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82-715: The 1,600 kilometres (990 mi) long Macquarie Fault Zone (also known as the Macquarie Ridge , its gazetted name since 2015, the Macquarie Ridge Complex or historically as the Macquarie Fault ) is a major right lateral-moving transform fault along the seafloor of the south Pacific Ocean which runs from New Zealand southwestward towards the Macquarie Triple Junction . It is also the tectonic plate boundary between

164-498: A complex form of the Love wave which, although a surface wave, he found provided a result more closely related to the mb  scale than the M s   scale. Lg waves attenuate quickly along any oceanic path, but propagate well through the granitic continental crust, and Mb Lg is often used in areas of stable continental crust; it is especially useful for detecting underground nuclear explosions. Surface waves propagate along

246-523: A component of convergence which increases as it approaches the South Island of New Zealand where it merges into the Alpine Fault which cuts across the continental crust of New Zealand's South Island. The northern and southern ends of the zone are believed to be converging at between 2 and 4 cm (1.6 in)/year presently. It has been observed that the south western portion of the zone has

328-498: A degree the apparent clash with evidence that the Puysegur subduction initiated at 20 million years ago is less challenging to explain. Compositional analysis on this single sample does not exclude the possibility of later oceanic island type origin even if it seems to fit with other samples from the zone. Seafloor spreading must have ceased by 10 million years ago on the central Macquarie segment containing Macquarie Island. Since then

410-401: A different scaling and zero point. K values in the range of 12 to 15 correspond approximately to M 4.5 to 6. M(K), M (K) , or possibly M K indicates a magnitude M calculated from an energy class K. Earthquakes that generate tsunamis generally rupture relatively slowly, delivering more energy at longer periods (lower frequencies) than generally used for measuring magnitudes. Any skew in

492-420: A given location. Magnitudes are usually determined from measurements of an earthquake's seismic waves as recorded on a seismogram . Magnitude scales vary based on what aspect of the seismic waves are measured and how they are measured. Different magnitude scales are necessary because of differences in earthquakes, the information available, and the purposes for which the magnitudes are used. The Earth's crust

574-409: A junction with another plate boundary, while transcurrent faults may die out without a junction with another fault. Finally, transform faults form a tectonic plate boundary, while transcurrent faults do not. Faults in general are focused areas of deformation or strain , which are the response of built-up stresses in the form of compression , tension, or shear stress in rock at the surface or deep in

656-701: A lowercase " l ", either M l , or M l . (Not to be confused with the Russian surface-wave MLH scale. ) Whether the values are comparable depends on whether the local conditions have been adequately determined and the formula suitably adjusted. In Japan, for shallow (depth < 60 km) earthquakes within 600 km, the Japanese Meteorological Agency calculates a magnitude labeled MJMA , M JMA , or M J . (These should not be confused with moment magnitudes JMA calculates, which are labeled M w (JMA) or M , nor with

738-571: A magnitude of at least M w 8.0 On 23 December 2004 a M w 8.1 event occurred about 150 km (93 mi) to the west of the plate boundary. This event to the west of the plate boundary was close to the transition from oblique subduction at the Puysegur Trough to strike slip, with some compression, on the Macquarie Ridge. It is believed these large events are because the transition to oblique convergence with subduction

820-584: A major central part of the plate convergent structures south of New Zealand that has become to be termed the Macquarie Ridge Complex as the geology has been better characterised. The northern part of this complex is an extension of New Zealand 's Alpine Fault which becomes the Puysegur Trench and ridge to its east. New Zealand is continental crust and the northern parts of the complex are associated with oceanic crust subducting under

902-466: A smaller section is also present in the Tasman District in the island's northwest. Other examples include: Seismic magnitude scales#Mw Seismic magnitude scales are used to describe the overall strength or "size" of an earthquake . These are distinguished from seismic intensity scales that categorize the intensity or severity of ground shaking (quaking) caused by an earthquake at

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984-418: A transform fault links a spreading center and the upper block of a subduction zone or where two upper blocks of subduction zones are linked, the transform fault itself will grow in length. [REDACTED] [REDACTED] Constant length: In other cases, transform faults will remain at a constant length. This steadiness can be attributed to many different causes. In the case of ridge-to-ridge transforms,

