横ずれ断層(transform faults): プレートが横にずれるとき

How Transform Boundary Motion Works

At a トランスフォーム断層境界2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。, two tectonic plates slide horizontally past each other along a vertical or near-vertical fault plane. Unlike the collision and subduction that occur at convergent boundaries, or the rifting and magma upwelling at divergent boundaries, transform motion is purely lateral — one side of the fault moves in one direction, the other side moves in the opposite direction, or both sides move in the same direction but at different speeds. This horizontal shearing motion generates characteristic 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。 earthquakes that are shallower than subduction events and rarely produce tsunamis (unless a secondary seafloor landslide is triggered), but can be extraordinarily destructive to cities built directly on or near the fault trace.

The Mechanics of Stick-Slip Motion

Transform faults do not slide smoothly and continuously. Instead, they exhibit what geophysicists call stick-slip behavior: stress accumulates as the plates try to move past each other but are held in place by friction along the 断層岩盤に沿って動きが生じた破断面。断層は数ミリメートルから数千キロメートルまで様々な長さがある。地震を引き起こす主要な断層は活断層と呼ばれる。 interface. This is the locked state. When the accumulated stress finally exceeds the frictional strength of the fault, it ruptures suddenly — the slip event — releasing elastic strain energy as seismic waves. The magnitude of the earthquake depends on the area of the 断層岩盤に沿って動きが生じた破断面。断層は数ミリメートルから数千キロメートルまで様々な長さがある。地震を引き起こす主要な断層は活断層と呼ばれる。 that ruptures and the amount of average slip断層に沿った変位の平均速度で、通常は年間ミリメートル単位で測定される。すべり速度が高いほど、一般に地震発生頻度とリスクが高いことを示す。. After the main rupture, 余震本震と同じ断層領域で本震の後に続く、より規模の小さい地震。余震活動は数週間から数年続くことがあり、最大の余震は通常本震よりマグニチュードが1.0〜1.2小さい。s occur as the fault system adjusts to its new stress state, following 大森公式（オモリ則）時間経過に伴う余震発生頻度の減衰を示す経験則で、余震の発生率は本震からの経過時間にほぼ反比例して減少する。 in their time decay.

The San Andreas Fault System

The San Andreas 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。 is the most studied transform fault on Earth and the dominant seismic structure of California. It forms the トランスフォーム断層境界2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。 between the Pacific Plate, moving roughly northwestward, and the North American Plate. The fault runs approximately 1,300 kilometers from the Salton Sea in the south to Cape Mendocino in the north, where it transitions to the Cascadia Subduction Zone. The system is not a single clean break but a complex network of parallel and branching faults. The 1906 San Francisco earthquake ruptured approximately 470 kilometers of the northern segment. The 1989 Loma Prieta earthquake (Mw 6.9) and the 1994 Northridge earthquake (Mw 6.7) both caused significant damage and loss of life, though they occurred on secondary structures rather than the main San Andreas trace.

The Creeping and Locked Sections

Not all parts of the San Andreas 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。 behave the same way. The central segment between Parkfield and San Juan Bautista creeps aseismically at several centimeters per year, releasing strain without generating large earthquakes. In contrast, the Carrizo Plain segment in southern California and the Peninsula segment near San Francisco are deeply locked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。, accumulating elastic strain for eventual catastrophic release. The seismic gap concept — a locked segment that has not ruptured in a long time — is directly applicable to the San Andreas. Scientists at the USGS estimate a significant probability of a major rupture on the southern San Andreas within the next few decades.

The North Anatolian Fault

The North Anatolian Fault in Turkey is one of the most active and historically deadly transform boundaries in the world. It forms a right-lateral 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。 stretching approximately 1,500 kilometers from eastern Turkey westward toward Greece, accommodating the westward escape of the Anatolian microplate as it is squeezed between the converging Eurasian and Arabian plates. The fault has produced a remarkable sequence of large earthquakes in the 20th century that appear to migrate westward along its length: the 1939 Erzincan earthquake (Mw 7.8), the 1944 Bolu, and a sequence of events through the 1990s. The 1999 Izmit earthquake (Mw 7.6) killed over 17,000 people near Istanbul. Scientists have interpreted this westward migration as a stress transfer process, where each rupture loads the adjacent locked segment to the west — a direct application of クーロン応力伝達地震が周辺の断層の応力状態を変化させ、将来の地震を誘発または遅延させうる過程。どの断層が破壊に近づいたかを予測するために用いられる。 analysis. The section nearest Istanbul is now considered among the most dangerous 断層岩盤に沿って動きが生じた破断面。断層は数ミリメートルから数千キロメートルまで様々な長さがある。地震を引き起こす主要な断層は活断層と呼ばれる。 segments on Earth.

