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discussed before the event, but, in the days immediately prior to October 17, there were no foreshocks. Just as the occurrence of this earthquake was anticipated, so were its principal effects. The extent of damage in San Francisco and Oakland, which lie approximately 60 miles from the epicenter, has many parallels with the 1985 tragedy in Mexico City caused by the magnitude 8.1 Michoacan earthquake that occurred more than 200 miles away from the capital. In both cases, the principal cause of damage was young, poorly Consolidated, water-saturated, fine-grained sediments that amplified the ground shaking and were susceptible to ground failure. The San Francisco Marina District tragedy is one of special irony; this community consists mostly of wood-frame buildings; such structures are flexible and ordinarily fare well even in strong earthquake shaking if there is solid ground beneath them. However, the Marina District is built on poorly consolidated artificial fill that was originally emplaced for the Panama-Pacific International Exposition, which celebrated San Francisco's recovery from the 1906 earthquake. This artificial fill amplified the shaking and failed massively and pervasively during the Loma Prieta earthquake. The Marina District fire resulted from a broken gas main, and the efforts to control the fire were hampered by broken water mains. The gas and water main breaks were a

result of ground failures. Similar problems also had occurred in artificial fills in the 1906 earthquake. The collapsed portion of the Cypress Street section of I-880 in Oakland was built on San Francisco Bay mud that amplified the shaking; the undamaged portions were built on firmer ground. These soft soil areas, as well as other areas that sustained significant but less severe damage, had been identified on USGS maps as having a high potential for damage. In the epicentral region, damage in the communities of Watsonville, Santa Cruz, and Los Gatos was most severe in unreinforced masonry buildings constructed before the modernization of California building codes. Here, as well as in the Marina District, major shaking damage occurred in structures having a poorly supported first floor that was unable to resist horizontal shear deformation; most structures built in the past few decades did not sustain major structural damage from the earthquake. Liquefaction, the transformation of loosely packed and fully saturated sediment into a fluid mass, was responsible for some of the most devastating damage caused by the earthquake. Liquefaction occurred in manmade fill around the margins of San Francisco Bay and in flood-plain deposits in the Salinas-Santa Cruz area. In the San Francisco Bay area, liquefaction-induced ground failure

Are You Prepared for the Next Big Earthquake?

A 24-page insert delivered in newspapers to some 2.4 million homes in the San Francisco Bay area starting September 9, 1990, asked that important question. The insert was prepared through an unprecedented cooperative effort by many local, State, and Federal agencies and private organizations, including the Bay Area Regional Earthquake Preparedness Project, Association of Bay Area Governments, California Division of Mines and Geology, California Office of Emergency Services, California Seismic Safety Commission, Earthquake Engineering Research Institute, Federal

Emergency Management Agency, Applied Technology Council, American Red Cross, United Way of Santa Clara County, and United Way of the Bay Area. Most of the cost of printing the newspaper insert was paid by the Northern California Earthquake Special Relief and Preparedness Project of the American Red Cross and the Northern California Disaster Relief Fund through the United Way of Santa Clara County and the United Way of the Bay Area. The earthquake awareness information in the full-color insert was organized and written by Peter Ward (on right in photograph), a USGS geophysicist in Menlo Park, Calif. Ward was recognized for his "imagination, leadership, hard work, and perseverance" with a public affairs award from the U.S. Department of the Interior. The award was presented to Dr. Ward by Dr. Harlan Watson, science advisor to the Secretary of the Interior. In presenting the award, Watson noted that, while scientists tend to communicate with a rather small group of fellow specialists, Ward, through this special publication, communicated useful scientific information to millions of people. Of the total 3.1 million copies of the booklet The Next Big Earthquake: Are You Prepared?, 2.4 million were distributed in

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Bay Area newspapers, and another 600,000 copies were sent on request to individuals, schools, libraries, and other interested parties throughout the country. A limited number of copies are available; write to Earthquakes, 345 Middlefield Rd., Menlo Park, CA 94025.


was most extensive in water-saturated manmade sand fills that subsequently had been paved over. Many of these areas also underwent liquefaction during the 1906 earthquake. Significantly, no obvious ground failure occurred in numerous other extensive bayshore land fills that apparently were better engineered. The Loma Prieta earthquake generated hundreds of landslides throughout a region of approximately 5,400 square miles (see map, p. 3). The epicentral region of the earthquake in the steep Santa Cruz Mountains historically has produced abundant landslides, both during earthquakes and during the rainy winters of the region. The largest October 17 landslide damaged dozens of residences. Other landslides, including rock falls, rock slides, and debris slides, occurred on coastal cliffs, steep hillsides, and along roads as far as 80 miles from the epicenter. Although most of the above effects were anticipated, the Loma Prieta earthquake did hold some surprises. The forecasts had predicted horizontal motion on a vertical fault, but the 6 feet of horizontal motion occurred on an inclined fault plane and was combined with 4 feet of reverse vertical slip on the same plane. In hindsight, this vertical movement agrees well with models of earthquake motion; however, accurate prediction of the motion depended on identifying the dip of the fault plane. Another surprising aspect was that the motion of the earthquake was not observed along a single fault break at the surface, which suggests that previous magnitude 7 earthquakes in this area may not have left direct evidence in the geologic record. This is an important issue because geological estimates of what has happened in the past form an important element of long-term forecasting and seismic hazard assessment. The motion on the fault was distributed across a broad zone of complex cracks and fractures. Individual cracks within this zone of unusual fractures generally follow, and may have controlled, the formation of the existing topography. These surface displacements have been explained as slope-related movements along the bedding plane faults that caused the strong shaking of the mainshock. Similar surface fissures were observed in this region after the 1906 earthquake. In the past, the occurrence and effects of a major earthquake generally were not predictable. The Loma Prieta earthquake is exceptional because the likelihood of its occurrence had been evaluated well in advance of the earthquake, and the worst damage and destruction occurred in areas known to be at greatest risk. The time duration of strong ground motion generated by

