Misplaced Pages

#224775

58-658: Convergent boundaries line the southern border including the New Britain subduction zone that contributed to the formation of New Britain and the Solomon Islands . Many earthquakes occur in this area particularly around New Britain, which has very complex tectonics and defining all the active plate boundaries has proved challenging. To the south of the South Bismark plate is the Solomon Sea plate which

116-763: A destructive boundary ) is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction . The subduction zone can be defined by a plane where many earthquakes occur, called the Wadati–Benioff zone . These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis , destruction of lithosphere , and deformation . Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types. Plate tectonics

174-737: A volcanic arc and are associated with extensional tectonics and high heat flow, often being home to seafloor spreading centers. These spreading centers are like mid-ocean ridges , though the magma composition of back-arc basins is generally more varied and contains a higher water content than mid-ocean ridge magmas. Back-arc basins are often characterized by thin, hot lithosphere. Opening of back-arc basins may arise from movement of hot asthenosphere into lithosphere, causing extension. Oceanic trenches are narrow topographic lows that mark convergent boundaries or subduction zones. Oceanic trenches average 50 to 100 km (31 to 62 mi) wide and can be several thousand kilometers long. Oceanic trenches form as

232-410: A large effect on the image created. For example, commonly used tomographic methods work by iteratively improving an initial input model, and thus can produce unrealistic results if the initial model is unreasonable. P-wave data are used in most local models and global models in areas with sufficient earthquake and seismograph density. S- and surface wave data are used in global models when this coverage

290-519: A model limits the resolution it can achieve. Longer wavelengths are able to penetrate deeper into the Earth, but can only be used to resolve large features. Finer resolution can be achieved with surface waves, with the trade off that they cannot be used in models deeper than the crust and upper mantle. The disparity between wavelength and feature scale causes anomalies to appear of reduced magnitude and size in images. P- and S-wave models respond differently to

348-480: A radial path through the Earth, and assumes this profile is valid for every path from the core to the surface. This 1984 study was also the first to apply the term "tomography" to seismology, as the term had originated in the medical field with X-ray tomography . Seismic tomography has continued to improve in the past several decades since its initial conception. The development of adjoint inversions, which are able to combine several different types of seismic data into

406-553: A result of bending of the subducting slab. Depth of oceanic trenches seems to be controlled by age of the oceanic lithosphere being subducted. Sediment fill in oceanic trenches varies and generally depends on abundance of sediment input from surrounding areas. An oceanic trench, the Mariana Trench , is the deepest point of the ocean at a depth of approximately 11,000 m (36,089 ft). Earthquakes are common along convergent boundaries. A region of high earthquake activity,

464-442: A result of thermal or chemical differences, which are attributed to processes such as mantle plumes, subducting slabs, and mineral phase changes. Larger scale features that can be imaged with tomography include the high velocities beneath continental shields and low velocities under ocean spreading centers . The mantle plume hypothesis proposes that areas of volcanism not readily explained by plate tectonics, called hotspots , are

522-418: A result of thermal upwelling within the mantle. Some researchers have proposed an upper mantle source above the 660km discontinuity for these plumes, while others propose a much deeper source, possibly at the core-mantle boundary . While the source of mantle plumes has been highly debated since they were first proposed in the 1970s, most modern studies argue in favor of mantle plumes originating at or near

580-432: A seismically active region with extensive permanent network coverage. These allow for the imaging of the crust and upper mantle . Regional to global scale tomographic models are generally based on long wavelengths. Various models have better agreement with each other than local models due to the large feature size they image, such as subducted slabs and superplumes . The trade off from whole mantle to whole Earth coverage

638-404: A single inversion, help negate some of the trade-offs associated with any individual data type. Historically, seismic waves have been modeled as 1D rays, a method referred to as "ray theory" that is relatively simple to model and can usually fit travel-time data well. However, recorded seismic waveforms contain much more information than just travel-time and are affected by a much wider path than

SECTION 10

#1732772075225

696-435: Is assumed by ray theory. Methods like the finite-frequency method attempt to account for this within the framework of ray theory. More recently, the development of "full waveform" or "waveform" tomography has abandoned ray theory entirely. This method models seismic wave propagation in its full complexity and can yield more accurate images of the subsurface. Originally these inversions were developed in exploration seismology in

