Aftershock monitoring along the Great Ocean Road

The foothills of the Otway Ranges, Gadubanud country, along the Great Ocean Road in south-east Victoria, where a magnitude 5.0 earthquake occurred in October 2023. Researchers from AuScope’s Subsurface Observatory continue to record aftershocks in the region, thanks to six sensors installed nearby. Image: Abraham Jones

Large earthquakes will always be followed by aftershocks, but without nearby sensors to record them, the true quantity and location of these aftershocks may never be known. Thanks to AuScope, the University of Melbourne now has six sensors operating within 15km of the magnitude 5.0 Apollo Bay Earthquake that shook residents along the Great Ocean Road on October 22nd, 2023.

Woken from their sleep

At 2:11 am on Sunday, October 22nd, 2023, residents in Apollo Bay and the Otway Ranges in Victoria were jolted awake by a magnitude 5.0 earthquake approximately 14km north-west of Apollo Bay. With strong shaking felt across the region, there was localised damage, with at least one house condemned from the quake, and many others reporting cracking, items thrown from shelves, damage to chimneys, and concrete water tanks being destroyed when the earthquake cracked their sides, spilling thousands of litres of precious water onto the ground. Felt reports submitted to Geoscience Australia show the event was felt across central and southwest Victoria and on King Island in Bass Strait.

Rapid Response

Soon after the earthquake struck, Victoria’s largest for two years since the Woods Point Earthquake in September 2021, the rapid response team from AuScope’s Subsurface Observatory got to work installing an aftershock monitoring network in the region. Led by seismologist Abraham Jones of the University of Melbourne, six sites were set up across the Otway Ranges, installed on farms and local properties to encircle the epicentre and enable aftershocks to be detected and accurately located.

University of Melbourne seismologist Abraham Jones attends to one of the aftershock monitoring sites and other rapidly deployed sites in the Otway Ranges. Image: Abraham Jones

When one earthquake occurs, expect more

Whenever a large earthquake occurs, we expect aftershocks to follow. While most aftershocks will follow soon after the main event, they can continue for weeks, months, and even years.

Initial large aftershocks included a magnitude 3.6 just 4 hours after the mainshock and a magnitude 3.7 the following day, both of which were felt strongly by residents in the region, with some reporting additional damage to already weakened structures.

In total, 5 aftershocks of magnitude 2.5 or larger occurred in the first 50 hours after the main quake. As expected, this activity slowed, but smaller aftershocks have continued, with over 60 detected, although most have been too small to feel.

With six temporary stations now installed within 15 km of the epicentre, the aftershock sequence is being recorded with a new level of accuracy and precision that may allow previously unmapped faults beneath the surface to be identified.

Location, location, location

The instruments have been collecting data for two months and will remain in place until early 2024, depending on how long the aftershocks continue. Data from these sensors is used first to detect that an earthquake has occurred, and then the seismic waves are used to locate precisely where in the earth’s crust it occurred. The more sensors you have close to the events, the more precise these locations can be, and the more small events detected. The current network has allowed many aftershocks smaller than magnitude 1.0 to be located, with the smallest magnitude recorded at 0.2.

Without these sensors installed close to the epicentre, many smaller aftershocks would have gone unnoticed and undetected.

Apollo Bay Earthquake (yellow star) and some preliminary aftershocks were located using data from installed aftershock sensors (red triangles). Known faults in the area are also shown, with arrows indicating the direction that these faults dip (angle) beneath the surface. Aftershocks from the first 48 hours before all UoM sensors were installed are not shown. Image: Abraham Jones

Fault detection

One of the key advantages of a high-precision aftershock monitoring network like this is its potential to identify previously unmapped faults. With the ability to detect small events that may have been overlooked, researchers can map seismic activity more comprehensively, contributing to a deeper understanding of the region's geological dynamics and ultimately improving Australia’s hazard models and maps.

Whilst it is too early to draw detailed conclusions, an interesting trend is beginning to emerge from within the cluster of located aftershocks. Aftershocks closer to the surface expression of the Castle Cove Fault are occurring deeper than those further away from this fault.

This response is the opposite of what we would expect if the Castle Cove fault was responsible for this aftershock activity. It suggests the possible existence of a previously undiscovered and unmapped fault between the Johanna and Castle Cove fault systems.

Are earthquakes on the rise?

Recent years have been busy for Victorian seismologists, with Victoria’s largest ever recorded earthquake in Woods Point in 2021, a magnitude 4.1 just 30km north of the Melbourne CBD in Sunbury in May 2023, the Apollo Bay Earthquake, and other felt events near Ferntree Gully and on the Mornington Peninsula. This has prompted many residents to ask, “Are earthquakes becoming more common?”.

On average, Australia experiences a magnitude 6.0 earthquake every 8-10 years and a magnitude 5.0 or larger on average every 1-2 years. The greater Melbourne region alone typically records one earthquake of magnitude 2.0 or larger every month. However, whether these events are felt depends on various factors, including earthquake magnitude, depth and population density around the epicentre.

These long-term averages have stayed the same in the last few years. The main difference recently is that more events have occurred close to population centres, so more people have felt them and they’ve attracted more media coverage.

Whilst the overall rate of earthquakes hasn’t increased in recent years, events close to population centres should remind Australians of the hazards that earthquakes pose. The more data we capture, the more we can understand this dynamic earth and prepare for the inevitable movement beneath our feet.

 
 
 

STORY IN A NUTSHELL
NCRIS enabled sensors track seismic aftershocks along the Great Ocean Road, Victoria, Australia.

AUTHORS
Abraham Jones, University of Melbourne
Edited by Philomena Manifold, AuScope

FURTHER INFORMATION

GeodesyAuScope