Alaskan Earthquake In 2002 Set Off Tremors On Vancouver Island

Perhaps it was just a matter of sympathy, but tremors rippled the landscape...

  Perhaps it was just a matter of sympathy, but tremors rippled the landscape of Vancouver Island, the westernmost part of British Columbia, in 2002 during a major Alaskan earthquake. Geoscientists at the University of Washington have found clear evidence that the two events were related.Tremor episodes have long been observed near volcanoes and more recently around subduction zones, regions where the Earth's tectonic plates are shifting so that one slides beneath another. Tremors in subduction zones are associated with slow-slip events in which energy equivalent to a moderate-sized earthquake is released in days or weeks, rather than seconds.Now researchers studying seismograph records have pinpointed five tremor bursts on Vancouver Island on Nov. 3, 2002, the result of a magnitude 7.8 earthquake on the Denali fault in the heart of Alaska.As surface waves, called Love waves, shook Vancouver Island they triggered tremors underneath the island in the subduction zone where the Explorer tectonic plate slides beneath the North American plate. The tremors were measured by seismometers along roughly the northern two-thirds of the island. "What we found is that when the waves pushed the North American plate to the southwest, the tremor episode turned on and when the motion reversed it turned off," said Justin Rubinstein, a UW postdoctoral researcher in Earth and space sciences and lead author of a paper describing the work published in the August 2 edition of Nature.Though the Denali quake was mostly felt in Alaska, its effects were apparent thousands of miles away. It sloshed lakes from Seattle to Louisiana, muddied wells as far east as Pennsylvania and triggered small earthquakes in seismic zones across the Western United States.Still, finding evidence of tremors on Vancouver Island was unusual."A few people have seen tremor episodes triggered by earthquakes, but not as clearly as we have. This is by far the clearest and easiest to interpret," said co-author John Vidale, a UW professor of Earth and space sciences and director of the Pacific Northwest Seismic Network."This shows us it's just like a regular fault -- you add stress and it slips," Vidale said. "It's like regular faulting but on a different time scale."Other authors are Joan Gomberg of the U.S. Geological Survey in Seattle and UW researchers Paul Bodin, Kenneth Creager and Stephen Malone.An earthquake typically will appear suddenly on a seismograph, while the much more subtle ground motion from a tremor burst gradually emerges from the background noise and then fades again, Rubinstein said.By comparison, tremors typically produce the strongest seismic signals in a slow-slip event, in which seismic energy is released very gradually during periods as long as three weeks.In this case, the authors suggest that the force of the Love waves induced slow slip on the interface between the North American and Explorer tectonic plates near Vancouver Island and triggered the tremor bursts, each lasting about 15 seconds."That made it easier for us to observe because there were these five distinct bursts," Rubinstein said. "Normally you are not going to feel these tremors. The shaking in the tremors we observed was 1,000 times smaller than the surface waves from the earthquake."Being able to spot the tremors was largely a matter of distance and timing, Vidale said."We were able to separate the tremor signal from that of the distant earthquake because the surface waves had traveled more than 1,200 miles, losing the high-frequency vibrations that would have masked the high-frequency tremor vibrations," Vidale said.While the tremors were recorded a great distance from the rupture that triggered the Denali earthquake, the scientists suggest the same process could occur closer to the fault and might actually be important in the rupture process.Seismograph data for the research came from the Canadian National Seismograph Network and was distributed by the Geological Survey of Canada.Climate change will inflict additional costs to the economy in the coming decades, according to a new report by the U.S. Government Accountability Office (GAO). Dealing with the effects of climate change will cost both citizens and businesses, especially insurance companies, but will cost the U.S. government even more — and before its emergency coffers are bankrupted, the government needs to take action, the report says.The U.S. government acts as the insurer of last resort. Various government programs, such as flood insurance, pay out when the weather damages personal property beyond what is covered by private insurance. From 1980 through 2005, private and federal insurers together paid more than $320 billion in claims from weather-related losses, according to the GAO report, released May 3. Such losses vary from drought-related crop losses to storm-related floods to hurricane-related damages. Whatever the cause, the government has paid out record amounts in recent years and those numbers are increasing, especially as more people move into hazard-prone areas, the report says. Furthermore, “rising temperatures are expected to increase the frequency and severity of damaging weather-related events, such as flooding or drought,” the report says. Private and federal insurers will both bear the costs of climate change-related damages over the coming decades. Unlike private insurers, the U.S. government does not have the funding to support such increases in insurance expenses, says Richard Murnane, program manager for the Risk Prediction Initiative and a senior research scientist at the Bermuda Institute of Ocean Sciences. “They’re setting themselves up for enormous problems,” he says. The government “needs to decide whether it wants to operate in a self-sufficient manner and charge adequate premium for its exposure, or whether it wants to operate in a reactionary manner and appropriate the additional funds required to pay for losses as they occur,” he says. “It’s really the unusual events, the low-probability, high-impact events, that drive the costs,” says Gregory van der Vink, a geologist at Princeton University in Princeton, N.J. Thus, he says, the question the government and private insurers need to ask is “how do you amortize the low-probability, high-impact event?” In 2004 and 2005, record numbers of hurricanes struck the United States, in record strengths. In 2006, despite contrary predictions, none struck the mainland. The combined impacts of shifting climactic zones, sea-level rise and development means that there will be more variability in recurrence intervals of low-probability, high-impact events, so the usefulness of past experience for projecting future losses is limited, van der Vink says. “We need improved, open modeling that will maintain public awareness and help insurance companies prepare” for these events. Although “rare for any specific town or city, when integrated across a nation for which large insurance companies and the federal government have responsibility, [low-probability, high-impact] events occur regularly and dominate the costs,” he says. It’s important that the government and private insurers examine climate’s effect on their costs, and thus the costs to consumers, van der Vink says, but unless people stop moving into hazard-prone environments, costs are only going to increase. And stopping developments in highly vulnerable areas is something that no one has come to terms with yet, he adds. Meanwhile, per the GAO recommendations, the departments of Agriculture and Homeland Security (which oversees the Federal Emergency Management Agency) will be further analyzing the long-term fiscal implications of climate change on the government’s insurance programs.