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When Cities Face Geologic Forces

Earthquakes, volcanic eruptions, landslides — these are some of the every day events of our active planet. When these events intersect with densely populated cities, they become urban problems. Many geologists now are working with their governments to help people accommodate and successfully live with an area’s natural geological tendencies. This success also means living with available resources, lest overtaxing a resource cause new geologic problems.

The challenge urban geology brings, as stated on the Natural Resources Canada Web site on urban geology, is: “to provide engineers, planners, decision-makers and the general public with the geoscience information required for sound regional planning of densely populated areas of the Country.”

Following are four stories of cities with unique urban geology problems. They offer only a sampling of the diverse set of issues every country faces as its cities grow.

Kobe: Preparing for the Next Quake
Las Vegas: A Thirsty, Sinking City
Goma: Mount Nyiragongo Lava Flows
Colorado Springs: Living with Landslides

Kobe: Preparing for the Next Quake

Almost eight years have passed since the Great Hanshin-Awaji Earthquake devastated Kobe, Japan. Striking directly below the bustling urban center on Jan. 17, 1995, the magnitude-7.0 earthquake killed more than 6,000 people, destroyed thousands of homes and buildings, and wrecked critical infrastructure systems. Kobe, Japan’s sixth largest city and second largest port, until then was considered one of the most quake-safe cities in Japan.

The great loss of life in what seemed to be an earthquake-ready city and country horrified the world and forced Japanese seismologists to re-evaluate their earthquake mitigation program and earthquake engineering practices.

The Kobe earthquake brought home a strong point to earthquake engineers and researchers: build with caution on filled land in seismically active areas. Earthquake vibrations can rupture the surface, bringing water-saturated soil up and turning solid ground into a liquefied mess. The Port of Kobe was one of the hardest hit areas, with significant sections of the port built before modern geotechnical practice permitted mitigation of liquefaction risk. After the Kobe quake, this crane collapsed due to liquefaction-related lateral movement of a quay wall on Rokko Island in Kobe. NISEE/EERC

“The Kobe quake was a turning point for earthquake monitoring in Japan,” says David Wald, a seismologist at the U.S. Geological Survey and developer of ShakeMap — a map of ground shaking intensity that automatically generates after an earthquake occurs (see ANSS feature).

Earthquakes are part of life in Japan, which sits at the intersection of four tectonic plates. Before Kobe, Japan was already a leader in earthquake monitoring, having invested more than $100 million a year in an earthquake prediction program. Despite that investment, the Kobe quake came with no warning, causing researchers to look more toward hazard mitigation rather than prediction. Now the country has more than a billion dollars in seismic instrumentation alone, Wald says.

Since 1997, Japan began to build up its network of seismic stations, boosting the number of stations from the hundreds to the thousands. The Japan Meteorological Agency (JMA) began taking real-time observational data from multiple sources, including the University of Tokyo and the Science Technology Agency — hoping to provide a better understanding of seismological activities in the country to the Ministry of Education, Culture, Sports, Science and Technology.

JMA operates a seismic network of 180 seismographs for continuous earthquake monitoring, along with 650 “seismic intensity meters,” strong-motion sensors that only take measurements during an event. Local agencies operate an additional 3,000 intensity meters that feed data to the network. The National Research Institute for Earth Science and Disaster Prevention (NIED) also has deployed more than a thousand seismic stations of its own, both weak and strong motion. “All data from short period and broadband seismographic stations are continuously telemetered to the NIED and JMA,” says Mizuho Ishida, director for earth science research at NIED.

Immediately after an earthquake, these agencies release the epicenter location, quake magnitude and distribution of intensity. “These data are distributed to all scientists by satellite. Archived data are also useable through the Internet from anywhere and by anybody,” Ishida says.

After Kobe, Japan developed a ShakeMap-type program of its own, a “prompt appraisal system” that allows emergency managers and researchers to locate earthquake damage. While in the United States, ShakeMap must fill in data gaps where instrumentation is poor, in Japan, the instrumentation speaks for itself. “Japan has so many instruments that interpolation is unnecessary,” Wald explains. “They know the seismic intensities.”

Lisa M. Pinsker

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Las Vegas: A Thirsty, Sinking City

In 1907, settlers in the two-year-old city of Las Vegas drilled the first well in the Las Vegas Valley and started pumping groundwater. Before then, the amount of groundwater leaving the natural aquifer through streams or evapotranspiration was balanced by how much entered from snowmelt in the nearby Spring Mountains. The aquifer beneath the valley used to supply surface springs and made the valley a desert oasis. Las Vegas indeed means “the meadows” in Spanish. Some geologists have estimated that about 25,000 to 35,000 acre-feet of water naturally recharge the aquifer every year (one acre-foot, about 325,000 gallons, can sustain a family of four for one year, according to the Colorado River Commission).

