Seismic Radiation During Slip Along a Bimaterial Fault: An Experimental Investigation

Tanner Shadoan, Brett M. Carpenter, Ze'ev Reches, Xiaofeng Chen, & Simon Zu

Published August 14, 2018, SCEC Contribution #8529, 2018 SCEC Annual Meeting Poster #175

Large slip along faults is likely to place crustal blocks of different lithological and mechanical properties against one another to form bimaterial-faults. For example, the Punchbowl fault in southern California is a bimaterial fault with igneous and metamorphic basement in contact with the Punchbowl Formation Sandstone. To explore frictional behavior and seismic radiation of such faults, we conducted shear experiments on bimaterial-faults composed of two blocks of either gabbro, limestone, or sandstone, with nine block combinations. The blocks were in contact along a ring-shaped surface, approximately 24 cm2 in area, and were loaded in a rotary shear apparatus (ROGA) at the University of Oklahoma. Shear rates and normal stresses ranged from 1µm/s to 1cm/s and 0.5 to 5 MPa, respectively. We performed constant velocity experiments and monitored the shear stress and seismic radiation along the fault. Seismic radiation was recorded at a rate of 1 MHz via four 3D accelerometers mounted ~2 cm away from the experimental fault.

The frequency spectrum data of the laboratory seismic events revealed differences in seismic behavior associated with rupture processes and ray path characteristics. For example, for an experimental fault composed of gabbro and limestone, the recorded events on the more compliant side (limestone) of the fault have high intensities at lower frequencies. We deduce that the ray path, which is controlled by the rock block properties, is responsible for this difference. When keeping the ray path constant (i.e. accelerometers mounted on gabbro) while changing the other fault block, to either gabbro, sandstone, or limestone, also indicate differences in the frequency content, this time due to rupture characteristics along the fault. By taking the spectral ratio of two events, we found that experimental faults with a compliant rock, e.g., sandstone, have lower energy at higher frequencies. This suggests that compliant rocks attenuate higher frequencies more than less compliant rocks, e.g. gabbro. In summary, the experiments show distinct dependence of both friction and seismic radiation on the mechanical properties of the three lithologies used here, and it is anticipated that related effects would occur along natural bimaterial faults.

Key Words
Bimaterial faults, acoustic emissions, seismic radiation, experimental faults

Citation
Shadoan, T., Carpenter, B. M., Reches, Z., Chen, X., & Zu, S. (2018, 08). Seismic Radiation During Slip Along a Bimaterial Fault: An Experimental Investigation. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)