断層の仕組み: 横ずれ、正断層、逆断層

Strike-Slip Fault: Horizontal Motion

A 横ずれ断層岩盤のブロックが水平方向に互いにすれ違う断層。サンアンドレアス断層と北アナトリア断層は、破壊的な地震を引き起こす主要な横ずれ断層である。 is characterized by predominantly horizontal relative motion along its plane. The two sides of the fault slide past each other laterally, with little or no vertical displacement at the surface. Geologists classify strike-slip faults as right-lateral (dextral) or left-lateral (sinistral) depending on the direction of apparent motion of the far block relative to the near block when viewed from above. Looking across the San Andreas Fault in California, the block on the far side appears to have moved to the right — making it a right-lateral fault. The North Anatolian Fault in Turkey is also right-lateral. The Alpine Fault in New Zealand is a complex structure with both strike-slip and reverse components. Strike-slip faults form at transform boundaries2枚のプレートが水平方向にすれ違うプレート境界。カリフォルニア州のサンアンドレアス断層は、トランスフォーム断層境界の最も有名な例である。 where plates slide past each other, but they also occur within plate interiors where crustal blocks are rotating or escaping due to distant convergence.

Recognizing Strike-Slip Faults

In the field, strike-slip faults are identified by offset features: streams, ridges, or road cuts that have been displaced horizontally by accumulated slip. Paleoseismological trenches across such faults reveal offset layers of soil and sediment, allowing scientists to reconstruct the history of past ruptures and estimate 地震再来間隔特定の断層における大地震と大地震の間の平均時間。古地震学や歴史記録から推定される。カスケード沈み込み帯の再来間隔は約500年である。s for major events.

Normal Fault: Extensional Forces

A 正断層断層面の上側の岩盤（上盤）が下側の岩盤に対して下方にずれる断層。リフト帯や発散型境界における引張力に関連して生じる。 forms where the crust is being pulled apart — in extensional tectonic settings. The defining characteristic is that the hanging wall (the block above the fault plane) has moved down relative to the footwall (the block below). Normal faults typically dip at 50 to 70 degrees from horizontal. They occur at divergent plate boundariesマントルから上昇したマグマによって新しい地殻がつくられながら、2枚のプレートが互いに離れていくプレート境界。中央海嶺が最も代表的な例。 such as mid-ocean ridges and continental rift zones, where the crust is being stretched. The East African Rift System — which may eventually split the African continent — is lined with normal faults that generate moderate earthquakes as the crust is pulled apart. The Basin and Range Province in the western United States is another classic extensional environment, where the crust has been thinned and faulted into a series of tilted blocks producing the alternating mountain ranges and valleys of Nevada and Utah.

Normal Fault Earthquakes

Earthquake focal mechanisms on normal faults show a characteristic pattern of tension: the compressional axis is near-vertical and the tensional axis is near-horizontal. Normal fault earthquakes are typically shallower and less prone to generating large tsunamis than subduction events, though notable exceptions exist. The 2009 L'Aquila earthquake in Italy (Mw 6.3), which killed 309 people, was a normal faulting event. Poorly constructed unreinforced masonry buildings suffered catastrophic collapse in that event, highlighting the interaction between building vulnerability特定の建物タイプについて、揺れの水準に応じたさまざまな被害段階の発生確率を示す数学的関数。損失推定モデルに不可欠な要素。 and shaking intensity.

Reverse Fault: Compressional Forces

A 逆断層（スラスト断層）圧縮力によって、上盤が下盤に対して上方にずれる断層。傾斜の緩い逆断層（スラスト断層）は、最大級の地震の原因となる。 is the compressional counterpart to a normal fault: the hanging wall moves up relative to the footwall, reflecting crustal shortening. Reverse faults dip at angles between 30 and 60 degrees. Thrust faults are a special case — low-angle reverse faults, often dipping less than 30 degrees. Reverse and thrust faults form at convergent plate boundaries2枚のプレートが互いに近づき合うプレート境界。海洋プレートと大陸プレートでは沈み込み帯を、大陸プレート同士では造山運動を、海洋プレート同士では深い海溝を生じる。 wherever the crust is being compressed. The Himalayan frontal thrust and the megathrust interfaces of subduction zones are all reverse or thrust faults. Reverse faulting earthquakes can be exceptionally destructive: the 1994 Northridge earthquake (Mw 6.7) on a blind thrust fault beneath the San Fernando Valley killed 57 people and caused $20 billion in damage.

