51.540°N 174.795°W depth=22.8km (14.2mi)
The Aleutian arc extends approximately 3,000 km from the Gulf of Alaska in the east to the Kamchatka Peninsula in the west. It marks the region where the Pacific plate subducts into the mantle beneath the North America plate. This subduction is responsible for the generation of the Aleutian Islands and the deep offshore Aleutian Trench.
The curvature of the arc results in a westward transition of relative plate motion from trench-normal (i.e., compressional) in the east to trench-parallel (i.e., translational) in the west, accompanied by westward variations in seismic activity, volcanism, and overriding plate composition. The Aleutian arc is generally divided into three regions: the western, central, and eastern Aleutians. Relative to a fixed North America plate, the Pacific plate is moving northwest at a rate that increases from roughly 60 mm/yr at the arc's eastern edge to 76 mm/yr near its western terminus. The eastern Aleutian arc extends from the Alaskan Peninsula in the east to the Fox Islands in the west. Motion along this section of the arc is characterized by arc-perpendicular convergence and Pacific plate subduction beneath thick continental lithosphere. This region exhibits intense volcanic activity and has a history of megathrust earthquakes.
The central Aleutian arc extends from the Andreanof Islands in the east to the Rat Islands in the west. Here, motion is characterized by westward-increasing oblique convergence and Pacific plate subduction beneath thin oceanic lithosphere. Along this portion of the arc, the Wadati-Benioff zone is well defined to depths of approximately 200 km. Despite the obliquity of convergence, active volcanism and megathrust earthquakes are also present along this margin.
The western Aleutians, stretching from the western end of the Rat Islands in the east to the Commander Islands, Russia, in the west, is tectonically different from the central and eastern portions of the arc. The increasing component of transform motion between the Pacific and North America plates is evidenced by diminishing active volcanism; the last active volcano is located on Buldir Island, in the far western portion of the Rat Island chain. Additionally, this portion of the subduction zone has not hosted large earthquakes or megathrust events in recorded history. Instead, the largest earthquakes in this region are generally shallow, predominantly strike-slip events with magnitudes between M5-6. Deeper earthquakes do occur, albeit rather scarcely and with small magnitudes (M<4), down to approximately 50 km.
Most of the seismicity along the Aleutian arc results from thrust faulting that occurs along the interface between the Pacific and North America plates, extending from near the base of the trench to depths of 40 to 60 km. Slip along this interface is responsible for generating devastating earthquakes. Deformation also occurs within the subducting slab in the form of intermediate-depth earthquakes that can reach depths of 250 km. Normal faulting events occur in the outer rise region of the Aleutian arc resulting from the bending of the oceanic Pacific plate as it enters the Aleutian trench. Additionally, deformation of the overriding North America plate generates shallow crustal earthquakes.
The Aleutian arc is a seismically active region, evidenced by the many moderate to large earthquakes occurring each year. Since 1900, this region has hosted twelve large earthquakes (M>7.5) including the May 7, 1986 M8.0 Andreanof Islands, the June 10, 1996 M7.9 Andreanof Islands, and the November 17, 2003 M7.8 Rat Islands earthquakes. Six of these great earthquakes (M8.3 or larger) have occurred along the Aleutian arc that together have ruptured almost the entire shallow megathrust contact. The first of these major earthquakes occurred on August 17, 1906 near the island of Amchitka (M8.3) in the western Aleutian arc. However, unlike the other megathrust earthquakes along the arc, this event is thought to have been an intraplate event occurring in the shallow slab beneath the subduction zone interface.
The first megathrust event along the arc during the 20th century was the November 10, 1938 M8.6 Shumagin Island earthquake. This event ruptured an approximately 300 km long stretch of the arc from the southern end of Kodiak Island to the northern end of the Shumagin Islands and generated a small tsunami that was recorded as far south as Hawaii.
The April 1, 1946 M8.6 Unimak Island earthquake, located in the central Aleutian arc, was characterized by slow rupture followed by a devastating Pacific-wide tsunami that was observed as far south as the shores of Antarctica. Although damage from earthquake shaking was not severe locally, tsunami run-up heights were recorded as high as 42 m on Unimak Island and tsunami waves in Hilo, Hawaii also resulted in casualties. The slow rupture of this event has made it difficult to constrain the focal mechanism and depth of the earthquake, though it is thought to have been an interplate thrust earthquake.
The next megathrust earthquake occurred along the central portion of the Aleutian arc near the Andreanof Islands on March 9, 1957, with a magnitude of M8.6. The rupture length of this event was approximately 1200 km, making it the longest observed aftershock zone of all the historic Aleutian arc events. Although only limited seismic data from this event are still available, significant damage and tsunamis were observed on the islands of Adak and Unimak with tsunami heights of approximately 13 m.