By 1968, Las Vegas residents were pumping an average 88,000 acre-feet every year. Also by that time, surveyors had noticed that the city was sinking.
Today, Las Vegas is the fastest growing metropolitan area in the United States. Between 1990 and 2000, more than 1.3 million people moved to Las Vegas, increasing its population to about 1.5 million. And about 30 million tourists pour into the city every year.

Under the Colorado River Commission, the State of Nevada is allotted 300,000 acre-feet a year from the river. Its allotment is much less than Arizona’s 2.8 million acre-feet and California’s 4.4 million. By the end of this year, Southern Nevada alone will have used 320,000 acre-feet, according to the Las Vegas Review-Journal. Within the next 20 years, the Review-Journal reports, the state will probably need 700,000 acre-feet a year. The Southern Nevada Water Authority is pursuing several options for acquiring more surface water or for banking groundwater in southern Nevada and Arizona.

The state is attempting to share both surface water and groundwater with other desert populations that are also growing quickly. “The biggest problem everywhere is groundwater depletion,” says Devin Galloway, a groundwater specialist for the USGS Western Region.

A side effect of subsidence is earth fissures, such as the one pictured here in the northwest Las Vegas Valley, where growth is particularly rapid. A few years ago, earth fissures in the north-central valley caused at least $6 million in damage to homes in one subdivision. The fissures often open near faults, says John Bell, engineering geologist with the Nevada Bureau of Mines and Geology. “Subsidence is preferentially reactivating these faults, and that’s where most fissures are occurring.” Developers must scout for faults before building in the city of North Las Vegas. “We don’t know where these things are until they break to the surface. We just know they’re near faults,” Bell says. “It’s a challenging urban geology problem.” Photo by John Bell.

What Las Vegas residents can’t get from surface water sources, they will get from their natural groundwater supply. The Southern Nevada Water Authority estimates that the Las Vegas Valley gets 88 percent of its water from the Colorado River and the remaining 12 percent from groundwater. But for decades, the amount of groundwater pumped from the aquifer has exceeded the amount that naturally recharges it. As water is pulled from the ground, the underlying sediments compact and sink. In many places, the city is still subsiding.

“What’s happened is they’ve got this allotment of Colorado River water and they’re pretty much up to that limit. If they can’t pump more groundwater they have to find it from somewhere,” Galloway says. “If they don’t get more imported water, they have to rely increasingly on the groundwater resources, and there’s going to be more subsidence.”

The state’s geological survey, the Nevada Bureau of Mines and Geology, monitors the subsidence with a mixture of ground-based and remote sensing tools. Some parts of the valley sunk as much as six feet between 1960 and 1990, says John Bell, a research engineering geologist with the Bureau.

A key tool for researchers studying subsidence in the valley is InSAR. Interferometric Synthetic Aperture Radar scans Earth’s surface from satellites and can assemble a picture of an entire area with a high resolution, “down to a parcel or even a house,” Bell says. With InSAR, Bell, Galloway and several other researchers have found that the central part of the valley beneath downtown Las Vegas is no longer subsiding, thanks to a program by the Las Vegas Valley Water Authority to artificially recharge the aquifer with Colorado River water. “Putting water back into the system to recharge it is repressurizing the system,” Bell says. He adds that a NASA grant is helping him train survey staffers in using InSAR to monitor the valley continuously so that they can help local agencies manage the groundwater.

The central valley rises in the winter during artificial recharge and falls in the summer when groundwater is mined again, according to other InSAR work published by Jörn Hoffmann of Stanford University, Galloway and others.

But, once compacted, not all the sediments can bounce back. “Only a small fraction of the subsidence that has occurred over the long-term can be recovered,” Galloway says. Delayed compaction of past mining still causes much of the city’s subsidence, says Randy Laczniak with the USGS in Las Vegas. That’s because thicker silts and clays in the aquifer continue to compact years or centuries after a decline in water level.

Artificial recharge, as long as it continues, might halt subsidence beneath downtown Las Vegas, the inelastic sediments can’t store as much water as they could before, making it easier for subsidence to start again if that recharge stopped, Hoffmann says.

The researchers also found that the northwest portion of the city continues to sink at about 2.5 to 3 centimeters per year, Bell says. Galloway suggests the Eglington fault running through the valley could be stopping the artificial recharge from flowing into the northwest portion of the city. Also, sediments there could respond more slowly to recharge, Hoffmann says.

The researchers agree that the city’s future depends on how much water it can garner from outside the valley. “I don’t think there’s enough water in Las Vegas Valley to sustain the economic activity taking place there,” Hoffmann says.

“They really are going to have to look for surface water. In the long run, the groundwater is limited by the natural recharge.”

