Kaikoura earthquake november 2016 Archives

A slashing blow by some mythical behemoth, knifing effortlessly through earth’s rocky foundations; a (seemingly) bottomless chasm, a canyon, with nothing but the wind between you and whatever lies below. A bit over the top perhaps, but canyons often spark the imagination – standing on the lip can feel like being perched on the edge of the world, vertiginous for some. Most canyons have been carved by the relentless churning of stream and river, incising the layers of rock and removing the sediment to distant shores.

Terrestrial canyons have their submarine counterparts, that transect the submerged, outer margins of continents and volcanic islands. Submarine canyons commonly mark the transition from shallow continental shelves and platforms to ocean basins, acting as conduits for sediment delivery from rivers, deltas and shallow seas, to the deep oceans. And like their land-based cousins, they are deep (1 to 2 km, or in the case of Grand Bahama Canyon, 5km), steep-sided incisions in the ocean floor. More than 600 submarine canyons have been identified world-wide from bathymetry maps.

The 3-dimensional bathymetric reconstruction of Monterey Canyon (top image), about 100km south of San Francisco, illustrates common attributes of these structures. The canyon cuts deeply into the break between the shelf and steeper marine slope – in this image the break is a definite line separating light blue from darker shades. The Monterey Canyon head encroaches onto the shallow shelf to within a few 100m of the shoreline; this is actually atypical of most other canyons where incision of the sea floor usually begins closer to the shelf edge. The canyon channel snakes down slope, eventually flattening out on the deep ocean floor; the main channel is joined by several smaller tributaries. Several smaller gullies are also incised into the shelf edge and slope.

The general opinion among earth scientists is that submarine canyons are formed by two main processes: Erosion by sea-floor hugging flows of mud and sand (given the general name sediment gravity flows), and by collapse of the steep margins, producing submarine landslides (and potentially, tsunamis). Common triggers are thought to include storm surges and earthquakes. The primary basis for this interpretation is abundant geological evidence of past events, combined with some experimental work, but it remains a largely theoretical interpretation because there have been very few direct observations of either process in action. The reasons for this disparity are that submarine flows of mud and sand are relatively rare events (at least on a human time scale), and because of the difficulties inherent in witnessing such processes in deep water. For this reason, recent events in Kaikoura Canyon (southeast New Zealand) have sparked significant international interest.

Kaikoura Canyon (New Zealand), 60km long and up to 1200m deep, is located along the tectonically active Hikurangi margin, close to the Alpine Fault system that transects northern South Island and the adjacent submarine shelf. At its deepest extent (about 2100m) the main canyon channel merges with Hikurangi Channel, which at more than 1500km, is one of the longest deep-sea channels in the world; Hikurangi channel wends its way across the more subdued ocean floor towards the abyssal Pacific Ocean. The submarine canyon head is an uncomfortable 1000m from the coast, a spitting distance that elevates the risk of destructive tsunamis that can evolve from submarine landslides along the canyon walls. November 2016, and the magnitude 7.8 Kaikoura earthquake, provided a rude reminder of the potential for disaster. The seismic jolt activated slope collapse and sediment movement down the canyon slopes and main channel; fortunately, the ensuing tsunami was small, but the bonus for science was huge. Mapping of the canyon head and main canyon channel, fortuitously three years before the earthquake and three months after the event, has enabled scientists to track the changes to channel morphology and sediment distribution that can be attributed solely to the earthquake

(The project was coordinated by New Zealand’s National Institute of Water and Atmospheric Research ( NIWA).

The first two images show before (2013) and after conditions at a location near the canyon head (closest to shore). Large swaths of muddy sediment were dislodged from the ridges and slopes, cascading into the main channel; most of the canyon head is now devoid of its sediment mantle. Parts of the canyon floor are 50m deeper than before the earthquake, because of erosion by the moving sediment.

The second set of images show before-and-after scenes of the canyon floor at 1800 to 2100m water depth. The striped pattern is formed by large, ripple-like gravel waves, or dunes, that under normal conditions would migrate slowly downslope. However, most of the gravel dunes were moved at least 500m downslope by the rapidly transiting muddy flow.

Much of the dislodged sediment continued as a turbulent muddy flow down the main canyon channel and thence to the deeper Hikurangi Channel; the flow had sufficient momentum to carry it more than 680km from its source. Evidence for this comes from deep-sea cores taken 4 days, 10 weeks, and 8 months after the earthquake. Cores were taken from the floors of both canyon channel and the more distant Hikurangi Channel, plus the flatter area, or overbank, beyond the channel banks (analogous to a river floodplain). The reasoning here is that, if sediment gravity flow deposits can be identified in the overbank region, it means that the flow itself was deeper than the channel and, given that we know how deep the channel is, an estimate can be made of the minimum depth of the actual flow. Overbank deposits were detected in cores, indicating that the moving flow was at least 180m thick, 680km from Kaikoura Canyon. As Joshu Mountjoy (one of the project leaders for NIWA) has pointed out, this has proved to be one of the few occasions in which actual flow dimensions in a deep-sea channel could be measured.

From the Kaikoura event we have confirmed that seismicity can trigger physical modifications to submarine canyons and submarine slopes, and that sediment is flushed from canyons to the deep ocean by far-travelled, turbulent muddy flows (i.e. sediment gravity flows). We have learned something of the stability of the canyon itself and the sediment that gradually mantles the sea floor. The legacy of the Kaikoura earthquake (or any major earthquake for that matter) is often voiced in terms of broken lives, disrupted highways, and the costs of rebuilding. There should be no attempt to minimise these outcomes, but we should also remind ourselves of the advances in scientific understanding of earthquakes, and the geological consequences that accrue from an event like this. We should applaud these gains in knowledge because ultimately such knowledge will help save lives and property.

