Infrasound and seismoacoustic signatures of the September 28 th 2018 Sulawesi super shear earthquake

Abstract. A magnitude 7.5 earthquake occurred on September 28th 2018 at 10:02:43 UTC near the city of Palu on the Indonesian island of Sulawesi. It was a shallow, strike-slip earthquake with fractures up to the surface and a rupture length of about 150 km. Moreover, this earthquake was identified as one of very few events having a super shear rupture speed. Clear and long-lasting infrasound signatures related to this event were observed by four infrasound arrays of the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty Organization as well as one national infrasound station in Singapore. Although these infrasound stations SING (Singapore), I39PW (Palau), I07AU (Australia), I40PG (Papua New Guinea) and I30JP (Japan) are located in large distances between 1800 km and 4500 km from the earthquake’s epicentral region, the observed infrasound signals associated to this event were intense, including both seismic and acoustic arrivals. The seismic-to-acoustic coupling at nearby terrain features is supposed to generate distinct infrasonic signatures clearly recordable at remote infrasound arrays. A detailed study of the event-related observations and the potential infrasound generation mechanisms is presented covering range- and time-dependent infrasound attenuation and propagation modeling, characterization of the atmospheric background conditions as well as identification of the regions of seismoacoustic activity by applying a backtracking method from the infrasound receivers to potential source regions. The back-projection of infrasonic arrivals allows to estimate that the main infrasound source region for the Sulawesi earthquake is related to the extended rupture zone and the nearby topography. This estimation and the comparison to other super shear as well as large regional earthquakes identifies no clear connection between the earthquake’s super shear nature and the strong infrasound emission.