The 7.8-magnitude quake (as per USGS) that rocked the South Island of New Zealand on Monday was centered 93 km north-east of Christchurch and 220 km south of Wellington at a depth of 23km. This quake was centered in a rural area, home to just a few thousand people and the energy was dispersed over a larger area.
The location of Monday’s quake largely helps explain why the damage was far less than the 2011 Christchurch earthquakes, even though the energy was about 30 times that of the 2011 February Christchurch quake that was located in a populated town Christchurch, meaning tens of thousands of people were exposed to the most violent shaking at the epicenter.
In Monday's event, multiple faults were ruptured and this was responsible for uplifting large stretches of the coastline, and also exposed a previously unknown fault near Waipapa Bay. The event also triggered a potential tsunami alert of about two meters on the coast of Kaikoura and Wellington, but no damage was caused by it.
Another interesting feature is that there were reports of 1220 aftershocks for Monday's event, with 32 registering magnitudes greater than 5.0 and two with magnitudes greater than 6.0 as of 11:00 hours UTC. The 2011 February Christchurch event recorded about 316 aftershocks (source GeoNet) within a week following the 6.3-magnitude earthquake.
The event is a reminder that the peak ground motions do not necessarily occur at the epicenter. The peak ground-motions for this event were recorded 130 km north-west of the quake's epicenter – close to where most of the slip did occur (source USGS). The ground-motions and hence also likely the associated damage at the epicenter were much lower.
The ruptures were mostly on a mountainous terrain with dense soil and rock. Hence liquefaction effects did not pose a risk, but about 80,000 to 100,000 landslides were triggered by the succession of quakes (source GeoNet). Hundreds of people remain stranded in the coastal town of Kaikoura, north of Christchurch, which was completely cut off by massive landslips and hampered relief operations.
Two lives were lost and throughout the region glass has been broken in buildings, chimneys have collapsed, roads and railway lines damaged and also some structural damage has occurred. Some structural and content damages were also experienced in regions as far as Wellington.
This event is a good example depicting that higher magnitude doesn't necessarily correlate to higher losses and earthquake dynamics are much more complex than just the location and magnitude.
Also, a well-placed response and mitigation measure in New Zealand certainly kept damage to life and property to a minimum when compared to a similar magnitude earthquake that hit Nepal in April 2015 which killed 9,000 people and flattened entire villages.
Category: Climate/natural disasters: Disaster risk, Earthquakes
Location: New Zealand