November 16, 2016
Complex, compound New Zealand earthquake – Part 1: Seismology by Night
Posted by Austin Elliott
Just after midnight last Sunday, the whole country of New Zealand was rocked by a massive earthquake at the north end of the South Island. From the outset, this earthquake was more confounding than most, and as more reports and data amass, we’re gaining a picture of a complicated earthquake that stemmed from the failure of several large faults in succession.
These successive failures may have resulted from structural linkages among the faults, or perhaps have been triggered remotely by large stress changes as all the stored elastic energy was being released on neighboring faults. Understanding how and how often faults operate together in such a way is paramount to assessing the hazard posed by known, mapped faults, and better estimating the locations of ones we don’t know about.
Figuring out which faults broke and how in the wake of a big quake takes coordination and cooperation from across the varied disciplines of earthquake science.
Part 1: Seismology by Night
The first clue that we had a complicated rupture on our hands came from seismic records–the first way* any earthquake is formally detected and measured. The earthquake’s moment tensor, an indication of which way the crust moved along which type of fault, showed a messy mixture of lateral strike-slip motion and convergent thrusting, neither tidily resolved. This suggested right away that multiple faults with different senses of motion were involved.
In addition to the strange mixture of directions of motion, the seismic energy was released in several pulses over a long period of time. Seismograms from around the world showed that the vast majority of the seismic energy was released 60 seconds into the quake! This later pulse was far more energetic than the quake’s original onset, and back-projecting the source of the energy shows that it came from far north of the epicenter.
Of course, the residents of New Zealand already knew this because this particular earthquake distinctively intensified over time, culminating rather surprisingly with very strong to severe shaking in its final seconds. These two fellows in a Wellington home-office understandably hesitate to do anything upon noticing the trembling, but they’re quick to take exemplary appropriate action when the shaking gets serious.
That video illustrates why it’s important to Drop, Cover, and Hold On as soon as you can: If you have time to ponder what to do, then it’s a big earthquake and there’s no predicting how badly it’s about to shake.
The next sign New Zealanders and global seismologists had that this was an exceptional quake was the profusion of large aftershocks, occurring in the minutes, then hours, and now days after the big event, across a swath of the country extending all the way up past Wellington.
Take a look – these were the earthquakes that happened in the 12 hours around last night’s big one. pic.twitter.com/98fE03hnQr
— Guardian World (@guardianworld) November 14, 2016
Aftershocks–somewhat of a misnomer as these are essentially new earthquakes in their own right–present one of the biggest threats after any earthquake. The initial fault rupture rapidly changes and redistributes huge tectonic stresses in the crust, spurring a spate of new fault failures in the immediate vicinity. That means that even if the epicenter is hundreds of kilometers away, after such a huge fault rupture new earthquakes may occur even closer to population centers that were originally marginally spared. This was the case around Wellington and across the northern South Island, where several of the “aftershocks” of the giant M7.8 had larger magnitudes themselves than the earthquake that devastated Christchurch in February 2011.
Then came the tsunami. Based on this earthquake’s size and the thrust component, seismologists sounded the alert that it may be tsunamigenic. In the 30 minutes after the earthquake, sea level plunged two meters on the tide gauge at Kaikoura, confirming this concern.
At a trough-to-crest amplitude of 4 meters, this was alarming. Sirens were sounded in coastal cities on both islands, warning people to evacuate inland.
New Zealand’s coasts have tsunami evacuation plans, recognizing their risk of megathrust earthquakes, and having fresh experience of tectonic disasters. In the end the tsunami was somewhat localized, reaching only fractions of a meter in other harbors along the coast. This rapid attenuation implies that the causative uplift (or landslide?) was confined to a relatively small portion of the seafloor and coastline. A detailed model of tsunami heights, although based on incomplete information about the changes that caused the wave, illustrates the confined extent of large wave heights.
Distribution of maximum #eqnz tsunami height,calculated by Mr. Aditya Gusman @CPPGeophysics @ALomaxNet @LastQuake pic.twitter.com/3LrDGMJrN6
— Muh Ma’rufin Sudibyo (@marufins) November 14, 2016
As you can see in the tide gauge data above, the occurrence of the tsunami just after low tide kept the crests barely higher than a high tide, but the problem with tsunamis is both their wavelength and their speed. Sea level changes that high in a matter of minutes induce huge currents in otherwise sheltered harbors, and the vast volume of uplifted water keeps spilling inland for a long time, funnelling up bays where the depth and speed of the current can become devastating. This happened locally along the South Island coast, devastating remote homesteads, but didn’t amount to anything too impressive in Lyall Bay, where a timelapse video shows the surges lapping like quick tides.
Overall the recognition that this was a major and dangerous event was swift, and warnings were sent and heeded appropriately despite the confusing source of the earthquake. The director of GeoNet New Zealand gave a nice synopsis of the response activities in that first night.
Many noticed conflicting and changing reports of the earthquake’s magnitude. This is still almost universal in large earthquakes because many methods of magnitude calculation saturate at large shaking amplitudes and cope poorly with long-duration, low-frequency ruptures. In part because of the bizarre nature of this rupture, it took a long time for seismic networks to parse the data necessary for a robust assessment of the earthquake’s energy release. GeoNet NZ summarizes this process nicely on their blog.
By the end of the night most seismic centers around the world agreed on a magnitude of approximately 7.8, really a very large earthquake. As day broke, it was time to see how the rupture really unfolded…
…see what was discovered when day broke in Part 2.
* Twitter has recently become faster than conventional seismic networks at formally identifying earthquakes in areas of sufficient Tweep population. Tweepulation?