scholarly journals 3D Seismic Traveltime Tomography of the  Central South Island, New Zealand

2021 ◽  
Author(s):  
◽  
Nicolas Eduard Alype Brikke
Keyword(s):  
Solution Model ◽  
The South ◽  
Low Velocity ◽  
3D Tomography ◽  
Alpine Fault ◽  
Australian Plate ◽  
Flexural Analysis ◽  
Model Features ◽  
Central South ◽  
Velocity Zone

<p>The three-dimensional (3D) evolution of the Australian-Pacifi c late boundary in the central South Island of New Zealand is investigated by analysing seismic data from the South Island GeopHysical Transect (SIGHT) project and by using a novel 3D tomography inversion method, FMTOMO. A 380 km-long, 350 km-wide and 56 km-deep 3D tomography image of the P-wave velocity structure and interface geometry of the crust and upper-mantle is constructed by inverting for 164,048 traveltime picks. The picks are both coincident (in-line) and oblique (cross-line) to the survey geometry. The traveltime picks and station elevations were static corrected and reduced to basement level, respectively, to eliminate the highly variable sedimentary component of the inversion process. Synthetic testing of the model space was carried out to help the interpretation of the solution model features. Some model features are consistent with previous results. Usual crustal velocities (5.5 km/s close to the surface and 6.3 km/s at the bottom of the crust) are found at distal ends of the collision zone. Lower velocities (5.7 km/s) intrude the mid-crust of the Australian plate to depths of about 20 km, which is consistent with the downward  flexure of the Australian plate. A low velocity zone (5.9 - 6.1 km/s) is situated to the southeast of the Alpine fault, which is consistent with the Alpine fault low velocity zone. Furthermore, a high-velocity body (6.3 km/s) is observed in the top 10 km of the upper-crust immediately above the thickened crust between the west coast of the South Island and the Main Divide of the Southern Alps. This body is interpreted as a drier, more rigid body of schist. A zone of low velocity (5.8 km/s reaching 8 km depth) is observed immediately to the southeast of the aforementioned high velocity body. The feature is interpreted as a back-shearing faulting structure through which fluid escape towards the surface. A flexural analysis of an apparent  flexure profile of the Australian Plate along SIGHT line 01 yielded a  flexural parameter, a, of 89 km, an elastic thickness, Te, of 14 km and a  flexural rigidity, D, of 1.5 : 10^(23) N.m. These results are consistent with results of a  flexural analysis of SIGHT line 02W [Harrison 1999]. The following features are derived from the solution model. An apparent gradient in uppermantle anisotropy is observed with seismic velocities increasing towards the south of the model. Also, the geometry of the Mohorovicic discontinuity is apparently smooth between the two main SIGHT transects. The tomography method used in this project proves to be complementary to other coarser-scale and finer-scale seismic studies of the region in that it brings out features that were not seen by them. Notwithstanding that the interface inversion process remains to be perfected in the software, the velocity inversion produced a satisfactory solution model.</p>

scholarly journals 3D Seismic Traveltime Tomography of the  Central South Island, New Zealand

2021 ◽  
Author(s):  
◽  
Nicolas Eduard Alype Brikke
Keyword(s):  
Solution Model ◽  
The South ◽  
Low Velocity ◽  
3D Tomography ◽  
Alpine Fault ◽  
Australian Plate ◽  
Flexural Analysis ◽  
Model Features ◽  
Central South ◽  
Velocity Zone

