Aseismic slip, which describes fault movement that is not associated with the production of elastic waves, is a phenomenon that is not fully understood. While potential explanations, such as the presence of serpentinite or lateral variations in normal stress on the fault have been suggested, the community has been unable to explain why one active fault exhibits aseismic behavior while another only behaves seismically. One key piece of knowledge that has not been fully addressed is whether aseismic slip is prevalent among the world's faults. Aseismic slip remains largely under-reported because of the difficulty in identifying fault creep in the field without installing local instrumentation (i.e. creepmeters) or by noting the deflection of urban structures from fault movement. The PI is evaluating the prevalence of aseismic slip around the globe using Interferometric Synthetic Aperture Radar (InSAR). This technique is ideally suited to identify fault creep and other aseismic transients on a global scale in a consistent manner. The growing catalogue of synthetic aperture radar data provides global coverage of the planet allowing for a systematic assessment of major fault systems. After a global catalogue of aseismic and seismic faults is established, we perform an analysis of fault related characteristics in an effort to identify which factors are most correlated with aseismic behavior. Characteristics of interest include near-fault geology, macroscopic geometry, as well as the orientation of the regional stress field. These comparisons allow the identification or exclusion of likely mechanisms that allow faults to creep and provide insight as to why one fault exhibits aseismic slip while others do not. The collection and synthesis of several global data sets provides an ideal opportunity to involve both undergraduate and graduate students in a large research project. The modular nature of the project allows students with a wide range of technical and geological skill sets to participate. To facilitate the training of students, a series of courses are developed that cover InSAR processing and theory, GIS skills for extracting fault characteristics, and focus groups where participants perform collaborative research on a particular geographic region.
The slip behavior on faults has been historically characterized as either seismic (i.e. produces earthquakes) or aseismic (i.e. creeping). However, there has been a growing appreciation in the scientific community for how faults exhibit a spectrum of slip behaviors. A litany of different names have been used to categorize these behaviors, including surface creep, slow slip, non-volcanic tremor, afterslip, low frequency earthquakes, and seismic rupture. This project explored the mechanics of aseismic slip on active faults in an effort to differentiate the fault processes or conditions that may be associated with specific fault behavior. Our research effort focused on three directions: 1) better quantify aseismic slip events on subduction zones and explore their mechanics, 2) synthesize the observations of surface creep on existing faults, such as the San Andreas, and 3) identify surface creep on new fault systems using InSAR as the observation tool. A catalog of aseismic slip events was created for subduction zones. The scaling relationships of these source parameters were explored, and compared to that found for earthquakes. A similar database was constructed for surface creep along the San Andreas fault, as well as other major strike slip faults. This project also included a major educational objective to engage students into the research tasks. Student learning is enhanced through the hands-on experience of working with real data. As part of this project, a course was designed around student research projects where the students process and interpret synthetic aperture radar (SAR) data. The students learned several skills that are critical to being a good scientist, including how to formulate a project and motivate the objectives. The skills that were taught in the course represented transferable skills. For example, many of the students were exposed to computer programming for the first time. Students also learned how best to convey their findings in both written and oral formats. For many undergraduates, this course represented their first research experience where they are working with real data (and all the uncertainties and complexities therein), and the first time they were attempting to answer an open-ended research question. This course allowed for the training of graduate and undergraduate students in the skills needed to execute the research tasks for the study of creep on faults. The course also allowed the students to explore other geological processes, including landslides, land subsidence, and volcanic inflation. The students also contributed to the creation of an online tutorial for the software used in the course.
