Large avalanches composed of ice and rock are a serious hazard to mountain communities and backcountry travellers. These processes shake the Earth and disturb the atmosphere, generating seismic waves and sound waves which can travel for hundreds of kilometers. In this study, we use the seismic waves and sound waves produced by two massive avalanches on a volcano in Alaska to reconstruct how the avalanches failed. Our method may assist with rapid emergency response to these global hazards.