1868年アリカ地震: 太平洋を横断した津波を起こしたM9.0メガスラスト

August 13, 1868: The Shaking Lasted 5 Minutes

The port city of Arica in 1868 was a Peruvian city — the boundary between Peru and Chile would not be redrawn until the War of the Pacific more than a decade later — and one of the most important Pacific ports of the South American coast. It sat at the edge of the hyperarid Atacama Desert, where the Andes drop precipitously to the sea, and served as the primary outlet for silver and other minerals from the Bolivian highlands.

On August 13, 1868, at approximately five in the afternoon, the ground began to shake. Contemporary accounts, written by survivors and by naval officers aboard ships in the harbour, describe shaking that lasted between four and five minutes — an extraordinarily long duration indicating a very large 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。. Buildings of adobe and masonry, which constituted virtually all of Arica's structures, collapsed almost universally. The city that had stood was, in minutes, a field of rubble. The coastal town of Iquique, some 400 kilometres to the south, suffered similarly. The earthquake was felt throughout Peru, Bolivia, Ecuador, and northern Chile, and even triggered ground motion perceptible in Buenos Aires, more than 3,000 kilometres from the 震央地下で地震が発生した震源の真上にあたる地表の地点。ニュース報道では地震の発生場所としてよく報じられる。.

Modern analysis estimates 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 the 1868 Arica earthquake at approximately 8.5 to 9.0 — placing it among the largest earthquakes in recorded South American history. At the time, no seismographs existed; the instrument had not yet been invented. The size of the 1868 event would not be estimated until 20th-century seismologists applied methods of 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。 and tsunami physics to reconstruct the source parameters. What happened after the shaking stopped was equally catastrophic and contributed significantly to the scientific legacy of the event.

Use Earthquake Energy Calculator to understand the energy release of a magnitude 8.5-9.0 megathrust earthquake and how it compares to modern instrumentally recorded events of similar size.

The Peru-Chile Trench: Megathrust Setting

The 1868 Arica earthquake occurred in one of the most seismically hazardous 収束型境界2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。 environments on Earth: the interface between the Nazca Plate and the South American Plate along what is now called the Peru-Chile Trench. Here, the Nazca Plate descends beneath South America at a rate of approximately seven to eight centimetres per year, locking with the overriding plate and accumulating enormous elastic strain over periods of decades to centuries.

This 沈み込み帯1枚のプレートがもう1枚のプレートの下にもぐり込み、マントルへと沈み込む領域。沈み込み帯は世界最大級の地震（M8.5以上）を引き起こし、深い海溝や火山弧を伴う。 has produced some of the largest earthquakes in human history. The 1960 Valdivia earthquake — at M9.5, the largest instrumentally recorded earthquake ever — struck the Chilean segment of this trench. The 2010 Maule earthquake (M8.8) struck the same region. Northward, in the segment encompassing Peru and northern Chile, the 1868 event represents one of the largest known historical ruptures of the past several centuries.

The section of the megathrust that broke in 1868 had apparently accumulated centuries of stress since its previous major rupture. [[Seismic-moment]] analysis based on the size of the resulting 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。 suggests the rupture area extended over at least 500 kilometres of the plate interface, with average slip of several metres. Such parameters are consistent with a 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。 in the range of 8.5 to 9.0, making the 1868 event a genuine megathrust earthquake comparable to the 2004 Indian Ocean and 2011 Tohoku events in physical scale.

The 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。 for earthquakes of this size in the northern Peru-Chile subduction segment appears to be on the order of several centuries, based on geological and historical evidence. This long recurrence interval is both reassuring in one sense — the next comparable event may be far in the future — and deeply concerning in another: the longer the interval, the more stress accumulates and the larger the potential 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。. The northern Peru-Chile segment had not, as of the early 2020s, produced a full-segment rupture comparable to 1868 since that date, making it one of the most discussed potential source zones in South American seismic hazard analysis.

The tectonic character of the northern Peru-Chile segment is also shaped by the subduction of the Inca Plateau and the Nazca Ridge — broad bathymetric highs on the Nazca Plate that affect the coupling geometry of the megathrust as they descend. Regions where such features are subducting tend to show higher locking and thus greater potential for large 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。 energy release. Understanding the heterogeneous coupling of the megathrust is an active research area that uses geodetic satellite data, 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。 measurements, and 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。 to map the spatial variation of stress accumulation along the trench.