754-405: Is driven by convection cells in the mantle. Convection cells are the result of heat generated by the radioactive decay of elements in the mantle escaping to the surface and the return of cool materials from the surface to the mantle. These convection cells bring hot mantle material to the surface along spreading centers creating new crust. As this new crust is pushed away from the spreading center by

812-444: Is likely that the plate may break along the boundary of continental and oceanic crust. Seismic tomography reveals pieces of lithosphere that have broken off during convergence. Subduction zones are areas where one lithospheric plate slides beneath another at a convergent boundary due to lithospheric density differences. These plates dip at an average of 45° but can vary. Subduction zones are often marked by an abundance of earthquakes,

870-412: Is most characteristic of oceanic volcanic arcs, though this is also found in continental volcanic arcs above rapid subduction (>7 cm/year). This series is relatively low in potassium . The more oxidized calc-alkaline series , which is moderately enriched in potassium and incompatible elements, is characteristic of continental volcanic arcs. The alkaline magma series (highly enriched in potassium)

928-418: Is not sufficient, such as in ocean basins and away from subduction zones. First-arrival times are the most widely used, but models utilizing reflected and refracted phases are used in more complex models, such as those imaging the core. Differential traveltimes between wave phases or types are also used. Local tomographic models are often based on a temporary seismic array targeting specific areas, unless in

986-415: Is scraped from the subducting lithosphere and emplaced against the overriding lithosphere. These sediments include igneous crust, turbidite sediments, and pelagic sediments. Imbricate thrust faulting along a basal decollement surface occurs in accretionary wedges as forces continue to compress and fault these newly added sediments. The continued faulting of the accretionary wedge leads to overall thickening of

1044-427: Is sometimes present in the deeper continental interior. The shoshonite series, which is extremely high in potassium, is rare but sometimes is found in volcanic arcs. The andesite member of each series is typically most abundant, and the transition from basaltic volcanism of the deep Pacific basin to andesitic volcanism in the surrounding volcanic arcs has been called the andesite line. Back-arc basins form behind

1102-593: Is subducting under New Britain at the New Britain Trench and a likely still active Trobriand plate which has fault zone relationships at the postulated plate boundary. GPS data shows the South Bismarck plate, though north of the Australian plate boundary, is being pushed by Australia in a northerly direction while the area of the historic North Bismarck plate is being dragged or pushed by

1160-473: Is the coarse resolution (hundreds of kilometers) and difficulty imaging small features (e.g. narrow plumes). Although often used to image different parts of the subsurface, P- and S-wave derived models broadly agree where there is image overlap. These models use data from both permanent seismic stations and supplementary temporary arrays. Seismic tomography can resolve anisotropy, anelasticity, density, and bulk sound velocity. Variations in these parameters may be

1218-502: The Wadati–Benioff zone , generally dips 45° and marks the subducting plate. Earthquakes will occur to a depth of 670 km (416 mi) along the Wadati-Benioff margin. Both compressional and extensional forces act along convergent boundaries. On the inner walls of trenches, compressional faulting or reverse faulting occurs due to the relative motion of the two plates. Reverse faulting scrapes off ocean sediment and leads to

SECTION 20

#1732772075225

1276-431: The underlying magmatic system . These images have most commonly been used to estimate the depth and volume of magma stored in the crust, but have also been used to constrain properties such as the geometry, temperature, or chemistry of the magma. It is important to note that both lab experiments and tomographic imaging studies have shown that recovering these properties from seismic velocity alone can be difficult due to

1334-400: The 1980s and 1990s and were too computationally complex for global and regional scale studies, but development of numerical modeling methods to simulate seismic waves has allowed waveform tomography to become more common. Seismic tomography uses seismic records to create 2D and 3D models of the subsurface through an inverse problem that minimizes the difference between the created model and

1392-564: The North and South Bismarck plates is called the Bismarck Sea Seismic Lineation (BSSL), and this line is ill-defined, but is associated with shallow earthquakes, with poor definition of BSSL associated earthquakes that become mixed with subduction associated earthquakes towards the southwest and New Ireland . These subduction associated earthquakes, unlike at the other plate boundaries, are often magnitude 7 or above and

