Summary of Recorded Ground Motions from the 2024 Muswellbrook, NSW, Earthquake Sequence

·        On 23 August 2024, the communities of Muswellbrook and Denman in New South Wales experienced a moment magnitude M 4.7 earthquake, with its epicentre near the Mount Arthur Coal Mine in the Upper Hunter region. ·        This earthquake was followed by numerous aftershocks, including a M 4.4 event on 24 August, a M 4.5 event on 7 September (AEST), and a local magnitude ML 4.1 event on 12 November. Geoscience Australia has recorded over 40 earthquakes south of Muswellbrook since 23 August 2024 (through to 31 October), though more events of a lower magnitude may have occurred. ·        The main shock on 23 August was strongly felt by the local community, and was felt as far as Wollongong, Tamworth and Dubbo, with over 3,600 felt reports being submitted through Geoscience Australia’s National Earthquake Alerts Centre. ·        In response to these earthquakes in the Upper Hunter, Geoscience Australia has deployed six monitoring kits that, together with the national seismometer network, are providing real-time data which can be used to determine more accurate information on the frequency, location, depth and magnitude of aftershocks. ·        The intensity of earthquake ground shaking can vary greatly over short distances. Shaking intensity is dependent on several factors, including the magnitude of the earthquake, the distance of the observer from the earthquake epicentre and the geology and topography at the observation site. ·        The Australian standard that is often used to assess the likelihood of building damage from ground vibrations is the “AS2187.2:2006 Explosives—Storage and Use Part 2: Use of Explosives”. This Standard specifies the lowest transient vibration value that has the potential of causing cosmetic damage to unreinforced or light-framed residential structures. This is estimated to be a peak velocity of 15 mm/s (recorded at a frequency of 4 Hz). ·        Analysis indicates that Geoscience Australia measured peak velocities approach values near 40 mm/s in the Muswellbrook region for the 7 September (AEST) M 4.5 earthquake. These measurements were observed at two sites (one located in Muswellbrook, approximately 10 km from the estimated earthquake locations) and may have been influenced by local geological and topographic effects at these sites. Geoscience Australia will undertake further research to characterise the local conditions at these sites in 2025.  ·        Ground motions from 23 August M 4.7 main shock would likely have been stronger than the recorded motions for the 7 September event. However, the shaking effects may vary based on the style of fault rupture and the consequent radiation pattern of seismic-wave energy  ·        A causal link between the recent earthquake sequence and the Mount Arthur mining operations cannot be demonstrated without further research.