<p>The three-dimensional (3D) evolution of the Australian-Pacifi c late boundary in the central South Island of New Zealand is investigated by analysing seismic data from the South Island GeopHysical Transect (SIGHT) project and by using a novel 3D tomography inversion method, FMTOMO. A 380 km-long, 350 km-wide and 56 km-deep 3D tomography image of the P-wave velocity structure and interface geometry of the crust and upper-mantle is constructed by inverting for 164,048 traveltime picks. The picks are both coincident (in-line) and oblique (cross-line) to the survey geometry. The traveltime picks and station elevations were static corrected and reduced to basement level, respectively, to eliminate the highly variable sedimentary component of the inversion process. Synthetic testing of the model space was carried out to help the interpretation of the solution model features. Some model features are consistent with previous results. Usual crustal velocities (5.5 km/s close to the surface and 6.3 km/s at the bottom of the crust) are found at distal ends of the collision zone. Lower velocities (5.7 km/s) intrude the mid-crust of the Australian plate to depths of about 20 km, which is consistent with the downward  flexure of the Australian plate. A low velocity zone (5.9 - 6.1 km/s) is situated to the southeast of the Alpine fault, which is consistent with the Alpine fault low velocity zone. Furthermore, a high-velocity body (6.3 km/s) is observed in the top 10 km of the upper-crust immediately above the thickened crust between the west coast of the South Island and the Main Divide of the Southern Alps. This body is interpreted as a drier, more rigid body of schist. A zone of low velocity (5.8 km/s reaching 8 km depth) is observed immediately to the southeast of the aforementioned high velocity body. The feature is interpreted as a back-shearing faulting structure through which fluid escape towards the surface. A flexural analysis of an apparent  flexure profile of the Australian Plate along SIGHT line 01 yielded a  flexural parameter, a, of 89 km, an elastic thickness, Te, of 14 km and a  flexural rigidity, D, of 1.5 : 10^(23) N.m. These results are consistent with results of a  flexural analysis of SIGHT line 02W [Harrison 1999]. The following features are derived from the solution model. An apparent gradient in uppermantle anisotropy is observed with seismic velocities increasing towards the south of the model. Also, the geometry of the Mohorovicic discontinuity is apparently smooth between the two main SIGHT transects. The tomography method used in this project proves to be complementary to other coarser-scale and finer-scale seismic studies of the region in that it brings out features that were not seen by them. Notwithstanding that the interface inversion process remains to be perfected in the software, the velocity inversion produced a satisfactory solution model.</p>

scholarly journals Fault Zone Guided Wave generation on the locked, late interseismic Alpine Fault, New Zealand

2021 ◽  
Author(s):  
JD Eccles ◽  
AK Gulley ◽  
PE Malin ◽  
CM Boese ◽  
John Townend ◽  
...  
Keyword(s):  
Fault Zone ◽  
Plate Boundary ◽  
Guided Wave ◽  
S Wave ◽  
Low Velocity ◽  
Catchment Scale ◽  
Near Surface ◽  
Low Velocity Zone ◽  
Alpine Fault ◽  
Velocity Zone

© 2015. American Geophysical Union. All Rights Reserved. Fault Zone Guided Waves (FZGWs) have been observed for the first time within New Zealand's transpressional continental plate boundary, the Alpine Fault, which is late in its typical seismic cycle. Ongoing study of these phases provides the opportunity to monitor interseismic conditions in the fault zone. Distinctive dispersive seismic codas (~7-35Hz) have been recorded on shallow borehole seismometers installed within 20m of the principal slip zone. Near the central Alpine Fault, known for low background seismicity, FZGW-generating microseismic events are located beyond the catchment-scale partitioning of the fault indicating lateral connectivity of the low-velocity zone immediately below the near-surface segmentation. Initial modeling of the low-velocity zone indicates a waveguide width of 60-200m with a 10-40% reduction in S wave velocity, similar to that inferred for the fault core of other mature plate boundary faults such as the San Andreas and North Anatolian Faults.

S-wave residuals in Canada

1980 ◽  
Vol 70 (3) ◽  
pp. 809-822
Author(s):  
A. J. Wickens ◽  
G. G. R. Buchbinder
Keyword(s):  
The United States ◽  
P Wave ◽  
The South ◽  
S Wave ◽  
Low Velocity ◽  
The West ◽  
Long Period ◽  
Seismograph Network ◽  
Velocity Region ◽  
Velocity Zone

abstract Relative S-wave station residuals have been determined over the Canadian long-period seismograph network and have been augmented by S residuals to the south into the United States taken from Poupinet (1977). The thick craton of the Canadian Shield is outlined by the S-residual contours with an inferred rapid thinning of the craton to the south except for a narrow central extension into Kentucky and Missouri. A comparison of these S residuals with P-wave residuals of Buchbinder and Poupinet (1977) reveals a low-velocity region in the west, a high-velocity zone centrally, and an area in the east where the S arrivals are late while the P arrivals are early.

scholarly journals Fault Zone Guided Wave generation on the locked, late interseismic Alpine Fault, New Zealand

2021 ◽  
Author(s):  
JD Eccles ◽  
AK Gulley ◽  
PE Malin ◽  
CM Boese ◽  
John Townend ◽  
...  
Keyword(s):  
Fault Zone ◽  
Plate Boundary ◽  
Guided Wave ◽  
S Wave ◽  
Low Velocity ◽  
Catchment Scale ◽  
Near Surface ◽  
Low Velocity Zone ◽  
Alpine Fault ◽  
Velocity Zone