1450-648: The Pacific plate to the west. The North Bismark plate does not have detectable independent motion to the Pacific plate and most believe it to be a relic plate. Accordingly, as the west-northwest motion of the North Bismarck microplate is similar to that of the Pacific plate, most of the Melanesian arc which is to the east of the New Ireland can be regarded as fixed on the Pacific plate. The line separating

1508-517: The Tethyan suture zone (the Alps – Zagros – Himalaya mountain belt). The oceanic crust contains hydrated minerals such as the amphibole and mica groups. During subduction, oceanic lithosphere is heated and metamorphosed, causing breakdown of these hydrous minerals, which releases water into the asthenosphere. The release of water into the asthenosphere leads to partial melting. Partial melting allows

1566-475: The accuracy of a model. As early as 1972, researchers successfully used some of the underlying principles of modern seismic tomography to search for fast and slow areas in the subsurface. The first widely cited publication that largely resembles modern seismic tomography was published in 1976 and used local earthquakes to determine the 3D velocity structure beneath Southern California. The following year, P-wave delay times were used to create 2D velocity maps of

1624-553: The area around southern New Ireland has a very high concentration of such. The plate boundaries to the east, mainly within New Guinea are also complex, although as for all the postulated plate boundaries, current shallow earthquake activity acts as a guide. New Guinea's Finistree Block is usually mapped into the plate. Sources This tectonics article is a stub . You can help Misplaced Pages by expanding it . Convergent boundary A convergent boundary (also known as

1682-457: The asthenosphere and volcanism. Both dehydration and partial melting occur along the 1,000 °C (1,830 °F) isotherm, generally at depths of 65 to 130 km (40 to 81 mi). Some lithospheric plates consist of both continental and oceanic lithosphere . In some instances, initial convergence with another plate will destroy oceanic lithosphere, leading to convergence of two continental plates. Neither continental plate will subduct. It

1740-504: The complexity of seismic wave propagation through focused zones of hot, potentially melted rocks. While comparatively primitive to tomography on Earth, seismic tomography has been proposed on other bodies in the solar system and successfully used on the Moon . Data collected from four seismometers placed by the Apollo missions have been used many times to create 1-D velocity profiles for

1798-437: The core-mantle boundary. This is in large part due to tomographic images that reveal both the plumes themselves as well as large low-velocity zones in the deep mantle that likely contribute to the formation of mantle plumes. These large low-shear velocity provinces as well as smaller ultra low velocity zones have been consistently observed across many tomographic models of the deep Earth Subducting plates are colder than

South Bismarck plate - Misplaced Pages Continue

1856-519: The dense oceanic lithosphere subducts beneath the less dense continental lithosphere. An accretionary wedge forms on the continental crust as deep-sea sediments and oceanic crust are scraped from the oceanic plate. Volcanic arcs form on continental lithosphere as the result of partial melting due to dehydration of the hydrous minerals of the subducting slab. Some lithospheric plates consist of both continental and oceanic crust. Subduction initiates as oceanic lithosphere slides beneath continental crust. As

1914-430: The differences in seismic waves recorded at different locations, it is possible to create a model of the subsurface structure. Most commonly, these seismic waves are generated by earthquakes or man-made sources such as explosions. Different types of waves, including P- , S- , Rayleigh , and Love waves can be used for tomographic images, though each comes with their own benefits and downsides and are used depending on

1972-485: The formation of an accretionary wedge. Reverse faulting can lead to megathrust earthquakes . Tensional or normal faulting occurs on the outer wall of the trench, likely due to bending of the downgoing slab. A megathrust earthquake can produce sudden vertical displacement of a large area of ocean floor. This in turn generates a tsunami . Some of the deadliest natural disasters have occurred due to convergent boundary processes. The 2004 Indian Ocean earthquake and tsunami

2030-420: The formation of newer crust, it cools, thins, and becomes denser. Subduction begins when this dense crust converges with a less dense crust. The force of gravity helps drive the subducting slab into the mantle. As the relatively cool subducting slab sinks deeper into the mantle, it is heated, causing hydrous minerals to break down. This releases water into the hotter asthenosphere, which leads to partial melting of