Warships Carried Inland: The USS Wateree Story

Within minutes of the earthquake, the ocean withdrew from the Arica harbour in the classic precursor to a large 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。. Sailors on vessels anchored in the port saw the seafloor exposed as the water receded, leaving ships grounded on mud and sand where open water had been moments before. This withdrawal — the trough of the first tsunami wave — was a warning that those who recognized it could use to flee to high ground. Many did not recognize it, or could not act quickly enough.

The tsunami waves arrived at Arica with terrifying speed. The first wave inundated the city, sweeping away what the earthquake had left standing. Subsequent waves reached reported heights of 12 to 15 metres above normal sea level, scouring the coastal lowlands and carrying ships far inland.

Among the vessels in Arica harbour were two American naval ships: the USS Wateree, a double-ended river gunboat, and the USS Fredonia, a supply vessel. The flat-bottomed hull of the Wateree — designed for riverine operations and thus extraordinarily stable in chaotic water — allowed it to ride the tsunami waves without capsizing. The ship was carried approximately three kilometres inland from the original shoreline and deposited on dry land several metres above sea level when the waters receded. Its crew, remarkably, survived almost entirely intact.

The Fredonia was less fortunate. The ship was capsized and destroyed, with survivors clinging to wreckage before eventually being rescued. Several other vessels were similarly grounded inland or destroyed. The Peruvian ironclad America survived by steaming into open water at full speed as the first wave approached — a manoeuvre that is now precisely what maritime tsunami protocols advise for vessels with sufficient time and sea room.

The USS Wateree sat beached in the Atacama Desert for decades, a bizarre landmark that became one of the most photographed and documented relics of the 1868 disaster. Its unusual survival story was widely reported in American newspapers, giving the Arica earthquake greater prominence in the English-speaking world than it would otherwise have received. The wreck eventually disintegrated from desert weathering, but photographs and accounts of the beached warship became iconic images in the history of Pacific tsunamis.

The testimony of naval officers, including detailed accounts of wave heights, timing, and the sequence of inundation, provided invaluable historical data. These first-hand records from trained observers — men accustomed to making precise measurements and writing official reports — are among the most reliable historical tsunami observations from any 19th-century Pacific event, and they have been used extensively in reconstructing the source parameters of the 1868 earthquake.

Pacific-Wide Tsunami: From Arica to Japan

The 1868 Arica 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。 was not a local event. [[Seismic-wave]]s radiated from the enormous 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。 in all directions, displacing the ocean surface across an area of millions of square kilometres. Within hours, the tsunami had propagated across the entire Pacific Ocean, striking Hawaii, New Zealand, Japan, and the western coast of North America.

In Hawaii, the tsunami arrived approximately fourteen hours after the earthquake. Reports from Honolulu described waves that inundated coastal areas and caused significant property damage. Several lives were lost. The Hilo bay area, which by virtue of its funnel-like geometry concentrates and amplifies tsunami waves from South American sources, experienced particularly strong run-up — a pattern that would be repeated in the 1960 Chile tsunami and that has shaped the design of Hawaii's tsunami warning and evacuation system.

In New Zealand, the tsunami reached the coast approximately twelve hours after the earthquake, producing flooding in coastal harbours and estuary areas. In Japan, the waves arrived more than twenty hours after the earthquake, producing run-up of one to two metres in some coastal locations — sufficient to be noticed and documented but not to cause casualties or major damage at such extreme distance. The Japanese measurements, made at tide gauges that were among the first modern instruments deployed in the Pacific basin, provided some of the earliest machine-readable tsunami records in history.

On the California coast, the tsunami produced measurable ocean level changes, with some contemporary accounts describing flooding of low-lying coastal areas in San Francisco Bay and elsewhere. The Pacific-wide propagation of the 1868 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。 provided one of the earliest demonstrations that tsunamis generated in South America could affect the entire Pacific basin — a lesson that would be reinforced repeatedly in subsequent events and that ultimately motivated the creation of the Pacific Tsunami Warning Centre after the 1960 Chile earthquake.

Pre-Instrumental Magnitude: Estimating Size from Tsunami Records

The 1868 Arica earthquake occurred nearly two decades before the invention of the seismograph. Seismologists studying this event cannot rely on instrumental recordings. Instead, they must reconstruct the earthquake's size from a combination of historical accounts, 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。 — the study of geological records of past earthquakes — and tsunami physics.

