November 18, 2016
Complex, compound New Zealand earthquake – Part 2: Faulting by Day
Posted by Austin Elliott
Reeling from the massive M7.8 earthquake at midnight, its relentless aftershocks, and the continuing coastal threat of tsunami, New Zealanders awaited daylight on Monday to see the full extent of the destruction. The bizarre seismic records observed overnight had raised confusion and speculation about what faults were to blame for this earthquake. With an epicenter on land but also a several meter tsunami, it was clear that some complicated combination of on- and offshore faults had slipped. In order to determine which, we needed a full picture of the landscape.
Part 2: Faulting by Day (catch up with the initial seismological observations in Part 1 here)
As dawn broke in New Zealand after a night of little, fitful sleep for New Zealanders–and the Earth beneath their feet–the faults that ruptured to produce the massive midnight earthquake would begin to be illuminated. Helicopter flights and ground crews departed for the epicentral region, aiming to find surface slip along the major, known faults on the northeast South Island in addition to assessing the extent of damage and dangers from landslides.
Landslides were pervasive, and a particularly Kiwi tableau soon became emblematic of the natural disaster wrought by the earthquake.
Shortly after we all lamented the landslide cows (they’ve been rescued by their rancher, you’ll be relieved to know), another striking scene was broadcast by Radio New Zealand–one that will become the quintessential image of strike-slip faulting.
This site lies alongside the Kekerengu River, on a rural coastal plain north of Kaikoura and Clarence. Clearly, it sits along the well mapped Kekerengu Fault. What you see there is astonishing for many reasons. The whole landscape has been sheared by a huge amount along this conspicuous fault. The dirt driveway that used to pass the house but now leads to its doorstep serves as the best marker of how much the fault slipped; it’s been offset by around 10 meters. This amount of coseismic offset is among the largest we ever observe on continental strike-slip faults. Offsets much higher than this are rare and controversial, so it’s impressive and important to see it fresh, having definitely occurred abruptly in a single seismic event. The huge amount of offset here, 120 km north of the quake’s epicenter, seemed to confirm the seismological inferences of a major sub-event late in the northward-propagating rupture. This much displacement is certainly compatible with the enormous release of energy experienced at the end of the quake.
Rarity aside, ~10 m of offset is about what we expect to see along future ruptures of major faults like the San Andreas, so this scene gives a good guide of what strike-slip faults can do. Which brings us to the next astonishing element of this scene: precise and unfortunate colocation of human and geologic structures. The fault passes directly beneath this house, and you can see that it has in fact sheared the foundation, juxtaposing interior flooring against grass from the other side of the fault. The house was anchored better to one side of the fault, so it was dragged along with the land on the south, ripped off its foundation which slid out from beneath it on the north.
Many earthquake-prone municipalities have ordinances and building codes meant to protect from this inevitable kind of destruction. But even in California, where urban surface ruptures in 1971 led to the Alquist-Priolo “fault setback zone” Act, the regulation exempts individual single family homes such as this one, in something of a build-at-your-own-risk approach meant to inhibit exploitation by big developers while relieving the up-front burden on individual property owners. Furthermore, documentation of the precise surface rupture risks remains patchy, and most places are still currently developing and honing sufficiently detailed fault maps to direct people where to set back from. Every new earthquake like this affords us new data to better understand how and where the ground surface breaks during fault rupture.
More images coming in from helicopter flights and people on the ground began to reveal some severe cases of coastal uplift. Vast tracts of the whole landscape were uplifted, but the effect is most evident to the eye at the shoreline, where relatively stable sea level provides a uniquely definite reference frame. Emergence and submergence due to vertical tectonic motions also cause a binary change in the environment of the terrestrial or aquatic ecosystems at a shoreline, becoming an issue of life or death.
Aerial photographs show the seabed uplift north of Kaikoura – estimated to be between 2 – 2.5 metres (Tonkin+Taylor) pic.twitter.com/JgdjC2GtX3
— Ramazan Demirtaş (@Paleosismolog) November 15, 2016
At the particular site pictured above, the coastal uplift was accompanied by a spectacularly sharp surface rupture along a fault whose presence had been marked by an abrupt change in the orientation of bedding in the seafloor rocks–evident now that they’re exposed. Where the fault crosses onto shore, it offsets a beach, then slices its way beneath road and rail before continuing off up into the mountains along the formerly suggested Papatea fault.
Notable in that Environment Canterbury video are the fault offsets themselves, as demarcated by the road and railroad tracks, which are left-lateral–not what we expect on the South Island, except that this fault is oriented quite differently from the regional tectonic fabric. What also shows up clearly is the broad zone of warping leading into the fault, as you can see in the graded railroad bed which is now curved into the abrupt shear zone at the main fault. From the ground, here’s how nearby offsets look:
If you want to know how much the South Island’s landscape has been affected by this week’s #EQNZ quake, just check this out: pic.twitter.com/uz7BQMc8Q1
— I. H. Laking (@IHLaking) November 16, 2016
At the same time as all of this low-altitude reconnaissance for ruptures was going on, Earth-observing satellites were sweeping overhead capturing images through the patchy clouds. Optical images didn’t do so well at first because of all the cloud cover, but radar satellites penetrated straight to the ground, and a multitude of groups started measuring the displacements that had occurred overnight.
Geocoded Sentinel-1 interferogram of New Zealand earthquake (20 cm deformation contours) kmz to follow @CopernicusEU @ESA_EO @NERC_COMET pic.twitter.com/bz3A27jh8d
— John Elliott (@jrelliott82) November 15, 2016
Many of these datasets have been posted for download into Google Earth, and they’re helping guide investigations into exactly how this fault system ruptured. Sure enough, displacement maps from both radar and image-matching have revealed rupture of a rather large number of separate faults that slipped on Sunday night.
.@theearthquakes @PeaseRoland the Sentinel-1 radar data show the earthquake ruptured a complex mixture of mapped and unmapped faults. pic.twitter.com/wCrhQWJ6Js
— Tim Wright (@timwright_leeds) November 15, 2016
Preliminary correlation of Sentinel-2A data – cloudy, but big displacements. @TTremblingEarth @timwright_leeds #eqnz pic.twitter.com/OkUiINEat0
— David Mackenzie (@MackenzieEarth) November 17, 2016
More images taken from helicopter flights have confirmed these space-borne observations. Environment Canterbury has posted photos from its overflight online, and they’re a treasure trove.
Meanwhile, researchers on the ground continue to inspect the complex array of faults that all have slipped. This nice report from Nicola Litchfield and Pilar Villanova of GNS summarizes what they observed in the first few days. All the additional knowledge these daylight inspections have given us have provided a more precise estimate of the shaking around New Zealand, with the multiple causative faults now incorporated in USGS Shakemaps, for example.
Together, after just 5 days of investigation, seismic data, GPS records, satellite observations, and on-the-ground field assessment have revealed what a complex beast of an earthquake assembled itself by rupturing a host of branching, anastomosing, neighboring faults. Although mostly known beforehand, the faults that ruptured Sunday night are largely subsidiary to the major throughgoing faults in the Marlborough region: the Hope, Clarence, and Awatare faults. As we keep finding with contemporary earthquakes, large hazard exists beyond the prime suspects.
Satellite view of the coastal ground uplift near Kaikoura :