Most of the information for this post is gleaned from NIWA news articles and publications, linked in the text above.

Seismic metaphors, or seismic as metaphor? Seismic, a word that geologists and geophysicists traditionally thought was reserved for their use, has been purloined by politicians and social scientists to describe momentous shifts in things like public attitudes and voter propensities. Seismic is the anglicized derivative of the Greek verb seien, (shake) and the word seismos (earthquake). Apparently, it first appeared in English language writing about the mid-19^th century. Personally, I find it satisfying that the social milieu sees fit to apply the scientific word in such a useful, metaphorical sense.

The word seismic is also a nice descriptor of our restless, physical world, especially the bits we live on. Most of the earth’s crust is under stress, some parts more than others. Most stresses are generated by the movement and jostling of tectonic plates, particularly at boundaries where plates converge, collide, or slide past one another. Different parts of the crust respond to stress by bending; this includes seemingly hard, immovable rock. If the stress is removed then the deformed rocks return to their original states; this is referred to as an elastic response. However, if the earth materials are bent too far or too fast, they will break. An interesting analogue for this process, and a historical one, is the collapse of a Tacoma suspension bridge in 1940.

Here, steel girders and tarmac bent and twisted under stress until the deformation reached a limit (called the elastic limit), at which point the bridge failed. When the earth’s crust fails, the seismic event, or earthquake, can be devastating. Earthquakes are caused by the sudden brittle failure of rock under stress; the failure takes place along a fault, across which land (or sea floor) is moved up, down, or sideways. The rapid displacement of rock masses produces pulses of energy, or seismic waves. It is these waves that do the damage.

There are two main kinds of seismic wave; body waves that propagate through the earth’s interior, and surface waves that move along the earth’s surface. Body waves include a primary, or P-wave, and a secondary or S-wave. P-waves travel fastest; they are also called compressional waves because they tend to push and pull materials as they propagate. Slower S-waves, or shear waves, produce a side-ways motion. Shear waves are not transmitted through fluids such as sea water, or the molten interior of the earth. The cartoon below illustrates how the earth reacts to these two wave types. Surface waves are the slowest to propagate but they are also the seismic pulses that do most of the damage during an earthquake. The animation shows the earth motion for one kind of surface wave;

Rayleigh Waves. Rayleigh Waves produce a circular, or orbital motion of earth materials at the surface, a bit like particle motion beneath sea waves.

Each type of seismic wave is identified by the speed at which it moves, and the kind of movement that sediment and rock are subjected to. These differences are expressed on seismograph recorders. On a typical seismogram (below), P-waves arrive first, followed by S-waves. S-waves tend to have lower frequencies than P-waves (more spread out on the graph), but higher amplitude. Surface waves usually have the highest amplitude.

Earthquake magnitude (M) reflects the severity of ground roll and shaking, and on seismograms corresponds to the amplitude of the signal (usually of surface waves). M is expressed as a number (M1.8, M4.6, M7.8) up to a maximum of 10 (10 might be caused by a large meteorite impact – when there isn’t much left). The numbers, and therefore the magnitude scale, are logarithmic, such that a magnitude of 4 (10⁴) is 10 times smaller than M5 (10⁵), and 100 times smaller than M6 (10⁶). We can also think of the changes in magnitude in terms of the energy released during a quake. It has been determined, (as an approximate empirical relationship) that for every unit increase in magnitude, there is a 27.5-fold increase in energy. The difference in energy is also logarithmic, such that an M8 event releases 571,914 times more energy than an M4 event (the magnitude is 10,000 times greater). These kinds of numbers demand a degree of respect for the twists and turns our earth can throw at us.

Seismograms also help seismologists determine the Epicenter of an earthquake; the epicenter is the map location rather than the actual location, or focus, at depth. The calculation makes use of the fact that P-waves are faster than S-waves, so that the distance to an epicentre is based on the difference in arrival times for each type of wave. Distance in this case is the radius of a circle centered on the seismograph location. To triangulate the epicentre, at least two more seismographs in different locations are needed – in reality there can be hundreds of seismographs that allow the calculation. If circles are drawn about each seismograph, each with a different radius, they will intersect at the epicentre. Variations in the structure of the earth mean that seismic wave velocities can vary, so that the circles may not all intersect at a single point. However, the large number of seismographs around the world means that location of the epicentre is usually accurate.

The November 14, 2016 earthquake in Kaikoura, New Zealand was a M7.8 event. We live about 500km north of the epicentre. Almost on the stroke of midnight we felt definite shaking (P-waves). A few seconds later the shaking increased significantly. At this point trees were swaying and water was slopping over the edge of the pool; this was due to ground roll from the slower surface waves. Other than an adrenaline rush, there was no damage for us; farther south it was a very different story. One spectacular outcome was the abrupt, lateral 6m shift along part of the New Zealand coastline; the same coast was also uplifted 1-2m.

I’ve always thought that New Zealand is a great place to witness geology in action. But sometimes it can go a bit too far.

An excellent technical paper on the Kaikoura event by Ian Hamling (GNS Science) and a host of co-authors, has been published in Science, March 26, 2017.

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Tag Archives: Kaikoura earthquake november 2016

Submarine mud flows and landslides modified Kaikoura canyon during the 2016 M7.8 earthquake

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