A microearthquake study of the plate boundary, North Island, New Zealand
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The seismicity, structure and tectonics of the Indian/Pacific plate boundary in the North Island of New Zealand have been studied by means of a microearthquake traverse oriented in the direction of dip of the subducted Pacific plate and extending for about 210 km. The geometry of the top of the Pacific plate is inferred from a band of concentrated microearthquake activity approximately 10 km thick which is identified with the crust of the plate. The Pacific plate has two knee-like bends, one where the top of the plate is about 25 km deep, the other below the volcanic front, where the plate is about 70 km deep. The shallower bend and subsequent restraightening of the plate are related to phase changes in the plate, the deeper bend to volcanism. Composite focal mechanisms indicate that seaward of the shallower bend the Pacific plate is being loaded by the Indian plate, whereas landward of this bend the Pacific plate is sinking under its own weight.Keywords:
Pacific Plate
Microearthquake
Convergent boundary
Eurasian Plate
North American Plate
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Composite plate
Microearthquake
Hypocenter
Eurasian Plate
Passive seismic
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Abstract With a small fraction of marginal subduction zones, the driving mechanism for the North American plate motion is in debate. We construct global mantle flow models simultaneously constrained by geoid and plate motions to investigate the driving forces for the North American plate motion. By comparing the model with only near‐field subducting slabs and that with global subducting slabs, we find that the contribution to the motion of the North American plate from the near‐field Aleutian, central American, and Caribbean slabs is small. In contrast, other far‐field slabs, primarily the major segments around western Pacific subduction margins, provide the dominant large‐scale driving forces for the North American plate motion. The coupling between far‐field slabs and the North American plate suggests a new form of active plate interactions within the global self‐organizing plate tectonic system. We further evaluate the extremely slow seismic velocity anomalies associated with the shallow partial melt around the southwestern North America. Interpreting these negative seismic shear‐velocity anomalies as purely thermal origin generates considerably excessive resistance to the North American plate motion. A significantly reduced velocity‐to‐density scaling for these negative seismic shear‐velocity anomalies must be incorporated into the construction of the buoyancy field to predict the North American plate motion. We also examine the importance of lower mantle buoyancy including the ancient descending Kula‐Farallon plates and the active upwelling below the Pacific margin of the North American plate. Lower mantle buoyancy primarily affects the amplitudes, as opposed to the patterns of both North American and global plate motions.
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North American Plate
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A tectonic state of a locked subduction is considered to be a possible source of a future interplate earthquake. Discriminating an actually locked state to verify its extent is therefore essential in constructing an accurate prospect against the forthcoming earthquake. Micorearthquake seismicity is an effective tool for such an analysis because it is considered to be a faithful indicator of the stress state, and is expected to exhibit a characteristic pattern in the area where the locked state in the subduction appears with a certain stress concentration. Focusing on the microearthquake seismicity around the Tokai district in central Japan, where a large interplate earthquake is feared to occur, we tried to identify such an area of locked subduction on the Philippine Sea plate, possibly related to the future earthquake. We investigated the microearthquake seismicity from various perspectives. First, the hypocenter distribution was analyzed to identify the extent of the locked area. The characteristic profile of the distribution was presumed to represent a stress concentrated area induced from the mechanical contact between both plates. The second approach is to interpret stress patterns reflected in focal mechanisms. The locked state was recognized and verified by a comparison of the P-axis distribution pattern with that expected from a model imaging a partially locked subduction. The third approach is to monitor the temporal change of the seismic wave spectrum. Analyzing predominant frequencies of P and S waves and monitoring their changes for a period of 10 years, we found a trend of gradual increase common to both waves. This means an increase of stress drop in microfracturings, and in its turn implies accumulation of stress around the focus area. The rate of the stress change converted from the frequency change was compared with the result derived from a numerical simulation. The simulation, performed on the basis of a constitutive friction law for a stick sliding on the plate interface, computed a changing rate of the maximum shear stress around the locked zone and showed its spatial variation along the subduction axis. Thus the simulated result indicated a certain compatibility with the observed one. Although ambiguities and uncertainties still exist in the study, all the results derived here seem to indicate an identical conclusion that the plate subduction is actually locked in this region at present.