2088-463: The geologic setting, seismometer coverage, distance from nearby earthquakes, and required resolution. The model created by tomographic imaging is almost always a seismic velocity model , and features within this model may be interpreted as structural, thermal, or compositional variations. Geoscientists apply seismic tomography to a wide variety of settings in which the subsurface structure is of interest, ranging in scale from whole-Earth structure to

2146-518: The interiors of other planetary bodies when only a single seismometer is available. For example, data gathered by the SEIS (Seismic Experiment for Interior Structure) instrument on InSight on Mars has been able to detect the Martian core. Global seismic networks have expanded steadily since the 1960s, but are still concentrated on continents and in seismically active regions. Oceans, particularly in

2204-616: The late-20th century, tomography is only capable of viewing changes in velocity structure over decades. For example, tectonic plates only move at millimeters per year, so the total amount of change in geologic structure due to plate tectonics since the development of seismic tomography is several orders of magnitude lower than the finest resolution possible with modern seismic networks. However, seismic tomography has still been used to view near-surface velocity structure changes at time scales of years to months. Tomographic solutions are non-unique. Although statistical methods can be used to analyze

2262-451: The location of the earthquake hypocenter . CT scans use linear x-rays and a known source. In the early 20th century, seismologists first used travel time variations in seismic waves from earthquakes to make discoveries such as the existence of the Moho and the depth to the outer core. While these findings shared some underlying principles with seismic tomography, modern tomography itself

2320-451: The mantle into which they are moving. This creates a fast anomaly that is visible in tomographic images. Tomographic images have been made of most subduction zones around the world and have provided insight into the geometries of the crust and upper mantle in these areas. These images have revealed that subducting plates vary widely in how steeply they move into the mantle. Tomographic images have also seen features such as deeper portions of

2378-432: The measured seismic waveform to be fit during the inversion. Seismic tomography is similar to medical x-ray computed tomography (CT scan) in that a computer processes receiver data to produce a 3D image, although CT scans use attenuation instead of travel-time difference. Seismic tomography has to deal with the analysis of curved ray paths which are reflected and refracted within the Earth, and potential uncertainty in

South Bismarck plate - Misplaced Pages Continue

2436-402: The moon, and less commonly 3-D tomographic models. Tomography relies on having multiple seismometers, but tomography-adjacent methods for constraining Earth structure have been used on other planets. While on Earth these methods are often used in combination with seismic tomography models to better constrain the locations of subsurface features, they can still provide useful information about

2494-468: The observed seismic data. Various methods are used to resolve anomalies in the crust , lithosphere , mantle , and core based on the availability of data and types of seismic waves that pass through the region. Longer wavelengths penetrate deeper into the Earth, but seismic waves are not sensitive to features significantly smaller than their wavelength and therefore provide a lower resolution. Different methods also make different assumptions, which can have

2552-417: The oceanic lithosphere subducts to greater depths, the attached continental crust is pulled closer to the subduction zone. Once the continental lithosphere reaches the subduction zone, subduction processes are altered, since continental lithosphere is more buoyant and resists subduction beneath other continental lithosphere. A small portion of the continental crust may be subducted until the slab breaks, allowing

2610-552: The oceanic lithosphere to continue subducting, hot asthenosphere to rise and fill the void, and the continental lithosphere to rebound. Evidence of this continental rebound includes ultrahigh pressure metamorphic rocks , which form at depths of 90 to 125 km (56 to 78 mi), that are exposed at the surface. Seismic records have been used to map the torn slabs beneath the Caucasus continental – continental convergence zone, and seismic tomography has mapped detached slabs beneath

2668-409: The reflection and refraction of these waves. The location and magnitude of variations in the subsurface can be calculated by the inversion process, although solutions to tomographic inversions are non-unique. Most commonly, only the travel time of the seismic waves is considered in the inversion. However, advances in modeling techniques and computing power have allowed different parts, or the entirety, of

2726-399: The result of internal deformation of the plate, convergence with the opposing plate, and bending at the oceanic trench. Earthquakes have been detected to a depth of 670 km (416 mi). The relatively cold and dense subducting plates are pulled into the mantle and help drive mantle convection. In collisions between two oceanic plates, the cooler, denser oceanic lithosphere sinks beneath

