Abstract
A non-contact, wideband method of sensing dynamic fault slip in laboratory geophysical experiments employs an inexpensive magnetoresistive sensor, a small neodymium rare earth magnet, and user built application-specific wideband signal conditioning. The magnetoresistive sensor generates a voltage proportional to the changing angles of magnetic flux lines, generated by differential motion or rotation of the near-by magnet, through the sensor. The performance of an array of these sensors compares favorably to other conventional position sensing methods employed at multiple locations along a 2 m long × 0.4 m deep laboratory strike-slip fault. For these magnetoresistive sensors, the lack of resonance signals commonly encountered with cantilever-type position sensor mounting, the wide band response (DC to ≈ 100 kHz) that exceeds the capabilities of many traditional position sensors, and the small space required on the sample, make them attractive options for capturing high speed fault slip measurements in these laboratory experiments. An unanticipated observation of this study is the apparent sensitivity of this sensor to high frequency electomagnetic signals associated with fault rupture and (or) rupture propagation, which may offer new insights into the physics of earthquake faulting.
Highlights
Many laboratory rock friction experiments have previously focused on steady-state or quasi-static fault motions, related to testing empirical rate- and state-dependent friction constitutive laws [1,2,3,4,5,6,7,8,9]which are widely used to model a variety of earthquake processes
Once the magnitude of the applied magnetic field exceeds the saturation threshold of the Anisotropic magnetoresistance (AM) material, and the path of current flow through the AM material is fixed, the electrical resistance of that AM material should only vary with the direction of the applied magnetic field, which is the principle behind the functionality of this AMR sensor
Calibration data from the AMR sensors were obtained from five separate sensor/magnet pairs
Summary
Many laboratory rock friction experiments have previously focused on steady-state or quasi-static fault motions, related to testing empirical rate- and state-dependent friction constitutive laws [1,2,3,4,5,6,7,8,9]which are widely used to model a variety of earthquake processes. A critical part of laboratory rock friction experiments such as these, is the ability to make detailed and accurate measurements of the fault slip. A variety of position sensing technologies have been employed in laboratory experiments to measure fault slip, including, but not limited to; linear-variable differential transformers (LVDT and DCDT), magnetostriction devices, eddy currents, linear capacitors, foil strain gages, and optical methods. All of these technologies can offer excellent accuracy and resolution performance. Each technology offers advantages and disadvantages with respect to the measurement range, bandwidth, ease of use, signal conditioning requirements, and cost. Experimental conditions impact sensor selection and performance with respect to mounting and space constraints, line of sight access for optical sensors, and sensitivity to temperature, shock, and vibration
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