By Jem Anshaw
It has been a week since the Muswellbrook area was literally rocked by a 4.7 magnitude earthquake on Friday, August 23, and there is every chance the aftershocks we have been experiencing will continue for months, if not longer.
“That’s quite typical of what we see, we see a bigger earthquake and it’s followed by many smaller earthquakes, and typically over time they happen less often and get smaller,” Jonathan Griffin, a senior seismologist for Geoscience Australia, said.
“The biggest event is what we call the main shock, and then events that occur after that, and within a certain close distance to where that main shock occurred, we classify as aftershocks.”
What is considered a bit unusual is to have had another large movement like the one on Saturday afternoon that registered at 4.5 magnitude, as generally aftershocks are at least one unit lower than the main event.
“The nearest seismometer is about 100 kilometres from these events, so we will send out more instruments as a temporary deployment and put them much closer to where these earthquakes have occurred and that will hopefully answer some of these questions,” Jonathan shared when discussing community speculation about the correlation between the epicentre and land use around that area.
“You do see the association there between these earthquakes pulling on top of the mine, but that actual question of causation is trickier to answer on any particular event. What we can say as a general statement is that any kind of human activity that modifies the stresses on the earth’s crust can potentially induce earthquakes. So it’s possible, but hard to then say yes, that one on Friday was or was not because of because of mining.”
In a final bit of interesting information from our chat with Jonathan he shared that there is a chance there have been more aftershocks than those that have been recorded. The distance between us and the closest instruments means they don’t register anything below a 2 magnitude movement, but unless you are right on top of the epicentre, neither would you.