1066-432: A wave, such as its timing, orientation, amplitude, frequency, or duration. Additional adjustments are made for distance, kind of crust, and the characteristics of the seismograph that recorded the seismogram. The various magnitude scales represent different ways of deriving magnitude from such information as is available. All magnitude scales retain the logarithmic scale as devised by Charles Richter , and are adjusted so

1148-399: Is "approximately related to the released seismic energy." Intensity refers to the strength or force of shaking at a given location, and can be related to the peak ground velocity. With an isoseismal map of the observed intensities (see illustration) an earthquake's magnitude can be estimated from both the maximum intensity observed (usually but not always near the epicenter ), and from

1230-401: Is actually a surface-wave magnitude. Other magnitude scales are based on aspects of seismic waves that only indirectly and incompletely reflect the force of an earthquake, involve other factors, and are generally limited in some respect of magnitude, focal depth, or distance. The moment magnitude scale – Mw or M w – developed by seismologists Thomas C. Hanks and Hiroo Kanamori ,

1312-510: Is based on an earthquake's seismic moment , M 0 , a measure of how much work an earthquake does in sliding one patch of rock past another patch of rock. Seismic moment is measured in Newton-meters (Nm or N·m ) in the SI system of measurement, or dyne-centimeters (dyn-cm; 1 dyn-cm = 10 Nm ) in the older CGS system. In the simplest case the moment can be calculated knowing only

1394-507: Is being created to change that length. [REDACTED] [REDACTED] Decreasing length faults: In rare cases, transform faults can shrink in length. These occur when two descending subduction plates are linked by a transform fault. In time as the plates are subducted, the transform fault will decrease in length until the transform fault disappears completely, leaving only two subduction zones facing in opposite directions. [REDACTED] [REDACTED] The most prominent examples of

1476-435: Is constantly created through the upwelling of new basaltic magma . With new seafloor being pushed and pulled out, the older seafloor slowly slides away from the mid-oceanic ridges toward the continents. Although separated only by tens of kilometers, this separation between segments of the ridges causes portions of the seafloor to push past each other in opposing directions. This lateral movement of seafloors past each other

1558-504: Is dissipated as friction (resulting in heating of the crust). An earthquake's potential to cause strong ground shaking depends on the comparatively small fraction of energy radiated as seismic waves, and is better measured on the energy magnitude scale, M e . The proportion of total energy radiated as seismic waves varies greatly depending on focal mechanism and tectonic environment; M e   and M w   for very similar earthquakes can differ by as much as 1.4 units. Despite

1640-639: Is located about 4.5 km (2.8 mi) east of the Australian Pacific plate boundary the transform of this to the north of the island is a 15 km (9.3 mi) wide zone of closely spaced NNE- to NE-trending faults while to the south of the island north of the island the zone narrows to as little as 8 km (5.0 mi) with NNE to NNW-trending faults. The fault zone is up to 40 km (25 mi) wide, elsewhere and it often has parallel eastern and western ridge crests, as well as multiple nearby fracture zones. The zone has in historic times been

1722-512: Is measured at periods of up to 30 seconds. The regional mb Lg scale – also denoted mb_Lg , mbLg , MLg (USGS), Mn , and m N – was developed by Nuttli (1973) for a problem the original M L scale could not handle: all of North America east of the Rocky Mountains . The M L scale was developed in southern California, which lies on blocks of oceanic crust, typically basalt or sedimentary rock, which have been accreted to

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1804-518: Is not particularly seismologically active presently. So while further studies take place the term Australian plate will be used to simplify the tectonics as of the present and this view is not inconsistent with the latest reference frames used in recent plate modelling. The oceanic crust of the Macquarie Fault Zone as found at Macquarie Island was generated however at the divergent Australian-Pacific plate boundary following break-up between

1886-556: Is postulated that because the lithosphere would cool and strengthen usually with mid oceanic ridge formation, that if such tectonic inversion is not rapid, it does not lead to the relative simplicity seen at say the Kermadec-Tonga subduction zone . This is not the case with the Macquarie ridge. The localised to segments transpression and compression seen in the Macquarie Ridge Complex took about 10 to 15 million years. The process

1968-708: Is putting the area of what has been called the Macquarie Block under significant strain that is eventually relieved by major failure reactivating past transform faults in the oceanic crust of the Tasman sea floor. The basalts which are oceanic in the zone have been dated. To the south east of the Puysegur Trench a seamount is as young as 1.59 ± 0.26 million years. It is not yet known for sure if this relatively recent date could be related to late mid ocean ridge spreading or back arc activity. The recent dating of