Kristina Bartlett

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Goma: Mount Nyiragongo Lava Flows

When it comes to a city’s volcanic risk, the threat depends on the neighboring volcano and what historically has vented out or fallen down: mudflows or lahars, pyroclastic flows, pahoehoe and `a`a lava flows, ash or toxic gases. With well over 50 million people worldwide living or working near a potentially dangerous volcano, it is difficult to single out a particular city at risk. Around the globe, 59 major cities conduct business as usual on top of historic lava flows or under the occasional dusting of ash, says Henry Gaudru, president of the European Volcanological Society’s International Commission on Mitigation of Volcanic Disasters in Geneva.

Almost seven months after the Jan. 17 eruption, steaming sinkholes such as this one are still opening up. This sinkhole developed on Aug. 12 in a prehistoric cinder cone above the main dike that fed the earlier eruption. Goma Volcano Observatory and USGS/OFDA Volcano Disaster Assistance Program

While the methods and techniques to monitor volcanoes around the world are fairly wellestablished, “other more immediate problems and dangers compete for the human and financial resources needed to carry out these investigations, particularly in developing countries,” Gaudru says. Goma in the Democratic Republic of the Congo (DRC) represents perhaps the worst-case scenario for a city struggling against other odds to manage a volcano observatory.

In 1994, as hundreds of thousands of Rwandan refugees fleeing genocide camped along the flanks of Mount Nyiragongo, the lava lake in the crater began spilling onto the crater floor. International concern for the refugees led Japanese and American teams of volcanologists to deploy seismometers around the volcano and provide financial aid. The Goma Volcano Observatory (GVO), established on a hill in the city, began to monitor earthquake activity. But over the years — with the transfer of the refugees off of the flanks, civil unrest within the country and a weak economy — all but two of the seismometers remained functioning. “It’s quite remarkable dedication that they were able to continue to keep anything operating,” says Chris Newhall of the University of Washington in Seattle.

Still working under adverse conditions and without pay, the seismologists at the GVO reported signs of active fumaroles in October 2001 and cracks in the inner walls of the crater, followed in January with seismic activity that remained high for weeks but then quieted to lower levels eight hours before a major eruption began on Jan. 17.

That morning, the lava lake at the top of Nyiragongo began to drain from fissures on the flanks of the volcano toward the city 18 kilometers away. While still faster than most lava flows because of its low silica content, which would otherwise polymerize in the melt and form chemical bonds that effectively increase viscosity, the flows were “generally not so fast that you couldn’t outrun them,” Newhall says.

Indeed, people living in the remote villages on the volcano’s lower flanks left toward Goma to escape, according to Peter Baxter of the University of Cambridge and Anne Ancia of the World Health Organisation (WHO) in their final report to WHO in June. “People in Goma were spectators to the event until lava flows began to enter the city towards the late afternoon,” Baxter says. “We were reliably informed that the mood of the population changed from expectancy to rapid exodus when lava vents were seen to develop within the city in advance of the main lava flow.”

The sudden appearance of the new vents, about 500 meters from the airport runway, spurred residents to evacuate. The main lava flow cut through the city and poured into Lake Kivu on the city’s southern shore. The second flow stopped short of trapping residents between the lava flows. Of the estimated 450,000 people living in Goma, 300,000 fled east to Rwanda and 100,000 went west, most on foot. All survived except for an estimated 70 people, and about 20 of those suspected deaths occurred during an explosion of a gas station when looters siphoning gasoline spilled the fluid onto the hot lava, Baxter reported.

“This is the first time in history a volcanic eruption producing only lava flows has impacted a city of such a size and made such a large number of people homeless,” says Patrick Allard, director of research at the National Center of Scientific Research in Yvette, France.

The Nyiragongo eruption lasted until Jan. 18, destroying about 13 percent of the city — including about 12,000 to 15,000 homes — and leaving approximately 80,000 to 100,000 people dependent on camps and host families for shelter. Since then, seismic activity at Nyiragongo continues to rattle the city. As of July, the international community has given approximately $35 million in response to the disaster, with about $7 million of that coming from the U.S. Agency for International Development. Most of those funds helped fill the immediate need for shelter assistance, water sanitation and health relief. But, at least some of the money is being used to evaluate continuing volcanic risks and provide for long-term monitoring.

The GVO scientists and technicians are again receiving a salary and have seven seismometer stations, each with a guardhouse to accommodate round-the-clock protection. The U.N. Office for the Coordination of Humanitarian Affairs provided $150,000 in telemetry equipment. The scientists in Goma are now tackling the chances that magma may break out again in the city or beneath Lake Kivu. With low areas known for collecting high concentrations of carbon dioxide, the scientists are also investigating gas emissions from the fissures.

