Google map showing the original interpreted location of the earthquake, just near Lake Coleridge. As you can see, over the ensuing hours the earthquake location and magnitude were both revised. Although the revised location is closer to the Alpine Fault, the reduced magnitude reduces the area-wide influence of this single event.
This morning 6th January 2015, at 06.48 am, an earthquake of M~6 shook the Southern Alps of New Zealand (see Geonet record here). It provided a nice example of the intricacies of earthquake science, and the need for the media and scientists to keep a fluid mindset in the first few hours after an event. Too much speculation soon after an event is nothing more than that - SPECULATION.
I was rung by a Christchurch Star reporter at ~ 10.15 am asking for comment on the Lake Coleridge earthquake. The major question on the reporter's lips was "How does this change the chances of an Alpine Fault earthquake?" I quickly checked Geonet and then provided a comment. However, within minutes of my check (while I was on the phone), and 3 hours 44 minutes after the earthquake, Geonet downgraded the magnitude of the earthquake from 6.4 to 6.0, and revised the location 40 km further west. The change, which reflected GNS seismologists evolving understanding of the earthquake based on new and better-analysed data, seriously impacted the validity of my comments to the reporter. I only discovered the change after hanging up and quickly rang back to tell the reporter of the change in earthquake location and magnitude and provide a revised statement.
The answer for the question regarding the Alpine Fault is clearly quite different for an earthquake situated 60 km from the fault and one at only 20 km from the fault. However, the revision of magnitude from 6.4 to 6.0 is also important. A 6.4 equates to an area of 150 km2 slipping 1 m. A 6.0 equates to about 65 km2 slipping about 0.5 m. Compare this with the Darfield earthquake, in which we saw slip at the surface of >5 m. The difference in radiated seismic energy is vast. While a 7.0 would supply New Zealand's energy needs (all fuel types, including electricity, oil, coal etc) for a day, a 6.4 only radiates enough energy to power much of the South Island for a day, and the 6.0 only radiates enough energy to power Christchurch for a day. The distance over which significant stress changes can occur is related to how the movement occurred (up the fault plane? along the fault plane?) and to how the slip is accommodated on the fault plane (Only a small part of the fault plane experiences the maximum slip) as well as the overall area of the rupture. It is therefore unwise to speculate at an early stage about how an earthquake will affect another fault when the earthquake in question has a location and magnitude that is still under investigation. Ultimately, public statements will be made by a spokesperson speaking for a group of seismologists, geologists and modellers, who will have made assessments based on the best data to hand. We, the public, should expect those assessments to change and evolve in response to improving data. The aftershock sequence for instance is a major data source but takes some time to play out.
So, what was my final comment? Something along the lines of: "The earthquake seems to have occurred about 20 km from the Alpine fault. People in Christchurch have a good idea of how earthquake effects fall off with distance for earthquakes around M6.0. At this stage it is unwise to speculate about the possible effects on the Alpine Fault. Remember, this is not that unusual, nor a great deal different to the Arthurs Pass earthquake of 1994."