Hokkaido Nansei-oki Earthquake of
July 12, 1993

Introduction

The Hokkaido Nansei-oki Earthquake (offshore of southwestern Hokkaido Island, Japan) had a magnitude (Mw) of 7.8, similar to the 1923 Great Tokyo (Kanto) Earthquake, which had a magnitude of 7.5 to 8.0. The recent earthquake affected a lightly populated part of the least populated major island of the Japanese archipelago, but it was a microcosm of what can happen to Tokyo; to other cities of Japan; and to North American cities like Portland, Seattle, and Vancouver. The primary lesson of the earthquake is that the triple hazards of earthquake shaking followed by tsunamis and fires can be devastating to modern cities.

The earthquake struck at 10:17 P.M., local time, on July 12, 1993. It affected the southwestern coastal area of the island of Hokkaido. The area most affected is about 100 by 125 km and is to the east of the island of Okushiri.

The main earthquake occurred at a depth of about 34 km in the Sea of Japan, north of Okushiri along a subduction zone that might be the boundary of the Eurasian and North American plates. The aftershock area around Okushiri extended over 150 km north to south and about 50 km east to west. As much as 150 km of faulting may have occurred along the causative fault zone.

Several strong-motion instruments in southwestern Hokkaido recorded 60+ seconds of strong ground motion. The highest recorded horizontal acceleration is reported to have been 0.50g and was at Kuromatsunai, west-southwest of Sapporo and about 80 km from the fault. The strongest ground motions probably occurred on Okushiri, where there were no instruments, but peak accelerations in excess of 0.40g have been estimated.

The earthquake caused a large tsunami, which hit the coast of Okushiri less than 5 minutes after the earthquake. The settled areas around the island were inundated by tsunami waves typically from 5 to 12 m high. The highest wave run-up along the southwest coast was about 30.5 m in a limited area. The tsunami caused extensive damage on the island and lesser but locally severe damage along the southwest coast of Hokkaido. Most of the populated areas worst hit by the tsunami were bounded by tsunami walls (up to about 4.5 m high), which may have moderated the overall tsunami effects but were ineffective for higher waves.

Most of the casualties in this earthquake were caused by the tsunami. As of July 24, the official death toll was 196, with about 50 people still missing. Most of the deaths were on Okushiri. Since the island is a tourist area, the number of missing might be higher than the estimate.

As of July 19, the University of Tokyo's International Center of Disaster Mitigation Engineering reported that 540 houses were destroyed by tsunami or fire, 154 were significantly damaged, and 1,826 were partially damaged. Thirty-one public buildings were damaged, some severely; railways were disrupted at 124 locations; and highways were damaged in at least 365 locations. Serious damage occurred to schools, industrial structures, bridges, port facilities, and all other types of infrastructure.

Most of the damage was caused by the tsunami. Ground shaking produced relatively light direct damage; however, extensive damage, as much as 90 km from the fault, was due to ground failures, including liquefaction, lateral spreading, settlement, and landsliding. Large landslides occurred throughout the strongly shaken areas on Okushiri and Hokkaido.

Earthquake

Shaking

Even though the earthquake shook a large area, the ground shaking intensities were generally moderate. This was probably due to the depth of the main shock; at 34 km the depth was triple that of a typical shallow earthquake along a lateral fault, such as California's San Andreas Fault. These moderate intensities caused widespread ground failures in poor soils and widespread minor direct structural damage. Pockets of severe structural damage were observed in areas of poor soils or in locations such as near edges of bluffs, where local topographically induced ground motion amplification may have occurred.

Landslides

As is typical in mountainous terrain, the earthquake caused hundreds of landslides throughout Okushiri Island and on Hokkaido. A large landslide buried a two-story hotel in the town of Okushiri, killing 11 of the 37 guests. Large landslides and rockfalls blocked and severely damaged numerous roads in the shaken area. At least one tunnel in Hokkaido collapsed due to a rockfall. Although many landslides did occur, they would have been even more numerous if the earthquake had been shallower, severely impeding the transportation system of the area.

Liquefaction and Other Ground Failures

The most obvious damage from the earthquake, aside from the tsunami and fire, was caused by ground failures such as liquefaction and lateral spreading. Both of these cases were typically exhibited as settlement and/or horizontal land movement. Ground failures severely affected many primary roads in Hokkaido, up to about 80 km from the faulting. The coastal stretch of National Highway 5 south of Oshamanbe, for example, was damaged at many locations, including long stretches of embankment near sea-level elevations. A section of Highway 5 collapsed north of Oshamanbe due to lateral spreading. The failure also damaged the nearby rail line.

