All about Alaska

You’ll notice from the map that the Sikuliaq seems more than a little off course at the moment. This is due to minor engine problem and has required us to call in at the remote but pretty town of Sand Point to collect a small electric motor, which is being flown in today. More about Sand Point in future though because this post is about the tectonics of Alaska, which of course is the reason we’re here in the first place.

 

Cruise_region_2
Map of our location and the instruments we’ve deployed. Only three more TRMs to go!

Deploying seismometers, GPS receivers and other geophysical equipment is an expensive, time consuming process. Yet Alaska is one of the most heavily instrumented places on Earth. Why is this so? It’s partly because of the state’s abundant petroleum resources, but also because it has much to teach us about plate tectonics.

Before we move to the tectonics, here are some fun facts about the people and geography of Alaska

  • It’s the largest state in the US, with twice the area of Texas and a longer coastline than the lower 48 combined.
  • The highest mountain in the U.S, Mt. Denali (6124m) is found here.
  • Some the Aleutian Islands fall beyond the International Dateline, making Alaska simultaneously the westernmost and easternmost state in the US.
  • At 1.2 inhabitants per square mile, it’s by far the most sparsely populated state in the US. Over half of the population lives in the Anchorage area.
  • More than 80% of the states’ revenues come from the petroleum products (oil and gas). Fishing makes up most of the rest.
  • The state neither levies sales tax nor income tax.
  • It has the highest number of pilots per capita of any US state (2006 data)

 

Geologically speaking, Alaska sits above a subduction zone where dense, thin, oceanic crust of the Pacific plate pushes against thicker, lower density crust of the North American continent. When this happens, the dense oceanic crust descends into the mantle. This process generates earthquakes at the interface between the two plates and volcanoes onshore where magma generated by melting of the mantle rises to the surface. Subduction has been ongoing on Alaska for more than 200 million years and is responsible for most of the large scale features we see today – the Aleutian island arc, high mountains along the coast, abundant volcanic activity and intense seismicity.

 

earthquake_map
All the earthquakes that have occurred in our study location since 1970. The dense clusters are aftershocks of large events. The earthquakes are colored by depth.

 

This particular project is most focused on the seismicity. All earthquakes occur due to the build-up and release of stress on faults. Distant from the fault, the rock on either side may be moving slowly due to accommodate the motion of tectonic plates. However, at the fault friction prevents this movement from taking place. This is known as fault locking. Over time, the rocks around the fault deform until the stress on the fault overcomes fiction and the fault slips suddenly. The energy released by this process manifests itself as seismic waves and is felt as an earthquake.

The largest and most dangerous type of earthquake is known as a megathrust event. These occur at subduction zones due to slip on the interface between the downgoing and overriding plates. Megathrust earthquakes can generate large tsunamis because they are associated with sudden deformation of the seafloor and hence uplift of the water column. The 1964 M9.2 Alaska event, the second largest in recorded history, is an example of such an earthquake. The earthquake generated a tsunami that was over 8 meters high in coastal Alaska and caused damage in Japan, Hawaii, Canada and the western US. Indeed, the Alaskan subduction zone has a history of generating such earthquakes, but only in specific areas. Other parts of the subduction zone appear to generate more frequent, smaller events and the reasons for these differences in rupture characteristics are poorly known. To understand them, it’s important to understand the structure of both the overriding and incoming plate around the region where these earthquakes happen. Since this region is offshore, ocean bottom instruments are a necessity for high-resolution studies. Hence one of the motivations for this study.

 

plates_diagram
Major features of the Alaskan subduction zone. Note how the distance between the Aleutian trench and the Aleutian volcanic arc increases towards the east. This is a sign that the dip of the subducting plate is decreasing.

 

Many other features of the Alaskan subduction zone are poorly understood. Solving their mysteries could have important implications for our understanding of plate tectonics and the hazards represented by volcanoes and earthquakes. Why, for example, does the chemistry and eruptive style of volcanoes change from west to east along the Aleutian Island arc? Why does the maximum depth at which earthquakes occur vary along the subduction zone? How has this changed over geological time?

These are tough questions. To answer them, we need to quantify the properties of the subduction zone: What materials are present there? What temperatures, pressures and stresses do they experience? How fast are they moving? Modeled and interpreted correctly, the seismic and GPS data collected from this experiment could go a long way towards producing the answers we’re looking for.

Robert Martin-Short, UC Berkeley Seismic Lab

 

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