2784-461: The rise of more buoyant, hot material and can lead to volcanism at the surface and emplacement of plutons in the subsurface. These processes which generate magma are not entirely understood. Where these magmas reach the surface they create volcanic arcs. Volcanic arcs can form as island arc chains or as arcs on continental crust. Three magma series of volcanic rocks are found in association with arcs. The chemically reduced tholeiitic magma series

2842-581: The southern hemisphere, are under-covered. Temporary seismic networks have helped improve tomographic models in regions of particular interest, but typically only collect data for months to a few years. The uneven distribution of earthquakes biases tomographic models towards seismically active regions. Methods that do not rely on earthquakes such as active source surveys or ambient noise tomography have helped image areas with little to no seismicity, though these both have their own limitations as compared to earthquake-based tomography. The type of seismic wave used in

2900-793: The subducting plate tearing off from the upper portion. Tomography can be used to image faults to better understand their seismic hazard . This can be through imaging the fault itself by seeing differences in seismic velocity across the fault boundary or by determining near-surface velocity structure, which can have a large impact on the magnitude on the amplitude of ground-shaking during an earthquake due to site amplification effects . Near-surface velocity structure from tomographic images can also be useful for other hazards, such as monitoring of landslides for changes in near-surface moisture content which has an effect on both seismic velocity and potential for future landslides. Tomographic images of volcanoes have yielded new insights into properties of

2958-523: The types of anomalies. Models based solely on the wave that arrives first naturally prefer faster pathways, causing models based on these data to have lower resolution of slow (often hot) features. This can prove to be a significant issue in areas such as volcanoes where rocks are much hotter than their surroundings and oftentimes partially melted. Shallow models must also consider the significant lateral velocity variations in continental crust. Because seismometers have only been deployed in large numbers since

SECTION 50

#1732772075225

3016-474: The upper few meters below the surface. Tomography is solved as an inverse problem . Seismic data are compared to an initial Earth model and the model is modified until the best possible fit between the model predictions and observed data is found. Seismic waves would travel in straight lines if Earth was of uniform composition, but structural , chemical, and thermal variations affect the properties of seismic waves, most importantly their velocity , leading to

3074-490: The validity of a model, unresolvable uncertainty remains. This contributes to difficulty comparing the validity of different model results. Computing power limits the amount of seismic data, number of unknowns, mesh size, and iterations in tomographic models. This is of particular importance in ocean basins, which due to limited network coverage and earthquake density require more complex processing of distant data. Shallow oceanic models also require smaller model mesh size due to

3132-447: The warmer, less dense oceanic lithosphere. As the slab sinks deeper into the mantle, it releases water from dehydration of hydrous minerals in the oceanic crust. This water reduces the melting temperature of rocks in the asthenosphere and causes partial melting. Partial melt will travel up through the asthenosphere, eventually, reach the surface, and form volcanic island arcs . When oceanic lithosphere and continental lithosphere collide,

3190-426: The wedge. Seafloor topography plays some role in accretion, especially emplacement of igneous crust. [REDACTED] Media related to Subduction at Wikimedia Commons Seismic tomography Seismic tomography or seismotomography is a technique for imaging the subsurface of the Earth using seismic waves . The properties of seismic waves are modified by the material through which they travel. By comparing

3248-473: The whole Earth at several depth ranges, representing an early 3D model. The first model using iterative techniques, which improve upon an initial model in small steps and are required when there are a large number of unknowns, was done in 1984. The model was made possible by iterating upon the first radially anisotropic Earth model , created in 1981. A radially anisotropic Earth model describes changes in material properties, specifically seismic velocity, along

3306-620: Was not developed until the 1970s with the expansion of global seismic networks. Networks like the World-Wide Standardized Seismograph Network were initially motivated by underground nuclear tests , but quickly showed the benefits of their accessible, standardized datasets for geoscience . These developments occurred concurrently with advancements in modeling techniques and computing power that were required to solve large inverse problems and generate theoretical seismograms, which are required to test

3364-552: Was triggered by a megathrust earthquake along the convergent boundary of the Indian plate and Burma microplate and killed over 200,000 people. The 2011 tsunami off the coast of Japan , which caused 16,000 deaths and did US$ 360 billion in damage, was caused by a magnitude 9 megathrust earthquake along the convergent boundary of the Eurasian plate and Pacific plate. Accretionary wedges (also called accretionary prisms ) form as sediment

#224775