Geotimes - July 2003 - Highlights

Geotimes

Highlights

Earthquakes
Susan Hough

The final few months of 2002 witnessed a dramatic verification of an old saying: Earthquakes don't kill people, buildings do. On Halloween day, a modest, magnitude-5.9 earthquake struck southeast of Rome, killing dozens of children in the village of San Giuliano di Puglia when part of a poorly built school building collapsed. Four days later, the largest earthquake of the year struck in central Alaska — a massive magnitude-7.9 earthquake that ruptured approximately 300 kilometers of the Denali Fault. Because there are so few buildings in that region, the earthquake caused only a single injury, no deaths, and (relatively) modest property damage.

Figure 1. Trans-Alaska Oil Pipeline. Photo courtesy of Susan Hough.

The earthquake at Denali can be considered a significant success story of earthquake-hazard mitigation efforts by virtue of what did not happen during the event. For half a century, the Denali Fault has been recognized as a major active fault and the Trans-Alaska Oil Pipeline, which crosses the fault, was engineered to withstand a major earthquake. Indeed, it did not break even though the fault moved more than four meters horizontally and almost one meter vertically at that location (Fig. 1). Mitigation efforts thus prevented an environmental catastrophe as well as a major economic loss.

The Denali earthquake also provided a guilt-free bounty of data for earth scientists. Although most of the investigations are still underway, the data are already proving invaluable for a wide range of studies, including earthquake rupture processes on major strike slip faults, stress transfer (Anderson and Chen, Geophysical Research Letters, v. 30, doi:10. 1029/2002GL016724, 2003), and earthquake triggering. The earthquake was clearly followed by remotely triggered earthquakes, even at distances of several thousand kilometers in places such as Yellowstone and the Cerro Prieto geothermal region in northern Mexico. As scientists endeavor to understand the mechanism by which large earthquakes trigger other earthquakes at such great distances — a phenomenon first recognized only following the 1992 Landers, Calif., earthquake — every new large earthquake provides precious new data.

Meanwhile last year, the attention of earth scientists was also focused on new data from so-called slow earthquakes (slip events that occur too slowly to release seismic waves) and on data from a previously unrecognized type of seismic source along subduction zones. K. Obara (Science, v. 296, p. 1679-1681) identified sustained, long-period "tremor" signals originating from the subduction zone in southwest Japan. Tremor has previously been seen only in volcanic regions and has been linked to the movement of underground magma. The new observations stand to provide important new clues about the processes at work along subduction zones.

Several studies elucidated the properties of slow earthquakes, showing that these events can apparently occur before large earthquakes (Melbourne and Webb, Geophysical Research Letters, v. 29, doi:10. 1029/2002GL015533, 2002) after large earthquakes (Melbourne and colleagues, Journal of Geophysical Research, v. 107, doi:10. 1029/2001JB000555, 2002), and in the interseismic period (for example, Dragert and others, Science, v. 292, p. 1525-1528; Ozawa and others, Science, v. 298, 1009-1012). Scientists have speculated for years about the possibility that aseismic slip events might precede large earthquakes, speculations that were fueled by a handful of anecdotal observations, including a pipe crossing the San Andreas Fault that broke just hours before the 1966 Parkfield earthquake. New data from GPS instruments and strainmeters are, therefore, of far more than academic interest. If precursory slow earthquakes occur commonly before large earthquakes, we can hold out a measure of hope that short-term earthquake prediction might not be impossible.

Another of the big seismology stories of 2002 holds much promise for the future: The Earthscope project, in the planning stages for many years, was funded by the National Science Foundation. This major new earth-science initiative comes with several components: The Plate Boundary Observatory will deploy hundreds of new GPS receivers to study the active boundary between the Pacific and North American plates, the San Andreas Fault Observatory at Depth (SAFOD) will literally probe the mysteries of the San Andreas Fault via a 4-kilometer-deep drill hole, and the USArray project will install seismometers across the United States to investigate continental and deep-earth structure. USArray will complement another major initiative, the Advanced National Seismic System (ANSS), which is now underway although not fully funded. When completed, ANSS will become the first integrated national earthquake recording system in the nation.

Several pilot projects upon which Earthscope will build are in progress or already completed. In 2002, a pilot SAFOD drill hole reached a depth of 2.2 kilometers, aimed at a section of the fault on which small repeating earthquakes are occurring (Fig. 2). A series of planning workshops will be held in 2003 and beyond to address both the scientific opportunities and the logistical challenges that Earthscope represents. The latter are substantial and daunting, but seismology has always been a data-driven science and the field is still barely a century old. With both Earthscope and ANSS on the horizon, we appear to be positioned for an exciting start to the second century of modern seismology.

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Hough is a research geophysicist with the U.S. Geological Survey in Pasadena, Calif. E-mail: hough@usgs.gov.

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