地震はどのくらい続くのか?

Earthquake Duration: From Seconds to Minutes

One of the most counterintuitive aspects of earthquakes is how briefly the actual ground shaking lasts. Most people, when asked, imagine earthquakes as prolonged events lasting several minutes. In reality, the vast majority of felt earthquakes shake the ground for just seconds. The duration of shaking depends primarily on the magnitude of the earthquake — or more precisely, on the length of the fault that ruptures.

Small earthquakes (magnitude 3–4) produce shaking that lasts only 1–3 seconds at locations near the epicentre. Moderate earthquakes (magnitude 5–6) may shake the ground for 10–30 seconds. Large earthquakes (magnitude 7–8) can produce shaking lasting 30 seconds to 2 minutes. The largest earthquakes on record, with magnitudes approaching 9.5, shook the ground for 3–5 minutes in the regions of strongest shaking. This scaling of duration with magnitude is not coincidental — it reflects the physics of fault rupture.

What Determines How Long Shaking Lasts

The duration of shaking at any particular location reflects two distinct contributions. The first is the source duration — how long it takes for the fault to finish rupturing. The second is the path effect — how long the 地震波地震や爆発によって発生し、地球内部を伝播する弾性波。地震波は、震源で放出されたエネルギーを遠方の地点まで運ぶ。s continue to reverberate after the rupture ends, especially in sedimentary basins and on soft soils.

For the source contribution, a small 断層破壊地震時に断層に沿って岩盤が破断し、蓄積された弾性エネルギーが地震波として放出される現象。破壊の長さは、小規模地震では数メートル、大地震では1,000km以上に及ぶこともある。 might propagate at about 3 km/s and be only a few kilometres long, completing in less than a second. A great earthquake rupture might propagate at similar speed but extend 1,000 km, requiring more than 5 minutes to complete. During this rupture process, 地震波地震や爆発によって発生し、地球内部を伝播する弾性波。地震波は、震源で放出されたエネルギーを遠方の地点まで運ぶ。s are radiated continuously, so the duration of source radiation scales directly with rupture length.

Once waves start reverberating in a sedimentary basin or soft soil site, they can continue shaking long after the primary wave train has passed. This resonance — related to 構造物の共振地震波の周波数が建物の固有振動数と一致したときに生じる、建物の揺れの増幅現象。低層建物は高周波数の波と、高層建物は低周波数の波と共振しやすい。 in buildings — means that soft-soil sites often experience longer shaking durations than adjacent rock sites, even for identical source earthquakes. The prolonged duration on soft soils contributes significantly to the higher damage levels observed at such sites.

Fault Rupture Length and Duration

The relationship between 断層破壊地震時に断層に沿って岩盤が破断し、蓄積された弾性エネルギーが地震波として放出される現象。破壊の長さは、小規模地震では数メートル、大地震では1,000km以上に及ぶこともある。 length and shaking duration follows from basic physics. Seismic rupture propagates along a fault at roughly 2.5–3.5 km/s (about 80–90 percent of the S-wave velocity in the surrounding rock). The total duration of source radiation is approximately equal to the rupture length divided by the rupture velocity.

Empirical scaling relations confirm this. For a magnitude 5.0 earthquake, the rupture length is typically about 2–5 km, giving a source duration of about 1–2 seconds. For magnitude 7.0, rupture lengths of 30–100 km give durations of 10–30 seconds. For the 2004 Indian Ocean earthquake (Mw 9.1–9.3), the rupture extended approximately 1,200 km along the Sunda Trench, with a total duration of about 500–600 seconds — nearly 10 minutes of continuous fault rupture, producing a 地震記象（地震波形記録）地震計が記録した出力で、地面の揺れを時間の関数として示したもの。地震学者はこの記録を解析し、地震の規模・深さ・位置を求める。 unlike anything recorded before or since.

The Longest Earthquake Shaking Ever Recorded

The instrumental records from the great 1960 Valdivia earthquake (Mw 9.5) show remarkable shaking durations. At stations across South America, the 地震記象（地震波形記録）地震計が記録した出力で、地面の揺れを時間の関数として示したもの。地震学者はこの記録を解析し、地震の規模・深さ・位置を求める。 shows coherent wave energy arriving for many minutes after the first arrivals. At teleseismic distances (on the other side of the world), the surface waves from this earthquake circled the globe multiple times and were recorded for days afterward, causing the entire Earth to ring like a bell — a phenomenon called free oscillations of the Earth.

In terms of local shaking duration — the duration experienced by people and structures in the affected region — the 1960 Valdivia earthquake produced violent shaking for approximately 3–4 minutes across a 500 km stretch of Chile. Survivors described the ground continuing to move in waves long after the initial violent shaking. This extended duration caused progressive structural failure in buildings that might have survived shorter, more intense shaking.

Duration vs Magnitude: The Relationship

The duration of shaking and the マグニチュード地震が放出した総エネルギー量を表す単一の数値。整数値が1増えるごとに、放出エネルギーはおよそ31.6倍になる。 of an earthquake are related but distinct quantities. Magnitude measures the total energy released; duration at any given site measures how long that energy arrives. A moderate, shallow earthquake near a city can produce shorter but more intense shaking than a large, deep earthquake far away. The total energy received at a given location — which determines cumulative structural damage — depends on both the intensity and the duration.

This is why 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。 standards for buildings specify not just the peak ground acceleration but also the duration of shaking. Long-duration shaking causes progressive degradation of structural connections, can liquefy saturated soils even at moderate accelerations, and accumulates more strain energy in flexible structures than brief, intense shaking of the same peak level. The 地震記象（地震波形記録）地震計が記録した出力で、地面の揺れを時間の関数として示したもの。地震学者はこの記録を解析し、地震の規模・深さ・位置を求める。 captures both the amplitude and the duration of ground motion, and engineers use the full record to assess damage potential.

Why Duration Matters for Building Damage

From a structural engineering perspective, duration is as important as 波の振幅地震波が静止位置から動く最大変位量。振幅は波が運ぶエネルギーと直接関係し、マグニチュードの算出に用いられる。 in determining building damage. Short intense shaking may stress a structure to near-failure without causing collapse; sustained shaking at somewhat lower intensity can cause the same structure to accumulate damage progressively until it fails. This phenomenon, called structural fatigue or cyclic degradation, is especially important for structures made of materials that weaken under repeated loading cycles, such as unreinforced masonry, concrete with inadequate shear reinforcement, and wood connections weakened by initial cycles.

The 2010 Chile earthquake (Mw 8.8) produced very long-duration shaking — approximately 90 seconds of strong ground motion in Santiago, 500 km from the epicentre — but caused proportionally less damage than the shorter, shallower 2010 Haiti earthquake because Chile's modern buildings are designed and constructed to perform in long-duration seismic environments. Duration-aware 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。 criteria, increasingly incorporated into performance-based design standards, require engineers to consider the number of strong shaking cycles, not just the peak values.