By Marisa Brennan
The more we know about offshore earthquakes, the better we can understand earthquake hazards onshore. With that aim in view, Natural Resources Canada (NRCan) earthquake scientists are now monitoring offshore earthquakes with new ocean floor seismographs, thanks to collaborative efforts with the NorthEast Pacific Time-Series Undersea Networked Experiment (NEPTUNE). NEPTUNE facilitates the collection of offshore earthquake data that is transmitted in real time by fibre optic cable to NEPTUNE’s shore station, and then on to the Geological Survey of Canada’s earthquake centre in Sidney, about 20 km north of Victoria on British Columbia’s Vancouver Island.
Earthquakes are caused by the interaction of tectonic plates, pieces of the earth’s surface up to 100 km thick. Moving at speeds comparable to those at which human nails grow (4 cm or so per year), tectonic plates grind against each other, sometimes storing energy for long periods of time, until the plates crack or suddenly slip past one another, releasing the stored-up energy in the form of an earthquake.
The largest earthquakes occur at the edges of tectonic plates as they slip past one another. “Think of two firm sponges being pressed together and forced to slide past one another,” explains Dr. Garry Rogers, a research scientist in NRCan’s Earth Sciences Sector (ESS) and a principal investigator on the NEPTUNE project. “They’ll initially stick together, then deform. When the pressure finally builds up too much, they’ll suddenly slide past one another, releasing the stored-up energy.”
The NEPTUNE project is concerned with the activity of two particular tectonic plates: the relatively small Juan de Fuca Plate, located off the coast of British Columbia, which is sliding under the western side of the much larger North American Plate. NEPTUNE, with a network of seismographs located directly on the Juan de Fuca plate, can detect more offshore earthquakes than land-based instruments. “NEPTUNE gives us up-close and personal data we couldn’t get otherwise,” says Garry. “Earthquakes related to motions between the plates can’t be accurately located from land, and tiny tremors can’t be detected from land.”
Building Safer Structures
Since the NEPTUNE Canada Seismograph Network taps into earthquake activity that was previously undetected, earthquake scientists expect to learn more about the locations and magnitudes of future earthquakes. And this new knowledge of where large-scale earthquakes are likely to occur can be used, among other purposes, to update building codes in areas of high seismic activity and to reinforce existing structures so that risks are minimized.
Onshore, new earthquake research is being used to increase opportunities for earthquake preparedness. Dr. John Cassidy, also an ESS research scientist, has discovered that the relative frequencies of soil and building motion have a cause-and-effect relationship. Tall buildings are affected by slow shaking of one cycle a second (1 Hz) or less, while low-rise buildings, such as single family homes, are affected by more rapid shaking of 10 cycles a second (10 Hz) or so. “If the frequency of the soil matches the frequency of the building, greater damage may result,” says John. “So researching soil, seismic shaking and building structures allows us to develop building standards that will provide better protection against earthquakes.”
The development of the NEPTUNE Canada project is funded by more than $100 million from the Canada Foundation for Innovation, an independent corporation created by the Government of Canada to fund research infrastructure, the British Columbia Knowledge Development Fund and industry contributors. NRCan scientists provided expertise for the design and location of the ocean floor seismograph network that will be used to reduce earthquake risks.
To read about related articles, see Natural Hazards