Category Archives: Earth science

The Silent Threat: When The Cascadia Fault Unzips California Will Be Inundated


PicMonkey Collage

Images from the 2004 Sumatra Tsunami

Earthquakes around the world, and in particular along the Ring of Fire that circles the Pacific are increasing. Within the last  few weeks  Japan, Myanmar, Ecuador and several other countries have been hit with powerful quakes that have caused loss of life and massive economic damage.

I want you to imagine a huge zipper about 800 miles (1300km) long. The teeth of the zipper are small protrusions of rock and they hold the two halves of the zipper together. When a few of those protrusions break off, the zip will fail and will unzip from that point.

This is the situation with the Cascadia fault line. It’s currently locked tight, and has been for 316 years, it last unzipped in 1700.

The Cascadia fault is a subduction zone, a region where one mass of land is forced under another at the leading edge of the tectonic plates that make up the earths surface. The Cascadia fault is caused by the Juan de Fuca plate moving underneath the North American plate. Unlike most faults Cascadia is silent, no minor quakes, no swarms to tell scientists something may be about to happen. It hasn’t budged for 316 years but for all of those years the pressure has been building, and one day, it will get too much and the fault will rupture.

In January 1700 a tsunami hit the coast of Japan, it came without warning. Japan is in a geologically unstable area, but there had been no earthquakes, nothing that could have warned people that a tsunami was rushing towards the coast. The huge waves wiped out whole villages and was documented in Japan having being named ‘The Orphan Tsunami’, a tsunami with no earthquake ‘parent’.

In 1987 scientist Brian Atwater was studying soil samples across the entire length of the Cascadia fault, he started near the ocean and worked inland…discovering sand, ocean sand way inland that could have only gotten there if a huge wave had washed it up. A tsunami had hit centuries before…three centuries before to be precise, at the same time as the Japanese Orphan Tsunami.

Based on the findings of Atwater et al the 1700 tsunami was found to have travelled 60 miles inland along the entire length of the fault. In 1700 hundred the area was scarcely populated, unlike now. San Francisco, Portland,Vancouver, Seattle and many other cities and towns of varying sizes cover the area where the 1700 tsunami hit.

The close proximity of the Cascadia fault to the West Coast means that the tsunami following the earthquake will arrive within 20-30 minutes. As the waves, which have crests that can be hundreds of miles apart, start to reach the continental shelf, the leading edge of the water slows down as it heads up the slope that marks the start of the continental land mass. The water behind it catches up and turns the wave into a wall of water that will destroy anything in its path. there is no way of knowing how high that wall will be but the Lituya Bay tsunami of 1958 reached a height of 1720 feet. The Lituya Bay quake was small in comparison to what the Cascade fault can produce.


Trees wiped out at Lityua Bay Alaska after a 1720ft tsunami hit

Mega thrust earthquakes, such as the Indonesian earthquake of 2004, only occur in subduction zones. The pressure on the rock builds up and suddenly ruptures, as the land on one side drops the whole water column above it is displaced and that is the start of the tsunami. The waves travel out in all directions, small close to the epicentre but increasing in size and speed as they move outwards, they pound the next solid ground they arrive at…with devastating consequences.

The Indonesian mega-thrust quake was a 9.3 and there is no reason to think that a similar force wouldn’t be generated by a failure of the Cascadia fault.

The entire West Coast of the United States would be devastated from a double whammy of a massive earthquake followed by a tsunami, and considering that the two events would happen within 30 minutes of each other the chance for escape is reduced massively.

The recovery time from a blow of this size will be considerable and although the cost in terms of human life is of paramount importance other things have to be considered. The infrastructure will be destroyed, the economic costs will be huge, farmland contaminated with saltwater will be out of commission for several seasons, food production will be reduced, medical care will have to be brought in from outside the disaster zone. The list goes on and on.

Planners and lawmakers are aware of the risks. buildings have to be built to strict codes in earthquake areas, giving those caught inside a greater chance of survival,  but little is said about tsunami safety. We can only hope that they implement safety measures such as vertical evacuation centres in a timely manner, because when cascade finally pops the whole of the West Coast is going to know the full power of mother nature.

Take care