I got my PhD studying tsunamis and working with other scientists who study tsunamis. One thing that almost every scientist studying tsunamis has in common is that they’ve never actually seen one.
For several of my former colleagues, this changed when the tsunami from Chile spread throughout the Pacific. Andy recorded 7 distinct waves using a ruler he’d just purchased at Home Depot in Santa Cruz harbor. Jody (my former advisor) and Tanya watched ice shift in tsunami waves in the frozen harbor of Petropavlovsk, Kamchatka (Russia). Jody has been studying tsunamis since the 80s. In an email she told us all, “I am SURE I eyewitnessed a tsunami, for the first time in my life!”
Tsunami scientist Tanya Pinegina watches gentle tsunami waves in the ice in Avacha Bay, Petropavlovsk, Russia.
Photo by Jody Bourgeois
All I can say is that I may have seen a tsunami, albeit a really small one. On a beach here in Seldovia I watched the dropping tide. Did it drop a little faster during the last 5 minutes? Has it slowed now? The tide gauge in town definitely saw something… a wave several inches tall with a period of 10 minutes or so (link here, many gauges on this page, including a more easily seen example of a tsunami recorded by a tide gauge in King Cove a few hundred miles southwest of Seldovia).
I also set up my camera. I took a timelapse video, accelerating reality by 300x. I’ve watched this video many times now, and I think I can see slight variations in how fast the tide drops down the beach near the center of the image. Then again, maybe not… you can judge for yourself. (The boat in the distance probably isn’t being moved by the tsunami, as the period of its motion is only 1-2 minutes, as opposed to about 10 minutes.)
What are the chances? Global occurrence of large earthquakes
The earthquake in Chile was a really big one. It’s amongst the largest ever measured, with the energy of a billion tons of TNT, enough to change the rotation of the earth. Decades pass without a single earthquake this large anywhere on the planet.
Does it seem like there are a lot of big earthquakes lately? Two recent deadly events, one in Haiti and one in Chile, have gotten a number people wondering if that is more than a coincidence.
In the case of Haiti and Chile, it almost certainly is just a coincidence. The earthquake in Haiti was a giant in terms of human tragedy, but as far as seismic energy, it was quite small in comparison to many earthquakes that have happened around the world lately. The USGS catalog records 16 earthquakes as large, or larger than the one in Haiti in the last year. The Haiti earthquake was large enough to increase the danger of other earthquakes on the same fault, but not large enough to influence tectonics a quarter of the way around the world in Chile.
However, the largest earthquakes – those over magnitude 8 – do seem to cluster in time. The three largest earthquakes in the 20th century, all magnitude 9 or more, occurred in 1952, 1960, and 1964. (Some catalogs list the 1957 Andreanof Islands earthquake as a 9.1, making four over 9 in that time range, but the USGS rates this one an 8.6.) After 1965, there were only two magnitude 8.3 earthquakes, and none higher until after the turn of the millennium. The statisticians have taken a look (Bufe and Perkins, 2005), and they don’t think that’s random. I plotted the data below, and you can judge for yourself.
We’ve been measuring earthquakes since 1900, and the recurrence of the largest ones doesn’t seem random. There’s a clump of large earthquakes in the ’50s and ’60s, and then a lull through the turn of the Millenia. Things have been more active again in the past decade.
Click the graphic for a larger version, as well as data and vector graphic file.
And the past decade has been a big one for earthquakes. There have been five earthquakes above 8.3, including the 2004 magnitude 9.1 earthquake in the northeastern Indian Ocean. Each increase of 0.2 in magnitude corresponds to a doubling in energy released, so the 2004 magnitude 9.1 released as much energy as 16 magnitude 8.3 earthquakes.
What does this mean? We only have a short instrumental record (since 1900) and there’s a lot of variability, so it’s impossible to know whether we have another magnitude 9 just around the corner. But it seems likely that the period of tectonic quiescence starting in 1965 and ending with the gradual increase in seismicity in the late 90s is gone. It’s no time to dally on the science, both old-fashioned paleoseismic studies, and maybe some new methods that can help warn of impending earthquakes. And it’s no time to skimp on education and good infrastructure that can save lives during an earthquake. Likely the biggest difference that led to far fewer people killed in Chile than in Haiti was better building standards.
Why do the biggest earthquakes come in clusters?
At this point we don’t really know.
For an earthquake on a fault to happen, there have to be two things in place: The fault has to be under stress so it can provide energy for an earthquake, and some point has to fail, triggering the fault to move and that energy to be released. Stress increases over time and eventual failure is inevitable, but exactly when it happens is dependent on that trigger, which can be very subtle. The point where failure begins is the hypocenter (directly beneath the epicenter on the surface of the earth) and the entire portion of the fault that moves is the rupture area.
One way to think of it is to imagine the fault as a large building. Perhaps it is an apartment building in Istanbul, and as new floors are illegally added the stress on the structure increases. This unstable structure has a lot of energy in it, all in the form of cement and other materials suspended high in the air by weak architecture. But when it finally collapses, that collapse starts somewhere. Perhaps a pillar designed for two stories and holding five collapses because someone uses it to tie up their dog. Now the nearby pillars and walls must suddenly bear more of the weight, and they collapse as well. The failure spreads from the original “epicenter” pillar, and consumes the entire building, analogous to the earthquake’s rupture area.
So was it the dog that caused the building to collapse, or the extra stories? I’d say the cause was the additional stress, while the trigger was the dog. Earthquakes are the same way… caused by gradually building stress, but triggered when some point gives way.
Bufe and Perkins, 2005, discuss how an earthquake in one area of the world might lead to another far away and years later. Their first possible explanation focuses on triggering, while the others suggest that stress might increase on distantly separated faults at the same time:
- Perhaps the vibrations from a distant earthquake increase the pressure of water trapped in pores in rock, and that pressure leads to a gradual weakening of the rock, initiating an earthquake.
- Maybe a slow pressure wave moves through the lower crust after a large earthquake, and this wave causes stress to increase on other faults that then fail.
- Perhaps large “silent” earthquakes, rarely detected because the deformation they cause is so slow, drive the system by running along plate boundaries and triggering their more violent kin.
- It could be that the changes in where and how much ice is in glaciers changes the stress on faults and leads to times of quiescence or greater activity.
- Or my favorite: Maybe the largest earthquakes change the stress on tectonic plates. As this stress moves around the planet it triggers more large quakes, until the crust has settled into a lower state of stress.