Stephen Hall-Jones, of Dunedin, asks :-
In 2015 Otago had two large earthquakes and neither had any aftershocks. Is this unusual? Also, what does it suggest to a seismologist?
Mark Stirling, a seismologist at the University of Otago's Geology department, responded.
Significant aftershock activity usually accompanies earthquakes of magnitude (M) greater than 6 or thereabouts. Earthquakes under that approximate size don't tend to produce a great deal of aftershocks, as the size of the fault and area impacted by the earthquake are relatively small.
The convention for describing earthquakes is based on the amount of energy released. Earthquakes less than Magnitude 5 are defined as "small", less than 7 as "moderate", less than 8 as "large", and 8 or more as "great".
By this definition, no large earthquakes have struck Otago in recent times, only small-to-moderate earthquakes such as the ones under Maungatua/Outram, and in the Wanaka area. These earthquakes were not unusual in the grand scheme of things. New Zealand lies astride the boundary between the Pacific and Australian tectonic plates, and although most of the deformation in the South Island is accommodated along the great Alpine Fault in Westland, a significant portion occurs to the east. This motion is expressed by fault lines that show geologic evidence of occasional movements through time (e.g. once every 5000 years or more), and by the occasional occurrence of earthquakes.
The earthquakes will always seem unusual as they seem to occur out of the blue in formerly quiet areas. However, this is a perception based on a short time window of observation. The longer time window of the 1-2 century long historical record of earthquakes shows lots of places where these earthquakes have happened. Its an ongoing thing, but seems unusual when they happen in our back yard.
More major events of the Canterbury and Cook Strait earthquake sequences occurred on significant faults and impacted significant areas. They produced a lot of aftershocks. All the Canterbury earthquakes are aftershocks of the mainshock Darfield earthquake of Sept 2010 (M7.1).