津波

Tsunami — Grokipedia Fact-checked by Grok 1 month ago Tsunami Ara Eve Leo Sal 1x A tsunami is a series of large ocean waves triggered by a sudden and large-scale displacement of seawater , most commonly from undersea earthquakes but also from volcanic eruptions, landslides, or rare events like asteroid impacts. [1] These waves differ from typical wind-driven waves due to their exceptionally long wavelengths—often spanning tens to hundreds of miles—and their ability to propagate across entire ocean basins with minimal energy loss. [2] In the deep ocean , tsunamis typically have amplitudes of less than 3 feet and travel at speeds exceeding 500 miles per hour, comparable to a commercial jet aircraft . [3] The primary cause of tsunamis is vertical displacement of the seafloor during large subduction zone earthquakes, where tectonic plates converge and one is forced beneath another, generating waves that radiate outward from the source. [4] Other mechanisms include submarine landslides, which displace water rapidly, and explosive volcanic activity that ejects material into the ocean, though earthquakes account for about 80% of all tsunamis. [5] Unlike tidal waves, which are driven by gravitational forces from the moon and sun, tsunamis are seismic or geological in origin and can strike without warning, often hours after the initial event depending on distance from the epicenter . As tsunamis approach shallower coastal waters, their speed decreases to 20–30 miles per hour while their height amplifies dramatically due to wave shoaling , sometimes exceeding 100 feet in extreme cases, leading to powerful inundation far inland. [6] The resulting impacts include catastrophic flooding, structural collapse from wave forces, erosion of coastlines, and hazards from strong currents and debris , which cause the majority of injuries and fatalities. [7] Over recorded history , more than 270 confirmed deadly tsunamis have claimed over 544,000 lives, with destruction amplified in densely populated coastal regions vulnerable to these events. [1] Among the most devastating tsunamis in modern history was the 2004 Indian Ocean event, triggered by a magnitude 9.1 earthquake off Sumatra , Indonesia , which killed approximately 230,000 people across 14 countries and caused billions in damages due to the absence of a regional warning system at the time. [8] The 2011 Tōhoku tsunami in Japan , generated by a magnitude 9.0 earthquake , resulted in nearly 18,000 deaths and the Fukushima nuclear disaster, highlighting vulnerabilities even in prepared nations. [9] These events underscore the global threat, particularly in the Pacific Ring of Fire , where about 80% of the world's earthquakes occur. [10] Modern mitigation relies on international warning networks, including NOAA's two Tsunami Warning Centers in Alaska and Hawaii , which monitor seismic activity and deploy Deep-ocean Assessment and Reporting of Tsunamis (DART) buoys to detect wave signatures in real time. These systems issue alerts—ranging from watches to warnings—within minutes, enabling evacuations that have saved countless lives since their expansion and improvements following the 2004 Indian Ocean tsunami and 2011 Tōhoku event in Japan. Enhanced forecasting accuracy has prevented unnecessary evacuations and provided critical lead time (hours for distant sources), dramatically reducing potential casualties in U.S. territories and coasts. Terminology Etymology and Definition The term "tsunami" originates from Japanese, where it combines the words tsu (meaning "harbor") and nami (meaning "wave"), literally translating to "harbor wave." [11] This etymology reflects the phenomenon's tendency to surge destructively into coastal harbors, and the word first appeared in English-language scientific literature in the late 19th century , following observations of seismic events in Japan . [12] A tsunami is defined as a series of ocean waves generated by large-scale disturbances of the sea, such as sudden vertical displacements of the ocean floor, which propagate across vast distances with significant energy . [13] Unlike wind-driven waves, which are powered by atmospheric friction on the surface, tsunamis derive their energy from the initial displacement and behave as shallow-water waves due to their extremely long wavelengths relative to the ocean depth—often spanning 100 to 300 kilometers or more. [14] This shallow-water characteristic governs their propagation speed according to the formula c = g h c = \sqrt{gh} c = g h ​ , where c c c is the wave speed, g g g is the acceleration due to gravity (approximately 9.81 m/s²), and h h h is the water depth. [15] The scientific community gradually adopted "tsunami" over earlier terms like "seismic sea wave" starting in the mid-20th century, with widespread standardization occurring through international efforts, including the establishment of the Intergovernmental Oceanographic Commission (IOC) of UNESCO's Pacific Tsunami Warning System