Strike-Slip Fault Mechanics

At the microscopic and mesoscopic scales, 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。 motion involves the grinding and fracturing of rock along the fault zone. The fault core — the narrow zone of intense deformation — contains highly comminuted rock called fault gouge. Surrounding it is the damage zone, a region of fractured rock that can extend hundreds of meters from the fault trace. The mechanical properties of the fault zone — particularly the coefficient of friction and the pore fluid pressure — control whether the fault is locked or creeping. Elevated pore pressures reduce effective normal stress across the fault, lowering the friction threshold and potentially promoting aseismic creep or induced seismicity. This is directly relevant to 誘発地震活動水圧破砕（フラッキング）、排水注入、採掘、貯水池の湛水など、人間活動によって引き起こされる地震。ほとんどは小規模（M4未満）だが、M5.5を超えたケースもある。 associated with wastewater injection near fault zones.

Surface Expressions of Strike-Slip Motion

Because transform faults accommodate horizontal motion, they leave characteristic surface expressions that can be mapped from satellite imagery and field surveys. Linear valleys, offset stream channels, and elongated sag ponds all mark the trace of a 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。. The San Andreas Fault is beautifully expressed in the Carrizo Plain as a series of offset drainage channels, providing direct measurement of cumulative slip over thousands of years. Where the fault bends, compressional or extensional jogs create local mountains (pressure ridges) or basins (pull-apart basins). The Salton Sea at the southern end of the San Andreas system occupies a large pull-apart basin formed by extension between overlapping fault strands.

Transform Fault Earthquakes vs Subduction

Transform and subduction earthquakes differ in several important ways. Transform events tend to be shallower — typically within the upper 20 kilometers of the crust — while subduction zone earthquakes can extend to 700 kilometers depth along the descending slab. Transform earthquakes rarely exceed Mw 8.0–8.2, while subduction megathrusts can reach Mw 9.5. Because transform faults produce primarily horizontal seafloor motion rather than vertical displacement, 津波海底地震時の海底の急激な変位によって発生する一連の海の波。津波はジェット機並みの速度（時速700km以上）で海洋全域を伝わることがある。 generation is far less common than at subduction zones. However, the proximity of many 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。 to major cities — Istanbul, San Francisco, Los Angeles — means their shaking hazard to built-up areas can be exceptionally high. The Seismic Risk Checker tool allows comparison of hazard across different fault types and regions.

Preparedness on Transform Boundaries

Cities situated on or near active transform boundaries face distinct preparedness challenges compared to those on subduction coasts. Because major transform 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 are often well-mapped and understood, municipal planners can identify the most hazardous corridors and apply 耐震補強既存の建物の耐震性を向上させるための強化工事。鋼製ブレースの追加、基礎の補強、構造物と基礎のボルト固定などが一般的な手法である。 programs to vulnerable structures such as 無補強組積造（URM）鉄筋補強のないレンガまたはブロック造の建物で、地震の揺れに極めて脆弱である。URM建物は、世界の地震死者数の大部分を占める。 buildings and ソフトストーリー（弱層）駐車場や店舗などの大きな開口部が原因で、上階に比べて著しく弱くなっている建物の階（通常は1階）。ソフトストーリーは最も一般的な倒壊メカニズムである。 apartment buildings. The city of Istanbul has invested heavily in identifying and strengthening buildings along the projected rupture corridor of the North Anatolian Fault beneath the Sea of Marmara. In California, mandatory retrofitting of soft-story wood-frame buildings in Los Angeles and San Francisco directly addresses the building types most vulnerable to the strong shaking generated by nearby 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。 ruptures. 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。 requirements, regular earthquake drills, and community 地震への備え家具の固定、連絡計画の作成、非常用物資の維持、訓練の実施など、地震の被害を最小限に抑えるための継続的な計画・準備の過程。 programs all contribute to reducing the human toll of future transform fault earthquakes.