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<p>The seismicity, structure and tectonics of the North Island plate boundary have been studied by means of a microearthquake traverse oriented in the direction of dip of the subducted Pacific plate and stretching from southern Hawke's Bay to northern Taranaki. The geometry of the top of the Pacific plate is inferred from a band of concentrated microearthquake activity which can be identified with the crust of the plate. The Pacific plate appears to have two knee-like bends, one between the east coast and the Ruahine Range, where the top of the plate is about 25 km deep, the other below the volcanic front, where it is about 70 km deep. The shallower bend and subsequent restraightening of the plate can be related to phase changes in the plate, while the deeper bend can be related to volcanism. Composite focal mechanisms indicate that seaward of its shallower bend the Pacific plate is being loaded by the Indian plate, whereas landward of this bend the Pacific plate is sinking under its own weight. Both composite focal mechanisms and the distribution of microseismicity in the Pacific plate suggest the existence of a major discontinuity striking down the dip of the plate and passing beneath the Tongariro volcanic centre. A conspicuous lack of microseismicity in the Indian plate in the eastern North Island revealed in this study can be related to the plates being unlocked in this region. A feature of the seismicity of the Indian plate in the region of the Wanganui Basin is the concentration of activity in the 25-42 km depth range, shallower activity being largely confined to the northeast edge of the basin, near Mt Ruapehu and Waiouru. Composite focal mechanisms suggest the 25-42 km deep activity reflects stresses set up by locking and unlocking of the plates, while the shallower activity reflects local stresses related to volcanic phenomena.</p>
Pacific Plate
Microearthquake
Eurasian Plate
Convergent boundary
North American Plate
Composite plate
Slab window
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<p>The seismicity, structure and tectonics of the North Island plate boundary have been studied by means of a microearthquake traverse oriented in the direction of dip of the subducted Pacific plate and stretching from southern Hawke's Bay to northern Taranaki. The geometry of the top of the Pacific plate is inferred from a band of concentrated microearthquake activity which can be identified with the crust of the plate. The Pacific plate appears to have two knee-like bends, one between the east coast and the Ruahine Range, where the top of the plate is about 25 km deep, the other below the volcanic front, where it is about 70 km deep. The shallower bend and subsequent restraightening of the plate can be related to phase changes in the plate, while the deeper bend can be related to volcanism. Composite focal mechanisms indicate that seaward of its shallower bend the Pacific plate is being loaded by the Indian plate, whereas landward of this bend the Pacific plate is sinking under its own weight. Both composite focal mechanisms and the distribution of microseismicity in the Pacific plate suggest the existence of a major discontinuity striking down the dip of the plate and passing beneath the Tongariro volcanic centre. A conspicuous lack of microseismicity in the Indian plate in the eastern North Island revealed in this study can be related to the plates being unlocked in this region. A feature of the seismicity of the Indian plate in the region of the Wanganui Basin is the concentration of activity in the 25-42 km depth range, shallower activity being largely confined to the northeast edge of the basin, near Mt Ruapehu and Waiouru. Composite focal mechanisms suggest the 25-42 km deep activity reflects stresses set up by locking and unlocking of the plates, while the shallower activity reflects local stresses related to volcanic phenomena.</p>
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Convergent boundary
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Abstract The configuration of mid-ocean ridges subducted below North America prior to Oligocene time is unconstrained by seafloor isochrons and has been primarily inferred from upper-plate geology, including near-trench magmatism. However, many tectonic models are permitted from these constraints. We present a fully kinematic, plate tectonic reconstruction of the NW Cordillera since 60 Ma built by structurally unfolding subducted slabs, imaged by mantle tomography, back to Earth’s surface. We map in three-dimensions the attached Alaska and Cascadia slabs, and a detached slab below western Yukon (Canada) at 400–600 km depth that we call the “Yukon Slab.” Our restoration of these lower plates within a global plate model indicates the Alaska slab accounts for Pacific-Kula subduction since ca. 60 Ma below the Aleutian Islands whereas the Cascadia slab accounts for Farallon subduction since at least ca. 75 Ma below southern California, USA. However, intermediate areas show two reconstruction gaps that persist until 40 Ma. We show that these reconstruction gaps correlate spatiotemporally to published NW Cordillera near-trench magmatism, even considering possible terrane translation. We attribute these gaps to thermal erosion related to ridge subduction and model mid-ocean ridges within these reconstruction gap mid-points. Our reconstructions show two coeval ridge-trench intersections that bound an additional “Resurrection”-like plate along the NW Cordillera prior to 40 Ma. In this model, the Yukon slab represents a thermally eroded remnant of the Resurrection plate. Our reconstructions support a “northern option” Farallon ridge geometry and allow up to ∼1200 km Chugach terrane translation since Paleocene time, providing a new “tomographic piercing point” for the Baja-British Columbia debate.
