UC Berkeley

Earthquakes are simulated in the laboratory using a homogeneous rock analog material, taking advantage of highly controlled conditions to investigate the source properties of asperity rupture. The frictional interface consists of two transparent Poly(methyl-methacrylate) (PMMA) blocks with self-similar roughness profiles imposed by sand blasting and laser engraving to match our understanding of rock-rock interfaces. The resulting interface is imagedby a pressure sensitive film revealing the real contact area spread over sparsely distributed 1 mm patches called asperities. Stick slip events on the direct shear fault are recorded by an array of absolutely calibrated, single component displacement acoustic emissions (AE) sensors. Rupture events which are fully contained within the fault surface have radiation patterns consistent with double couple source mechanisms aligned with the fault plane and strike, making them earthquake-like events at the millimeter scale. A catalog is developed containing 50 Mw-7.5 to Mw-6.5 events which exhibit self-similar scaling properties in estimated magnitude and rupture area, which connects these events to previously observed earthquake scaling behavior at laboratory and natural scales.

The acoustic emissions sensors are absolutely calibrated by a method combining well characterized, broadband calibration sources with a Green’s function model for wave propagationthrough the PMMA base plate. The Green’s functions are carefully tuned for the samplespecific material properties of the plate, including anelastic attenuation. Each sensor is individually calibrated and found to have a sensitivity on the order of 1 V/nm. Displacement records for the experimental stick-slip events have sudden onsets, with first arriving pulses around 0.01 - 1 nm in amplitude and 1 - 4 μs in duration. The arrival pulses are often complex in shape, such as having two peaks rather than one smooth pulse. Displacement arrival pulses are directly proportional to the moment-rate function which describes the distribution of slip during the event source. Furthermore, the magnitude analysis of the events indicates a rupture area at the appropriate scale but generally larger than any one asperity observed on the fault surface. Together, these observations indicate that rupture events recorded likely have coseismic slip spreading over multiple nearby asperities.

In order to study the complex, multiple-asperity sources, a finite source inversion methodfollowing Hartzell and Heaton [1] is developed. The method is tested on forward modeled synthetic asperity ruptures, beginning with simple ruptures on single, circular asperity models. Although a brief attempt is made at applying the method to recorded experimental data, more testing and sensitivity analysis is required before the method may be reliably used. Continued work in laboratory scale seismic inversions shows great promise in providing a highly controlled environment for detailed source study of earthquake-like events.