2050-546: Is related to the Macquarie Triple Junction which was created about 47.91 million years ago. The Macquarie Ridge is a very long, but incompletely studied extinct mid-ocean ridge system which has since experienced complex tectonic processes over about 30 million years that have differed down its length as it became the Australian–Pacific plate boundary. Not all timings derived from different methodologies concord as well as they do for easier to study tectonic structures. It

2132-436: Is stressed by tectonic forces. When this stress becomes great enough to rupture the crust, or to overcome the friction that prevents one block of crust from slipping past another, energy is released, some of it in the form of various kinds of seismic waves that cause ground-shaking, or quaking. Magnitude is an estimate of the relative "size" or strength of an earthquake , and thus its potential for causing ground-shaking. It

2214-441: Is summarised as: This last stage has been difficult to characterise as the usual approach through magnetic anomaly studies only allowed a good time for the central McDougall and Macquarie segments spreading ceasing by 24 million years ago and either end was only constrained to the 30 million years ago estimate. It is now known to have occurred differentially, with cessation of sea floor spreading happening at 25.9 million years ago in

2296-994: Is the San Andreas Fault on the Pacific coast of the United States. The San Andreas Fault links the East Pacific Rise off the West coast of Mexico (Gulf of California) to the Mendocino Triple Junction (Part of the Juan de Fuca plate ) off the coast of the Northwestern United States , making it a ridge-to-transform-style fault. The formation of the San Andreas Fault system occurred fairly recently during

2378-527: Is the mantle magnitude scale, M m . This is based on Rayleigh waves that penetrate into the Earth's mantle, and can be determined quickly, and without complete knowledge of other parameters such as the earthquake's depth. M d designates various scales that estimate magnitude from the duration or length of some part of the seismic wave-train. This is especially useful for measuring local or regional earthquakes, both powerful earthquakes that might drive

2460-435: Is where transform faults are currently active. Transform faults move differently from a strike-slip fault at the mid-oceanic ridge. Instead of the ridges moving away from each other, as they do in other strike-slip faults, transform-fault ridges remain in the same, fixed locations, and the new ocean seafloor created at the ridges is pushed away from the ridge. Evidence of this motion can be found in paleomagnetic striping on

2542-611: The Australian plate to the northwest and the Pacific plate to the southeast. As such it is a region of high seismic activity and recorded the largest strike-slip event on record up to 23 May 1989, of at least M w 8.0 The Macquarie Fault Zone/Ridge extends south of New Zealand on the South Pacific Ocean 's seafloor except where it surfaces as Macquarie Island . Some of it is on Tasman Sea sea floor, north of

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2624-506: The Campbell Plateau and Resolution Ridge between the middle Eocene about 40 million years ago and what was formally thought to be the late Miocene. However this cessation of seafloor spreading generally propagated from south at 25.9 million years ago to north along the ridge where it is now known that it might have stopped as recently as 1.6 million years ago although an earlier date remains possible. Many researchers conclude that

2706-705: The Oligocene Period between 34 million and 24 million years ago. During this period, the Farallon plate , followed by the Pacific plate, collided into the North American plate . The collision led to the subduction of the Farallon plate underneath the North American plate. Once the spreading center separating the Pacific and the Farallon plates was subducted beneath the North American plate,

2788-567: The Shindo intensity scale .) JMA magnitudes are based (as typical with local scales) on the maximum amplitude of the ground motion ; they agree "rather well" with the seismic moment magnitude M w   in the range of 4.5 to 7.5, but underestimate larger magnitudes. Body-waves consist of P-waves that are the first to arrive (see seismogram), or S-waves , or reflections of either. Body-waves travel through rock directly. The original "body-wave magnitude" – mB or m B (uppercase "B") –

2870-474: The adakitic Little Solander Island volcanics to the zone's far east to about 50,000 years ago suggests back arc activity from the subducted oceanic crust in the northern part of the zone may not have ceased. A northern sea mount of the McDougall Fault Zone is 10.9 ± 0.26 million years old while at the southern end there is one dated at 18.34 ± 0.53 million years old. A sea mount at the north of

2952-420: The Earth's subsurface. Transform faults specifically accommodate lateral strain by transferring displacement between mid-ocean ridges or subduction zones. They also act as the plane of weakness, which may result in splitting in rift zones . Transform faults are commonly found linking segments of divergent boundaries ( mid-oceanic ridges or spreading centres). These mid-oceanic ridges are where new seafloor