Blind Thrust Fault: Hidden Dangers

A ブラインドスラスト断層地表に達しないスラスト断層で、地表からは見えず検出が困難である。1994年のノースリッジ地震はブラインドスラスト断層で発生した。 is a reverse fault that does not break the surface. Its upper tip remains buried beneath overlying sediment or rock, leaving no surface fault trace to alert planners or seismologists to its existence. These hidden faults pose a particularly insidious hazard because they can only be inferred from subtle surface topography — broad anticlinal folds, uplifted terraces — and geophysical surveys. The 1983 Coalinga earthquake (Mw 6.5) and the 1994 Northridge earthquake in California were both caused by blind thrust faults. In the Los Angeles Basin, numerous blind thrust systems have been identified beneath the urban area using subsurface geology and seismic reflection surveys. The discovery of the Puente Hills Fault beneath downtown Los Angeles revealed a structure capable of producing a Mw 7.0–7.5 earthquake directly under one of the world's most densely populated urban centers.

Fault Creep vs Locked Faults

Not all faults rupture in discrete earthquakes. Fault creep顕著な地震を発生させることなく、断層に沿ってゆっくりと継続的に生じる動き。サンアンドレアス断層の一部区間は、年間2〜3cmの速度でクリープしている。 refers to slow, continuous slip along a fault without producing significant seismic waves. Creeping faults release stress gradually and tend to generate fewer large earthquakes than locked faults摩擦によって動きが妨げられ、応力が蓄積している断層区間。固着断層がついに破壊すると、大地震を引き起こすことがある。, but they can cause progressive damage to roads, buildings, and infrastructure that cross the fault. The central section of the San Andreas Fault near Parkfield creeps at approximately 25 millimeters per year. The Calaveras and Hayward faults in the San Francisco Bay Area also creep measurably, producing small earthquakes and slow deformation. In contrast, a 固着断層摩擦によって動きが妨げられ、応力が蓄積している断層区間。固着断層がついに破壊すると、大地震を引き起こすことがある。 has zero or near-zero creep — all plate motion must eventually be accommodated by sudden slip in earthquakes. The contrast between creeping and locked behavior reflects differences in fault zone composition, temperature, and pore fluid pressure.

Slow Slip and Silent Earthquakes

Between fully locked and continuously creeping behavior, some fault segments undergo slow slip events — transient episodes of fault slip that release stress over days to weeks without generating felt earthquakes. These were first detected using GPS networks and are now monitored on subduction zones and major strike-slip faults worldwide. Slow slip events may interact with the locked zones of faults, transferring stress and potentially influencing the timing of future large earthquakes, though the relationship is complex and an active area of research.

Mapping Fault Lines Worldwide

Understanding which 断層線地表に現れた断層の痕跡で、線状または破砕された岩石の帯として視認できる。地質学者は活断層線を地図化し、周辺地域の地震リスクを評価する。s exist, where they are located, and how active they are is foundational to seismic hazard assessment. Fault mapping uses multiple techniques: geological field surveys that trace fault scarps and offset features; aerial and satellite imagery that reveals linear topographic features; subsurface geophysical surveys using seismic reflection and refraction; 古地震学断層トレンチ、隆起した段丘、津波堆積物といった地質学的証拠を通じて、先史時代の地震を研究する学問。地震の記録を数千年前まで遡らせる。 trenching studies; and GPS測地学全地球測位システムの受信機を用いて、プレートの動きや地殻変動をミリメートル単位の精度で測定する手法。地震と地震の間に断層に歪みがどのように蓄積するかを明らかにする。 measurements that detect surface deformation. National geological surveys — notably the USGS in the United States — compile fault databases that form the backbone of national hazard maps特定の期間内に地震の揺れが指定の水準を超える確率を示した地図。技術者・都市計画者・保険会社が地震リスクを評価するために用いる。. Globally, the International Seismological Centre and the GEM Foundation maintain databases of active faults that support 地震リスク評価特定の地域や構造物について、地震ハザード・建物の脆弱性・想定される損失を評価する過程。ハザードマップ、建物台帳、被害モデルを組み合わせて行われる。 in earthquake-prone regions worldwide.