© 2015. American Geophysical Union. All Rights Reserved. Fault Zone Guided Waves (FZGWs) have been observed for the first time within New Zealand's transpressional continental plate boundary, the Alpine Fault, which is late in its typical seismic cycle. Ongoing study of these phases provides the opportunity to monitor interseismic conditions in the fault zone. Distinctive dispersive seismic codas (~7-35Hz) have been recorded on shallow borehole seismometers installed within 20m of the principal slip zone. Near the central Alpine Fault, known for low background seismicity, FZGW-generating microseismic events are located beyond the catchment-scale partitioning of the fault indicating lateral connectivity of the low-velocity zone immediately below the near-surface segmentation. Initial modeling of the low-velocity zone indicates a waveguide width of 60-200m with a 10-40% reduction in S wave velocity, similar to that inferred for the fault core of other mature plate boundary faults such as the San Andreas and North Anatolian Faults.

Partial melting and the low-velocity zone

1970 ◽  
Vol 4 (1) ◽  
pp. 62-64 ◽  
Author(s):  
Don L. Anderson ◽  
Hartmut Spetzler
Keyword(s):  
Partial Melting ◽  
Low Velocity ◽  
Low Velocity Zone ◽  
Velocity Zone

Nature of the low velocity zone in Cascadia from receiver function waveform inversion

2012 ◽  
Vol 337-338 ◽  
pp. 25-38 ◽  
Author(s):  
Ralf T.J. Hansen ◽  
Michael G. Bostock ◽  
Nikolas I. Christensen
Keyword(s):  
Receiver Function ◽  
Waveform Inversion ◽  
Low Velocity ◽  
Low Velocity Zone ◽  
Velocity Zone

Focal mechanisms of earthquakes related to the 29 November 1975 Kalapana, Hawaii, earthquake: The effect of structure models

1981 ◽  
Vol 71 (3) ◽  
pp. 713-729 ◽  
Author(s):  
R. S. Crosson ◽  
E. T. Endo
Keyword(s):  
Main Shock ◽  
Horizontal Layer ◽  
Focal Mechanisms ◽  
Local Network ◽  
The South ◽  
Low Velocity ◽  
Tectonic Model ◽  
Low Velocity Layer ◽  
Using Data ◽  
Velocity Layer

abstract Initial focal mechanism determinations for the 29 November 1975 Kalapana, Hawaii, earthquake indicated discrepancy between the mechanism determined from teleseismic data by Ando and the mechanism determined using data from the local U.S. Geological Survey network surrounding the epicenter region. The resolution of this difference is crucial to correctly understand this earthquake, as well as to understand the tectonics of the south flank of Kilauea volcano. When a model with a low-velocity layer at the base of the crust is used for projection back to the focal sphere for the local network mechanisms, the discrepancy vanishes. To further investigate this result, focal mechanisms were determined using several contrasting models for a set of well-recorded earthquakes. A large number of these earthquakes have mechanisms identical to the main shock when the low-velocity layer model is used. Dispersion of P and T axes is also minimized by use of this model. A low-angle slip direction, favored for the main shock and typical of most other solutions, exhibits remarkable stability normal to the east rift zone of Kilauea. Our results suggest a tectonic model, similar in nature to that proposed by Ando, in which the south flank of Kilauea consists of a mobile block of crust which is relatively free to move laterally on a low-strength zone at about 10 km depth. Forceful injection of magma along the rift zones provides the loading stress which is released by catastrophic failure in the weak, horizontal layer in a cycle of perhaps 100 yr.

scholarly journals Delineation of Weathered Layer Using Uphole and Surface Seismic Refraction Methods in Parts of Niger Delta, Nigeria

2021 ◽  
Vol 26 (1) ◽  
pp. 58-66
Author(s):  
Mfoniso Aka ◽  
Okechukwu Agbasi
Keyword(s):  
Niger Delta ◽  
Average Velocity ◽  
Seismic Refraction ◽  
Low Velocity ◽  
Engineering Structures ◽  
Experimental Proof ◽  
Seismic Surveys ◽  
Shot Point ◽  
Weathered Layer ◽  
Velocity Zone

Uphole and surface seismic refraction surveys were carried out in parts of the Niger Delta, Nigeria, to delineate weathering thickness and velocity associated with aweathered layer. A total of twelve uphole and surface seismic refraction surveyswere shot, computed and analyzed. The velocity of the uphole seismic refraction ranged from 344.8 to 680.3 m/s with a thickness of 5.45 to 13.35 m. Surface seismic refraction ranged from 326.6 to 670.2 m/s and 4.30 to 12.0 m, respectively. The average velocity and thickness ranged from 559.6 to 548.0 m/s and 9.43 to 8.63m with differences of 11.6 m/s and 0.83 m respectively. The VW/VS ratios ranged from 0.955 to 1.059. This indicates that the uphole velocity is higher than the surface refraction velocity leading to low VW/VS values. This is a direct experimental proof of a low velocity zone, confirming the weathered nature of the area. The results of both refraction methods are reliable; the differences in surface refraction values are due to shot point offsets. Based on these findings, it is recommended that shots for seismic surveys should be located above 15.0 m in the area to delineate the effects associated with weathered layers to ensure that will be competent to withstand engineering structures.  