3034-562: The Earth's surface, and are principally either Rayleigh waves or Love waves . For shallow earthquakes the surface waves carry most of the energy of the earthquake, and are the most destructive. Deeper earthquakes, having less interaction with the surface, produce weaker surface waves. The surface-wave magnitude scale, variously denoted as Ms , M S , and M s , is based on a procedure developed by Beno Gutenberg in 1942 for measuring shallow earthquakes stronger or more distant than Richter's original scale could handle. Notably, it measured

3116-461: The Earth's surface. Geophysicist and geologist John Tuzo Wilson recognized that the offsets of oceanic ridges by faults do not follow the classical pattern of an offset fence or geological marker in Reid's rebound theory of faulting , from which the sense of slip is derived. The new class of faults, called transform faults, produce slip in the opposite direction from what one would surmise from

3198-646: The IASPEI in 1967; this is the basis of the standardized M s20 scale (Ms_20, M s (20)). A "broad-band" variant ( Ms_BB , M s (BB) ) measures the largest velocity amplitude in the Rayleigh-wave train for periods up to 60 seconds. The M S7 scale used in China is a variant of M s calibrated for use with the Chinese-made "type 763" long-period seismograph. The MLH scale used in some parts of Russia

3280-538: The Pacific plate interacting with an area of the Indo-Australian plate termed the Macquarie microplate that has rotated independent of the Australian plate starting 6.24 million years ago. The best fit modelling of recent tectonics requires this microplate and there is evidence for a diffuse zone of deformation associated with compression between this microplate and the Australian plate but this diffuse zone

3362-695: The San Andreas Continental Transform-Fault system was created. In New Zealand , the South Island 's Alpine Fault is a transform fault for much of its length. This has resulted in the folded land of the Southland Syncline being split into an eastern and western section several hundred kilometres apart. The majority of the syncline is found in Southland and The Catlins in the island's southeast, but

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3444-452: The amount of slip, the area of the surface ruptured or slipped, and a factor for the resistance or friction encountered. These factors can be estimated for an existing fault to determine the magnitude of past earthquakes, or what might be anticipated for the future. An earthquake's seismic moment can be estimated in various ways, which are the bases of the M wb , M wr , M wc , M ww , M wp , M i , and M wpd scales, all subtypes of

3526-426: The amplitude of surface waves (which generally produce the largest amplitudes) for a period of "about 20 seconds". The M s   scale approximately agrees with M L   at ~6, then diverges by as much as half a magnitude. A revision by Nuttli (1983) , sometimes labeled M Sn , measures only waves of the first second. A modification – the "Moscow-Prague formula" – was proposed in 1962, and recommended by

3608-556: The body-wave (mb ) or the seismic energy (M e  ) is there a difference comparable to the difference in damage. Rearranged and adapted from Table 1 in Choy, Boatwright & Kirby 2001 , p. 13. Seen also in IS 3.6 2012 , p. 7. K (from the Russian word класс, 'class', in the sense of a category ) is a measure of earthquake magnitude in the energy class or K-class system, developed in 1955 by Soviet seismologists in

3690-456: The constancy is caused by the continuous growth by both ridges outward, canceling any change in length. The opposite occurs when a ridge linked to a subducting plate, where all the lithosphere (new seafloor) being created by the ridge is subducted, or swallowed up, by the subduction zone. Finally, when two upper subduction plates are linked there is no change in length. This is due to the plates moving parallel with each other and no new lithosphere

3772-534: The continent. East of the Rockies the continent is a craton , a thick and largely stable mass of continental crust that is largely granite , a harder rock with different seismic characteristics. In this area the M L scale gives anomalous results for earthquakes which by other measures seemed equivalent to quakes in California. Nuttli resolved this by measuring the amplitude of short-period (~1 sec.) Lg waves,

3854-631: The continental crust of Zealandia . This north Macquarie Ridge Complex has to its west the Resolution Ridge which separates the oceanic crust of the north and south Tasman basins and defines the western margin of the Macqurie Block. The southern part of the Macquarie Ridge Complex is the Hjort Trench which also has a ridge to its east. Some authors split the central part of the Macquarie Ridge Complex into two central segments where

