A seismoacoustic study of the 2011 January 3 Circleville earthquake

Abstract

SUMMARY Wereportonauniquesetofinfrasoundobservationsfromasingleearthquake,the2011January 3Circlevilleearthquake(Mw 4.7,depthof8km),whichwasrecordedbynineinfrasoundarrays inUtah.Basedonananalysisofthesignalarrivaltimesandbackazimuthsateacharray,wefind thattheinfrasoundarrivalsatsixarrayscanbeassociatedtothesamesourceandthatthesource location is consistent with the earthquake epicentre. Results of propagation modelling indicate that the lack of associated arrivals at the remaining three arrays is due to path effects. Based on these findings we form the working hypothesis that the infrasound is generated by body waves causing the epicentral region to pump the atmosphere, akin to a baffled piston. To test this hypothesis, we have developed a numerical seismoacoustic model to simulate the generation of epicentral infrasound from earthquakes. We model the generation of seismic waves using a 3-D finite difference algorithm that accounts for the earthquake moment tensor, source time function, depth and local geology. The resultant acceleration–time histories on a 2-D grid at the surface then provide the initial conditions for modelling the near-field infrasonic pressure wave using the Rayleigh integral. Finally, we propagate the near-field source pressure through the Ground-to-Space atmospheric model using a time-domain Parabolic Equation technique. By comparing the resultant predictions with the six epicentral infrasound observations from the 2011 January 3, Circleville earthquake, we show that the observations agree well with our predictions. The predicted and observed amplitudes are within a factor of 2 (on average, the synthetic amplitudes are a factor of 1.6 larger than the observed amplitudes). In addition, arrivalsarepredictedatallsixarrayswheresignalsareobserved,andimportantlynotpredicted at the remaining three arrays. Durations are typically predicted to within a factor of 2, and in some cases much better. These results suggest that measured infrasound from the Circleville earthquake is consistent with the generation of infrasound from body waves in the epicentral region.