In 1700, a magnitude 9 earthquake struck the northwest coast of America. Why did this earthquake happen? Could it happen again? When will the next one happen and how big will it be? To try and help answer some of these questions, I’m heading on a cruise in the Pacific Ocean to join a team of U.S. earth scientists working in the region. |
A 1000 km (630 mile) long geological fault lies off the west coast of North America. It runs from Cape Mendocino (northern California) in the south to Vancouver Island (Canada) in the north. This giant fracture draws the battle lines in a titanic struggle between two tectonic plates: one oceanic; the other continental. North America wins the battle as the oceanic Juan de Fuca plate slowly sinks into Earth’s mantle. We call this collision and sinking of plates subduction.
A major battle line in Earth’s crust
The Cascadia subduction zone is not alone. Subduction takes place all around the Pacific Rim. Along shallow parts of these slanting subduction faults, friction between the two plates is high. The locking together of these two plates and the builds up of stress along the fault for over hundreds of years. Eventually, the stress becomes too large and the fault suddenly unzips - a giant megathrust earthquake. Megathrust earthquakes can often exceed magnitude 8, which is why subduction zones are highly hazardous. Because these faults tend to lie beneath the oceans, they can cause large, damaging tsunamis. Since the last decade, large megathrust earthquakes in Indonesia (2004), Chile (2010) and Japan (2011) and associated tsunamis have reminded us of their destructive power.
The last known great earthquake to hit the northwest coast of America was in 1700 - some 300 years ago. Evidence suggests that these type of earthquakes have taken place seven times in the past 3,500 years. This history indicates that earthquakes along the Cascadia margin tend to occur every 400–600 years. The Cascadia subduction zone has been eerily quiet for a while – this could suggest that stress is building along the fault and the two plates may be almost fully locked against each other. Based on historical data, there is nearly a 40% chance of a major earthquake occurring in this region within the next 50 years.
Listening to crackles on the seabed
With large centers of population in the Pacific northwest living close to the coast – cities such as Seattle and Portland - authorities aren’t taking any chances. The US government has funded a large-scale geophysical experiment to monitor and understand seismic hazard in Cascadia. The $10m Cascadia Initiative aims to deploy GPS and seismic stations to monitor any changes in movement along the fault. The findings from this research will help to understand the frictional properties of the megathrust fault, crucial to understand how and why it may host future large earthquakes.
Over the next three weeks, I’ll be taking to the seas offshore of Oregon and California on Research Vessel Oceanus to collect seismometers that have been lying at the seabed for many months. These highly sensitive instruments will record any small vibrations in the ground caused by earthquakes. We hope to make some preliminary measurements from the recorded data to try and the nature of earthquakes in the region. Understanding the small earthquakes may be key to understanding the big one that happens in the future. We will also deploy further seismometers on the seabed. I’ll be part of a team led by Chief Scientists Anne Trehu from Oregon State University and Dean Livelybrooks from the University of Oregon. In total, over the three weeks, we have 45 ocean bottom seismometers to deploy and decommission, so we going to be working around the clock to stick to schedule.
Over the next three weeks, I’ll be taking to the seas offshore of Oregon and California on Research Vessel Oceanus to collect seismometers that have been lying at the seabed for many months. These highly sensitive instruments will record any small vibrations in the ground caused by earthquakes. We hope to make some preliminary measurements from the recorded data to try and the nature of earthquakes in the region. Understanding the small earthquakes may be key to understanding the big one that happens in the future. We will also deploy further seismometers on the seabed. I’ll be part of a team led by Chief Scientists Anne Trehu from Oregon State University and Dean Livelybrooks from the University of Oregon. In total, over the three weeks, we have 45 ocean bottom seismometers to deploy and decommission, so we going to be working around the clock to stick to schedule.
Map showing our cruise plan. The orange and black circles are ocean bottom seismometer sites that need to have instruments deployed / decommissioned. Our deepest ocean seismometer will be placed some 4000 m (1300 ft) below sea level and our furthest seismometer lies 350 km (220 mi) from the nearest piece of coastline.
I would be lying if I said I wasn't nervous. I have very little experiencing of travelling at sea, let alone working at sea. Seasickness could be a huge concern for me. This trip will be a big challenge and I'm sure there'll be some hiccups along the way. Nevertheless, it will be a great experience and a good chance to learn a new method of studying earthquakes.
I will try to keep you informed of our progress by using this blog and Google Maps I will also be regularly posting mini-updates to Twitter.
I will try to keep you informed of our progress by using this blog and Google Maps I will also be regularly posting mini-updates to Twitter.