3936-485: The continental crust. All these problems prompted the development of other scales. Most seismological authorities, such as the United States Geological Survey , report earthquake magnitudes above 4.0 as moment magnitude (below), which the press describes as "Richter magnitude". Richter's original "local" scale has been adapted for other localities. These may be labelled "ML", or with

4018-399: The correlation can be reversed to predict tidal height from earthquake magnitude. (Not to be confused with the height of a tidal wave, or run-up , which is an intensity effect controlled by local topography.) Under low-noise conditions, tsunami waves as little as 5 cm can be predicted, corresponding to an earthquake of M ~6.5. Another scale of particular importance for tsunami warnings

4100-561: The different relative heights of the abutting plates as well as the component of compression between the plates. The namesake Macquarie Island , named after Lachlan Macquarie lies atop a segment of the Macquarie Ridge. The Australia-Pacific plate boundary is now understood to be along the crest of the ridges rather than in the troughs with a zone of deformation up to 100 km (62 mi) wide. This central area has had up to 290 km (180 mi) right lateral displacement since ocean floor spreading ceased at about 10 million years ago. In

4182-505: The duration of shaking. This is why, in the 1989 Loma Prieta earthquake , the Marina district of San Francisco was one of the most damaged areas, though it was nearly 100 km from the epicenter. Geological structures were also significant, such as where seismic waves passing under the south end of San Francisco Bay reflected off the base of the Earth's crust towards San Francisco and Oakland. A similar effect channeled seismic waves between

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4264-401: The extent of the area where the earthquake was felt. The intensity of local ground-shaking depends on several factors besides the magnitude of the earthquake, one of the most important being soil conditions. For instance, thick layers of soft soil (such as fill) can amplify seismic waves, often at a considerable distance from the source, while sedimentary basins will often resonate, increasing

4346-647: The far south subduction is possibly initiating at the Hjort Trench. However this subduction has been described as atypical as lighter oceanic crust generated from the Southeast Indian Ridge is subducted under heavier oceanic crust from the extinct spreading center of the Australia-Pacific plate, where as normal subduction has the heavier component going under. This regions previous tectonic evolution has also been studied in detail as it

4428-442: The fault changes from a normal fault with extensional stress to a strike-slip fault with lateral stress. In the study done by Bonatti and Crane, peridotite and gabbro rocks were discovered in the edges of the transform ridges. These rocks are created deep inside the Earth's mantle and then rapidly exhumed to the surface. This evidence helps to prove that new seafloor is being created at the mid-oceanic ridges and further supports

4510-415: The fault zone here is an incipient subduction zone, with oblique motion corresponding to the transition from lateral (strike-slip) motion. In the area known as the Puysegur Trench , the Indo-Australian plate appears to be starting to sink beneath the Pacific plate, the reverse of what is occurring off of New Zealand's North Island (see Kermadec-Tonga Subduction Zone ). The Macquarie Ridge represents both

4592-470: The generic M w scale. See Moment magnitude scale § Subtypes for details. Seismic moment is considered the most objective measure of an earthquake's "size" in regard of total energy. However, it is based on a simple model of rupture, and on certain simplifying assumptions; it does not account for the fact that the proportion of energy radiated as seismic waves varies among earthquakes. Much of an earthquake's total energy as measured by M w  

4674-525: The lateral offset between segments of divergent boundaries , forming a zigzag pattern. This results from oblique seafloor spreading where the direction of motion is not perpendicular to the trend of the overall divergent boundary. A smaller number of such faults are found on land, although these are generally better-known, such as the San Andreas Fault and North Anatolian Fault . Transform boundaries are also known as conservative plate boundaries because they involve no addition or loss of lithosphere at

4756-781: The line between the south of the Auckland Islands and Tasmania . To its west on the sea floor is the South East Tasman Basin and to its east the Solander Trough and Emerald Basin. Technically the plate boundary becomes part of the Emerald Fracture Zone to the south of the Hjort Trench, so the zone does not quite reach the Macquarie Triple Junction. The Macquarie Fault Zone term is also used often now to refer to

4838-400: The maximum amplitude of the ground shaking, without distinguishing the different seismic waves. They underestimate the strength: The original "Richter" scale, developed in the geological context of Southern California and Nevada, was later found to be inaccurate for earthquakes in the central and eastern parts of the continent (everywhere east of the Rocky Mountains ) because of differences in

