Like all earthquakes, last week’s magnitude 6.0 earthquake in California’s Napa Valley came suddenly and without warning. And while predicting earthquakes is still a seismologist’s dream, our understanding of them is growing, if in intriguing ways.
Last Sunday’s quake occurred near the north shore of San Pablo Bay. The bayshore areas in the San Francisco Bay region are underlaid by landfill and bay mud so they experience greater damage during earthquakes. The damage is caused by soil failure in the fills while the soft mud amplifies ground shaking. That’s a head’s up for similar situations in our own region when the next big quake comes.
There were no fatalities but several hundred people needed hospital attention. There was a lot of damage to older buildings and highways. Gas and water mains were ruptured, fires triggered, and power knocked out for thousands. As people watch the familiar fallout, those ‘what-if’ questions surface for our region.
Earthquakes are complicated affairs and their reach isn’t just the immediate surroundings. Japanese researchers recently published a paper in the journal Science that detailed their observations of the response of volcanoes to the seismic waves produced by the great Tohoku-oki earthquake in 2011. They showed how earthquakes can impact volcanoes and an earthquake can help assess the risk of massive volcanic eruptions worldwide. That should be a head’s up for us given our high seismic landscape and active volcanoes to the south.
The Tohoku-oki earthquake was a magnitude 9.0 undersea megathrust quake 70 kilometres off the coast at a depth of 30 kilometres. Scientists analyzed over 70 terabytes of seismic data. In computer geek speak, one terabyte equals 1000 gigabytes. From all that data they found that the regions where the disturbances of the Earth’s crust were the greatest were not those where the shocks were the strongest. They were localized under volcanic regions, in Japan’s case under Mount Fuji. The findings support theories that the last eruption of Mount Fuji in 1707 could have been triggered by the giant 8.7 magnitude Hoei earthquake which took place 49 days before the eruption.
At the Scripps Institution of Oceanography in San Diego, scientists have discovered the first evidence that deep earthquakes – those rupturing at over 400 kilometres below the surface – can break much faster than ‘ordinary’ earthquakes. The findings have given seismologists new clues about the forces behind deep earthquakes as well as fast-breaking earthquakes striking near the surface.
“Supershear” earthquakes have rupture speeds of four kilometres per second (9,000 miles/hour) or more. The rupture travels faster than the shear waves of seismic energy it radiates.
Last year, the scientists discovered the first deep supershear earthquake while examining the aftershocks of a M8.3 earthquake off the Russian mainland. An aftershock of M6.7 ruptured at 640 kilometres below Earth’s surface at a speed of 8 kilometres per second.
Understanding these deep, super-speed earthquakes may provide better understanding of supershear earthquakes close to the surface. Apparently the 1994 Northridge earthquake in California was similar in size and geometry to the Russian aftershock. Supershears have potential for more shaking and damage.
Slipping around beneath our feet are tectonic plates setting us up for the Big One. From northern Vancouver Island to the Queen Charlotte Islands, the Pacific plate is sliding to the northwest at about 6 cm/year relative to the North America plate. The boundary between these two plates is the Queen Charlotte fault which is Canada’s equivalent of the San Andreas fault. Four large earthquakes have occurred along the fault line in the last hundred years including a M7 in 1929, a M8.1 in 1949 (causing a 500-kilometre segment of the fault to break), a M7.4 in l970 and a M7.7 in 2012.