3:00 PM - 3:15 PM
[U15-06] Damage to Wooden Houses Caused by the 2024 Noto Peninsula Earthquake and Preliminary Countermeasures Required in the Future
★Invited Papers
Keywords:Seismic Retrofitting, Seismic Diagnosis, New Earthquake Resistance Standard, Seismic Grade
On January 1, 2024, an earthquake with a maximum seismic intensity of 7 on the Noto Peninsula caused the collapse of many wooden houses and other serious damage, resulting in the loss of more than 240 human lives, most of which were caused by collapsed houses.
Japan is one of the most earthquake-prone countries in the world, and the revisions of earthquake resistance standards have been repeatedly reflected in past earthquake damages. Firstly, the changes in the Japanese Building Standard Law (BSL) are summarized.
BSL was enacted in 1950. Subsequently, there was a major revision in 1981, and since then it has been referred to as the New Earthquake Resistance Standard. The 1995 Hyogo-ken Nanbu Earthquake occurred in 1995, and countermeasures based on the findings of this damage survey were incorporated in the 2000 revision. Therefore, to understand the reasons for earthquake damage to wooden houses, it is necessary to distinguish between (1) those built before May 1981 (Old Standard), (2) those built between June 1981 and May 2000(New Standard), and (3) those built after June 2000 (Current Standard).
A survey report by the Architectural Institute of Japan(AIJ) on the damage caused to wooden houses by the 2016 Kumamoto earthquake indicates that most of the damage was concentrated in (1) (collapse rate of about 27.9%), followed by (2) (collapse rate of about 8.7%) and (3) (collapse rate of about 2.2%). (2) is ambiguously positioned, not meeting the current standard but not the old standard, and is referred to as a "gray zone" among experts.
The revision of BSL in 2000 specifies the method of connections between columns and beams. This requires that the ends of columns must be connected with hardware of sufficient strength to prevent them from pulling out. The Japan Building Disaster Prevention Association has prepared a method for verifying the seismic performance of houses in the gray zone, "Seismic Performance Verification Method for Wooden Houses under the New Earthquake Resistance Standards". Owners of houses built before May 2000 are encouraged to implement this method.
The total cost of seismic retrofitting of a house is often around 1 to 2 million yen, which is not an inexpensive purchase, but subsidies may be available from the local government for seismic diagnosis and retrofitting. However, this subsidy is mainly for (1) and may not include (2). In light of the damage caused by recent major earthquakes, there is a movement to make (2) also eligible for subsidies. The retrofitting of the gray zone in (2) is considered an issue that must be addressed urgently in the future.
It is also recommended that newly constructed houses obtain "Seismic Grade" under the Housing Performance Indication System; it is reported that damage to houses that obtained seismic grade 3 (the highest rank) was particularly small during the 2016 Kumamoto earthquake. The Housing Performance Indication System was implemented in 2000, and the current utilization rate is about 30%.
A comprehensive survey of the damage caused by the 2024 Noto Peninsula earthquake is being conducted mainly by AIJ, and the results of this survey should be closely checked. At this stage, as in the case of the 2016 Kumamoto earthquake, most of the damage was caused by old earthquake-proof houses or houses in the gray zone, but some houses built after 2000 also seem to have been damaged. On the other hand, some relatively new wooden houses remained undamaged even in the area of the severe earthquake (Photo 1). New houses appropriately designed and constructed have not been damaged in recent major earthquakes.
The Noto Peninsula has been seismically active in recent years. BSL defines a seismic zone factor Z according to the frequency of past earthquakes or the risk of occurrence, and the strength can be reduced in areas with low risk. The standard for general areas is Z = 1.0, but Z = 0.9 in Wajima City and Suzu City on the Noto Peninsula (Z = 0.7 to 1.0 under BSL). In Shizuoka Prefecture, where the Nankai Trough earthquake is feared to occur, a voluntary standard of Z=1.2 (1.32 for wooden structures) has been mandated.
