Published: Fri Sep 4 2020 9:00 AM
Ten years ago, today, Cantabrians were jolted awake by a magnitude 7.2 earthquake.
It became known as the Darfield earthquake – as the epicentre was near to Darfield, a small rural town 40 km west of Christchurch – and triggered an extensive aftershock sequence across the Canterbury region, including the devastating February 22 Christchurch earthquake.
The September 4, 2010 earthquake began at about 10 kilometres deep within the earth and then broke the ground all the way to the surface. Shaking from the earthquake was felt widely throughout the South Island and into the North Island. The earthquake struck at 4.35 am, damaging brick chimneys, displacing train tracks, and causing power outages across the region.
Buildings were damaged, particularly older, unreinforced structures. Masonry fell from historic churches and some buildings partially collapsed. Roads cracked open and paddock fences and shelter belts were displaced horizontally by up to five metres.
The earthquake also produced liquefaction – where normally solid sandy ground or soil, becomes saturated with water, turning it into a silty, watery mess. This caused damage to roads, pipes, and building foundations.
Around 100 people were injured in the Canterbury region, two of them seriously. Thankfully there were no fatalities. Most people were safely tucked up in bed sleeping at the time of the quake.
If the earthquake had occurred during the daytime when the streets were busy, there would likely have been more casualties.
Despite the relatively low number of injuries, the earthquake was a significant shock for people.
Many personal stories have been captured in Quake Stories.
It was, at the time, the strongest ground-shaking ever recorded in New Zealand (later pipped by future earthquakes). The maps below show Peak Ground Accelerations (PGA) caused by the mainshock. These were used by GeoNet duty officers to determine the earthquake’s location and magnitude. Note in particular the reading of 125%g near the epicentre; this represents an extremely rare seismic recording made near a fault rupture. 125%g means an acceleration 1.25 times that of acceleration due to gravity.
The Darfield earthquake was also the most damaging earthquake in New Zealand since the 1931 Hawkes Bay earthquake.
Although most of the movement from the Darfield earthquake was along the Greendale Fault – a previously hidden faultline, the earthquake began on a smaller fault called the Charing Cross Fault, which then triggered several other faults, including the Greendale Fault.
The Greendale Fault is buried under the gravels that were deposited on the Canterbury plains at the end of the last glaciation. Scientists believe that it is likely the fault had not ruptured for about 20,000 years as the gravels showed no sign of having been disturbed. The earthquake produced approximately 30 km of surface rupture along the Greendale Fault, and was the first known earthquake in New Zealand to produce a surface rupture since the 1987 Edgecumbe earthquake.
The Darfield earthquake triggered a sequence of thousands of aftershocks, that would disrupt Canterbury for years. Some of these were major, including the devastating February 22, 2011 aftershock.
This is the first in a series of stories we will post, over the next few months, marking the ten-year anniversary of the Canterbury earthquakes.
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