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Guam Earthquake of August 8, 1993


Even though damage to some structures from the 1993 Guam Earthquake was severe, little human injury and no fatalities occurred. Hardest hit were the island's hotels, which had the same design weaknesses typical to many high-rise hotels damaged in earthquakes worldwide.

Introduction

At 6:34 P.M. local time on Sunday, August 8, 1993, a magnitude 8.1 earthquake occurred approximately 50 km south of the island of Guam. Varied damage to structures and lifelines occurred throughout the island. Injuries were generally minor and there were no fatalities. Given the severity of damage to some structures, it is surprising that fatalities and more severe injuries did not occur. Loss estimates (very approximate) provided by the government 10 days after the event were $4 million for homes, $113 million for private businesses, $40 million for government buildings, and $130 million for naval facilities.

Guam encompasses about 600 km2 and is home to approximately 130,000 people. This includes some 20,000 U.S. military personnel serving the naval and air force bases, which are among the largest in the Pacific. Guam has very little industry; its main source of revenue is tourism.

Typhoons frequently affect Guam and cause damage. Last year, Typhoon Omar caused approximately $250 million in damage. At the time of the 1993 earthquake, tourism was just recovering from a slowdown that followed Omar. As the earthquake occurred, Tropical Storm Steve was subsiding.

The island escaped disastrous damage partly because of the depth of the earthquake (about 65 km beneath the ocean floor) and the generally competent volcanic and limestone foundation materials (rock). Unfortunately, none of the strong-ground-motion instruments on the island were operational, so there is no quantitative measure of the severity of the shaking on the island. Estimates of peak horizontal ground accelerations have been made at about 0.25g with a duration of 30 to 60 seconds.

Geologic Setting

The island of Guam is a volcanic formation. The mountainous southern half of the island consists of a thin layer of soil over volcanic rock, with a few alluvial bands along the coast. The northern half is almost entirely a limestone (old coral) plateau. Most of the developed areas of the island are founded on firm soil or rock. Only in the port area, where widespread liquefaction of alluvium and fill occurred, did soils contribute significantly to damage. Relatively minor soil-induced failures, including broken underground utilities and small relative settlement of different portions of structures, were observed elsewhere.

Structures

Nearly all building structures on the island are constructed of reinforced concrete or masonry. Construction in Guam is governed by the Uniform Building Code, the same standard used in most seismically active regions of the continental United States. Guam is classified as Seismic Zone 3 (of 4), similar to Seattle, Sacramento, Salt Lake City, and Memphis. Zone 4 includes San Francisco and Los Angeles. Materials of construction and structural detailing are similar in Zones 3 and 4. However, the required design earthquake loading in Zone 3 is 75% of that in Zone 4.

Buildings in Guam are also designed to resist typhoon-level wind gusts of 70 m/s (155 mph). In many instances, this additional requirement provides substantially greater lateral strength than would be required for seismic resistance alone. The severe-wind design requirement contributed significantly to the relatively good seismic performance of most structures observed, despite the general lack of proper earthquake design details.

Low-rise Buildings

The large majority of low-rise buildings escaped serious damage. Many of the commercial buildings that were damaged had an open storefront configuration.

Mid- and High-rise Buildings

The mid- and high-rise buildings (mostly hotels) sustained the greatest damage. Most of Guam's few dozen hotels range from about 10 to 20 stories in height, and nearly all high-rise buildings are less than 20 years old. Essentially all mid- and high-rise buildings sustained minor damage, in the form of cracking and spalling in concrete-block infill as well as in concrete walls, beams, and columns. Several hotels had damage estimated at between 10% and 20% of their replacement costs. A few had structural damage severe enough to justify demolition. Virtually all of the hotels were closed for two or more days due to loss of power and water, and to permit damage evaluation. The one steel high-rise structure sustained little damage.

The worst and most dramatic damage involved a recently opened 12-story concrete-frame resort building with masonry infill walls. The resort had been open for only 18 days when the earthquake struck. In the section adjacent to the beach, columns supporting one side of the third floor collapsed, causing the structure to tilt several degrees. This portion of the resort will be demolished. Although many guests were temporarily trapped in the hotel, no serious injuries were reported.

This damage was partially the result of structural irregularities between the two lowest floors, which house public areas and therefore have large open spaces, and the upper portion, which is made up of smaller guest rooms with numerous interior walls. In effect, this and many other hotel high-rise buildings are made up of a lower, relatively flexible (soft) structure and an upper, very stiff structure. The failure occurred at this interface and was typical of severe damage to reinforced concrete hotels in many recent earthquakes.

Another severely damaged building was the four-story hotel across the street from the 12-story resort. This building is mostly concrete-frame construction, although there were many bays with concrete-block infill at the back and sides. The hotel was displaced laterally by approximately 20 cm (9 inches) across the first story. The concentration of damage at the first level appeared to be caused by a soft story combined with building torsion (twisting).

