How do Faults Slip: Earthquakes versus Episodic Tremor and Slip

Mike Brudzinski

Miami University, Oxford, OH, brudzimr@muohio.edu

Sophomore-Junior level undergraduates, interested in Geosciences, but not necessarily a major

Students would have had an introductory geology course and need to understand:

• what a seismogram is
• what GPS data is and how to interpret a GPS time series for motion
• what a dipping fault is

This is an activity I run at the end of my Earthquakes unit (second of five units in the course). Plate tectonics, including the use of GPS data, is the first unit.

Students would understand how seismic and GPS data is used to explain how faults move.

Students would understand how to use data to interpret a physical concept.

Despite what we have learned from the theory of plate tectonics, the specifics of how those plate motions contribute to movement along faults remain a matter of much debate. Since the discovery of plate tectonics, scientists have recognized that earthquake activity, both the orientation and magnitude, is related to plate motions. However, efforts to total up the motion simply associated with earthquakes often falls far short of the plate motions. This suggests that plates have a way to slide past one another along faults without generating earthquakes, and discovering what controls whether faults produce earthquakes is critical for better characterizing seismic hazards around the world. Scientists are using a combination of GPS and seismometer recordings to investigate this issue. Some portions of a fault reveal traditional earthquake stick-slip behavior where gradual GPS motions show the fault is locked for a long time while plate motions cause stress to accumulate at the fault until the rocks break and the fault moves over the span of minutes generating large seismic signals and an abrupt GPS motion. In 2003, researchers discovered that portions of a fault also release accumulated stress more gradually over the course of several weeks in the form of a slow slip event that is accompanied by weak seismic tremors observed in a narrow frequency range that requires specific filtering to observe. These new phenomena are described as episodic tremor and slip as they recur on nearly an annual basis, much more frequently than large earthquakes which can have recurrence intervals of 50-5000 years. To better understand how faults move, this activity will examine both GPS and seismic data in the Cascadia region to identify key observations and build interpretation from them.

This assignment includes 32 multiple choice questions. A more open-ended version of the questions could be derived from the current set of questions.