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On Sunday 5 November at 6.51pm Fosterville Gold Mine (FGM) recorded a Mine Induced Seismic (MIS) event, which was felt and heard by the surrounding communities. At the time of reporting, FGM’s underground seismic monitoring system recorded this event as a magnitude 2.5Mw. As part of our investigation of this event, FGM engaged with Geoscience Australia to better understand the differences between the reported magnitudes from our system and theirs.

At all times FGM strives to provide both accurate and timely information to the community. While the information we published on 5 November was the magnitude recorded at the time of reporting; these further investigations have led to a reclassification of the event magnitude.

More about FGM’s underground Seismic Monitoring System

FGM maintains a dedicated underground seismic monitoring system to identify and monitor all mine induced seismic events. A third-party specialist, the Institute of Mine Seismology (IMS), is engaged to monitor and process all data recorded by the underground monitoring system.

The FGM underground seismic monitoring system has sensors located in available underground drives, spaced out as far as practicable from the main mining areas where MIS occurs, to observe both small and large events. The FGM underground seismic monitoring system generates a Magnitude measurement, specifically Moment Magnitude (Mw) from the waveforms detected by the system.

Our investigations have identified that for larger events felt on the surface, there are limitations to the accuracy of the Magnitude measurement being recorded by our system. In order to achieve a more accurate measurement, we need to use data from geophones that are much further away.

Following the event, IMS seismologists manually integrated the data from FGM’s surface vibration monitoring network, which is located further away from the active mining area, to generate a more precise measurement of Moment Magnitude for the event.

As a result of this comprehensive investigation and the integration of additional data, IMS has reclassified the MIS event on 5 November from a 2.5Mw event to a 3.1Mw event.

Local Magnitude (ML) vs Moment Magnitude (Mw)

Geoscience Australia and the Seismology Research Centre both publish Local Magnitude when reporting seismicity detected by their monitoring network, which will often be a different measurement to Moment Magnitude that our system reports.

What Next?

We acknowledge the impact of the MIS event on 5 November and the confusion caused by the different magnitudes reported by FGM, Geoscience Australia and the Seismic Research Centre. FGM is committed to the provision of timely information; and we will continue to notify community of MIS events felt on surface via our website (fgmcommunity.com.au) and provide the most accurate information available at the time of reporting. However, there will likely remain some variances in future reporting between FGM and external agencies due to the differences between Moment Magnitude and Local Magnitude.

We will continue to work with IMS on the processing and integration of surface monitoring data to improve the accuracy and timeliness of our reported information.

Should you have any questions or concerns regarding the information provided above or would like to discuss the event further, please contact a member of the FGM Community Team via 03 5439 9000 or fgm.community@agnicoeagle.com.

In addition, you will also find below a link to two resources published by FGM regarding Mine Induced Seismicity.