Evidence Of Nonlinear Infrasound Propagation From The 2010 El Mayor Rupture

In recent years, a large number of surface and atmospheric sources have generated infrasound energy that has been recorded at source-receiver distances over 1000 km at the USArray, a large transportable array making its way across the United States. The sources, including surface explosions, large bolides, mining events and a space shuttle, combined with the very high density of recording sites, have contributed to an understanding of the broad range of infrasound signals that can be recorded at seismic sensors. One finding from these studies is that infrasound propagation modeling using both ray theory and linear full-waveform synthesis methods fails to predict the full range of arrivals observed at the recording sites. This failure reflects inaccuracies either in the atmospheric specifications - the wind and static sounds - that guide acoustic propagation, or in the assumptions used in linear propagation methods. The effect of nonlinearity on infrasound propagation modeling is investigated for the 7.2 El Mayor earthquake that occurred on 4 April, 2010. This event generated infrasound at frequencies of several Hertz that was recorded over 200 km away at sites to the north and west of the epicentral region, despite stratospheric winds that favor propagation in an eastward direction. The observations of higher frequency infrasound suggest non-linear steepening, either at stratospheric or thermospheric altitudes. The locations of the epicenter, aftershocks, and surface rupture suggest that the rupture had a spatial extent of roughly 75 km in length, i.e nearly a line source. We compute waveforms with and without the effects of nonlinearity from a line source for a wind and sound speed model relevant to the El Mayor rupture, and compare these to waveforms recorded for this event.