in 1965 and subsequent refinements in terminology by 1969. [16] This adoption helped distinguish tsunamis from misnomers like "tidal waves," which incorrectly imply a connection to tides . [17] Misnomers and Related Terms The term "tidal wave" is frequently misused to describe tsunamis, implying a connection to tidal forces driven by gravitational interactions between the Earth , Moon , and Sun, whereas tsunamis result from sudden water displacement unrelated to tides . [18] This misnomer persists in popular media and historical accounts but has been rejected by scientists, as it misleads the public about the phenomenon's causes and mechanics. [19] Another outdated term, "seismic sea wave," was commonly used in scientific literature before the mid-20th century to highlight earthquake-generated tsunamis, but it inaccurately limits the scope since tsunamis can also arise from landslides, volcanic activity, or other non-seismic events. [18] The U.S. established the Seismic Sea Wave Warning System in 1949 following the 1946 Aleutian Islands tsunami, which was renamed the Pacific Tsunami Warning System in 1968 to adopt the broader Japanese term "tsunami," reflecting international standardization around 1963. [20] [21] Terms like " rogue wave " and " sneaker wave " describe distinct ocean phenomena often confused with tsunamis due to their sudden and destructive nature, but they differ fundamentally in generation and propagation. Rogue waves are localized, nonlinear surface waves exceeding twice the height of surrounding seas, typically formed by wind-driven interactions or wave focusing far from shore, lasting mere seconds and not traversing oceans like tsunamis. [22] Sneaker waves, meanwhile, are unexpectedly large coastal surges that run up beaches without prior warning, caused by distant storms or wave sets rather than seismic displacement, and pose risks through rapid inundation but lack the long-wavelength propagation of tsunamis. [23] [24] Cultural terms for tsunamis vary linguistically, often reflecting local perceptions of sea motion. In Hawaiian, "kai eʻe" translates to "mounting sea " or " sea swell," capturing the rising surge of distant tsunamis, while "kai mimiki" refers specifically to local ones generated nearby. [25] In Spanish-speaking regions, "maremoto" literally means " sea movement" or " sea quake," historically encompassing both the triggering earthquake and resulting waves, though it is now largely synonymous with tsunami in modern usage. [26] Historical Overview Major Historical Events One of the earliest recorded tsunamis struck the eastern Mediterranean on July 21, 365 AD, originating from a powerful earthquake near Crete , Greece , which generated waves that devastated coastal areas including Alexandria , Egypt , where the sea retreated before surging back to inundate the city and destroy ports across northern Africa. [27] [28] Historical accounts estimate thousands of deaths, with ancient sources like Ammianus Marcellinus describing the destruction of entire populations in affected harbors. [29] On November 1, 1755, the Lisbon earthquake, estimated at magnitude 8.5–9.0, triggered a transatlantic tsunami that reached heights of up to 6 meters in Lisbon , Portugal , and propagated across the Atlantic to impact the Caribbean islands, with waves reported as far as the British Isles and North Africa . [30] The event contributed to an overall death toll exceeding 60,000 in the region, though tsunami-specific fatalities were concentrated along the Iberian Peninsula and Morocco, where run-up exceeded 15 meters at Cape St. Vincent . [31] In the 19th and 20th centuries, several devastating tsunamis highlighted the hazards in volcanic and seismic zones. The August 27, 1883, eruption of Krakatoa in Indonesia produced waves up to 40 meters high in the Sunda Strait , destroying 165 coastal villages on Java and Sumatra and causing over 36,000 deaths, with more than 34,000 attributed directly to the tsunami. [32] Similarly, the April 1, 1946, magnitude 8.6 earthquake in the Aleutian Islands , Alaska , generated a Pacific-wide tsunami with run-up heights reaching 42 meters on Unimak Pass and 17 meters in Hilo, Hawaii , resulting in 167 deaths and over $26 million in damages (in 1946 dollars). [33] The deadliest tsunami in recorded history occurred on December 26, 2004, following a magnitude 9.1 earthquake off Sumatra , Indonesia , which unleashed waves up to 30 meters high across the Indian Ocean , affecting 14 countries and killing 227,898 people, with the majority in Indonesia , Sri Lanka , India , and Thailand . [34] More recently, the March 11, 2011, magnitude 9.0 Tōhoku earthquake off Japan's Honshu coast produced tsunami waves up to 40 meters that inundated coastal areas, causing 19,759 deaths and triggering the Fukushima Daiichi nuclear disaster, which led to widespread evacuations and long-term radiological contamination. [9] [35] On January 15, 2022, the eruption of the Hunga Tonga–Hunga Ha'apai