Strong Earthquake strikes South Island

Published: Mon Aug 12 2019 11:10 PM
Updated: Tue Aug 13 2019 10:15 AM

The M5.5 earthquake occurred 15 km north of Milford Sound at 12km deep, 10:35pm on Monday 12th August 2019.

The earthquake was felt widely throughout the South Island of New Zealand and lower North Island, and there were more than 6,800 felt reports. 

Felt reports from the M5.5 earthquake

Felt reports from the M5.5 earthquake

We spoke to Elizabeth Abbott who is the Duty Seismologist this week, to give us some more information on the event:

The earthquake was initially posted as a magnitude 6.2, why did this change? The Magnitude 6.2 was a preliminary location posted shortly after the shaking, once we had more information we were able to give a more acurate size, depth and location. With larger earthquakes such as this one, you do have to wait a bit longer to get a reliable magnitude as more data is required.

Which fault was this earthquake on? Like the majority of earthquakes in New Zealand, this event did not occur on a fault that we were previously aware of. Fiordland is a seismically active region, so earthquakes like this are not unexpected.

Has this earthquake had any impact on the Alpine Fault? The way that earthquake stresses around faults interact is very complex. At this stage we are unsure of how the Alpine Fault may have been affected, though it is likely that any effect would be very small.

How many aftershocks have there been, and is this normal? We have currently located 63 aftershocks, with 16 being magnitude 3 and above, this is normal aftershock behaviour following an event of this size.

Map showing the M5.5 event and aftershocks

Map showing the M5.5 event and aftershocks

Why was the event felt far away? Many people said they felt the earthquake as a long rolling motion, this is due to large shallow earthquakes creating surface waves. You need to be further away from the earthquake to feel this rolling motion.

Is the earthquake related to the recent large quakes South of New Zealand? All earthquakes have some effect on their surroundings. Although there may be some relation, this earthquake and the recent earthquakes south of New Zealand are far enough apart that relevant stress changes from these earthquakes are likely to be small. We wrote about those recent earthquakes last week and you can read more about them here

Media enquiries: media@gns.cri.nz or 021 574 541

Attributable to - Elizabeth Abbott – GNS Science, Seismic Hazard Specialist


Remember in an earthquake to Drop, Cover and Hold

Drop, Cover and Hold

Drop, Cover and Hold

Want to know more about what to do before, during and after an earthquake? Check out the Ministry of Civil Defence and Emergency Management's website for more information.

For advice on how to make your home safe in a natural disaster, check out EQCs website Be Prepared.