The damage to wooden houses caused by the 2024 Noto Peninsula earthquake highlighted the current lack of progress in retrofitting older wooden houses against earthquakes. It is strongly hoped that more generous subsidies for retrofitting houses in the gray zone are promoted, and the construction of new houses with higher seismic grades.
Japan is one of the most earthquake-prone countries in the world, and the revisions of earthquake resistance standards have been repeatedly reflected in past earthquake damages. Firstly, the changes in the Japanese Building Standard Law (BSL) are summarized.
BSL was enacted in 1950. Subsequently, there was a major revision in 1981, and since then it has been referred to as the New Earthquake Resistance Standard. The 1995 Hyogo-ken Nanbu Earthquake occurred in 1995, and countermeasures based on the findings of this damage survey were incorporated in the 2000 revision. Therefore, to understand the reasons for earthquake damage to wooden houses, it is necessary to distinguish between (1) those built before May 1981 (Old Standard), (2) those built between June 1981 and May 2000(New Standard), and (3) those built after June 2000 (Current Standard).
A survey report by the Architectural Institute of Japan(AIJ) on the damage caused to wooden houses by the 2016 Kumamoto earthquake indicates that most of the damage was concentrated in (1) (collapse rate of about 27.9%), followed by (2) (collapse rate of about 8.7%) and (3) (collapse rate of about 2.2%). (2) is ambiguously positioned, not meeting the current standard but not the old standard, and is referred to as a "gray zone" among experts.
The revision of BSL in 2000 specifies the method of connections between columns and beams. This requires that the ends of columns must be connected with hardware of sufficient strength to prevent them from pulling out. The Japan Building Disaster Prevention Association has prepared a method for verifying the seismic performance of houses in the gray zone, "Seismic Performance Verification Method for Wooden Houses under the New Earthquake Resistance Standards". Owners of houses built before May 2000 are encouraged to implement this method.
The total cost of seismic retrofitting of a house is often around 1 to 2 million yen, which is not an inexpensive purchase, but subsidies may be available from the local government for seismic diagnosis and retrofitting. However, this subsidy is mainly for (1) and may not include (2). In light of the damage caused by recent major earthquakes, there is a movement to make (2) also eligible for subsidies. The retrofitting of the gray zone in (2) is considered an issue that must be addressed urgently in the future.
It is also recommended that newly constructed houses obtain "Seismic Grade" under the Housing Performance Indication System; it is reported that damage to houses that obtained seismic grade 3 (the highest rank) was particularly small during the 2016 Kumamoto earthquake. The Housing Performance Indication System was implemented in 2000, and the current utilization rate is about 30%.
A comprehensive survey of the damage caused by the 2024 Noto Peninsula earthquake is being conducted mainly by AIJ, and the results of this survey should be closely checked. At this stage, as in the case of the 2016 Kumamoto earthquake, most of the damage was caused by old earthquake-proof houses or houses in the gray zone, but some houses built after 2000 also seem to have been damaged. On the other hand, some relatively new wooden houses remained undamaged even in the area of the severe earthquake (Photo 1). New houses appropriately designed and constructed have not been damaged in recent major earthquakes.
The Noto Peninsula has been seismically active in recent years. BSL defines a seismic zone factor Z according to the frequency of past earthquakes or the risk of occurrence, and the strength can be reduced in areas with low risk. The standard for general areas is Z = 1.0, but Z = 0.9 in Wajima City and Suzu City on the Noto Peninsula (Z = 0.7 to 1.0 under BSL). In Shizuoka Prefecture, where the Nankai Trough earthquake is feared to occur, a voluntary standard of Z=1.2 (1.32 for wooden structures) has been mandated.
The damage to wooden houses caused by the 2024 Noto Peninsula earthquake highlighted the current lack of progress in retrofitting older wooden houses against earthquakes. It is strongly hoped that more generous subsidies for retrofitting houses in the gray zone are promoted, and the construction of new houses with higher seismic grades.