Christchurch quakes within aftershock probabilities

Published: Mon Jul 22 2019 1:35 PM

Today’s M3.9 earthquake in Christchurch was the largest in the city in the past 12 months and the third quake in the wider city area of between M3 and M4 this month.

More than 5000 people submitted ‘felt reports’ to the GeoNet website from as far south as Leeston to the north at Kaiapoi, and including a few on Banks Peninsula.

Chch Quake-July2019

Chch Quake-July2019

During May 2019 there were four earthquakes in the wider Christchurch area between M3 and M4. This earthquake activity fits with expected aftershock sequence for the Canterbury region in the wake of the 2010-2011 Canterbury earthquakes. The current aftershock forecast notes there is a 43% likelihood of a magnitude 5.0-5.9 earthquake in the greater Christchurch area in a year. GNS Science updates these aftershock probability figures annually.

Smaller earthquakes – between M3 and M4 – are not included in the aftershock probabilities for Canterbury. But in typical aftershock sequences, there will be many more magnitude 3s and 4s than magnitude 5s. The aftershock probabilities for the Canterbury region can be found here

The Canterbury aftershock sequence is expected to last for some years until Christchurch returns to pre-2010 levels of earthquake activity.

It's possible that today's earthquake and Saturday's M3.5 quake occurred on the same fault to the south of the city as they were very close together. However, there are multiple faults in the area, and 1 kilometre is at the limit of our location accuracy.

Of the 229 earthquakes that GeoNet has recorded in the greater Christchurch area in the past 12 months, 15 have been between M3 and M4. The rest were smaller, with a number too small to be felt. We recommend that being prepared is a sound strategy for natural disasters. There is more information on preparedness on the website of the Ministry of Civil Defence and Emergency Management.

The above can be attributed to GNS Science Duty Seismologist Caroline Little