The easternmost megathrust earthquake was the March 28, 1964 M9.2 Prince William Sound earthquake, currently the second largest recorded earthquake in the world. The event had a rupture length of roughly 700 km extending from Prince William Sound in the northeast to the southern end of Kodiak Island in the southwest. Extensive damage was recorded in Kenai, Moose Pass, and Kodiak but significant shaking was felt over a large region of Alaska, parts of western Yukon Territory, and British Columbia, Canada. Property damage was the largest in Anchorage, as a result of both the main shock shaking and the ensuing landslides. This megathrust earthquake also triggered a devastating tsunami that caused damage along the Gulf of Alaska, the West Coast of the United States, and in Hawaii.
The westernmost Aleutians megathrust earthquake followed a year later on February 4, 1965. This M8.7 Rat Islands earthquake was characterized by roughly 600 km of rupture. Although this event is quite large, damage was low owing to the region's remote and sparsely inhabited location. A relatively small tsunami was recorded throughout the Pacific Ocean with run-up heights up to 10.7 m on Shemya Island and flooding on Amchitka Island.
Although the Aleutian arc is highly active, seismicity is rather discontinuous, with two regions that have not experienced a large (M>8.0) earthquake in the past century: the Commander Islands in the western Aleutians and the Shumagin Islands in the east. Due to the dominantly transform motion along the western arc, there is potential that the Commander Islands will rupture in a moderate to large strike-slip earthquake in the future. The Shumagin Islands region may also have high potential for hosting a large rupture in the future, though it has been suggested that little strain is being accumulated along this section of the subduction zone, and thus associated hazards may be reduced.
East of the Aleutian arc along the Gulf of Alaska, crustal earthquakes occur as a result transmitted deformation and stress associated with the northwestward convergence of the Pacific plate that collides a block of oceanic and continental material into the North America plate. In 2002, the Denali Fault ruptured in a sequence of earthquakes that commenced with the October 23 M6.7 Nenana Mountain right-lateral strike-slip earthquake and culminated with the November 3, M7.9 Denali earthquake which started as a thrust earthquake along a then unrecognized fault and continued with a larger right-lateral strike-slip event along the Denali and Totschunda Faults.
More information on regional seismicity and tectonics
27.016°N 65.547°E depth=15.0km (9.3mi)
No fewer than four major tectonic plates (Arabia, Eurasia, India, and Africa) and one smaller tectonic block (Anatolia) are responsible for seismicity and tectonics in the Middle East and surrounding region. Geologic development of the region is a consequence of a number of first-order plate tectonic processes that include subduction, large-scale transform faulting, compressional mountain building and crustal extension.
Mountain building in northern Pakistan and Afghanistan is the result of compressional tectonics associated with collision of the India plate moving northwards at a rate of 40 mm/yr with respect to the Eurasia plate. Continental thickening of the northern and western edge of the India subcontinent has produced the highest mountains in the world, including the Himalayan, Karakoram, Pamir and Hindu Kush ranges. Earthquake activity and faulting found in this region, as well as adjacent parts of Afghanistan and India, are due to collisional plate tectonics.
Beneath the Pamir-Hindu Kush Mountains of northern Afghanistan, earthquakes occur to depths as great as 200 km as a result of remnant lithospheric subduction. Shallower crustal earthquakes in the Pamir-Hindu Mountains occur primarily along the Main Pamir Thrust and other active Quaternary faults, which accommodate much of the region's crustal shortening. The western and eastern margins of the Main Pamir Thrust display a combination of thrust and strike-slip mechanisms.
Along the western margin of the Tibetan Plateau, in the vicinity of southeastern Afghanistan and western Pakistan, the India plate translates obliquely relative to the Eurasia plate, resulting in a complex fold-and-thrust belt known as the Sulaiman Range. Faulting in this region includes strike-slip, reverse-slip and oblique-slip motion and often results in shallow, destructive earthquakes. The relatively fast moving left-lateral, strike-slip Chaman Fault system in southeastern Afghanistan accommodates translational motion between the India and Eurasia plates. In 1505, a segment of the Chaman Fault system near Kabul, Afghanistan ruptured causing widespread destruction of Kabul and surrounding villages. In the same region, the more recent 30 May 1935, M7.6 Quetta, Pakistan earthquake, occurred within the Sulaiman Range, killing between 30,000 and 60,000 people.
