Energy Geosciences Division researchers  are preparing to deploy new instrumentation developed at Berkeley Lab’s Geosciences Measurement Facility in a 500-meter deep borehole drilled across portions of the San Andreas Fault near San Juan Bautista, California, this summer. The goal of the project, being carried out in collaboration with Emily Brodsky and Andrew Barbour of UC Santa Cruz and the USGS, is to document the complex interplay between variations in fault hydro-geologic properties and active fault movements related to plate boundary tectonics.

Spanning the length of California, the San Andreas Fault (SAF) zone is formed along the transform plate boundary between the Pacific and North American plates as they slide past one another. Over time, the plates themselves move slowly – about 5 centimeters per year on average, however in some years or locations there is no movement at all. Eventually the built-up strain of the plates pushing against one another breaks the rock along the fault, causing earthquakes and changes in hydrogeology.

To prepare for their work there and select an optimal drilling site location, Craig Ulrich, an EESA principal scientific engineering associate, built a local geological model to include geological, hydrogeological and seismic data (see image and video about the model building). EGD staff scientist Yves Guglielmi and GMF team are leading similar EGD research explorations on inactive faults at the Sanford Underground Research Laboratory in South Dakota, and Mont Terri, in Switzerland. In these stable fault systems, the team employs the SIMFIP probe developed at Berkeley Lab to artificially inject fluid pressure in order to activate the inactive faults or the fracture movements at very high (large 106 Pascal) fluid pressures representative of deep crust conditions.

In the project exploring the San Andreas Fault, it was necessary to develop a new probe to passively monitor fault displacement and pressure at natural ambient pressures without fluid injection. Also developed at GMF by Paul Cook and Florian Soom, this probe is more appropriate for investigating faults in active tectonic regions. Indeed, the instrumentation is capable of detecting fast (seconds to hours) three-dimensional movements on the order of micrometers to millimeters, caused by small earthquakes or creep events. It also can autonomously readjust its measurement range in order to continue reading long-term slow (days to years) displacement activity and has integrated fluid-pressure sensors in order to explore how the fault-pore pressure changes with displacements.