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While researching across the Australian Earthquake Engineering Society archive about Seismology, I came across an earthquake map of the Australian island state of Tasmania (below):

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I am aware that Australia experiences intraplate earthquakes, so I am not surprised the earthquakes occurring. However, what I am wondering is, is there a geological explanation for the hook-like pattern of earthquakes occurring off the north-east coast of Tasmania?

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    Having dates for the earthquakes would be interesting, in case there are any trends. Overlaying a map of regional faults would also be interesting. The other interesting this is why are there only lower magnitude quakes in the southern part of the island & why are there fewer quakes in the south compared to the north? Looking at the picture there appears to be a pattern of quakes on the continental shelf, on the eastern side – Fred Jul 21 '16 at 13:11
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    Not to spoil your fun, but is there any statistical analysis confirming this is an actual pattern? Humans often see patterns that aren't there, and I'm not convinced about your 'observation' of a hook-like pattern. How about saying there's a half-island wide 'bar', tilted WSW-ENE, over the northern half of the island (Strahan - St Marys - Hunter Island - Cape Barren Island)? – Jan Doggen Jul 21 '16 at 13:31
  • @JanDoggen if you wish to edit the wording, go for it –  Jul 21 '16 at 23:05

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I can't prove it, but I strongly suspect that most, if not all, of the earthquakes in that north-south lineation along the eastern Bass Strait are caused by slumping of sediments in a series of listric faults. That whole continental edge is a very thick heap of deltaic and continental sediments of about Kimmeridgian (late Jurassic)to late Eocene age. There is a brief summary in: Submarine canyons of the continental margin, East Bass Strait (Australia).

As to 'the hook', it is surely no coincidence that this exactly coincides with 'Flinders Canyon', which you can see on Google Earth. What you can't make out on G.E. is the enormous size of the Canyon and its sedimentary catchment area. The sediments here are up to about 3 kilometers thick, and a massive submarine amphitheater funnels sediment into the canyon in periodic surges. I would bet a month's beer money that the 'hook earthquakes' were caused by occasional turbidite collapses of sediment down this steep canyon. The magnitudes, location, slope and approximate frequency are all spot on. If you include the smaller canyons along the same continental edge then it is estimated that some 80,000 to 90,000 cubic kilometers of sediment have collapsed from this continental edge since the Late Eocene, i.e. over the last 40 million years.

Ken Graham
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Gordon Stanger
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  • Awesome, this explanation does go a substantial way in making sense of what appears to be a signature of a structure. –  Jul 21 '16 at 23:06
  • Due to their (what I believe) association to an earthquake swarm I am not convinced that they might be caused by turbidite collapses. I would greatly appreciate a reference for your claim that the magnitudes are spot for such origin. – wienein Jul 22 '16 at 13:17
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    The classic example is the Grand Banks landslide which turned into a turbidite. See http://gps-prod-storage.cloud.caltech.edu.s3.amazonaws.com/people_personal_assets/kanamori/HKbssa87.pdf This was about a magnitude 7 event, and is exceptional because of that. The vast majority of turbidites would be less spectacular, and triggered by much smaller events - in the region of magnitude of magnitude 4 to 5. – Gordon Stanger Jul 23 '16 at 00:29
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Almost all of the earthquakes in that area are associated to an earthquake sequence between 1883-1892 (some are described for example in: Michael-Leiba, 1989). The seismicity did not cease after 1892, the largest recorded earthquake in Tasmania in the 20th century happened in the same region (Michael-Leiba and Jensen, 1993)

If you tinker around with the Geoscience Australia earthquake database no such pattern is visible, the following map shows the 73 earthquakes M>3 around Tasmania which happened between 1955 and today.

Map of earthquakes

Michael-Leiba, 1989: Macroseismic effects, locations and magnitudes of some early Tasmanian earthquakes. Journal of Australian Geology & Geophysics, 11, 89-99

Michael-Leiba & Jensen, 1993: The West Tasman Sea (Flinders Island) earthquake of 14 September 1946. Journal of Australian Geology & Geophysics, 13, 369-372

wienein
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