4920-523: The measurement procedures and equations for the principal magnitude scales, M L  , M s  , mb , mB  and mb Lg  . The first scale for measuring earthquake magnitudes, developed in 1935 by Charles F. Richter and popularly known as the "Richter" scale, is actually the Local magnitude scale , label ML or M L . Richter established two features now common to all magnitude scales. All "Local" (ML) magnitudes are based on

5002-846: The mid-oceanic ridge transform zones are in the Atlantic Ocean between South America and Africa . Known as the St. Paul, Romanche , Chain, and Ascension fracture zones, these areas have deep, easily identifiable transform faults and ridges. Other locations include: the East Pacific Ridge located in the South Eastern Pacific Ocean , which meets up with San Andreas Fault to the North. Transform faults are not limited to oceanic crust and spreading centers; many of them are on continental margins . The best example

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5084-513: The mid-range approximately correlates with the original "Richter" scale. Most magnitude scales are based on measurements of only part of an earthquake's seismic wave-train, and therefore are incomplete. This results in systematic underestimation of magnitude in certain cases, a condition called saturation . Since 2005 the International Association of Seismology and Physics of the Earth's Interior (IASPEI) has standardized

5166-505: The other continent. In his work on transform-fault systems, geologist Tuzo Wilson said that transform faults must be connected to other faults or tectonic-plate boundaries on both ends; because of that requirement, transform faults can grow in length, keep a constant length, or decrease in length. These length changes are dependent on which type of fault or tectonic structure connect with the transform fault. Wilson described six types of transform faults: Growing length: In situations where

5248-402: The other major faults in the area. An earthquake radiates energy in the form of different kinds of seismic waves , whose characteristics reflect the nature of both the rupture and the earth's crust the waves travel through. Determination of an earthquake's magnitude generally involves identifying specific kinds of these waves on a seismogram , and then measuring one or more characteristics of

5330-630: The remote Garm ( Tajikistan ) region of Central Asia; in revised form it is still used for local and regional quakes in many states formerly aligned with the Soviet Union (including Cuba). Based on seismic energy (K = log E S , in Joules ), difficulty in implementing it using the technology of the time led to revisions in 1958 and 1960. Adaptation to local conditions has led to various regional K scales, such as K F and K S . K values are logarithmic, similar to Richter-style magnitudes, but have

5412-453: The ridge is to the west and the trench is to the east. Slightly confusingly the 5,500 m (18,000 ft) deep McDougall Trough is the trench to the east of the ridge of the Macquarie Fault Zone, with the other northern fault zone to the troughs north being named the McDougall Fault Zone. The trough has a step over basin to the Macquarie Fault Zone which commences where the Jurru fault zone in

5494-407: The same location, but twice as deep and on a different kind of fault, was felt over a broad area, injured over 300 people, and destroyed or seriously damaged over 10,000 houses. As can be seen in the table below, this disparity of damage done is not reflected in either the moment magnitude (M w  ) nor the surface-wave magnitude (M s  ). Only when the magnitude is measured on the basis of

5576-444: The seafloor. A paper written by geophysicist Taras Gerya theorizes that the creation of the transform faults between the ridges of the mid-oceanic ridge is attributed to rotated and stretched sections of the mid-oceanic ridge. This occurs over a long period of time with the spreading center or ridge slowly deforming from a straight line to a curved line. Finally, fracturing along these planes forms transform faults. As this takes place,

5658-522: The segments accommodated the two plates convergence by oceanic-oceanic subduction at the southern Hjort segment and at the northern Puysegur segment and by predominant strike-slip motion in the two central segments of the Macquarie Ridge structure. Maps of this tectonic activity have been modelled from 42 million years ago to the present showing the evolving spreading center, its extinction and fault zone relationships. Transform fault Most such faults are found in oceanic crust , where they accommodate

5740-507: The seismometer off-scale (a problem with the analog instruments formerly used) and preventing measurement of the maximum wave amplitude, and weak earthquakes, whose maximum amplitude is not accurately measured. Even for distant earthquakes, measuring the duration of the shaking (as well as the amplitude) provides a better measure of the earthquake's total energy. Measurement of duration is incorporated in some modern scales, such as M wpd   and mB c  . M c scales usually measure

5822-399: The smaller definition of the Macquarie Fault Zone is 14.1 ± 1.2 million years old and one beyond Macquarie Island to its south is 25.82 ± 0.18 million years old. Macquarie Island itself has multiple ages in the relatively limited range from 10.34 to 10 million years ago. A seamount near the eastern far end of the Hjort Trench is 25.90 ± 0.23 million years old. The Macquarie Fault Zone includes