scholarly journals Paleomagnetic Rotation Study of Woodlark-Australia plate motions in the Woodlark Rift, SE Papua New Guinea

2021 ◽  
Author(s):  
◽  
Elizabeth Ann Cairns
Keyword(s):  
Papua New Guinea ◽  
Vertical Axis ◽  
The South ◽  
Clockwise Rotation ◽  
Plate Motions ◽  
Continental Extension ◽  
Woodlark Basin ◽  
Australian Plate ◽  
Anticlockwise Rotation ◽  
Oceanic Spreading

<p>The Woodlark Rift in SE Papua New Guinea is a continental rift to the west of active oceanic spreading in the Woodlark Basin, which separates the Australian Plate to the south from the relatively anticlockwise rotating Woodlark Plate to the north. During Pliocene to Recent times the Woodlark Rift has been the setting for rapid exhumation of the world’s youngest UHP rocks (Baldwin et al., 2004, 2008; Gordon et al, 2012; Little et al., 2011), and is currently one of few places on the globe where active continental breakup is occurring ahead of a propagating oceanic spreading centre. While the Woodlark Basin contains a record of oceanic spreading since ˜6 Ma (Taylor et al., 1999), and GPS data describe present-day crustal motions (Wallace et al., manuscript in review), the Neogene temporal and kinematic evolution of continental extension in the Woodlark Rift is less well constrained. We compare Characteristic magnetization directions for six formations, Early Miocene (˜20 Ma) to Late Pliocene (3 ± 0.5), with contemporaneous expected field directions corresponding to Australian Plate paleomagnetic pole locations. We interpret declination anomalies (at 95% confidence) to estimate finite vertical-axis rotations of crustal blocks with respect to a fixed Australian Plate. Temporal and spatial relationships between declination anomalies for six formation mean directions, across four paleomagnetic localities, provide new evidence to constrain aspects of the Miocene to Recent history of the Woodlark Rift.  We obtained 250 oriented core samples from Miocene to Pliocene aged rocks at four localities in the Woodlark Rift. Components of Characteristic Remanent Magnetization (ChRM) have been determined from step-wise thermal and alternating field demagnetization profiles of >300 individual specimens. A total of 157 ChRM components contribute to the calculation of representative paleomagnetic directions for six formations, which have undergone vertical-axis rotations with respect to the Australian Plate associated with development of the Woodlark Rift.  Pliocene volcanic rocks at two key localities near the northern extent of the rift record that: 1) The Amphlett Islands has experienced 10.1 ± 7.6° of anticlockwise rotation since 3 ± 0.5 Ma; 2) NW Normanby Island has undergone a 16.3 ± 9.5° clockwise rotation during the same time interval. Sedimentary rocks at Cape Vogel Peninsula on the northern coast of the mainland Papuan Peninsula, record variable anticlockwise finite rotations of 28.4 ± 10.9° and 12.4 ± 5.5° for Early and Middle Miocene rocks respectively, in contrast to a younger clockwise rotation of 6.5 ± 11.2° for Late Miocene rocks. At the Suau Coast locality, on the south eastern coast of the Papuan Peninsula, Late Miocene dikes record 22.7 ± 13.3° of anticlockwise rotation.  At the Amphlett Islands and NW Normanby localities paleomagnetic data are consistent with current GPS plate motions, suggesting the current kinematics in the rift were established by at least ˜3 Ma. The Amphlett Islands result is consistent with the rate of Pliocene sea floor spreading in the Woodlark Basin, suggesting that locality can be considered as fully on the Woodlark Plate. The clockwise rotation indicated at NW Normanby Island may record development of an incipient dextral transfer fault within an active part of the Woodlark Rift.  Time-varying declination anomalies from the Cape Vogel Peninsula suggest that rifting began there by ˜15 Ma, 7 Ma earlier than previously inferred based on stratigraphic evidence. Furthermore, paleomagnetic data from the south coast of the Papuan Peninsula suggests that early rifting extended further south, and has since contracted to where continental extension is currently accommodated north of the Papuan Peninsula.</p>

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