Off the south coast of Pakistan and southeast coast of Iran, the Makran trench is the present-day surface expression of active subduction of the Arabia plate beneath the continental Eurasia plate, which converge at a rate of approximately 20 mm/yr. Although the Makran subduction zone has a relatively slow convergence rate, it has produced large devastating earthquakes and tsunamis. For example, the November 27, 1945 M8.0 mega-thrust earthquake produced a tsunami within the Gulf of Oman and Arabia Sea, killing over 4,000 people. Northwest of this active subduction zone, collision of the Arabia and Eurasia plates forms the approximately 1,500-km-long fold and thrust belt of the Zagros Mountains, which crosses the whole of western Iran and extends into northeastern Iraq. Collision of the Arabia and Eurasia plates also causes crustal shortening in the Alborz Mountains and Kopet Dag in northern Iran. Eastern Iran experiences destructive earthquakes that originate on both strike-slip and reverse faults. For example, the 16 September 1978 M7.8 earthquake, along the southwest edge of the Dasht-e-Lut Basin killed at least 15,000 people.
Along the eastern margin of the Mediterranean region there is complex interaction between the Africa, Arabia and Eurasia plates. The Red Sea Rift is a spreading center between the Africa and Arabia plates, with a spreading rate of approximately 10mm/yr near its northern end, and 16mm/yr near its southern end (Chu, D. and Gordon, R. G., 1998). Seismicity rate and size of earthquakes has been relatively small along the spreading center, but the rifting process has produced a series of volcanic systems across western Saudi Arabia.
Further north, the Red Sea Rift terminates at the southern boundary of the Dead Sea Transform Fault. The Dead Sea Transform is a strike-slip fault that accommodates differential motion between the Africa and Arabia plates. Though both the Africa plate, to the west, and the Arabia plate, to the east, are moving in a NNE direction, the Arabia plate is moving slightly faster, resulting in the left-lateral, strike-slip motion along this segment of the plate boundary. Historically, earthquake activity along the Dead Sea Transform has been a significant hazard in the densely populated Levant region (eastern Mediterranean). For example, the November 1759 Near East earthquake is thought to have killed somewhere between 2,000-20,000 people. The northern termination of the Dead Sea Transform occurs within a complex tectonic region of southeast Turkey, where interaction of the Africa and Arabia plates and the Anatolia block occurs. This involves translational motion of the Anatolia Block westwards, with a speed of approximately 25mm/yr with respect to Eurasia, in order to accommodate closure of the Mediterranean basin.
The right-lateral, strike-slip North Anatolia Fault, in northern Turkey, accommodates much of the westwards motion between the Anatolia Block and Eurasia Plate. Between 1939 and 1999, a series of devastating M7.0+ strike-slip earthquakes propagated westwards along the North Anatolia Fault system. The westernmost of these earthquakes was the 17th August 1999, M7.6 Izmit earthquake, near the Sea of Marmara, killed approximately 17,000 people.
At the southern edge of the Anatolia Block lies the east-west trending Cyprian Arc with associated levels of moderate seismicity. The Cyprian Arc represents the convergent boundary between the Anatolia Block to the north and the Africa Plate to the south. The boundary is thought to join the East Anatolia Fault zone in eastern Turkey; however no certain geometry or sense of relative motion along the entire boundary is widely accepted.
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15.851°S 74.562°W depth=45.8km (28.5mi)
The South American arc extends over 7,000 km, from the Chilean margin triple junction offshore of southern Chile to its intersection with the Panama fracture zone, offshore of the southern coast of Panama in Central America. It marks the plate boundary between the subducting Nazca plate and the South America plate, where the oceanic crust and lithosphere of the Nazca plate begin their descent into the mantle beneath South America. The convergence associated with this subduction process is responsible for the uplift of the Andes Mountains, and for the active volcanic chain present along much of this deformation front. Relative to a fixed South America plate, the Nazca plate moves slightly north of eastwards at a rate varying from approximately 80 mm/yr in the south to approximately 65 mm/yr in the north. Although the rate of subduction varies little along the entire arc, there are complex changes in the geologic processes along the subduction zone that dramatically influence volcanic activity, crustal deformation, earthquake generation and occurrence all along the western edge of South America.
Most of the large earthquakes in South America are constrained to shallow depths of 0 to 70 km resulting from both crustal and interplate deformation. Crustal earthquakes result from deformation and mountain building in the overriding South America plate and generate earthquakes as deep as approximately 50 km. Interplate earthquakes occur due to slip along the dipping interface between the Nazca and the South American plates. Interplate earthquakes in this region are frequent and often large, and occur between the depths of approximately 10 and 60 km. Since 1900, numerous magnitude 8 or larger earthquakes have occurred on this subduction zone interface that were followed by devastating tsunamis, including the 1960 M9.5 earthquake in southern Chile, the largest instrumentally recorded earthquake in the world. Other notable shallow tsunami-generating earthquakes include the 1906 M8.5 earthquake near Esmeraldas, Ecuador, the 1922 M8.5 earthquake near Coquimbo, Chile, the 2001 M8.4 Arequipa, Peru earthquake, the 2007 M8.0 earthquake near Pisco, Peru, and the 2010 M8.8 Maule, Chile earthquake located just north of the 1960 event.