5904-423: The source of some very large earthquakes and up until 1993 at least 20 over M w 6.5 had been recorded. The currently seismically active area is the whole of the zone and parts to the west of its northern aspects in the northern South East Tasman Basin. However both west of Macquarie Island and to its east towards the southern tip of the Campbell Plateau there has been some historic earthquake activity outside

5986-406: The south and possibly as recently as 1.6 million years ago near New Zealand. However it is known from elsewhere that basalt eruptions can continue for up to 3.5 million years after spreading stops so sea floor spreading in the north actually likely stopped sometime between 5 million and 1.6 million years ago if the single seamount sampled to date in this region is of mid oceanic ridge origin. And so to

6068-440: The southern Tasman Basin intercepts the plate boundaries. In the middle of the Macquarie Fault Zone is Macquarie Island . Its rocks like that of all the zone are mid-ocean ridge basalt or ocean island basalt . The central part of Macquarie Ridge Complex has a maximum height of 433 m (1,421 ft) at Macquarie Island but the troughs to the east drop to 6,000 m (20,000 ft) below sea level. At Macquarie Island which

6150-429: The spectral distribution can result in larger, or smaller, tsunamis than expected for a nominal magnitude. The tsunami magnitude scale, M t , is based on a correlation by Katsuyuki Abe of earthquake seismic moment (M 0  ) with the amplitude of tsunami waves as measured by tidal gauges. Originally intended for estimating the magnitude of historic earthquakes where seismic data is lacking but tidal data exist,

6232-634: The standard interpretation of an offset geological feature. Slip along transform faults does not increase the distance between the ridges it separates; the distance remains constant in earthquakes because the ridges are spreading centers. This hypothesis was confirmed in a study of the fault plane solutions that showed the slip on transform faults points in the opposite direction than classical interpretation would suggest. Transform faults are closely related to transcurrent faults and are commonly confused. Both types of fault are strike-slip or side-to-side in movement; nevertheless, transform faults always end at

6314-698: The standardized mB BB   scale. The mb or m b scale (lowercase "m" and "b") is similar to mB , but uses only P-waves measured in the first few seconds on a specific model of short-period seismograph. It was introduced in the 1960s with the establishment of the World-Wide Standardized Seismograph Network (WWSSN); the short period improves detection of smaller events, and better discriminates between tectonic earthquakes and underground nuclear explosions. Measurement of mb  has changed several times. As originally defined by Gutenberg (1945c) m b

6396-408: The theory of plate tectonics. Active transform faults are between two tectonic structures or faults. Fracture zones represent the previously active transform-fault lines, which have since passed the active transform zone and are being pushed toward the continents. These elevated ridges on the ocean floor can be traced for hundreds of miles and in some cases even from one continent across an ocean to

6478-474: The usefulness of the M e   scale, it is not generally used due to difficulties in estimating the radiated seismic energy. Two earthquakes differing greatly in the damage done In 1997 there were two large earthquakes off the coast of Chile. The magnitude of the first, in July, was estimated at M w  6.9, but was barely felt, and only in three places. In October a M w  7.1 quake in nearly

6560-515: The zone. In the Hjort Trench region the plate boundary is definitely under the stress condition of transpression as earthquake focal mechanisms are both thrust and dextral strike-slip types as found in such areas. To the south of the Hjort Trench there is relatively low grade seismic activity in the Emerald Fault Zone and towards the Macquarie Triple Junction. The largest strike-slip event till then on record occurred May 23, 1989, and had

6642-436: Was based on the maximum amplitude of waves in the first 10 seconds or more. However, the length of the period influences the magnitude obtained. Early USGS/NEIC practice was to measure mb  on the first second (just the first few P-waves ), but since 1978 they measure the first twenty seconds. The modern practice is to measure short-period mb  scale at less than three seconds, while the broadband mB BB   scale

6724-496: Was developed by Gutenberg 1945c and Gutenberg & Richter 1956 to overcome the distance and magnitude limitations of the M L   scale inherent in the use of surface waves. mB  is based on the P- and S-waves, measured over a longer period, and does not saturate until around M 8. However, it is not sensitive to events smaller than about M 5.5. Use of mB  as originally defined has been largely abandoned, now replaced by

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