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Geometrical Implications for Calibrating Acoustic Sensors for Lab-Generated Earthquake Sources

Session: New Insights Into the Preparatory Phase of Earthquakes From Tectonic, Field and Lab Experiments I

Type: Oral

Date: 4/19/2021

Presentation Time: 10:45 AM Pacific

Description: 

Spontaneous stick-slip motion and associated acoustic emissions (AEs), generated in laboratory frictional sliding experiments, are useful analogues for natural earthquake behaviour. In particular, they have been shown to be an important means of understanding earthquake nucleation and source properties. However, AE recording systems are usually uncalibrated and can only provide limited information, such as number and relative amplitude of AEs. Calibrating AE recording systems is advantageous as the absolute amplitude, time history and spectral character of AEs can be determined. We use the absolute acoustic sensor calibration method, based on momentum from a ball drop, by McLaskey et al. (2015), to estimate the seismic moment of lab-generated spontaneous stick-slip and acoustic emissions. A suite of different balls, ranging from 1mm to 17mm diameter, are used to calibrate acoustic (piezoelectric) sensors within two experimental setups: 1) a steel plate and 2) a seismic sample assembly used within triaxial deformation apparatus for frictional sliding experiments. In order to see the effect of geometry on instrument response and path effects, spectra from ball drop experiments are compared. Significant spectral differences between the two setups, caused by geometrical complexities of the apparatus, highlight the importance of accurately quantifying the instrument-apparatus response. Without this, the true source characteristics of lab-generated earthquakes may be masked in high frequency oscillations caused by the geometry of the seismic sample assembly. AE waveforms will be measured during frictional sliding experiments within the seismic sample assembly on Perspex, at confining pressures ranging from 10-40MPa. Initial results reflect natural earthquake behaviour: for example, as confining pressure is increased, the amplitude of stick-slips increases. Using the calibrated sensors, future work will look to obtain source properties of lab-generated AEs, particularly those which precede the main stick-slip, to better understand earthquake nucleation and propagation.

Presenting Author: Louisa O. Brotherson

Student Presenter: Yes

Authors