Geotimes - June 2002 - Geophenomena

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Geophenomena

Earthquake Stresses High water in Venice

Earthquake Stresses
Stress is accumulating on portions of the San Andreas Fault and other faults in southern California, caused by earthquakes hundreds of miles away, according to a new model of how earthquakes transfer energy through the mantle. Generally earthquakes are considered stress relief for a region, as aftershocks and other earthquakes immediately following the break in the fault relieve tension in the vicinity.

But the new model shows that the stress relieved in one region may transfer through the mantle to another area more so than seismologists previously thought. This stress transfer increases the likeliehood of earthquakes for several fault blocks along the San Andreas Fault Zone that have not ruptured in over a century.

Andrew Freed of the Carnegie Institution of Washington and Jian Lin of the Woods Hole Oceanographic Institution in Woods Hole, Mass., have shown in their model results, published in the June Geology, that how much stress stays in a region depends on how viscous the mantle is under the fault. Higher viscosity regions of the mantle will flow until the stress is relieved — and transferred to other portions of the crust, perhaps hundreds of miles away. Freed and Lin have projected their models out over the next two decades.

They began by looking at the impact of the Landers earthquake in 1992, which Ross Stein, a seismologist at the U.S. Geological Survey, recently determined had contributed to the nearby Hector Mine earthquake in 1999. Freed and Lin have shown that, in turn, the stress from the Hector Mine earthquake continues to travel through the mantle. “What we find is that the stress increase is continuing to rise, creating a pulse or wave of stress moving towards the Los Angeles region,” Freed says. Their model shows that four earthquakes in the Mojave Desert in the 1990s will increase stress for decades to come along several far away fault zones — including the San Bernardino Mountain segment on the San Andreas and the San Jacinto and Elsinore faults further south.

“It’s important and exciting in the sense that they added to the picture the relaxation of the lower crust,” Stein says. He points out two shortcomings of Freed and Lin’s results however. The modeled pressures might be too high, he says, and the San Andreas earthquake record itself does not document an increase in the small earthquakes that would be expected from a wave of increased stress.

More than one version of the authors’ model fits the surface strain documented by GPS measurements of Earth’s surface deformation. But no matter how the authors changed variables, their model shows that the stress around the San Bernardino Mountain segment increases by at least 1 bar over 20 years, the equivalent of the pressure at the bottom of the shallow end of a swimming pool. This accumulation of stress occurs incrementally. “The stress changes we’re talking about are really small, tenths of a bar,” Freed said. “What we don’t know is what level of stress it takes for these faults to fail.”

The San Bernardino Mountain segment last failed in 1812, and the estimated seismic cycle of the fault is 130 years. Another version of their model predicts that, by 2020, the stress change could be more than enough to cause failure along that faultline.

Naomi Lubick
Geotimes contributing writer

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High Water in Venice

Heavy rains in June inundated the lagoon city of Venice and aided tides in setting record flooding levels for that time of year. Rarely have summer tourists waded though the iconic flood waters in Saint Mark's Square as they did on June 6, when the water reached 121 centimeters above average sea level.

The lowest point in Venice, St. Mark’s Square begins to flood when the tide reaches 70 centimeters above sea level. Photo courtesy of Miroslav Gacic

As the lowest point in the city, the square begins to flood when tides reach 70 centimeters above average sea level. Since the 1950s the number of floods above 80 centimeters has increased from about 20 a year to about 50 a year. The largest flood to hit Venice occurred in November 1966 after a great storm diverted the Sile River into the lagoon and raised tidal flooding to a record 194 centimeters. The winter high tides make November and December the worst months for flooding.

As sea level is expected to rise over the next 100 years, high tides are anticipated to push stormwater floods to record levels with more frequency. In order to protect the city from the onslaught of the Adriatic Sea, the Italian government decided last December to move forward on a $2.6 billion dollar project to construct a series of mobile gates at the three inlets into the Venetian Lagoon over the next eight years.

In the May 14 issue of Eos, Transactions of the American Geophysical Union, scientists debated the soundness of the mobile gate plan, called MOSE for Experimental Electromechanical Module, and its long-term capabilities.

The 79 gates are currently designed to rise from a resting position at the bottom of the lagoon to an angle that would protect the inlets from the lapping sea only during periods of threatening high tide. Compressed air would be injected into the hollow gates when tides brought water levels 110 centimeters above normal. Critics of the project are concerned that the protection the gates provide may in turn hamper navigation and cause potential environmental impacts to the natural flushing of the lagoon if sea-level rise forces them to close often.

With frequent closures, "there is a danger that the lagoon will not be ventilated enough," says Miroslav Gacic of the National Institute of Oceanography and Experimental Geophyiscs in Trieste, Italy. To address this concern, Gacic and colleagues are conducting a two-year study measuring the tidal exchange through the three inlets where the gates will be built. They reported preliminary results from last summer showing that the entire 550 million cubic meters of the lagoon is flushed in a single day, primarily through tides. Even infrequent openings of the gates, of only a few hours a day, might dispose most of the collected waste water out to sea, Gacic says. Still he adds, "other measures have to be taken together with construction of the gates, such as prevention of the lagoon pollution by industrial and city waste waters."

But Paolo Pirazzoli of the French Centre National de la Recherche Scientifique contends that, "apart from the degree of water pollution in the lagoon, the main concern is the duration of closures, because the gates, as projected, are not watertight." While the barrier will allow water to move between individual gates, Pirazzoli's concern is with the oscillation of the individual gates as they move with the waves and the gaps that oscillation might create when neighboring gates are out of phase. In the same issue of Eos, he argues that other techniques such as raising street-level elevations and increasing water resistance at the inlets by decreasing water depth would "return the frequency of flooding to the very acceptable level of about one century ago." He says such techniques would provide "a few decades," giving scientists time to better estimate sea-level rise and design a more effective, watertight barrier between the lagoon and the sea.

The final design of the gate plan, however, is still under consideration. And Rafael Bras of the Massachusetts Institute of Technology says that the redesign of the gates makes the walls thicker, preventing any gaps from occurring when the gates move out of phase under the waves. "Water tightness is not an issue," he says. Bras and colleagues at MIT and the University of Padova, Italy, wrote in the same Eos issue a response to Pirazzoli. "The issue in the face of the worst-case scenario of a 50-centimeter rise in the next 100 years is that the barriers would have to be closed very often. This would have a significant impact on navigation to the Port of Venice unless a large ship lock is built at Malamocco, and this is being considered for the final design."

Christina Reed

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