Originally, the main aim of seismic research and anti-earthquake measures in Japan was to predict major earthquakes. Consequently, seismometers and other monitoring equipment were mainly concentrated in areas such as the Tōkai region on the Pacific coast of central Honshū, where a major earthquake has long been expected. This changed in 1995, when the Great Hanshin-Awaji Earthquake devastated the city of Kōbe in Hyōgo Prefecture and surrounding areas.
Originally, the main aim of seismic research and anti-earthquake measures in Japan was to predict major earthquakes. Consequently, seismometers and other monitoring equipment were mainly concentrated in areas such as the Tōkai region on the Pacific coast of central Honshū, where a major earthquake has long been expected. This changed in 1995, when the Great Hanshin-Awaji Earthquake devastated the city of Kōbe in Hyōgo Prefecture and surrounding areas. This unexpected major earthquake brought about a swift shift in strategy. From that point, the focus shifted to putting together a policy based on understanding the mechanism of earthquakes first, before trying to predict where they might occur.
Japan’s High-Density Seismic Observation Network
The government’s first step was to establish a nationwide earthquake observation network. Using GPS technology, the Geospatial Information Authority of Japan monitors plate and land surface movements in approximately 1,000 locations around the country. The National Research Institute for Earth Science and Disaster Prevention has set up devices to measure seismic intensity at more than 1,800 locations nationwide, providing seismic monitoring on a continual basis. Today, it is possible to measure almost every earthquake that occurs, however small—even down to minor events of magnitude 0.3–1.2. This data is published over the Internet, making it available almost instantly to researchers around Japan, greatly speeding up the progress of seismological research and disaster prevention efforts.
The detailed seismic information obtained through this system, together with research on ocean trench earthquakes and active faults, makes it easier to predict the likelihood that a major earthquake will occur. A detailed survey of ground conditions throughout the country also makes it possible to measure the levels of seismic activity across the affected area when an earthquake does occur. The Japanese Seismic Intensity Scale measures (in units called Shindo) how strongly earthquake tremors are felt in a particular area, regardless of the actual magnitude of the quake at the epicenter. The government also publishes a map showing the probability that each area of Japan will experience tremors equivalent to a rating of Shindo Lower 6 or higher within the next 30 years. (At this level, furniture shifts and most people find it difficult to remain on their feet.)
Probability of Shindo Lower 6 Tremors Within the Next 30 Years
Japan Seismic Hazard Information Station (J-SHIS)
Map data showing predicted seismic activity is based on research data obtained by the National Research Institute for Earth Science and Disaster Prevention. The map can be enlarged to show detailed information for any location in Japan.
Constructing a Disaster Prevention System Based on Anticipated Damage
Anticipated damage is assessed based on the predicted seismic intensity figures for each area after an earthquake. These predictions not only assess primary damage caused directly by the tremors of the earthquake itself, such as earth liquefaction and landslides, ruptures in the ground surface, collapsing buildings, and human injuries and fatalities, but also secondary damage, including earthquake-induced fires and the impact on daily lives of interrupted electricity and water supplies, as well as tertiary damage caused by the crippling of urban functions and interruptions to economic activity.
Detailed studies of conditions on the ground during the Great Hanshin-Awaji Earthquake played a major role in improving the accuracy of these assessments of anticipated damage. Previous damage predictions were carried out logically on the basis of engineering factors such as the estimated strength of buildings, but today the accumulation of detailed data makes it possible to carry out more accurate assessments based on actual figures.
The government conducts detailed assessments of anticipated damage in the case of five major earthquake hotspots. These include the Tōkai region and an earthquake widely expected to occur directly under Tokyo at some stage in the future. Based on these assessments of anticipated damage, the government sets concrete targets determining the extent to which anticipated damage can reasonably be reduced within the next ten years, and also establishes a strategy for implementing these reductions.
The same information is also used to plan emergency responses for when a major earthquake disaster does strike. Assessments of anticipated damage are also used to determine the volume of emergency relief supplies necessary and the routes that rescue teams can take in the event of a major earthquake. Manuals based on these assessments are drawn up for use in the event of a real-life emergency. Follow-up studies are then carried out every three years to ascertain the extent to which the predicted damage has been reduced and the progress that has been made with disaster-response measures.
Japan’s Earthquake Resistance Standards for Buildings
Japan’s earthquake resistance standards for buildings have been adopted and revised several times over the years in the aftermath of a major earthquake. Particularly important were the revisions made in the New Building Standards Act enacted in 1981, introducing new earthquake resistance standards. In 1978, a magnitude 7.4 earthquake struck off the coast of Miyagi Prefecture, registering as high as Shindo 5 on the Japanese seismic intensity scale. In addition to causing 28 fatalities and injuries to more than 10,000 people, the earthquake resulted in widespread damage to housing and other buildings. Some 7,400 houses were either wholly or substantially destroyed, with lesser damage done to a further 86,010 houses and buildings. Based on the lessons learned from this disaster, the building standards laws were substantially revised. The new regulations made it a legal requirement for all new houses, apartment buildings, and high-rise buildings to be designed to sustain only slight cracks in the event of an intermediate-scale earthquake (up to around Shindo Upper 5 on the Japanese seismic intensity scale), and to not collapse even in the event of a major earthquake (Shindo Upper 6 or Shindo 7).
Of the 6,434 people who lost their lives in the Great Hanshin-Awaji Earthquake in 1995, almost 80 percent were killed instantly when the building they were in collapsed or when they were crushed by heavy items such as moving furniture. But of buildings constructed since the enactment of the 1981 law, some 80 percent suffered only minor damage or none at all—and only 1 percent of them collapsed.
In December 1995, the Law for Promotion of Seismic Retrofit of Buildings was passed. Since this law came into effect, regular inspections have been carried out and improvements made to the earthquake resistance features of buildings that did not meet the requirements of the 1981 standards. A further amendment to this law was enacted in 2006 to make buildings even more resistant to major seismic events. These measures have been further enhanced by the Japan Building Disaster Prevention Association, which guarantees liabilities and provides information on earthquake preparedness, and through the introduction of a system of tax deductions for improvement work on buildings to make them more resistant to earthquakes.
Emergency Earthquake Early Alert
Japan’s emergency earthquake early alert system has been running officially since 2007. There are two main kinds of seismic waves: P-waves (primary waves; 5–7 km/sec), which cause the initial minor tremors, and S-waves (secondary waves; 3–4 km/sec), which cause the bigger tremors known as major seismic motion. The emergency earthquake early alert system picks up P-waves close to the epicenter and automatically calculates the epicenter and scale of the earthquake, as well as the predicted seismic intensity in various locations. If the predicted maximum seismic intensity is Shindo 3 or more, or the predicted magnitude is 3.5 or more, the system automatically sends a signal to public facilities, factories, and transportation networks. At predicted seismic intensities at or above Shindo Lower 5, the system alerts households and other regular users. The purpose of the system is to give people time to take evasive action before the major seismic motion begins.
Some people have criticized the technical limitations of the emergency earthquake early alert system, claiming that the short interval between the P- and S-waves close to the epicenter makes it difficult for the system to provide a warning quickly enough for it to be effective. But there can often be as much as ten seconds or more between the warning and the onset of major seismic motion—enough time for people to take cover and protect their heads from falling objects.