Although this building was only five years old, it did not appear to have the well-known structural details that enable a building to sustain large deformations without collapse. The four-story hotel was vacated immediately and scheduled for demolition.

One of the older damaged hotel buildings was a seven-story structure designed by a California firm in the 1970s. Constructed of stacked precast-concrete bearing walls seven stories high and cast-in-place floor slabs, the building sustained extensive damage to concrete surrounding welded connections between its precast elements. Poorly detailed wall piers along an exterior balcony line were shattered at their bases. An entire line of exterior architectural columns was severely fractured because the columns interacted with a recent low-rise addition.

Other structural damage of concern, noted at several locations, included pounding of adjacent portions of structures, cracking of shear walls, and cracking of concrete spandrels or link beams that connected adjacent walls and were not detailed to adequately withstand seismic loads. Poor construction quality, such as inadequately grouted concrete blocks and misplaced reinforcement, appeared to play a major role in some damage.

Nonstructural Damage

Some nonstructural damage occurred that could have been life-threatening. An elevator counterweight in one high-rise hotel was reported to have fallen from its supports into the elevator cab below. In other instances, unnecessarily heavy architectural building elements fell from upper floors or were damaged at their supports.

Assessing structural damage was of immediate concern to determine whether buildings were safe. It became apparent, however, that repairs of nonstructural damage, especially tile and architectural panels, could contribute significantly to business interruption and repair costs.

Schools

The safety of schools was a sensitive local issue after the earthquake. Several schools were damaged. A government-sponsored inspection program after the earthquake determined that portions of three schools were structurally unsafe. These schools were constructed between 1965 and 1986 and were all low-rise buildings. Engineering inspections revealed poor construction practices in some instances. Bond issues for constructing new or replacement schools are being developed.

The two-story concrete-frame Inarajan High School sustained the most severe damage. This school, located on the edge of a bluff on the southern end of the island and closer to the epicenter, may have experienced stronger ground motion because of its location. One of its classroom buildings had severe damage to all of its second-story columns. The damage was due to poor detailing, interaction with infill walls (short column effect), and an excessively heavy roof. The concrete-frame gym had severe damage to its columns, probably as the result of large forces introduced by the extremely (unnecessarily) heavy architectural roof.

Lifelines

Guam's electric power system is jointly operated by the Guam Power Authority and the U.S. Navy. There is an island-wide wastewater system, and a potable water system supplied primarily from wells. There is no central gas system; fuel for cooking is supplied from on-site propane tanks. The Guam Telephone Authority operates local phone service, and long-distance service is provided by major international carriers. Transportation between towns is over well-maintained, two-lane highways. The island is served by one commercial airport and one commercial seaport.

Electric Power

Electric power is produced on the island by a combination of oil-fired steam plants, diesel generators, and gas-turbine generators. The total generating capacity for the island is about 380 megawatts (MW). Minor damage to the power distribution system prevented immediate restoration of power after the earthquake.

About 70 MW of generating capacity was lost because of damage to the navy-operated Piti Steam Plant, located in the port area. The unanchored and unrestrained emergency batteries of the Piti plant fell over causing loss of dc power, which resulted in severe bearing and turbine shaft damage on two of the operating unit's generators. Station transformers moved on their foundation pads, and one unanchored transformer tipped over. Damage to the generators from potential support settlement was also feared. Other damage included rupture of buried piping and overturned equipment.

With the Piti plant off-line, the island had little reserve generating capacity. Rotating power outages were common in the weeks immediately after the earthquake.

Two large unanchored fuel tanks, one each at the naval base and Guam Power Authority, leaked their contents of fuel oil and diesel into their containment dikes. Fortunately, no fires occurred.

Water

Primary raw water sources are a system of 94 wells spread throughout the island and a single large reservoir. Raw water is typically drawn from depths of about 120 km through motor-driven pumps and then chlorinated. Potable water is stored in ground-mounted, welded steel tanks near population centers around the island and is distributed through about 800 km of buried piping of various types and sizes. The more heavily developed northern half of the island is served by a total of five sewage treatment plants where wastewater is chlorinated and routed through settling basins prior to discharge offshore.

All wells, pumps, storage tanks, and treatment plants appeared to have survived the earthquake without significant damage. Restoration of service, however, was limited by delays in restoration of electric power and by damage in buried piping systems. Water was distributed by supply trucks for the first few days after the earthquake. Within a week after the earthquake, more than 100 breaks had been reported in water mains.

Transportation and Communications

The road system experienced rockslides and minor localized slumping in pavement. It was not necessary to close any roads due to damage, but three of the island's concrete bridges were temporarily closed due to settlement at the abutments and minor cracking of concrete piers.

