North Avalon rockfall triggers local 'earthquake'
A large rockfall at Avalon, on Sydney’s Northern Beaches on Friday afternoon may have been powerful enough for Geoscience Australia’s network of seismometers to detect an earthquake of 1.8 on the Richter scale.
The rockfall occurred at Avalon Headland around 5:19pm, just south of Bangalley Headland - the highest point on the Northern Beaches coastline where the cliffs reach 116m above sea level. The area consists of layered Narrabeen shales, capped by Hawkesbury sandstone.
The entire suburb of North Avalon shook in response to the event, with additional reports coming in from as far north as the Central Coast.
It is still unclear whether the rockfall was triggered by a local earthquake, or whether the rockfall occurred naturally, thereby triggering an “apparent” earthquake. Seismometers pinpointed the site of the quake to a position approximately 300m north the rockfall.
However, preliminary Geoscience Australia data reports that the “earthquake” had a depth of 0km, which suggests that the rockfall itself may have triggered a localised earthquake-like event.
In the last thirty days, almost ninety earthquakes have been detected across Australia however Friday afternoon's incident at Avalon is the only event that occured at a depth of 0km.
An earthquake of magnitude 1.8 is classified as being a 'micro' event, with its effects being "not felt, or felt rarely, (but) recorded by seismographs".
Both scenarios are possible; we’ll seek confirmation from Geoscience Australia early next week.
The size of the rockfall can be shown in the image below - see the person with the white shirt for scale.
Photo Greg Webber
Images taken from South Avalon also show a considerable amount of rockfall extending well east of the headland, well above the water line.
Photo Tim Wreyford
This is not the first time in recent history Avalon Headland has experienced a significant geological disruption.
St Michaels’s Cave is a vertical doleritic dyke located near the rockfall, extending some 110 metres into the cliff face. Its roof partially collapsed in 1983, forcing the Warringah Shire Council to fence it off to the general public.
A 2002 Plan of Management by the Pittwater Council for the Bangalley Reserve and North Avalon Headland reserves stated that “underpinning of potentially unstable rock is planned.”
So, the big question for surfers is: will we see a change in the surf at Avalon?
The predominant swell direction across Sydney’s northern beaches is from the south, and with the rockfall located just north of North Avalon Headland, it’s unlikely that this will affect southerly or south-easterly swells at North or South Avalon.
To that end, Whale Beach will remain unaffected due to the influence of Bangalley Headland to the north.
As for east or north-easterly swells, we may see a change in the surf at North Avalon - and if so, it’ll be apparent during the next swell originating from this direction.
North Avalon isn’t a classic point break per se, that is, the predominant wave at North Av does not peel from the tip of the northern point, along its rocky perimeter before finishing on the beach. Most waves at North Avalon are sand and rip dependent. There is a chance that fresh rockfall east of the Avalon headland may attenuate swells as they approach from the east or north-east. Whether this is detrimental or not to the quality of the surf remains to be seen.
Photo Tim Wreyford