ダム安全と地震リスク

Why Dam Safety Is an Earthquake Priority

Dams represent a unique category of civil infrastructure where seismic failure creates 二次的地震災害揺れそのものではなく、揺れによって引き起こされる災害——津波・地すべり・液状化・火災・ダム決壊・化学物質の流出などを指す。揺れそのものより大きな被害をもたらすことが多い。 of potentially catastrophic scale. A dam failure releases not just the structure itself but the entire reservoir behind it — millions to billions of cubic meters of water that inundates downstream communities with little warning. The combination of structural failure and sudden flood creates a compound disaster that can kill thousands and devastate entire valleys.

The 最大想定地震（MCE）地質学的証拠に基づき、特定の断層または地域で合理的に発生しうる最大規模の地震。ダムや原子力発電所など重要施設の設計に用いられる。 (Maximum Considered Earthquake) concept is central to dam safety regulation worldwide. Unlike most buildings where the design earthquake represents a probability of exceedance over a 50-year building life, dam design often uses deterministic maximum credible earthquake scenarios that consider the largest physically possible earthquake on capable faults that could affect the dam site. The asymmetry of consequences — dam failure releasing a catastrophic flood — justifies this extreme conservatism in hazard characterization.

Dam Types and Their Seismic Behavior

Concrete dams — gravity dams, arch dams, and buttress dams — respond to earthquakes differently from earthfill dams. Concrete gravity dams rely on their mass and base friction to resist both water pressure and earthquake forces. Their seismic vulnerability concentrates in the concrete's tensile capacity, which is limited, and in the base contact zone, where sliding or rocking under dynamic loading can develop. Arch dams transfer reservoir loads to canyon walls through arching action and are designed for flexibility, but large earthquakes can damage the canyon rock abutments that provide the reaction for arch forces.

Earthfill dams are the most common dam type worldwide and have distinctive seismic vulnerabilities. Saturated zones within the embankment or foundation can liquefy under earthquake loading, causing sudden loss of shear strength that allows the embankment to flow or slide. 液状化水を含んだ緩い土壌が強い揺れによって一時的に強度を失い、液体のように振る舞う現象。建物が地面に沈下・傾斜・崩壊することがある。 in dam foundations caused the failure of the Lower San Fernando Dam in the 1971 San Fernando earthquake: the upstream slope of the dam slid 33 feet, leaving only a few feet of freeboard and requiring emergency evacuation of 80,000 downstream residents. Had the dam failed completely, the consequences would have been catastrophic.

Liquefaction: The Critical Dam Seismic Hazard

液状化水を含んだ緩い土壌が強い揺れによって一時的に強度を失い、液体のように振る舞う現象。建物が地面に沈下・傾斜・崩壊することがある。 of saturated loose cohesionless soils (sands and silts) occurs when earthquake shaking increases pore water pressure to the point where effective stress drops to zero and the soil loses its strength. Within an earthfill dam or its foundation, liquefaction can trigger flow slides, lateral spreading, and large permanent deformations. Modern dam design requires thorough geotechnical investigation to identify potentially liquefiable materials and either remove them, treat them through densification or grouting, or design the embankment to remain stable despite potential liquefaction.

The 耐震設計地震力に耐えられるよう構造物を設計する手法。現代の耐震設計は、大地震における構造被害はある程度許容しつつ、倒壊の防止と人命の保護を目指す。 of earthfill dams for liquefaction uses several approaches. Computer-based dynamic analysis models the soil's cyclic behavior, tracking pore pressure development and identifying zones at risk. Post-liquefaction stability analysis evaluates whether the dam can remain stable with liquefied zones providing only residual undrained strength. Deformation analysis estimates total movements and determines whether the crest settles below the reservoir level, which would trigger overtopping and failure.

Seismic Safety Evaluation Programs

Most dam safety programs worldwide require periodic seismic safety evaluations that apply current methods and hazard information to existing dams. The Federal Energy Regulatory Commission (FERC) in the United States requires hydropower dam owners to evaluate seismic safety, and the Association of State Dam Safety Officials (ASDSO) promotes systematic state programs for evaluating non-federal dams.

