We run shearing experiments on a Geocomp direct shear apparatus. Currently, our focus is on defining strength characteristics of subduction zone materials (primarily quartz, illite, and smectite) of the Nankai Margin. Primarily, we want to see how adsorbed water, pore pressure, and fabric affect shear stress values along subduction faults.

Our Geocomp direct shear apparatus shears a consolidated sediment sample that is a rectangular prism of dimensions 10 cm x 5 cm x 2cm. This relatively large size allows detailed visual inspection, and scanning electron microscope images, of sheared fabric. In turn, we correlate observed fabric characteristics to fault strength values (i.e. yield, peak, and residual strength).

Another goal of our shearing experiments is to constrain the effect of transient pore pressure fluctuations with incremental changes in shearing velocity. Preliminary results show that either: (1) transient pore pressure has a minor effect on overall fault strength, or (2) transient pore pressure changes are concentrated in a small, localized area of the shear zone that we haven't been able to measure. Right now we are working on our pore pressure reading instrumentation to adequately sample pore pressures in the shear zone. Our future work is going to concentrate on shearing subduction fault materials at high temperatures (100 - 300 C), and noting its effect on friction/strength values. These experiments will bring us closer to our goal of mimicking natural conditions present in subduction faults (i.e. temperature, pressure, composition) in a laboratory setting. We are currently in the process of designing the instrumentation to do these shearing experiments.

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Geocomp Large ShearTrac II Specifications:

Capacity	supports loads up to 250 kN
Strain rate	varies between 0.00003 to 15 mm/minute
Power	110/220 V, 50/60 Hz, 1 Phase
Horizontal and Vertical Travel	100 mm
Included Software	measures vertical and horizontal displacement, and vertical and horizontal stress