Large intermediate-depth earthquakes (those occurring between depths of approximately 70 and 300 km) are relatively limited in size and spatial extent in South America, and occur within the Nazca plate as a result of internal deformation within the subducting plate. These earthquakes generally cluster beneath northern Chile and southwestern Bolivia, and to a lesser extent beneath northern Peru and southern Ecuador, with depths between 110 and 130 km. Most of these earthquakes occur adjacent to the bend in the coastline between Peru and Chile. The most recent large intermediate-depth earthquake in this region was the 2005 M7.8 Tarapaca, Chile earthquake.
Earthquakes can also be generated to depths greater than 600 km as a result of continued internal deformation of the subducting Nazca plate. Deep-focus earthquakes in South America are not observed from a depth range of approximately 300 to 500 km. Instead, deep earthquakes in this region occur at depths of 500 to 650 km and are concentrated into two zones: one that runs beneath the Peru-Brazil border and another that extends from central Bolivia to central Argentina. These earthquakes generally do not exhibit large magnitudes. An exception to this was the 1994 Bolivian earthquake in northwestern Bolivia. This M8.2 earthquake occurred at a depth of 631 km, making it the largest deep-focus earthquake instrumentally recorded, and was felt widely throughout South and North America.
Subduction of the Nazca plate is geometrically complex and impacts the geology and seismicity of the western edge of South America. The intermediate-depth regions of the subducting Nazca plate can be segmented into five sections based on their angle of subduction beneath the South America plate. Three segments are characterized by steeply dipping subduction; the other two by near-horizontal subduction. The Nazca plate beneath northern Ecuador, southern Peru to northern Chile, and southern Chile descend into the mantle at angles of 25° to 30°. In contrast, the slab beneath southern Ecuador to central Peru, and under central Chile, is subducting at a shallow angle of approximately 10° or less. In these regions of “flat-slab” subduction, the Nazca plate moves horizontally for several hundred kilometers before continuing its descent into the mantle, and is shadowed by an extended zone of crustal seismicity in the overlying South America plate. Although the South America plate exhibits a chain of active volcanism resulting from the subduction and partial melting of the Nazca oceanic lithosphere along most of the arc, these regions of inferred shallow subduction correlate with an absence of volcanic activity.
27.263°N 65.587°E depth=14.8km (9.2mi)
The September 28, 2013 M6.8 earthquake in south-central Pakistan occurred as the result of oblique-strike-slip motion at shallow crustal depths. This earthquake is an aftershock of the September 24, 2013 M7.7 event, which struck with a similar faulting mechanism approximately 30 km to the south-southwest. As with the September 24 event, the September 28 earthquake mechanism and location are consistent with rupture within the Eurasia plate above the Makran subduction zone. The event occurred within the transition zone between northward subduction of the Arabia plate beneath the Eurasia plate and northward collision of the India plate with the Eurasia plate.
30.956°S 178.244°W depth=34.8km (21.6mi)
The eastern margin of the Australia plate is one of the most sesimically active areas of the world due to high rates of convergence between the Australia and Pacific plates. In the region of New Zealand, the 3000 km long Australia-Pacific plate boundary extends from south of Macquarie Island to the southern Kermadec Island chain. It includes an oceanic transform (the Macquarie Ridge), two oppositely verging subduction zones (Puysegur and Hikurangi), and a transpressive continental transform, the Alpine Fault through South Island, New Zealand.
Since 1900 there have been 15 M7.5+ earthquakes recorded near New Zealand. Nine of these, and the four largest, occurred along or near the Macquarie Ridge, including the 1989 M8.2 event on the ridge itself, and the 2004 M8.1 event 200 km to the west of the plate boundary, reflecting intraplate deformation. The largest recorded earthquake in New Zealand itself was the 1931 M7.8 Hawke's Bay earthquake, which killed 256 people. The last M7.5+ earthquake along the Alpine Fault was 170 years ago; studies of the faults' strain accumulation suggest that similar events are likely to occur again.
North of New Zealand, the Australia-Pacific boundary stretches east of Tonga and Fiji to 250 km south of Samoa. For 2,200 km the trench is approximately linear, and includes two segments where old (>120 Myr) Pacific oceanic lithosphere rapidly subducts westward (Kermadec and Tonga). At the northern end of the Tonga trench, the boundary curves sharply westward and changes along a 700 km-long segment from trench-normal subduction, to oblique subduction, to a left lateral transform-like structure.
