There was a clear sequence of seismic events that led up to last night's M6.9 earthquake.
 
The first event of this seismic sequence occurred on 22 April with a magnitude 4.9 quake. The following day, there were many more small and moderate sized events, including a M5.9 quake. We call this sequence a seismic swarm because it contained many similar sized events occurring in the same location and within a short time of each other. There is some indication a number of these events may have occurred on splay faults that branch from the main subduction fault (see cross-section below).

We now have deployed 23 and recovered 17 ocean bottom seismometers, leaving only three more to deploy and one to recover. This means we are due to finish two days earlier than planned (on schedule for arrival back into port on Wednesday evening). The weather has taken a slight turn for the worse again, and I have suffered from a couple bouts of seasickness. Fortunately, we have been blessed with amazing sunrises and sunsets.

Now that we have recovered some seismometers, we have been able to look at some initial results from the 12 months that the seismometers have been at the seabed. We have been able to find some micro-earthquakes at the southern end of the Cascadia subduction zone. We also found that the seismometers were able to record the seismic waves from a damaging magnitude 6.0 earthquake that hit Napa, California on 24 August 2014. Our initial results show that the recording quality of the instruments is good and they will provide crucial information to unravel the character of the Cascadia megathrust fault and may provide an insight into the regional tectonic structure of the Western United States and Eastern Pacific rim.

Ship time is expensive. Pots of money for scientific research are rarely bottomless. For us scientists, time on board is precious. To keep to schedule, we have been forced to work at all hours of the day. Each team of crew, technicians and scientists assigns watch duties to each member. Yesterday, I was lucky enough to have my watch in the early hours of the morning. The fog was thick and visibility minimal. Fortunately, the deck is well lit by floodlights. Here are several photos showing the atmospheric conditions we sometimes get on deck.

Just a quick update as we've re-docked to pick up more gear for the next phase of the ocean-bottom seismometer deployment. As we arrived into the harbour, the fog was dense and visibility was minimal. There was a somewhat eerie feeling; buoys ringing in the distance and the ship's foghorn calling across the estuary.

We have been temporarily blessed with fast internet, so I've been able to upload a video of one of our deployments from the past few days:

There are several different teams on board. Of course, we have 15-strong ship's crew. They are all hugely friendly, and the cook is renowned across the marine geophysics community for whipping up some great meals. It is clear we are going to be well looked after. There is also a team of technicians fromLamont-Doherty Earth Observatory at Columbia University. These guys will be dealing with the technical aspects of ensuring that the ocean bottom seismometers are ready to be deployed. Finally, there are the scientists - that's us. There are 7 of us in total - all from the States, apart from me. I will introduce you some of my fellow hands in the next few posts.

Check out our progress. 
Yellow circle = current location

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.

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.

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.

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.