Tsunami-based magnitude estimation works on a straightforward principle: the size of a tsunami depends on the amount of seafloor deformation caused by the earthquake, which is related to the size of the 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。 area and the amount of slip. By modelling what fault parameters would produce the observed tsunami run-up heights at multiple locations across the Pacific, seismologists can work backward to estimate the 地震モーメント断層面積・平均変位量・岩石のせん断弾性率の積として算出される、地震が放出した総エネルギーの尺度。モーメントマグニチュードの算出基盤となる。 and thus 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 the parent earthquake.

For the 1868 event, researchers have compiled historical run-up data from Peru, Chile, Ecuador, Hawaii, New Zealand, Japan, and California. These data, entered into numerical tsunami propagation models, consistently suggest a source earthquake in the magnitude 8.5 to 9.0 range. The uncertainty arises from the quality of the historical records — many run-up estimates are qualitative descriptions rather than surveyed measurements — and from the simplifying assumptions required in source modelling. Different research groups, using different model geometries and different subsets of the observational data, arrive at somewhat different magnitude estimates, but all agree the event was among the largest in the historical South American catalogue.

[[Paleoseismology]] provides a complementary line of evidence. Studies of coastal sediments in northern Chile and southern Peru have identified tsunami deposits — layers of marine sand and shell material deposited inland by historical tsunamis — that can be dated and correlated with historical events. The 1868 deposit is one of the most distinctive in the Holocene record of this coastline, consistent with a very large event and providing geological confirmation of the historical accounts. In some coastal locations, the 1868 sand layer is intercalated with the deposits of earlier tsunamis, allowing researchers to construct a multi-millennial record of megathrust recurrence in this segment.

Historical Earthquakes in Modern Hazard Assessment

The 1868 Arica earthquake demonstrates why historians and seismologists must work together in assessing earthquake hazard. Modern instrumental records span only about 120 years — far too short to capture the full recurrence cycle of the largest earthquakes, which may have return periods of hundreds to thousands of years. For 沈み込み帯1枚のプレートがもう1枚のプレートの下にもぐり込み、マントルへと沈み込む領域。沈み込み帯は世界最大級の地震（M8.5以上）を引き起こし、深い海溝や火山弧を伴う。 environments like the Peru-Chile Trench, the great earthquakes that pose the most extreme hazard may be precisely those that fall outside the instrumental record.

The cities that sit in this hazard zone — Arica (now Chilean), Iquique, Antofagasta, and Tacna — have grown enormously since 1868. Arica's population has expanded from a few thousand to over 230,000. Modern construction, 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。 warning systems, evacuation routes, and public education programmes are far more developed than in 1868. But the physical hazard is unchanged, and the tsunami that would follow a comparable rupture today would reach populated coastlines within minutes — leaving almost no time for warning-based evacuation in the near field.

For communities within a few tens of kilometres of the trench — including small fishing villages that sit directly on the coast — the standard guidance is to treat strong earthquake shaking as the only reliable warning and to begin moving to high ground immediately, without waiting for any official alert. This "self-evacuation" protocol, promoted by tsunami education programmes throughout the Pacific, is the direct operational legacy of events like the 1868 earthquake, where the 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。 is long, the warning time is negligible, and survival depends entirely on individual preparedness.

The effectiveness of public education in tsunami-prone coastal areas depends critically on the public's understanding of the connection between earthquake shaking and tsunami arrival. The 1868 Arica event pre-dates any formal warning infrastructure, and even today the near-field warning time for the northern Chile coast would be measured in minutes — insufficient for technology-based warning to save lives in the most exposed areas. The only effective mitigation is vertical evacuation of people to high ground before the waves arrive, driven by the immediate recognition that strong shaking in a coastal zone means an incoming 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。. This is a straightforward behavioural protocol, but one that must be drilled repeatedly to become automatic. The 1868 event's magnitude and the speed of its tsunami propagation define the upper bound of what the population must be prepared to respond to.

The 1868 Arica earthquake is thus simultaneously a historical artifact and a template for what the region's hazard planners must prepare for. By studying the evidence it left behind — in sediment cores, in colonial archives, in the propagation records across the Pacific — modern scientists have constructed a picture of one of history's most powerful earthquakes and used it to build more realistic 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。s for the millions of people who currently live along the northern Andean coast.

