Ancient B.C. tsunamis etched stories into Vancouver Island's sediment

Dig 100 centimetres into the soil inspecific spots along Vancouver Island's coast and the striations of greys and browns tell a story about an ancient tsunami dating back to 1700 A.D.to scientistswho know what they are looking for.

Dr. Peter Bobrowsky of the Geological Survey of Canada says the study of paleotsunamisis a new field started in the 1990s.

Soon, he hopes, they will be able to help coastal communities plan by being able to predict just how often and how hard tsunamis typically hit B.C.'s coast.

The information could be used to help cities like Port Hardy, Port Alberni and Tofino plan for catastrophic events.

"No reason for people to be afraid. The tsunami modellers have done a great job estimating worst case scenarios ... So we know pretty well along the coast what the potential impact should be. More importantly, we should be promoting public education and awareness.All of us living on the West Coast have accepted a level of tolerable personal risk," saidBobrowsky.

Reading the soil tells the story

Researchers look for peat bogs and read the different coloured sediments like timelines.

"It's difficult, but it's fun at the same time," said Bobrowsky who carries a shovel to dig into bogs where sediment has been captured in banded layers of browns and greys.

He said scientists learned where to look throughtrial and error and came to know the difference between storm deposits from floods from rivers and creeks and water that washed in from deep ocean.

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They can now date the sediments with radio carbon dating, which can measure up to 50,000 years ago as close to the target as 50 years.

The main concern is gathering enough information to determine how often big waves hit the coast.

Sharing data with seismologists, researcherswere able to find the 1700 A.D. event about 25 years agoand more recently returned with an international team to learn more.

Tsunamis from near and far

Some of the earthquakes are local.

The South Coast of B.C. is situated in what's known as theCascadiasubductionzone, where the ocean crust is trying to slide under the plate on which sitsNorth America. This often causes smaller quakes. The "big one" that everybody worries about would be caused bya section ofthese two plates becominglocked.

Stress has been building up for centuries, and at some point, Bobrowsky says, that stress will be alleviated bya megathrust earthquake of magnitude 9.0 or higher.

"Now we've found evidence ofa number of thesesubductionevents and a number of tsunamis from other areas from Japan or Alaska inbetweenthese great earthquake events," said Bobrowskywho hopes research can help gauge how much damage smaller tsunamis could do to modern Vancouver Island.

"Of course we should be concerned about a magnitude 9 hitting," he said, adding that smaller quakes are also a concern.

Researchers correlate what they find with records of quakes globally.

"Our concern here is the side effects, the impact of how big is that tsunamithat'simpacted," he said.

The bands of colour in the soil can reveal how far inland debris was carriedand how energetic or turbulent the event was.

In the image of the 100-centimetre pit dug in Tofino B.C.the soil colours indicate:

• 0-15 (dark brown) modern day soil
• 15-28 (light brown) intertidal sediments
• 28-38 (light grey) tsunami sand
• 38-45 (dark brown) fossil soil
• 45-60 (light brown) intertidal sediments
• 60-65 (light grey) tsunami sand
• 65-80 (grey) fossil soil
• 80-85 (grey) intertidal sediments

Bobrowsky explained:

"The 1700 AD great earthquake resulted in the ground surface dropping below sea level (38 cm)and immediately it was covered by the tsunami (28-38). Overtime the intertidal sediments accumulated (15-28) and the land surface gradually uplifted above sea level. "

"With additional time new soil was formed (0-15). Similar events occurred much earlier in time, hence we have a series of the same features below the 1700 AD event. Old soils, subsidence and tsunamis.This cycle was likely repeated dozens of times since the glaciers disappeared 12,000 years ago. Our challenge is the lack of preservation of such geological events."