitoring temperatures, and throughout the first two years there really wasn't much activity. Then in May 2009 there was a swarm of quakes at Bombay Beach, which is very close to this spot; a couple of magnitude 5s. I got really excited because I have eight little metal loggers that we put into the mud. I thought, 'This is great; we've got decent size earthquakes nearby.' So, we went out there and we were not able to recover any of them. All eight of them had been basically melted off the wires. One possibility is that there actually was a huge pulse that melted the ties that we had, but we don't have the data, so we don't really know. We purchased more loggers and put them back in," to continue long-term study. Thus far, he said there isn't much correlation between temperature changes and smaller magnitude quakes. However, "I was down there the last week of May to look at a field of vents that has recently become exposed on dry land as the water level in the Salton Sea has been dropping. I threw a couple of temperature probes in and found temperatures boiling at 100 degrees Celsius, which is 30 degrees hotter than we've measured at other places in the area. This new spot is significantly hotter with a lot more gas coming out." In earlier years when water was present, workers and lake visitors reported seeing bubbles there, so this now-exposed ground presents a new study site. Onderdonk's work has taken him to colder climates as well. After earning his bachelor's in physics from Principia College in Illinois and his master's and Ph.D. in geology from UC Santa Barbara, he did postdoctoral research at the University of Oslo, Norway, and has studied in Greenland. He spent part of summer 2004 working on Spitsbergen with two colleagues from Oslo to study landslides that occurred 120 million years ago. "They're preserved in thick layers and are dramatic looking. Big blocks slipped down and they're now exposed along a coastal cliff along the eastern side of the island," he said. "Depending on how you interpret what the causes of these landslides were, that will change our ideas of what the geologic units look like offshore at that point where they've been exploring and drilling for oil," he explained. "Before, they've been interpreted as slides at the edge of the continental shelf, analogous to what you see at the mouth of the Mississippi River delta. It's a very shallow sloping offshore environment. Much of the data that I've collected suggested that these are actually due to a fault. They're landslides that are falling off the fault scarp. That's significant because we weren't aware that there were active faults at that time in that area, which would change the interpretation of offshore locations. The oil industry always wants to understand the architecture of the basin where the old rivers were coming out, because oil collects in these offshore environments," he said. Onderdonk called the project both fun and challenging. "We actually lost about five or six days of working time because of polar bears," that could present a danger while the scientists worked along confined cliff spaces. Weather delays in an area where the field season is only about six weeks long didn't help, either. Nevertheless, their efforts resulted in an article in the December 2010 issue of Marine and Petroleum Geology.

********** Published: August 18, 2011 - Volume 10 - Issue 18