表面波: ラブ波とレーリー波の説明

Love Waves: Horizontal Ground Shearing

表面波地球内部ではなく地表に沿って伝わる地震波。実体波より速度は遅いが、振幅が大きく継続時間が長いため、被害が大きくなる傾向がある。s are 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 that travel along the Earth's surface rather than through its interior. They form when energy from P波（初期微動）岩盤中も液体中も伝わる、秒速5〜8kmの最も速い地震波。P波はスリンキーのように進行方向に対して物質を圧縮・伸長させながら伝わり、地震観測点に最初に到達する。s and S波（主要動）進行方向に対して垂直に岩盤を揺らしながら伝わる地震波で、P波の後に到達する。S波は液体中を伝わることができず、この性質から地球の外核が液体であることが証明された。s, travelling upward through the crust, reaches the surface and becomes trapped, propagating horizontally outward like ripples on a pond. Though they travel more slowly than body waves, surface waves carry enormous amounts of energy and are responsible for much of the damage in large, distant earthquakes.

ラブ波地面を水平方向にせん断させる表面波の一種。数学者A.E.H.ラブにちなんで名付けられ、建物の基礎に特に大きな被害を及ぼす。s are the faster of the two surface wave types. Named for the British mathematician A.E.H. Love, who predicted their existence in 1911, Love waves produce purely horizontal ground motion — the rock shears back and forth in the horizontal plane perpendicular to the direction of wave propagation. Stand on the ground during a Love wave and you will feel a side-to-side swaying, similar to standing on a swinging bridge. Love waves exist because the Earth's layered structure allows shear waves to be guided along the surface; they require a velocity gradient with depth and cannot exist in a perfectly uniform half-space.

Rayleigh Waves: The Rolling Motion

レイリー波海の波に似た楕円運動で地面を動かす表面波。レイリー卿にちなんで名付けられ、地震時に感じる「ローリング」のような揺れの原因となることが多い。s, predicted mathematically by Lord Rayleigh in 1885 and later confirmed observationally, produce an elliptical retrograde rolling motion — the rock traces an ellipse, moving forward at the top of the ellipse and backward at the bottom, opposite to the direction the wave is travelling. If you have ever watched ocean swells pass beneath a floating object, you have seen exactly this motion: the object traces a backward ellipse as the wave passes.

The vertical and horizontal components of Rayleigh wave motion are always 90 degrees out of phase. This distinctive signature makes Rayleigh waves easy to identify on 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。s: the horizontal and vertical channels show the same frequency content but shifted in time. Rayleigh waves travel slightly slower than Love waves, typically at about 90 percent of the S-wave speed. Like Love waves, their speed is frequency-dependent — a property called dispersion — which allows seismologists to infer shear-wave velocity structure at depth by analysing how different frequency components travel at different speeds.

Why Surface Waves Cause More Damage Than Body Waves

For many earthquakes, especially large or distant ones, 表面波地球内部ではなく地表に沿って伝わる地震波。実体波より速度は遅いが、振幅が大きく継続時間が長いため、被害が大きくなる傾向がある。s cause more structural damage than the P波（初期微動）岩盤中も液体中も伝わる、秒速5〜8kmの最も速い地震波。P波はスリンキーのように進行方向に対して物質を圧縮・伸長させながら伝わり、地震観測点に最初に到達する。s and S波（主要動）進行方向に対して垂直に岩盤を揺らしながら伝わる地震波で、P波の後に到達する。S波は液体中を伝わることができず、この性質から地球の外核が液体であることが証明された。s that arrive first. Several factors explain this. First, surface waves decay more slowly with distance than body waves. While body wave amplitudes decrease roughly as the square of the distance, surface wave amplitudes decrease only as the square root of the distance. This means that at large distances from the epicentre, surface waves dominate 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。 and the actual ground shaking.

Second, surface waves have longer 波の周期地震波の連続する波の頂点間の時間間隔。長周期波（10〜20秒）はより遠くまで伝わり、表面波マグニチュードの算出に用いられる。s — they oscillate more slowly than the high-frequency body waves. Many building types, especially tall or flexible structures, have natural resonance frequencies in the range of 0.5–2 seconds, which overlaps the dominant period of surface waves from large earthquakes. This 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。 can amplify the building's response dramatically beyond what the ground motion alone would suggest. Third, surface waves produce sustained shaking over longer durations, giving more time for fatigue to accumulate in structural connections.

How Surface Wave Magnitude Is Calculated

The 表面波マグニチュード（Ms）周期約20秒のレイリー波の振幅に基づくマグニチュード尺度。浅発地震にはよく機能するが、マグニチュード8.0以上では飽和する。 scale (Ms) was developed in the 1940s as an improvement over the リヒタースケール1935年にチャールズ・リヒターが考案した、局地地震のマグニチュードを測定するための最初の対数マグニチュード尺度。現在はモーメントマグニチュードに大きく置き換えられているが、報道では今も広く使われている。 for large earthquakes. It measures the 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。 of Rayleigh waves at a period of approximately 20 seconds, recorded on 地震計地震波によって生じる地面の揺れを検知し記録する装置。現代のデジタル地震計は、ナノメートル未満の変位も検出できる。 instruments at teleseismic distances (beyond about 2,000 km). The 20-second period was chosen because it is the dominant period of surface waves for most damaging earthquakes, and it is in the frequency band where classical long-period seismometers were most sensitive.

Ms worked well for shallow earthquakes but still suffers from saturation above about magnitude 8.0–8.5, as the very longest-period energy from great earthquakes is not captured by the standard measurement period. This is one reason the seismological community ultimately transitioned to モーメントマグニチュード断層面積・平均すべり量・岩石の剛性の積である地震モーメントに基づく、地震規模を測定する現代の標準的な尺度（Mw）。あらゆる規模の地震に対して精度が高い。 as the standard reporting scale. Nevertheless, Ms remains in use for historical comparison and for certain applications where its calibration against the historical record is valuable.

Surface Waves and Building Damage Patterns

The spatial pattern of surface wave damage helps engineers and seismologists understand which structural configurations are most vulnerable. Low-frequency ラブ波地面を水平方向にせん断させる表面波の一種。数学者A.E.H.ラブにちなんで名付けられ、建物の基礎に特に大きな被害を及ぼす。s and レイリー波海の波に似た楕円運動で地面を動かす表面波。レイリー卿にちなんで名付けられ、地震時に感じる「ローリング」のような揺れの原因となることが多い。s preferentially excite the fundamental mode of vibration in tall structures — the swaying back and forth that you can see in videos of skyscrapers during major earthquakes. High-rise buildings in Mexico City suffered catastrophic damage in the 1985 Michoacán earthquake (magnitude 8.1) largely because the soft lake bed sediments amplified surface waves to dominant periods near 2 seconds, precisely matching the natural period of the city's medium-height buildings of 8–14 stories.

Buildings supported by soft sediments also experience longer-duration shaking from surface waves because the sediments can trap and reverberate seismic energy. This 地盤増幅（サイト効果）軟弱な土壌や堆積層が地震波を増幅させることによって生じる、揺れの強さの増大。軟弱地盤上の構造物は、基盤岩上の構造物に比べて2〜10倍強い揺れを経験することがある。 effect transforms what might be moderate surface wave shaking in adjacent rock into severe, sustained shaking on soft soils. The interaction between surface wave characteristics, local soil profiles, and building resonance periods forms one of the central concerns of earthquake engineering, because addressing just one of these three factors can substantially reduce damage even without changing the others.