The commercial port handles all bulk material (and about 80% of total goods) in and out of the island. Spreading of fill adjacent to the three container-handling gantry cranes misaligned the rails over more than half of their length, severely reducing available travel distance. The port was informed by its insurers that using the cranes prior to a damage assessment and completion of necessary repairs would invalidate the port's insurance. Consequently, two large cargo ships en route to Guam were sent to Taiwan to load their containers into smaller ships with off-loading cranes. Piers on Guam servicing these smaller ships escaped serious damage.

The U.S. Naval Station piers also sustained extensive damage from lateral spreading of the fills behind older pier bulkheads. The newer piers performed well. Extensive liquefaction occurred around older fills and on natural terrain near an enlisted men's club. The club, which was an unreinforced masonry building, collapsed. Numerous other buildings on the base were damaged, but overall damage to the buildings was light.

The telephone system and the airport escaped with little damage. Both have emergency generators, which supplied power after the earthquake.

Recovery

Within a week after the earthquake, life appeared to have returned to normal for much of the population. The generally minor damage to most homes did not displace many people. Primary longer-term effects include periodic local power blackouts, caused by the lack of reserve generation capacity, and absence of water service in remote areas, caused by buried pipe damage. Shortages of some products, including fresh produce, resulted from damage at the port. At least one of the piers is expected to be operational within a few months.

The longest-lasting effect of the earthquake may be decreased tourist revenue due to hotel damage. Worker layoffs at some of the severely damaged hotels were reported within the first week, and one major hotel is expected to remain partially closed at least until late December.

Lessons Learned

  • Hotel design in many seismic regions needs to be reevaluated.
  • The poor performance of hotel buildings in this earthquake has commonly been observed in recent large earthquakes throughout the world. High-rise, and even mid-rise, hotels have had a poor record in the last eight years-since the Mexico Earthquake of 1985. The reasons for this poor record worldwide include investment in architectural features at the expense of structural elements, poor compliance with building codes, designs by engineers unfamiliar with seismic issues, and lack of adequate inspection.

    In the 1993 Guam Earthquake, much of the damage was attributable to irregular structural configurations and the participation of architectural elements (infill block walls) in the structural system, which was not anticipated by the designers. Many of the designs were performed in Japan or the continental United States. This probably resulted in limited surveillance of the construction process by the design professionals, contributing to poorer construction quality.

  • Design for strong wind loads does not automatically ensure good earthquake performance.
  • Design for strong winds, even in regions subjected to hurricanes and typhoons, does not ensure good earthquake performance. Earthquake loads on structures tend to be far larger than those produced by the worst storms. Superior earthquake performance can best be achieved by providing continuous systems, regular configurations, toughness through proper detailing, structural redundancy, and proper construction inspection.

  • New buildings are not always adequately earthquake resistant.
  • The economic loss of one wing and the extensive damage to the remaining wings of the newly opened 12-story luxury resort graphically demonstrate the importance of investing design effort in structural integrity as well as in architectural features.

  • Insurance claims for earthquakes may be more extensive than for windstorms.
  • Building owners were surprised that earthquake damage was different from past typhoon damage. Structures that received only minor typhoon damage were sometimes seriously damaged by the earthquake, and vice versa. It was reported that although there were only about half as many insurance claims for the earthquake as for Typhoon Omar of 1992, the claims tended to be for larger amounts, indicating the more serious structural and architectural nature of the damage.

    Observations/Conclusions

    Most of the earthquake damage observed was explainable after a cursory review by experienced earthquake engineers. Although structures in Guam are nominally designed in accordance with modern U.S. standards, both attention to structural earthquake details and construction quality appeared lax in many structures.

    The island was fortunate because the earthquake was both distant and deep, resulting in only moderately strong ground motion. If the earthquake had been somewhat closer and/or shallower, the ground motion could have been much stronger, causing more extensive damage and building collapses. Damage could have been compounded by the general lack of adequate structural details, and the life and property losses could have been substantial.

    Many regions of the United States are in the process of adopting earthquake-resistive design standards for the first time. An important lesson from this earthquake is that adoption of reliable codes does not by itself ensure good performance. Design professionals, the construction industry, and building officials also have to be trained to design and construct structures of appropriate quality.

    This article was written by EQECAT engineers Ronald Hamburger, Stephen K. Harris, David McCormick, Sam Swan, and Peter Yanev, all of whom made an earthquake reconnaissance visit to Guam within days after the event. EQECAT's Stan Siler and Anthony Hitchings also visited the earthquake site to gather information. EQECAT sent this team of engineers to evaluate structural, geotechnical, port, equipment, industrial, and lifeline damage and to assist in insurance claims adjustment.

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