地震は断層に沿ってのみ起きるのか?

The Myth: Earthquakes Only Happen Along Fault Lines

Ask most people where earthquakes happen and they will point to the San Andreas Fault in California, the Cascadia Subduction Zone in the Pacific Northwest, or the fault systems of Japan and Indonesia. These are real, well-mapped seismic hazards, and their fame has created a mental model: earthquakes happen on faults, faults are mapped, therefore places without mapped faults are safe. This mental model is dangerously incomplete. While it is true that most earthquakes occur on fault structures, the distribution of seismic hazard is far broader and less tidy than the "just avoid fault lines" model suggests.

What Faults Are — and What We Don't Know

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。 is a fracture or zone of fractures in the earth's crust along which past displacement has occurred. Major, frequently active faults like the San Andreas are well-mapped from surface expression, 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, and instrumental seismicity. But faults do not always break the surface. [[Blind-thrust-fault]] structures — reverse faults that terminate below the surface and produce folds in overlying rock — generate large earthquakes without any surface trace that traditional mapping methods could detect. The 1994 Northridge earthquake in Los Angeles (M6.7, 57 deaths) occurred on a previously unrecognized blind thrust fault that did not appear on any seismic hazard map at the time.

More fundamentally, faults exist at a range of scales. The US Geological Survey fault database contains thousands of entries, and detailed regional mapping continuously discovers previously unknown faults. In densely vegetated, covered, or urbanized terrain, fault mapping is extraordinarily difficult. The number of unmapped or unknown faults certainly exceeds the number of mapped ones globally.

[[Plate-boundary]] vs. Intraplate Hazard

The most seismically active zones on Earth correspond to プレート境界2枚のテクトニックプレートが接する境目。ほとんどの地震・火山噴火・造山運動はプレート境界で発生する。収束型・発散型・トランスフォーム型の3種類がある。 settings: subduction zones where oceanic crust dives beneath continental or island arc crust, transform boundaries where plates slide past each other, and continental collision zones. These boundaries host the great majority of large earthquakes. The 環太平洋火山帯世界の地震のおよそ90%が発生する、太平洋を取り囲む馬蹄形の地帯。総延長は40,000kmに及び、452の火山を含む。 around the Pacific Ocean is essentially a map of these plate boundaries.

But intraplate earthquakes — earthquakes occurring far from active plate boundaries, in the interiors of tectonic plates — represent a genuine and often underestimated hazard. The New Madrid Seismic Zone in the central United States, centered in Missouri, Arkansas, and Tennessee, produced a sequence of three M7.2-8.1 earthquakes in 1811-1812, among the largest instrumentally uncorrected events in US history. Memphis, Tennessee, and St. Louis, Missouri, are built largely on the Quaternary alluvial sediments that would experience severe 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。 in a large New Madrid event. Yet neither city sits near a dramatic active fault like the San Andreas.

The Charleston Earthquake Warning

The 1886 Charleston, South Carolina, earthquake (estimated M7.0) struck a region with no previous instrumental seismicity record, on a fault system still not fully characterized more than a century later. South Carolina was not on anyone's earthquake risk map in 1885. The earthquake killed nearly 100 people, destroyed hundreds of buildings, and was felt from Cuba to New York. Seismological and geodetic research since then has documented ongoing strain accumulation in the region, making another large Charleston-area earthquake a genuine long-term hazard.

Charleston illustrates a fundamental point: intraplate seismic hazard zones are defined not by fault-line proximity but by the deep geological history of continental crust, ancient sutures and failed rifts, and stress concentrations in the lithosphere that are poorly understood compared to plate boundary settings.

[[Induced-seismicity]] Anywhere There Are Wells

As discussed extensively in the induced seismicity guide, human activities can create seismic hazard where little natural hazard existed. Oklahoma's earthquake rate transformation from near-zero to hundreds of M3+ events per year demonstrates that 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。 activation by wastewater injection can occur in geologically "quiet" regions if pre-existing faults are present — and pre-existing faults are ubiquitous in continental crust.

The central United States, which hosts both the New Madrid Seismic Zone's ancient rift structures and extensive oil and gas operations, represents a zone of both natural and induced seismic hazard far from any stereotypical "earthquake zone." The USGS one-year seismic hazard forecasts now explicitly include induced seismicity alongside natural tectonic hazard for precisely this reason.

Why Risk Mapping Must Be Probabilistic, Not Binary

The map of earthquake hazard is not a binary "safe/unsafe" overlay. It is a continuous probability surface, with the highest probabilities concentrated near active プレート境界2枚のテクトニックプレートが接する境目。ほとんどの地震・火山噴火・造山運動はプレート境界で発生する。収束型・発散型・トランスフォーム型の3種類がある。 faults but non-negligible probabilities nearly everywhere. The USGS National Seismic Hazard Maps express hazard as the probability of exceeding a given peak ground acceleration level in 50 years — and these maps show elevated hazard in many areas people assume are earthquake-free.

The appropriate mental model is not "faults are dangerous, everywhere else is safe" but rather "active plate boundaries have the highest hazard, intraplate zones have intermediate hazard with less certainty, and induced seismicity can elevate hazard temporarily in any region with subsurface fluid injection." This more accurate picture argues for universal baseline seismic design requirements, not just fault-proximity regulations.

The Design Implications

[[Building-code]] seismic provisions apply not only to coastal California and Pacific Northwest cities but across the entire country, with requirements scaled to local hazard levels. Cities in the New Madrid zone, Charleston, and other intraplate hazard areas have design requirements reflecting their specific risk. Understanding that you can face earthquake risk without living next to a famous fault should motivate engagement with local building codes and emergency preparedness regardless of perceived fault-line proximity.