by Michael W. Hamburger and Walter D. Mooney Thursday, January 5, 2012
In the aftermath of the devastating magnitude-9.0 earthquake and tsunami that struck the Tohoku region of Japan on March 11, attention quickly turned away from a much smaller, but also highly destructive earthquake that struck the city of Christchurch, New Zealand, just a few weeks earlier, on Feb. 22. Both events are stark reminders of human vulnerability to natural disasters and provide a harsh reality check: Even technologically advanced countries with modern building codes are not immune from earthquake disasters. The Christchurch earthquake carried an additional message: Urban devastation can be triggered even by moderate-sized earthquakes.
As seismologists and engineers sort through the aftermath of Christchurch’s earthquake, they are already revealing some crucial lessons that might help us learn how to better prepare for such urban earthquakes. The magnitude-6.1 Christchurch earthquake was dwarfed by the Japan earthquake, which carried some 22,000 times more energy. It was even overshadowed by the much larger magnitude-7.0 earthquake that struck just 45 kilometers away from Christchurch on Sept. 3, 2010 (the Christchurch quake is widely considered to be an aftershock of the September quake). Nonetheless, the Christchurch event inflicted considerable damage: It killed nearly 200 people and caused more than $12 billion in losses (the September quake didn’t kill anyone and caused only a quarter of the damage). The damage was far greater than would have initially been expected for an event of this size.
Why? The damage was considerable because the earthquake originated only six kilometers from Christchurch’s population center and parts of Christchurch’s urban area were as close as one kilometer from the fault rupture. Its relatively shallow depth (only 5 kilometers beneath Earth’s surface) produced extraordinarily strong shaking at the surface. The earthquake occurred at about 1 p.m. on a busy workday, so many more people were in downtown Christchurch and thus exposed to the hazards of collapsing tall buildings; half the deaths from this earthquake occurred when one building collapsed. Finally, the effects of the seismic vibrations were probably amplified by the thick sedimentary layers on which the city is built.
Early reports from Christchurch found patterns of damage that are familiar to seismic engineers: the destruction of older, unreinforced masonry structures (like the Christchurch Cathedral) and the collapse of a handful of high-rise buildings that were built prior to modern seismic engineering standards. But it also appears that there were ground accelerations recorded in the Christchurch area that exceeded the design specifications of more recent buildings. Furthermore, there are clear indications of earthquake-induced liquefaction, or ground failure, and flooding that exacerbated impacts and hampered disaster response.
Earthquakes the size of Christchurch’s temblor occur globally about 100 times per year, or a couple of times each week; statistical analysis of the earthquake record indicates that for every magnitude-9.0 Tohoku-sized earthquake, there are about 1,000 Christchurch-sized earthquakes of magnitude 6.0. Fortunately, most of the moderate earthquakes don’t wind up in newspaper headlines. They occur in remote, unpopulated areas, are under water, or are deep enough that they don’t produce significant societal impacts. As the Christchurch earthquake demonstrated, however, when one of these events takes place close to a populated area, its effects can be devastating.
Many urban planners in the U.S. are appropriately concerned about the potential occurrence of a Christchurch-type earthquake — a relatively moderate magnitude-6.0 to -6.5 earthquake — directly beneath an American city.
Many American cities are vulnerable to damage in much the same way as Christchurch. A number of U.S. cities — notably Charleston, Memphis, Los Angeles, Portland, Salt Lake City, San Francisco and Seattle — are built dangerously close to known faults that have produced large earthquakes in the past, and their growing urban population centers are encroaching on hazardous areas. Most of our cities situated in seismic zones — particularly those in the central and eastern U.S. — have a large inventory of old, unreinforced masonry structures that are subject to damage or collapse, even from moderate earthquakes. And even more recent buildings constructed in the 1960s and early 1970s were built without seismic-resistant design.
Furthermore, the ground beneath many of our cities is underlain by unconsolidated sediments and thus is subject to seismic wave amplification and liquefaction. Add in the vulnerability of critical facilities — such as dams, nuclear power plants and chemical storage facilities — to the cascading secondary effects of earthquakes, including strong ground shaking, liquefaction, landslides, tsunamis and flooding, and you have the makings of a disaster.
Earth scientists understand that earthquakes are an inevitable consequence of geological processes, but we also know that earthquake disasters are not. Renowned 20th century author Will Durant wrote in “The Story of Civilization”: “Civilization exists by geological consent, subject to change without notice.” If we are to address his prophetic challenge, we must seek innovative ways to help prepare for and mitigate the urban disaster that is likely to happen during our lifetimes.
That disaster is most likely to manifest itself, not in the form of a massive, Tohoku-style earthquake, but as a moderate-sized Christchurch-style event, in which its proximity to an urban center results in damage far out of proportion to its size. While we cannot forecast which city will be the next victim of such an earthquake, we can mitigate potential impacts. For the most part, we already know what needs to be done. Now the challenge is transforming this geoscience knowledge into action. Let’s not be caught unaware — or unprepared.
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