Australia-Pacific convergence rates increase northward from 60 mm/yr at the southern Kermadec trench to 90 mm/yr at the northern Tonga trench; however, significant back arc extension (or equivalently, slab rollback) causes the consumption rate of subducting Pacific lithosphere to be much faster. The spreading rate in the Havre trough, west of the Kermadec trench, increases northward from 8 to 20 mm/yr. The southern tip of this spreading center is propagating into the North Island of New Zealand, rifting it apart. In the southern Lau Basin, west of the Tonga trench, the spreading rate increases northward from 60 to 90 mm/yr, and in the northern Lau Basin, multiple spreading centers result in an extension rate as high as 160 mm/yr. The overall subduction velocity of the Pacific plate is the vector sum of Australia-Pacific velocity and back arc spreading velocity: thus it increases northward along the Kermadec trench from 70 to 100 mm/yr, and along the Tonga trench from 150 to 240 mm/yr.
The Kermadec-Tonga subduction zone generates many large earthquakes on the interface between the descending Pacific and overriding Australia plates, within the two plates themselves and, less frequently, near the outer rise of the Pacific plate east of the trench. Since 1900, 40 M7.5+ earthquakes have been recorded, mostly north of 30°S. However, it is unclear whether any of the few historic M8+ events that have occurred close to the plate boundary were underthrusting events on the plate interface, or were intraplate earthquakes. On September 29, 2009, one of the largest normal fault (outer rise) earthquakes ever recorded (M8.1) occurred south of Samoa, 40 km east of the Tonga trench, generating a tsunami that killed at least 180 people.
Across the North Fiji Basin and to the west of the Vanuatu Islands, the Australia plate again subducts eastwards beneath the Pacific, at the North New Hebrides trench. At the southern end of this trench, east of the Loyalty Islands, the plate boundary curves east into an oceanic transform-like structure analogous to the one north of Tonga.
Australia-Pacific convergence rates increase northward from 80 to 90 mm/yr along the North New Hebrides trench, but the Australia plate consumption rate is increased by extension in the back arc and in the North Fiji Basin. Back arc spreading occurs at a rate of 50 mm/yr along most of the subduction zone, except near ~15°S, where the D'Entrecasteaux ridge intersects the trench and causes localized compression of 50 mm/yr in the back arc. Therefore, the Australia plate subduction velocity ranges from 120 mm/yr at the southern end of the North New Hebrides trench, to 40 mm/yr at the D'Entrecasteaux ridge-trench intersection, to 170 mm/yr at the northern end of the trench.
Large earthquakes are common along the North New Hebrides trench and have mechanisms associated with subduction tectonics, though occasional strike slip earthquakes occur near the subduction of the D'Entrecasteaux ridge. Within the subduction zone 34 M7.5+ earthquakes have been recorded since 1900. On October 7, 2009, a large interplate thrust fault earthquake (M7.6) in the northern North New Hebrides subduction zone was followed 15 minutes later by an even larger interplate event (M7.8) 60 km to the north. It is likely that the first event triggered the second of the so-called earthquake "doublet".
12.280°N 141.693°E depth=104.0km (64.6mi)
The Philippine Sea plate is bordered by the larger Pacific and Eurasia plates and the smaller Sunda plate. The Philippine Sea plate is unusual in that its borders are nearly all zones of plate convergence. The Pacific plate is subducted into the mantle, south of Japan, beneath the Izu-Bonin and Mariana island arcs, which extend more than 3,000 km along the eastern margin of the Philippine Sea plate. This subduction zone is characterized by rapid plate convergence and high-level seismicity extending to depths of over 600 km. In spite of this extensive zone of plate convergence, the plate interface has been associated with few great (M>8.0) ‘megathrust’ earthquakes. This low seismic energy release is thought to result from weak coupling along the plate interface (Scholz and Campos, 1995). These convergent plate margins are also associated with unusual zones of back-arc extension (along with resulting seismic activity) that decouple the volcanic island arcs from the remainder of the Philippine Sea Plate (Karig et al., 1978; Klaus et al., 1992).
South of the Mariana arc, the Pacific plate is subducted beneath the Yap Islands along the Yap trench. The long zone of Pacific plate subduction at the eastern margin of the Philippine Sea Plate is responsible for the generation of the deep Izu-Bonin, Mariana, and Yap trenches as well as parallel chains of islands and volcanoes, typical of circum-pacific island arcs. Similarly, the northwestern margin of the Philippine Sea plate is subducting beneath the Eurasia plate along a convergent zone, extending from southern Honshu to the northeastern coast of Taiwan, manifested by the Ryukyu Islands and the Nansei-Shoto (Ryukyu) trench. The Ryukyu Subduction Zone is associated with a similar zone of back-arc extension, the Okinawa Trough. At Taiwan, the plate boundary is characterized by a zone of arc-continent collision, whereby the northern end of the Luzon island arc is colliding with the buoyant crust of the Eurasia continental margin offshore China.