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Hurricane Hugo, September 1989

By R. Erik Schuck-Kolben and

Lionel Kaufman n September 1989, Hurricane Hugo, a I very powerful and destructive hurricane having winds in excess of 130 miles per hour (see Saffir-Simpson scale, p. 8), hit the U.S. Virgin Islands, eastern Puerto Rico, and the coast of South Carolina. This hurricane was one of the most destructive storms to hit the Caribbean and the East Coast of the United States in the 20th century. Although rainfall totals associated with this storm were relatively low (4 to 10 inches in the most affected areas), high winds and storm-surge flooding along the coastal areas caused severe damage. The maximum storm surge measured in South Carolina was one of the highest ever recorded anywhere on the East Coast of

the United States. At least 50 persons lost their lives as a result of the storm, and storm damage is estimated to be about $10.4 billion. U.S. Virgin Islands.-Rainfall may have produced localized flooding in the islands, but total rainfall associated with Hurricane Hugo generally was less than 10 inches, which is relatively minor in comparison with rainfall totals commonly associated with hurricanes. Most of the flood damage in the U.S. Virgin Islands occurred in coastal areas as a result of tidal flooding. Storm-tide elevations ranged from about 3 to l l .5 feet on St. Croix and from 4.5 to 6.5 feet on St. Thomas and St. John. USGS personnel located and surveyed elevations of high-water marks throughout the U.S. Virgin Islands, Puerto Rico, and other nearby islands. Damage from Hurricane Hugo was severe throughout the U.S. Virgin Islands, particularly in St. Croix. Approximately 65 percent of the buildings in St. Croix were destroyed, leaving about 20,000 people homeless. All public utility services were disrupted.

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About 90 percent of all power lines on the island were downed by the storm, two of three desalination units were out of service, one 10-million-gallon water storage tank was destroyed and another severely damaged, and the sewage treatment system was out of service. A USGS employee, Bruce Green, living and working in St. Croix, played a key role in establishing emergency water supplies (see box, p. 8).

Damage in the U.S. Virgin Islands from Hurricane Hugo is estimated to total about $2 billion. The Official death toll for this storm is 14.

Puerto Rico.—Rainfall associated with Hurricane Hugo exceeded 10 inches in 48 hours near the town of Naguabo in eastern Puerto Rico. Total rainfall, however, was between 4 and 8 inches over much of the east. The amount and intensity of rainfall were substantially less than those associated with other large hurricanes. Inland flooding, however, did occur along some small streams. Storm-tide elevations along the eastern and northern coasts of Puerto Rico ranged between 4 and 10 feet but exceeded 12 feet near San Juan. Coastal flooding occurred in some beach and low-lying areas.

The Luquillo Experimental Forest, also known as the Caribbean National Forest, was severely damaged. The area in and around the forest, where rainfall amounts and intensities were high, was also the site of more than 200 mostly shallow landslides on the steep and highly dissected mountain slopes. Half of the landslides were associated with highway construction and road cuts.

Other damage from the hurricane included the loss of fish and shellfish from lagoons along the coast as a result of drastic changes in water quality associated with the storm. Within several weeks after the hurricane, USGS teams sampled and tested the water quality of Laguna de Piñones, Laguna La Torrecellia, and Laguna San Jose. The most significant changes in water quality were noted in Laguna de Piñones where dissolved solids concentrations, which normally range from 14,000 to 32,000 milligrams per liter, had been reduced to 2,600 milligrams per liter by the freshwater flowing into the lagoon as a result of the heavy rains. Dissolvedoxygen concentrations, which normally exceed 6 milligrams per liter, were less than 3 milligrams per liter, and concentrations of sulfide, normally less than 0.5 milligram per liter, had increased to more than 10 milligrams per liter.

Property damage in Puerto Rico is estimated to be about $2.5 billion. Only two deaths were directly attributed to the hurricane, but six employees of the power authority

were killed while repairing downed power lines.

South Carolina. Rainfall produced by Hugo over the State of South Carolina ranged from a maximum of 10 inches south of Charleston to 2 inches in the upland part of the State; more than 4 inches occurred only in the southern coastal area. Rainfall from Hugo was much less than expected, and no serious flooding of inland rivers occurred. Severe coastal flooding occurred along much of the South Carolina coast. The high-water elevation at the Charleston tide gage peaked at about 10 feet above sea level shortly before 1:00 a.m. on September 22 when the hurricane came ashore. This peak was about 8 feet higher than the normal (predicted) tide stage.

