NZ Anniversaries: 06/01/2015 Mw 5.6 Wilberforce Quake

The 2015 Wilberforce Earthquake

A simplified felt report map of the January 2015 Mw 5.6 Wilberforce Earthquake on the South Island of New Zealand (Source: GeoNet - Imagery: Google Earth)

Introduction

During 2019 I will be writing blog posts on the anniversaries of the major earthquakes to strike New Zealand since European colonisation began - the first quake chronologically occurred in 1848. Each quake will get a small piece on the cause of the earthquake and it's effects & impacts (both societal and environmental) - for some of these events there will be far more information owing to their recent occurrence, greater impact or greater potential impact if they were to recur in the future.

My aim with this exercise is to hopefully better communicate the seismic hazard in New Zealand to the public - some areas have surprisingly had significant and damaging earthquakes, but these and the risk they pose may not be known to many people besides academics.

Links to sources for the information shared are provided throughout the texts of these posts & the others I produce (including my 2019 >/=Ml 4.0 quakes & 2019 widely felt quakes pages).

Wilberforce

In the early morning of January 6th 2015, at 6:48am, a sharp earthquake rocked much of the South Island. It's origin was approximately 9 kilometres beneath the Southern Alps, in the uppermost streams of the Wilberforce River, Canterbury Province.  The quake most likely took no more than 5 seconds to rupture a small (~5km) fault, but in doing so it produced a moment magnitude 5.6 (local magnitude 6.0) earthquake. 

The earthquake was initially located beneath Lake Coleridge, which led to very confused reactions from people living there who had not experienced the quake to a severe intensity (MM5 or moderate intensity only). This was subsequently revised to it's correct position.

As can be seen in the pictured at the top of the article, the quake was felt from New Plymouth in the north down to Dunedin and Invercargill in the south; it was felt most strongly on the west coast of the South Island (from Fox Glacier up to Westport) and also in the westernmost & southernmost areas of the Canterbury Plain (Timaru, Temuka, Geraldine, Methven, Oxford).

A seismogram of the 2015 Wilberforce Earthquake from the Waitara Valley seismograph station (filtered 1-5 Hz - Source: GeoNet)

The earthquake occurred on a strike-slip fault - that is where two slabs of rock slide past one another laterally, with the fault which slipped being vertical. The below focal mechanism shows the two possible fault planes it could have occurred on - a NNW-SSE sinistral (left-lateral motion) fault or a WSW-ENE dextral (right-lateral motion) fault. GeoNet aftershock locations do not allow an easy analysis of which of these two fault strikes (alignments) the quake occurred on as they follow a NW-SE alignment themselves. The largest aftershocks (Mw 4.5 & Mw 4.6) occurred 6 and 11 hours after the mainshock - both of these were also strike-slip.

The focal mechanism for the 2015 Wilberforce Earthquake shows it was the result of strike-slip faulting (Source: GeoNet)

The primary impact from this quake was the number of landslides which occurred within the Southern Alps around the epicentre. An aerial survey by GNS identified 263 landslides, the second largest of which (25km away from the epicentre) temporarily dammed the Arahura River.

This earthquake has probably been forgotten by most people because it's societal impact was negligible, but this is the most recent example of a sizeable Southern Alps earthquake. The Southern Alps plays host to many different strike-slip and thrust faults which accommodate some of the relative plate motion of the obliquely colliding Australian & Pacific Plates - most of this is accommodated by the Alpine Fault.

Quakes have occurred on faults in this area in the past - the 1929 Arthur's Pass, 1994 Arthur's Pass and 1995 Cass earthquakes are largest to have occurred within the past 170 years and will be discussed further later in the year.

The earthquake was too small really to change the stress regime in the Southern Alps - small segments of the adjacent Alpine Fault were stressed towards failure and destressed away from failure, but the changes exerted were too small to really change the likelihood of an Alpine Fault quake in the short-term or long-term.

This article was written by J H Gurney at 18:00 GMT on the 5th January 2019 (07:00 NZDT on the 6th January 2019).