September 17th, 2024
As the residents of Canterbury continue to live with the consequences of the September 2010 Darfield Earthquake, understanding the probabilities of future aftershocks is crucial. GeoNet, a partnership between Toka Tū Ake EQC, GNS Science, and Land Information New Zealand (LINZ), is at the forefront of providing aftershock forecasts using computer models. These models are continually updated based on the behaviour of aftershock sequences worldwide and the specific knowledge that scientists have of the Canterbury aftershock sequence since September 2010.
The aftershock probabilities for the next year:
Magnitude 5.0-5.9: 34% probability, with 0-2 events expected
Magnitude 6.0-6.9: 4% probability, considered very unlikely
Magnitude ≥7.0: <1% probability, deemed extremely unlikely
A dynamic and evolving aftershock landscape
It is important to note that aftershock probabilities change over time. In general, the chance of further earthquakes decreases as time passes. The table provided by GeoNet shows how the probability of larger magnitude aftershocks (e.g., over M7.0) is lower than smaller magnitude aftershocks (e.g., M5.0-5.9). A magnitude increase of one means a rate decrease of roughly ten times, making a magnitude 7 earthquake about 100 times less likely than a magnitude 5 earthquake.
However, if another large aftershock occurs, it can re-energise the system and spark a resurgence of earthquake activity for a month or so. This phenomenon was observed with both the February and June 2011 magnitude 6.3 earthquakes.
Staying prepared and resilient
Given the uncertainties surrounding earthquake probabilities, it is vital for residents and authorities to be prepared for potential future events. The National Emergency Management Agency (NEMA) provides valuable information on what to do before, during, and after an earthquake. Some key steps include:
In case of a strong or long-lasting earthquake near the coast, move to high ground or as far inland as possible to avoid potential tsunamis
Prepare homes and buildings by following the guidelines provided by Toka Tū Ake EQC, which focus on making homes safer and stronger for earthquakes
Peak Ground Accelerations: A measure of earthquake impact
GeoNet's table of peak ground accelerations (PGA) for aftershocks of magnitude 5.5 or above offers a detailed overview of significant aftershocks that have occurred since September 2010. The table presents information such as the date and time of the earthquake, magnitude, location, depth, largest recorded PGA, and the distance from the station recording the highest PGA to the epicentre.
The road ahead
As Canterbury residents forge ahead into an uncertain future, it is crucial for individuals, communities, and authorities to remain vigilant and prepared for potential aftershocks. By staying informed and adhering to the guidelines provided by organisations like GeoNet, NEMA, and Toka Tū Ake EQC, the people of Canterbury can continue to build resilience and adapt to the ever-changing landscape of aftershock probabilities.