A classic case of a true 'megathrust'

Historic events like natural disasters often serve as reference points in our lives.

And so it is with the March 11, 2011 earthquake off the coast of Japan that triggered a devastating tsunami.

“We all feel a responsibility to know what is going on in these events,” says Dr. John Gosse, who holds the Canada Research Chair in Earth Systems Evolution at Dalhousie.

Earthquakes, Volcanoes and Natural Disasters

Students who sign up for his course this fall will learn about the causes of the 2011 Sendai earthquake. Earthquakes, Volcanoes and Natural Disasters (ERTH 1060) is a class he developed in 2003. Arts and science students benefit from first-hand instruction by an internationally recognized expert in geomorphology, the study of processes that shape landforms.

“This is a classic — it is a textbook case of a true megathrust with a magnitude of nine — and that is rare in global and historic terms,” he says.

In the past century, less than 10 earthquakes measuring 8.8 or higher on the  moment magnitude scale (Mw) have shaken the Earth. The location of such violent events is limited to subduction zones, the  margins where lithospheric plates collide.

The Pacific ‘ring of fire’ is notorious for producing this type of strong earthquake, including a pair of the largest in a hundred years — Chile in 2010 (Mw 8.8) and Japan in 2011 (Mw 9.0). Year after year, the Pacific plate inexorably advances northwest about eight centimetres closer to Japan.

The Sendai epicentre is found offshore near the Japanese trench, a deep gash in the ocean floor formed by the imperfect meeting of the Eurasian and Pacific plates.

In such zones a dense iron-rich and cold portion of a lithospheric plate is forced beneath the comparatively less dense Eurasian plate. For relative scale, imagine the collision of a cast iron skillet with an aluminum cookie sheet with similar thickness, he suggests. The dense skillet will sink under the cookie sheet. To extend the analogy, neither the sheet nor the skillet is coated with Teflon so there is friction between them. At the scale of hundreds of kilometres along the collisional boundary between lithospheric plates, zones can be lubricated by water and sediment, but other zones can become stuck. There, the friction generates increasing strain and then violent release.

'Exactly as expected'

“This earthquake did what it could have been expected to do, being a large but shallow quake in a subduction zone — there was a big vertical motion offshore as the ocean floor dropped,” says Dr. Gosse. “The orientation of motion was exactly as expected, following that segment of the trench.”

Depth is crucial to how an earthquake is experienced. If a Mw 9.0 quake happens far below the surface energy will be dissipated as it travels upward, he explains. The Sendai event originated just 130 kilometres off Japan’s east coast and at a relatively shallow depth of 32 kilometres.

Earthquake duration is a significant factor as well. While a magnitude four earthquake could be expected to end after a dozen seconds, the Sendai earthquake unleashed intense force for about one minute and had a duration closer to three minutes. Dozens upon dozens of strong aftershocks — some with magnitudes of sixes and sevens — can be expected to continue for many months, if not years, he says.

Elsewhere in the world a different kind of earthquake is caused by plates slipping along one another, such as the magnitude seven earthquake that devastated Haiti in 2010. After being locked together for more than 250 years, tectonic plates along the Enriquillo Plantain Garden Fault (or nearby faults) yielded to massive stress and slipped horizontally nearly two metres. Haiti is categorized as a ‘strike-slip‘ system, so while this earthquake may have had a little vertical component of strain, the movement was mostly horizontal. For this reason, it did not generate a tsunami.

'A series of long-period signals'

The Sendai earthquake generated a significant tsunami based on the size of the area affected by the quake and its vertical drop of four metres.

“A tsunami is not one wave, rather it is a series of long-period signals,” says Dr. Gosse.

Shocking video images were relayed around the world as the tsunami reached the coast of Japan. While the tsunami was measured at more than four metres in height along the Japanese coast, damage was exacerbated by repeated incursions of these high waters. Just as earlier waves began to retreat, subsequent waves of water moving inland created complex currents.

“Tsunami are really seen as a temporary sea level change that lasts dozens of minutes, rather than a sharp crested wave,” he says.

Nine hours later, echoes of the tsunami swelled the ocean surface along Canada’s western coastline. Tsunami reached above two metres in height as it ran up beaches of California.

“We probably experienced effects from this tsunami in the North Atlantic, but at a much diminished scale,” he says.

In years to come, references to Sendai will remind us of where we were and what we were doing at the moment we first learned about this natural disaster.

Photo by Twitter user @_mego of the affects of the 2011 Sendai earthquake and tsunami in Miyagi Prefecture near Sendai.