While the residents of Goma knew to escape, most also returned home within a few days, the hot lava still cooling and dangerous. For scientists to better understand the current threat to Goma, the lesson of the January eruption is not to focus on the natural hazard alone, but to communicate with social planners, health providers and community leaders to gain a broader perspective of the risks the city is willing to face and prepare for. “Databases tend to focus on the volcanoes and not on the cities,” says volcanologist Bruce Houghton of the University of Hawaii at Manoa and a member of the organizing committee for a Cities on Volcanoes meeting in July 2003. “Really successful management of crises and humanitarian assistance requires a wide range of skills. Science is often pigeonholed. We need as much social science input as we need physical science input.”

Christina Reed

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Colorado Springs: Living with Landslides

In April and May of 1999, several days of high rainfall triggered landslides in the western section of Colorado Springs, Colo., uprooting foundations of homes and tearing apart driveways. The damage to homes exceeded $75 million. The Federal Emergency Management Agency (FEMA) spent $4.7 million to acquire 25 of the worst-hit homes.

Geology and topography make landslides an ever-present risk in Colorado Springs — a risk that becomes more dangerous as people continue to move to the foothills west of the city center. These higher elevations are prime real estate, offering spectacular views of the city.

“Thousands of Colorado Springs residents live in areas susceptible to landslides,” says David Noe, chief of engineering geology at the Colorado Geological Survey.

The wrecked foundation of a home is all that remains after a 1995 landslide broke off from the hillside and smashed into the house below. Heavy rains in 1999 re-activated the 4-acre landslide and put the house on top of the hill in jeopardy. The photo was taken April 24, 2001. Mark Squire, Colorado Springs Office of Emergency Management

Colorado Springs sits at the boundary of the Great Plains and the Rocky Mountains. The foothills have many steep slopes underlain by weak Cretaceous claystones that give way easily.

The topography in the foothills forms through landslides, Noe says. “What we have is a number of mesas that are eroding back through time.” Indeed, many of the homes are built on deposits formed from landslides.

When it rains, water seeps through the alluvium that caps the claystones. The water adds weight to the soil and bedrock, and reduces the friction between soil and rock particles. A landslide begins when the force of gravity exceeds the frictional force holding the soil and bedrock in place.

Development can exacerbate slope instability. Irrigating, cutting slopes and adding weight to the tops of slopes can all increase the chance of landslides.
The 1999 landslides ranged in size from less than an acre to 440 acres. They lumbered through the foothills with a “fast creeping motion, on the order of several inches per day, commencing with the rainstorms and continuing for several weeks,” Noe says.

The Colorado Geological Survey recently completed a landslide susceptibility map for the region, funded jointly by the Colorado Springs Planning Department and FEMA. The map delineates which areas of the city are prone to slope failure and which are not.

The planning department uses the map as a reference tool. “When the city gets a development proposal, they can say, right off the bat: ‘You are in a possible susceptible area, so you will really need to do a slope-stability analysis,’” says T.C. Wait, an engineering geologist at the Colorado Geological Survey who co-authored the map along with Survey engineering geologist Jonathan White.

In 1996, the city passed an ordinance requiring developers to address geologic hazards on any proposed site and to engineer ways to mitigate those hazards. The planning department often turns to the Colorado Geological Survey to determine whether the proposals are sound.

The impetus for the ordinance came in 1995 when a wet spring season triggered several landslides in the city. The landslides did not cause as much damage as those in 1999, but they did open some eyes to the potential dangers.

One popular way to reduce the chance of a landslide is to install drain pipes. Draining water out of the soil and bedrock can keep those layers from becoming heavy enough to slip. Another approach is to install retaining walls or buttresses that hold entire slopes in place.

Development pressure within landslide-susceptible areas is high. The Colorado Geological Survey reviews approximately one proposal a week for a new development in the foothills — from a single home to whole residential complexes. Almost all of the proposals pass the review, as long as they include an effective plan to mitigate the potential hazards.

Requirements for passing range from simply notifying possible owners that landslides are a risk to installing extensive mitigation systems, depending on results from site-specific geotechnical studies.

The drought this summer has prevented landslides from moving much, showing how sensitive the landslides are to climate. Vicki Cowart, director of the Colorado Geological Survey, notes that the 1999 landslides occurred in part because of conditions created by El Niño.

“A two-year period, wetter than average, saturated the materials. Then there was a big rain event, and subsequent to that event, there was massive sliding, flooding and debris flows,” Cowart says.

While this summer’s drought reduced landslide movement, it poses another risk: out-of-sight, out-of-mind.

“When the ground dries up and things stop moving, people forget. New people move in and new planners move in, and it doesn’t take long for people to say ‘What landslide problem?’” Cowart says.

Greg Peterson

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