A seismic safety evaluation typically begins with seismic hazard characterization: identifying all capable faults within a broad radius, developing probabilistic seismic hazard analysis (確率論的地震ハザード解析（PSHA）起こりうるすべての地震の発生源・マグニチュード・地震動レベルを考慮し、特定の揺れの水準を超える確率として結果を表現する、地震ハザードの定量評価手法。) results for the site, and defining the design earthquake as either a probabilistic level (e.g., 10,000-year return period) or the 最大想定地震（MCE）地質学的証拠に基づき、特定の断層または地域で合理的に発生しうる最大規模の地震。ダムや原子力発電所など重要施設の設計に用いられる。 deterministic scenario. The evaluation then examines the dam's structural response to this hazard through analysis appropriate to the dam type and failure modes.

For concrete dams, linear and nonlinear dynamic analysis examines stress distributions, joint opening and sliding, and stability under the design earthquake. For earthfill dams, nonlinear dynamic analysis tracks pore pressure development, identifies liquefaction potential, and estimates deformations. The evaluation identifies whether the dam meets current safety standards and, if not, what modifications are required.

二次的地震災害揺れそのものではなく、揺れによって引き起こされる災害——津波・地すべり・液状化・火災・ダム決壊・化学物質の流出などを指す。揺れそのものより大きな被害をもたらすことが多い。 Downstream

Dam failure 二次的地震災害揺れそのものではなく、揺れによって引き起こされる災害——津波・地すべり・液状化・火災・ダム決壊・化学物質の流出などを指す。揺れそのものより大きな被害をもたらすことが多い。 are among the most severe consequences of major earthquakes. A dam failure during or immediately after an earthquake combines the direct casualties of the earthquake with the additional casualties and destruction from the resulting flood wave. Historical dam failures — Vajont in Italy (1963), Banqiao and Shimantan in China (1975) — demonstrate that flooding casualties can far exceed those of the triggering event.

The 2008 Sichuan earthquake damaged hundreds of dams in China, including the Tangjiashan landslide dam — a natural dam created when the earthquake triggered a massive landslide that blocked a river. The resulting lake threatened 1.3 million people downstream. Emergency mechanical excavation to lower the dam crest and control the outflow required three weeks of intensive work. Managing the cascade of dam safety threats in a post-earthquake environment, where infrastructure is damaged and access is limited, is a major challenge for emergency managers.

Planning for dam failure scenarios requires mapping inundation zones, establishing warning systems, and developing evacuation plans for downstream communities. Federal guidelines in the United States require Emergency Action Plans (EAPs) for all significant and high-hazard dams, specifying the actions to be taken if dam failure becomes imminent. These plans include notification procedures, pre-calculated inundation maps, and coordination with local emergency management.

Seismic Upgrade Strategies

When seismic evaluation identifies deficiencies in existing dams, a range of remediation strategies is available. For earthfill dams with liquefaction risk in the foundation, ground improvement through dynamic compaction, vibro-compaction, or permeation grouting can densify or strengthen susceptible soils. Upstream blankets and drainage layers manage pore pressure buildup during earthquake shaking.

Raising the dam crest to increase freeboard — the distance between the water surface and the crest — provides safety margin against deformation. If the crest settles during an earthquake but remains above the reservoir level, overtopping and failure are prevented. This approach accepts some embankment deformation as tolerable while ensuring that the ultimate failure mode (overtopping) does not occur.

For concrete dams, post-tensioned anchors can improve resistance to sliding and overturning at the base. Drainage improvements reduce uplift water pressure. Detailed nonlinear analysis sometimes reveals that existing dams are more capable than simplified analysis suggested, reducing the apparent need for structural modifications.

The 耐震設計地震力に耐えられるよう構造物を設計する手法。現代の耐震設計は、大地震における構造被害はある程度許容しつつ、倒壊の防止と人命の保護を目指す。 of new dams incorporates lessons from historical failures and advances in analysis methods, site characterization, and construction quality control that make modern dams substantially more reliable than their predecessors. But the legacy inventory of older dams worldwide represents an enormous ongoing seismic safety challenge that will require sustained investment and systematic evaluation over many decades.