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In order to evaluate past and present earthquake activity along the East Anatolian Fault between Karliova and Maras, the distribution in time and space of earthquakes was investigated. The investigation dealt with an interval of 75 years for largemagnitude earthquakes, and with a period of 105 days for microearthquakes occurring in a limited area. The most active regions of the fault zone are around Bingöl in the north, the Lake Hazar in the middle, and Adiyaman in the south. Along the fault zone the average Richter magnitude of earthquakes is about 5, the hypocentral depth about 25 kilometers, the period of occurrence (determined by spectral analysis) about 11 years, and the b-value for the magnitude range of 7>M>4 approximately 0.5. The microearthquake recordings around Lake Hazar indicated a daily average of 5 events with magnitudes less than 3. The composite fault plane solutions confirmed a lefthand strikcslip motion for most of the secondary faults. The average daily energy release of 1016 erg and explicit alignment of strain release contours with the fault are some of the indications of the continuous tectonic activity in this region even in this quiet period. An 11 and 27 days periodicity of microearthquake activities has been obtained from spectral analysis. A close correspondence in epicentral distribution exists between major earthquakes and the present microearthquakes.
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Fault plane
Seismotectonics
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North Anatolian Fault
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The horizontal velocities of 111 stations selected at random from 432 stations issued by IERS under the ITRF reference frame in 2001 are used to obtain the velocity field of the plate movement around the globe. The movements of Eurasian Plate(EA), North American(NA) and North Pacifica Plate(NP) in the north Hemisphere are shown by the velocity field, taking the Atlantic Ridge as axis, the EA moves northeastwards, eastwards and then southeastwards; the NA moves northwestwards, westwards and then southwestwards; so they show “sheep horn like” rifting movement between each other, the northward movements of the mantle shown by them across the North Pole to the Aleus Arc. The velocity of the NP moving to the NWW is about 6 times as much as those of the EA, NA etc; the collisional relationship between the NP and EA is shown by the deep subduction of the oceanic plate and thrusting of the continental plate; and the relationship between the NP and NA is the transtension shown by the movement of the San Andrews strike slip Fault which is the boundary between them. The plate movements of South American Plate (SA), African Plate (AF), Arabian Plate (AR), Indian—Australian Plate (IN—AU) and Southeast Pacific Plate (SP) in the south Hemisphere are also obtained: taking the southeast Pacific ridge as axis, the Nazcar Plate(NZ) to the east of the axis is moving by high speed to the east, however the SA, AF, AR and IN—AU to the east of the NZ are moving to the NE—NNE, their velocities become faster and faster eastwards; these phenomena may result from converging and opposite converging movements of the mantle imposed by the rifting of the Atlantic Ridge, Indian Ridge and Red Ocean Ridge among them. So strong collision and strike slip movements occur between the southwestern part of Pacific and the east and northeast edges of the AU; and the movements between the NZ and NA show the form of the “run and catch” type. The mantle under the Antarctica Plate (AP) is moving southwards from peri Antarctic Ridge in the southeastern Pacific Ocean, continuing moving to the north across the South Pole, then to Africa and the Indian Ocean, which also shows somewhat dextral rotation. The plate movements mentioned above also indicate the disharmony state of movements between the north and south hemispheres; there are 13 large en echelon active faults in this disharmony zone. Now, the latitudinal component of movement of the EA, NA and NP in the north hemisphere is more significant. However, except the SP, several large continental plates in the south hemisphere are moving to the northeast north northeast, namely the longitudinal component of movement is more important. And the Equator fault zone separates these two types of movements. Moreover, the mantle in the south and north hemispheres does not only flow along latitude and longitude separately; so the problems about the combined latitudinal and longitudinal mantle flow, the controls on the mantle flow and the four tectonic systems around the globe by the double asymmetry of the north/south hemispheres and the 0°/180°hemispheres need to be considered.
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African Plate
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Convergent boundary
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The seismicity, structure and tectonics of the Indian/Pacific plate boundary in the North Island of New Zealand have been studied by means of a microearthquake traverse oriented in the direction of dip of the subducted Pacific plate and extending for about 210 km. The geometry of the top of the Pacific plate is inferred from a band of concentrated microearthquake activity approximately 10 km thick which is identified with the crust of the plate. The Pacific plate has two knee-like bends, one where the top of the plate is about 25 km deep, the other below the volcanic front, where the plate is about 70 km deep. The shallower bend and subsequent restraightening of the plate are related to phase changes in the plate, the deeper bend to volcanism. Composite focal mechanisms indicate that seaward of the shallower bend the Pacific plate is being loaded by the Indian plate, whereas landward of this bend the Pacific plate is sinking under its own weight.
Pacific Plate
Microearthquake
Convergent boundary
Eurasian Plate
North American Plate
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Composite plate
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