Along its western margin, the Philippine Sea plate is associated with a zone of oblique convergence with the Sunda Plate. This highly active convergent plate boundary extends along both sides the Philippine Islands, from Luzon in the north to the Celebes Islands in the south. The tectonic setting of the Philippines is unusual in several respects: it is characterized by opposite-facing subduction systems on its east and west sides; the archipelago is cut by a major transform fault, the Philippine Fault; and the arc complex itself is marked by active volcanism, faulting, and high seismic activity. Subduction of the Philippine Sea Plate occurs at the eastern margin of the archipelago along the Philippine Trench and its northern extension, the East Luzon Trough. The East Luzon Trough is thought to be an unusual example of a subduction zone in the process of formation, as the Philippine Trench system gradually extends northward (Hamburger et al., 1983). On the west side of Luzon, the Sunda Plate subducts eastward along a series of trenches, including the Manila Trench in the north, the smaller less well-developed Negros Trench in the central Philippines, and the Sulu and Cotabato trenches in the south (Cardwell et al., 1980). At its northern and southern terminations, subduction at the Manila Trench is interrupted by arc-continent collision, between the northern Philippine arc and the Eurasian continental margin at Taiwan and between the Sulu-Borneo Block and Luzon at the island of Mindoro. The Philippine fault, which extends over 1,200 km within the Philippine arc, is seismically active. The fault has been associated with major historical earthquakes, including the destructive M7.6 Luzon earthquake of 1990 (Yoshida and Abe, 1992). A number of other active intra-arc fault systems are associated with high seismic activity, including the Cotabato Fault and the Verde Passage-Sibuyan Sea Fault (Galgana et al., 2007).
Relative plate motion vectors near the Philippines (about 80 mm/yr) is oblique to the plate boundary along the two plate margins of central Luzon, where it is partitioned into orthogonal plate convergence along the trenches and nearly pure translational motion along the Philippine Fault (Barrier et al., 1991). Profiles B and C reveal evidence of opposing inclined seismic zones at intermediate depths (roughly 70-300 km) and complex tectonics at the surface along the Philippine Fault.
Several relevant tectonic elements, plate boundaries and active volcanoes, provide a context for the seismicity presented on the main map. The plate boundaries are most accurate along the axis of the trenches and more diffuse or speculative in the South China Sea and Lesser Sunda Islands. The active volcanic arcs (Siebert and Simkin, 2002) follow the Izu, Volcano, Mariana, and Ryukyu island chains and the main Philippine islands parallel to the Manila, Negros, Cotabato, and Philippine trenches.
Seismic activity along the boundaries of the Philippine Sea Plate (Allen et al., 2009) has produced 7 great (M>8.0) earthquakes and 250 large (M>7) events. Among the most destructive events were the 1923 Kanto, the 1948 Fukui and the 1995 Kobe (Japan) earthquakes (99,000, 5,100, and 6,400 casualties, respectively), the 1935 and the 1999 Chi-Chi (Taiwan) earthquakes (3,300 and 2,500 casualties, respectively), and the 1976 M7.6 Moro Gulf and 1990 M7.6 Luzon (Philippines) earthquakes (7,100 and 2,400 casualties, respectively). There have also been a number of tsunami-generating events in the region, including the Moro Gulf earthquake, whose tsunami resulted in more than 5000 deaths.
30.752°S 178.439°W depth=146.5km (91.0mi)
35.487°N 23.261°E depth=36.2km (22.5mi)
The Mediterranean region is seismically active due to the northward convergence (4-10 mm/yr) of the African plate with respect to the Eurasian plate along a complex plate boundary. This convergence began approximately 50 Ma and was associated with the closure of the Tethys Sea. The modern day remnant of the Tethys Sea is the Mediterranean Sea. The highest rates of seismicity in the Mediterranean region are found along the Hellenic subduction zone of southern Greece, along the North Anatolian Fault Zone of western Turkey and the Calabrian subduction zone of southern Italy. Local high rates of convergence at the Hellenic subduction zone (35mm/yr) are associated with back-arc spreading throughout Greece and western Turkey above the subducting Mediterranean oceanic crust. Crustal normal faulting throughout this region is a manifestation of extensional tectonics associated with the back-arc spreading. The region of the Marmara Sea is a transition zone between this extensional regime, to the west, and the strike-slip regime of the North Anatolian Fault Zone, to the east. The North Anatolian Fault accommodates much of the right-lateral horizontal motion (23-24 mm/yr) between the Anatolian micro-plate and Eurasian plate as the Anatolian micro-plate is being pushed westward to further accommodate closure of the Mediterranean basin caused by the collision of the African and Arabian plates in southeastern Turkey. Subduction of the Mediterranean Sea floor beneath the Tyrrhenian Sea at the Calabrian subduction zone causes a significant zone of seismicity around Sicily and southern Italy. Active volcanoes are located above intermediate depth earthquakes in the Cyclades of the Aegean Sea and in southern Italy.