Ground failures disrupted underground utilities throughout the affected region. At the time of ABS Consulting's reconnaissance visit, repair work was observed in every coastal settlement visited along Uchi-ura Bay and the west coast of Hokkaido facing Okushiri Island. Damage was observed to water, sewage, and fuel lines. Ground failures also damaged aboveground and underground piping to large storage tanks as far as 80 to 100 km from the faulting.

At least one 7-span, two-lane, steel girder, concrete column bridge near Esashi in Hokkaido was severely damaged. The 2-m-diameter reinforced concrete columns failed in shear. Most bridges in the strongly shaken area sustained minor damage such as embankment and abutment fill settlement.

Port facilities were severely damaged because of ground failure throughout the affected area. The major port at Hakodate, about 100 km from the fault, sustained extensive damage to its waterfront facilities and breakwaters. Extensive port damage also occurred at Aonae, the town of Okushiri, and elsewhere on Hokkaido.

Structures

With a few exceptions, the structural damage caused by shaking was generally moderate to light. The more interesting structural failures occurred on Okushiri Island. Numerous small unreinforced masonry farm silos were damaged or collapsed. Most buildings on the island were typical Japanese two-story wood-frame buildings. These generally sustained light damage.

Several larger commercial structures in and around Aonae on Okushiri Island were severely damaged. The Aonae lighthouse tower collapsed as a unit onto the adjacent structure. The reinforced concrete tower rotated about 45o after failing its anchorage to the foundation.

A 20-m steel-frame tower supporting a cement hopper at the Okushiri Cement Plant buckled and collapsed vertically about 50 cm. The hopper had adequate diagonal bracing for earthquake loads but inadequate vertical columns. The buckled columns were built-up cold-formed steel sections. The columns buckled at their bases, near the diagonal bracing. The anchorage of two other hoppers at the plant failed, which resulted in anchor bolt breakage and local plate buckling of the tank skirts. Overall damage at the plant was estimated at about 20% of the total replacement value.

The main three-story, older, reinforced concrete-frame classroom building of the Aonae Elementary School had severe shear cracking in spandrels with the damage progressing into the main structural columns. This damage is typical of past earthquake damage to Japanese schools and commercial buildings of reinforced concrete built before about 1973. A number of such structures have collapsed in Japan, as well as elsewhere, in recent earthquakes.

All of the above structures were located either on soft soils (cement silos, school) or near the edge of a bluff, where local ground motion amplification appeared to have occurred (the lighthouse).

Tsunami

Eyewitness accounts collected by the University of Tokyo indicated that the island of Okushiri was battered by a series of tsunamis less than 5 minutes after the shaking. The western coast of Hokkaido, across from Okushiri, was also inundated by tsunami. Water run-up in the area of Aonae was from 10 to 20 m high-equivalent to the height of three- to five-story buildings. The highest recorded run-up in Okushiri was about 30.5 m-roughly equivalent to a 10-story building.

In Aonae (population about 1,500), half of the 690 houses were washed away by tsunamis even though most were bounded by recently constructed, massive concrete tsunami walls. Generally, the damage to wood-frame buildings nearest the coast was total. Many concrete foundations could be seen afterwards with sill anchor bolts but without a shred of wood-either sills or vertical framing. Several concrete and steel buildings were also damaged. Typically, in these more substantial buildings, the interiors and the architectural finishes on the ground floor, and sometimes higher, were destroyed. The damage potential of a major tsunami to light structures in its path far exceeds that of either an earthquake or a windstorm.

The tsunami also caused extensive and severe damage to port facilities and all other infrastructure in its direct path, including water, power, transportation, telecommunications, and sewage. For example, all power lines for several kilometers along the west coast of Okushiri within the inundated area were completely destroyed. The tsunami damaged roads extensively in some areas, completely stripping bituminous and concrete paving and depositing it far inland.

The sewage treatment plant at Aonae was not damaged directly by the tsunami because it was naturally protected by high sand dunes. The plant had ground settlement up to about 30 cm, light structural damage, and tipping of unanchored electrical equipment.

The plant was knocked out of service, however, by flooding from the tsunami. The flooding occurred when one or more tsunami waves forced water into the basement of the plant through the outflow pipe. The basement was flooded to a height of about 1.25 m, and various pumps and electrical equipment were inundated. This damage points out the need for protecting large-bore piping in important facilities against the effects of tsunami.

Typically, the settled coast of Okushiri was bounded by recently constructed tsunami walls, which ranged up to about 4.5 m high. Similar walls have been constructed in and around Tokyo and other metropolitan areas of Japan. The southernmost part of Aonae was completely surrounded by such a wall. The tsunami washed right over the wall and destroyed all wood-frame structures in the area. The tsunami wall may have been partially effective in slowing down and moderating the height of the tsunami but it was ineffective (in this, as well as other areas) in protecting people and property.