Many mountain roads in eastern Puerto Rico were blocked by the more than 200 landslides that were a result of Hurricane Hugo, which hit the island on September 18, 1989.


SAFFIR-SIMPSON HURRICANE SCALE Hurricane Hugo was one of the most destructive storms to hit the Caribbean and

East Coast of the United States during this

Wind speed Storm surge

Category !" ". ...'..., Evacuation century. The devastating effects from Hugo No p 74—94 4.5 No. underscore the need to continue efforts to No. 96–110 6–8 Some shoreline residences and low-lying study and understand the mechanisms and areas, evacuation required. potential effects of hurricanes and other No. 3 111–130 9–12 Low-lying residences within several coastal storms. By being prepared as best as blocks of shoreline, evacuation possi- possible for nature's capricious action, we can bly required. - help to reduce a disaster's toll. No. 131–155 13–18 Massive evacuation of all residences on low ground within 2 miles of shore. No. 155 18 Massive evacuation of residential areas

on low ground within 5–10 miles of
shore possibly required.


Water-surface elevations related to the storm surge were even higher in other areas along the South Carolina coast. Water-surface elevations of 12 to 16 feet above sea level were common in much of the area from Myrtle Beach southward to Sullivans Island east-northeast of Charleston. High-water elevations, based on more than 300 flood marks, were located and surveyed by USGS personnel within a few weeks after the storm.

The maximum water-surface elevations associated with the storm occurred in Bull Bay. The absence of barrier islands and the trapping effect of the bay on waves driven by extremely high onshore winds resulted in peak water-surface elevations of about 20 feet above sea level. Very few storms have ever produced storm surges of this magnitude along the East Coast of the United States (see “Hurricane Hugo and the South Carolina Coast,” p. 11).

Damage to property along the South Carolina coast was severe and is estimated to be about $5.9 billion; 29 deaths have been attributed directly or indirectly to the storm.

Hurricane Hugo and Puerto Rico

By Rafael W. Rodriguez and
Richard M.T. Webb

Co." resources as diverse as offshore
sand deposits, recreational beaches, and
coral reefs were affected severely when Hurri-
cane Hugo struck Puerto Rico. The Puerto
Rico Marine Geology Program, a cooperative
study between the Puerto Rico Department of
Natural Resources and the USGS, assessed the
impact of the storm and monitored the recov-
ery of these resources in the months following
the storm.
Offshore sand deposits are of major
importance to Puerto Rico because onshore
Sources suitable for use in the construction
industry and in beach replenishment projects
have been depleted. Escollo de Arenas, the
largest offshore sand deposit (more than 3
billion cubic feet), was severely degraded by
Hugo. The Escollo de Arenas is a trailing
edge sand deposit, which lies off the north-
west coast of Vieques Island. Soil cores and
aerial photography show a loss of between
3.5 and 7 million cubic feet of sand from this
deposit. The sand from the Escollo de Arenas

USGS Employee Restores Water Supply to St. Croix Disaster Relief Center

By Sandra L. Holmes

During the late evening and early morning hours of September 17–18, 1989, Hurricane Hugo, a category 4 storm, hit St. Croix in the U.S. Virgin Islands with sustained winds of 140 miles per hour and gusts of 200 miles per hour. Hugo battered St. Croix for 8 hours, damaged 90 percent of all homes and other buildings, destroyed the main power station, and cut off all sources of freshwater supplies to the island.

The USGS established a field office in December 1988 on St. Croix to support water-resources activities with the principal local cooperating agency, the Virgin Islands Water and Power Authority (VIWPA). Bruce K. Green, a USGS hydrologic technician stationed on St. Croix, had recently completed a project with the VIWPA in support of the siting and development of new public watersupply wells.

Green and his family rode out Hurricane Hugo along with fellow residents. In the aftermath, Green recognized the need for immediate disaster relief, especially the need for potable water for basic sustenance and disease control. Cutting his way with a chain saw to a passable road, he arrived via four-wheel-drive vehicle at the center for emergency relief operations. At the center, Green was asked by personnel of the U.S. Army Corps of Engineers, the lead agency for recovery operations, to serve on the formal Emergency Response Team.

Aided by his knowledge of the quantity and quality of the ground water in the newly developed well fields, Green directed his attention to the problem of water supply. Working through Army supply channels, he obtained miles of PVC pipe and many gasoline-powered generators and pumps.

After cutting and clearing the way to the remote well fields, Green directed the Construction of a temporary water-supply line to the disaster relief center. These efforts restored at least limited public water supply to island residents long before any other utility was available after the devastating storm. It took 5 months to repair the structure of and reinstall telephone and electrical lines to the U.S. Department of Agriculture building that had housed the USGS field office before the hurricane. During this time, Green operated the USGS field office out of the carport of his home.

For his dauntless courage and unquestioned leadership in an uncertain and potentially life-threatening situation, Green received the Valor Award of the U.S. Department of the Interior.

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