In the Mediterranean region there is a written record, several centuries long, documenting pre-instrumental seismicity (pre-20th century). Earthquakes have historically caused widespread damage across central and southern Greece, Cyprus, Sicily, Crete, the Nile Delta, Northern Libya, the Atlas Mountains of North Africa and the Iberian Peninsula. The 1903 M8.2 Kythera earthquake and the 1926 M7.8 Rhodes earthquakes are the largest instrumentally recorded Mediterranean earthquakes, both of which are associated with subduction zone tectonics. Between 1939 and 1999 a series of devastating M7+ strike-slip earthquakes propagated westward along the North Anatolian Fault Zone, beginning with the 1939 M7.8 Erzincan earthquake on the eastern end of the North Anatolian Fault system. The 1999 M7.6 Izmit earthquake, located on the westward end of the fault, struck one of Turkey's most densely populated and industrialized urban areas killing, more than 17,000 people. Although seismicity rates are comparatively low along the northern margin of the African continent, large destructive earthquakes have been recorded and reported from Morocco in the western Mediterranean, to the Dead Sea in the eastern Mediterranean. The 1980 M7.3 El Asnam earthquake was one of Africa's largest and most destructive earthquakes within the 20th century.
Large earthquakes throughout the Mediterranean region have also been known to produce significant and damaging tsunamis. One of the more prominent historical earthquakes within the region is the Lisbon earthquake of November 1, 1755, whose magnitude has been estimated from non-instrumental data to be about 8.0. The 1755 Lisbon earthquake is thought to have occurred within or near the Azores-Gibraltar transform fault, which defines the boundary between the African and Eurasian plates off the west coast of Morocco and Portugal. The earthquake is notable for both a large death toll of approximately 60,000 people and for generating a tsunami that swept up the Portuguese coast inundating coastal villages and Lisbon. An earthquake of approximately M8.0 near Sicily in 1693 generated a large tsunami wave that destroyed numerous towns along Sicily's east coast. The M7.2 December 28, 1908 Messina earthquake is the deadliest documented European earthquake. The combination of severe ground shaking and a local tsunami caused an estimated 60,000 to 120,000 fatalities.
9.765°N 124.022°E depth=56.8km (35.3mi)
6.487°S 154.928°E depth=54.1km (33.6mi)
The Australia-Pacific plate boundary is over 4000 km long on the northern margin, from the Sunda (Java) trench in the west to the Solomon Islands in the east. The eastern section is over 2300 km long, extending west from northeast of the Australian continent and the Coral Sea until it intersects the east coast of Papua New Guinea. The boundary is dominated by the general northward subduction of the Australia plate.
Along the South Solomon trench, the Australia plate converges with the Pacific plate at a rate of approximately 95 mm/yr towards the east-northeast. Seismicity along the trench is dominantly related to subduction tectonics and large earthquakes are common: there have been 13 M7.5+ earthquakes recorded since 1900. On April 1, 2007, a M8.1 interplate megathrust earthquake occurred at the western end of the trench, generating a tsunami and killing at least 40 people. This was the third M8.1 megathrust event associated with this subduction zone in the past century; the other two occurred in 1939 and 1977.
Further east at the New Britain trench, the relative motions of several microplates surrounding the Australia-Pacific boundary, including north-south oriented seafloor spreading in the Woodlark Basin south of the Solomon Islands, maintain the general northward subduction of Australia-affiliated lithosphere beneath Pacific-affiliated lithosphere. Most of the large and great earthquakes east of New Guinea are related to this subduction; such earthquakes are particularly concentrated at the cusp of the trench south of New Ireland. 33 M7.5+ earthquakes have been recorded since 1900, including three shallow thrust fault M8.1 events in 1906, 1919, and 2007.
The western end of the Australia-Pacific plate boundary is perhaps the most complex portion of this boundary, extending 2000 km from Indonesia and the Banda Sea to eastern New Guinea. The boundary is dominantly convergent along an arc-continent collision segment spanning the width of New Guinea, but the regions near the edges of the impinging Australia continental margin also include relatively short segments of extensional, strike-slip and convergent deformation. The dominant convergence is accommodated by shortening and uplift across a 250-350 km-wide band of northern New Guinea, as well as by slow southward-verging subduction of the Pacific plate north of New Guinea at the New Guinea trench. Here, the Australia-Pacific plate relative velocity is approximately 110 mm/yr towards the northeast, leading to the 2-8 mm/yr uplift of the New Guinea Highlands.