The destructiveness of this tsunami and the speed with which it inundated Okushiri raise concerns about other counter-earthquake measures undertaken in the more populous parts of Japan, the U.S. and Canadian Pacific Northwest, and other areas where subduction earthquakes occur. Because of the ineffectiveness of the tsunami walls at Okushiri, a reexamination of engineering assumptions concerning future tsunami effects to these areas is warranted.

Fire

Aonae was also the area most affected by fire. Aonae is densely built with narrow streets and buildings, generally of one- or two-story, wood post-and-beam construction. After tsunami, fire following earthquake was the most damaging agent in the July earthquake, destroying more than 300 homes in an 11-hour blaze.

Shortly after the earthquake, personnel from the 38-person fire department made a quick circuit of the town looking for fires. Seeing none and concerned about tsunami, the fire fighters returned to the fire station, which is on top of a bluff, to await the need for emergency service.

The fire department reported that about 20 minutes after the earthquake and 15 minutes after the first tsunami, it received a citizen alarm of a fire in the main area of town. Fire fighters immediately responded, but encountered debris (the remains of a temple) blocking the street. Fire fighting was therefore from hand lines supplied from the pumpers, drafting from bluff-top cisterns.

The initial ignition source is uncertain, but fuel spills were caused by the earthquake and tsunami, including residential and business kerosene tanks (for heating), propane tanks, and the fuel from many boats deposited ashore by the tsunami. Fire progress was relatively slow but was aided by the flammables stored in each house. Every time the fire department seemed to be gaining headway, the fire would flare up again, probably due to these tanks.

Fire spread was southward at about 35 meters per hour, with fire fighting on the downwind edge. Two hours into the fire, a second fire ignited behind the fire line. At about 4:00 A.M., available water from the cisterns was exhausted. Citizen volunteers assisted in moving the hose over debris from the cisterns to the port, where the two pumpers drafted from the harbor. At this point, the advancing firefront was about 90 m wide. The fire department used equipment to push aside debris and two buildings, creating a firebreak. Using four hand lines from the drafting pumpers, they were able to successfully stop the fire at about 9:00 A.M., saving several dozen remaining houses.

Conclusions

The Hokkaido Nansei-oki Earthquake occurred close to the 70th anniversary of the Great Tokyo (Kanto) Earthquake of September 1, 1923. The Hokkaido Nansei-oki Earthquake, however, is unique in recent history because of its effects. It combined, within a few minutes, the triple hazards of earthquake shaking, tsunami, and fire following.

On the island of Okushiri, one can observe what could happen on a vastly larger scale to a modern metropolitan area. Tokyo can be used as one example of a city sitting astride a major subduction zone.

Tokyo Bay is ringed by heavy industrial, power, and petrochemical plants. Thousands of tanks storing toxic chemicals, various fuels, and other flammable fluids are interspersed throughout the vast complex. All of this is built on either fill or poor soils, both of which have a history of inadequate performance during strong shaking.

Some new technologies being employed are believed capable of controlling liquefaction and preventing serious damage in parts of Japan. This belief has allowed building construction in some possibly dangerous locations from a risk perspective. Until recently, the same confidence of protection existed about tsunami walls. That confidence is being questioned after the walls failed to protect Okushiri, however.

The soils around Tokyo Bay, from Yokohama to Chiba and beyond, could fail extensively in a major earthquake. Many large tanks in the area could fail as a result of such soil effects. A few ignitions would undoubtedly occur, and conflagrations might not be preventable. In addition, it is generally believed that tsunamis cannot strike Tokyo Bay. The Hokkaido Nansei-oki Earthquake has refuted many previous beliefs, however, and whether or not the bay is vulnerable to tsunami remains to be seen.

Past estimates of possible damage to the Tokyo area from an earthquake and its effects need to be reevaluated in light of the Hokkaido Nansei-oki Earthquake. The earthquake risk remains high, and according to experts, there is a probability that a major earthquake could strike the Tokyo area within the next 50 years.

Every major metropolitan area around the world at risk from a subduction earthquake should reexamine any counter-earthquake measures, building codes, and emergency planning currently in place. Experts may also have to revise estimates of potential earthquake damage using data gathered from the Hokkaido Nansei-oki Earthquake.

Dr. Charles Scawthorn and Mr. Peter Yanev visited the earthquake-affected area on July 21-24 as part of the U.S. team investigating the earthquake. The team was led by the National Institute of Standards and Technology. Dr. Scawthorn and Mr. Yanev were sponsored by the Earthquake Engineering Research Institute and by the National Center for Earthquake Engineering Research, respectively. They received extensive support from the Japanese authorities, led by Dr. Yutaka Iita and Dr. Kazuhiro Kawashima of Japan's Public Works Research Institute.