The Colonial Record as Seismological Archive

The 1868 earthquake sits near the middle of the documented historical period for the northern Andean coast, which extends back to the Spanish colonial era in the 16th century. Colonial administrative records — mining reports, viceregal correspondence, church records, and chronicles — provide accounts of several significant earthquakes in the Peru-Chile region before the instrumental era, including events in 1604, 1615, 1647, 1687, 1725, and 1746. The systematic compilation and analysis of these historical accounts — a painstaking process of archival research conducted by historians and seismologists working in collaboration — has produced a historical earthquake catalogue for South America that extends the record of seismicity back several centuries beyond the instrumental period.

The 1868 event occupies a special place in this historical catalogue because it is documented by an unusually rich set of sources. The simultaneous presence of American and other naval vessels in Arica harbour, with their trained officers and official reporting obligations, produced multiple independent first-hand accounts of the earthquake and tsunami that are more reliable than most 19th-century earthquake descriptions. The Pacific-wide reach of the 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。 means that the event is also documented in tide gauge records, newspaper accounts, and government reports from Hawaii, California, New Zealand, and Japan — a geographically distributed corroboration network that allows cross-checking of the source parameters derived from any individual set of records.

This rich documentation has made the 1868 event one of the best-constrained pre-instrumental earthquakes in the South American catalogue, and has established the methodology of multi-source historical 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。 as a standard approach for characterizing large earthquakes that occurred before the 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。 era.

The 2014 Iquique Earthquake: A Partial Release

On April 1, 2014, a magnitude 8.2 earthquake struck offshore northern Chile, in the northern Peru-Chile subduction zone — the same regional setting as the 1868 Arica earthquake. The 2014 Iquique earthquake broke a portion of the subduction interface in the area that had been identified as a seismic gap — a section of the fault that had not produced a large earthquake in over a century and was therefore thought to have accumulated significant stress. The earthquake was the largest in the world in 2014 and generated a 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。 that was observed across the Pacific.

However, the 2014 earthquake broke only a fraction of the length that ruptured in 1868. Geodetic analysis and seismological modelling suggested that significant stress remained unresolved on adjacent portions of the interface, and that a larger event remained possible. The partial nature of the 2014 release — providing some relief but not resolving the accumulated stress from more than a century of locked plate convergence — exemplifies the complexity of the 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。 concept in segmented subduction zones.

The 1868-to-2014 record for the northern Chile segment illustrates what seismologists call the "earthquake cycle" in an unusually clear way: stress accumulates over a long period of locked plate convergence, is released in one or more large events, and then the cycle begins again. The challenge for hazard assessment is that the cycle is not perfectly regular, that single events may or may not release the full accumulated stress, and that the next major event may break a different combination of 断層セグメント特有のすべり挙動を持つ、より大きな断層系の中の明確な区間。異なるセグメントが独立して、あるいは連鎖的に破壊することがあり、地震の規模に影響する。s than any previous historical rupture. The 1868 Arica earthquake anchors the long end of this historical record and defines the upper bound of what the northern Andean 沈み込み帯1枚のプレートがもう1枚のプレートの下にもぐり込み、マントルへと沈み込む領域。沈み込み帯は世界最大級の地震（M8.5以上）を引き起こし、深い海溝や火山弧を伴う。 is capable of producing.

Arica's Vulnerability Today

The modern city of Arica sits in a tectonic setting essentially unchanged from that of 1868. The Nazca Plate continues to converge with South America at the same rate. The locked zone of the megathrust continues to accumulate stress. The coastal topography — with steep cliffs rising immediately behind a narrow coastal strip — gives the city limited high ground for tsunami evacuation. The run-up from a 1868-scale event would potentially inundate the entire low-lying port area and extend up the valley of the Lluta River, which drains the Atacama behind the city.

Chile's National Seismological Centre and the Pacific Tsunami Warning Centre both classify the northern Chile coast as a high-priority area for tsunami preparedness. Arica has vertical evacuation structures — reinforced buildings designated as last-resort refuges for people who cannot reach high ground in time — and extensive public education about tsunami self-evacuation. The city conducts regular tsunami drills. Emergency managers and seismologists monitor the region closely for signs of accelerating stress accumulation that might precede a major rupture.

None of this guarantees safety. 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 and 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。 from a future M8.5-9.0 earthquake in the northern Chile segment would strike with little warning time, overwhelming even the best-prepared response system. The lesson of 1868 — that this coast is exposed to one of the most extreme natural hazards on Earth — remains as relevant as it was 150 years ago, and the city that rebuilt from the catastrophe of that August afternoon carries that lesson in every evacuations map and every coastal land use decision it makes.