Whereas the northern band of deformation is relatively diffuse east of the Indonesia-Papua New Guinea border, in western New Guinea there are at least two small (<100,000 km²) blocks of relatively undeformed lithosphere. The westernmost of these is the Birds Head Peninsula microplate in Indonesia's West Papua province, bounded on the south by the Seram trench. The Seram trench was originally interpreted as an extreme bend in the Sunda subduction zone, but is now thought to represent a southward-verging subduction zone between Birds Head and the Banda Sea.
There have been 22 M7.5+ earthquakes recorded in the New Guinea region since 1900. The dominant earthquake mechanisms are thrust and strike slip, associated with the arc-continent collision and the relative motions between numerous local microplates. The largest earthquake in the region was a M8.2 shallow thrust fault event in the northern Papua province of Indonesia that killed 166 people in 1996.
The western portion of the northern Australia plate boundary extends approximately 4800 km from New Guinea to Sumatra and primarily separates Australia from the Eurasia plate, including the Sunda block. This portion is dominantly convergent and includes subduction at the Sunda (Java) trench, and a young arc-continent collision.
In the east, this boundary extends from the Kai Islands to Sumba along the Timor trough, offset from the Sunda trench by 250 km south of Sumba. Contrary to earlier tectonic models in which this trough was interpreted as a subduction feature continuous with the Sunda subduction zone, it is now thought to represent a subsiding deformational feature related to the collision of the Australia plate continental margin and the volcanic arc of the Eurasia plate, initiating in the last 5-8 Myr. Before collision began, the Sunda subduction zone extended eastward to at least the Kai Islands, evidenced by the presence of a northward-dipping zone of seismicity beneath Timor Leste. A more detailed examination of the seismic zone along it's eastern segment reveals a gap in intermediate depth seismicity under Timor and seismic mechanisms that indicate an eastward propagating tear in the descending slab as the negatively buoyant oceanic lithosphere detaches from positively buoyant continental lithosphere. On the surface, GPS measurements indicate that the region around Timor is currently no longer connected to the Eurasia plate, but instead is moving at nearly the same velocity as the Australia plate, another consequence of collision.
Large earthquakes in eastern Indonesia occur frequently but interplate megathrust events related to subduction are rare; this is likely due to the disconnection of the descending oceanic slab from the continental margin. There have been 9 M7.5+ earthquakes recorded from the Kai Islands to Sumba since 1900. The largest was the great Banda Sea earthquake of 1938 (M8.5) an intermediate depth thrust faulting event that did not cause significant loss of life.
26.268°N 110.178°W depth=8.0km (5.0mi)
Located atop three of the large tectonic plates, Mexico is one of the world's most seismically active regions. The relative motion of these crustal plates causes frequent earthquakes and occasional volcanic eruptions. Most of the Mexican landmass is on the westward moving North American plate. The Pacific Ocean floor south of Mexico is being carried northeastward by the underlying Cocos plate. Because oceanic crust is relatively dense, when the Pacific Ocean floor encounters the lighter continental crust of the Mexican landmass, the ocean floor is subducted beneath the North American plate creating the deep Middle American trench along Mexico's southern coast. Also as a result of this convergence, the westward moving Mexico landmass is slowed and crumpled creating the mountain ranges of southern Mexico and earthquakes near Mexico's southern coast. As the oceanic crust is pulled downward, it melts; the molten material is then forced upward through weaknesses in the overlying continental crust. This process has created a region of volcanoes across south-central Mexico known as the Cordillera Neovolcánica.
The area west of the Gulf of California, including Mexico's Baja California Peninsula, is moving northwestward with the Pacific plate at about 50 mm per year. Here, the Pacific and North American plates grind past each other creating strike-slip faulting, the southern extension of California's San Andreas fault. In the past, this relative plate motion pulled Baja California away from the coast forming the Gulf of California and is the cause of earthquakes in the Gulf of California region today.
Mexico has a long history of destructive earthquakes and volcanic eruptions. In September 1985, a magnitude 8.1 earthquake killed more than 9,500 people in Mexico City. In southern Mexico, Volcán de Colima and El Chichón erupted in 2005 and 1982, respectively. Paricutín volcano, west of Mexico City, began venting smoke in a cornfield in 1943; a decade later this new volcano had grown to a height of 424 meters. Popocatépetl and Ixtaccíhuatl volcanos ("smoking mountain" and "white lady", respectively), southeast of Mexico City, occasionally vent gas that can be clearly seen from the City, a reminder that volcanic activity is ongoing. In 1994 and 2000 Popocatépetl renewed its activity forcing the evacuation of nearby towns, causing seismologists and government officials to be concerned about the effect a large-scale eruption might have on the heavily populated region. Popocatépetl volcano last erupted in 2010.
23.591°N 121.443°E depth=12.0km (7.5mi)
30.315°S 71.733°W depth=10.7km (6.6mi)
19.158°S 172.642°W depth=10.0km (6.2mi)
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