scholarly journals Crust and Upper Mantle Inhomogeneities Beneath Western North Island, New Zealand: Evidence from Seismological and Electromagnetic Data.

2021 ◽  
Author(s):  
◽  
Michelle Linda Salmon
Keyword(s):  
Electrical Resistivity ◽  
New Zealand ◽  
Melting Temperature ◽  
Wave Attenuation ◽  
Receiver Function ◽  
Crustal Thickness ◽  
P Wave ◽  
Magnetotelluric Data ◽  
Crust And Upper Mantle ◽  
Southern Limit

<p>Three geophysical techniques have been used to investigate the location and the nature of a large-scale change in crust and uppermost mantle properties below the western North Island of New Zealand. Receiver function analysis reveals a step like change in crustal thickness from ~ 25 km below the northwestern North Island to ≥ 32 km in the southwestern North Island. P-wave attenuation is elevated north of this change in crustal thickness (1000/Qp ≈ 1.9 for α = 0) and is compatible with a wet mantle at near solidus temperatures (T ≈0.97 melting temperature). Attenuation decreases by at least a factor of 2 for the southwestern North Island to values closer to those expected for normal continental lithosphere (1000/Qp ≤ 1 for α = 0). A region of extremely high attenuation (1000/Qp ≈ 5 for α = 0) is observed below the Central Volcanic Region. This value of attenuation is compatible with a wet mantle at temperatures just above melting (T ≈ 1.02 melting temperature). Finally 2D modelling of magnetotelluric data reveals a region of low electrical resistivity (100 Ωm) in the mantle below the region of thinned crust. Like the P-wave attenuation, this region of low resistivity can be explained by a water-saturated mantle at near solidus temperatures (T=0.88-0.97 melting temperature). The changes in crustal thickness, attenuation and electrical resistivity are all coincident with the southern limit of volcanism (~ 39.3°S) at a boundary that runs approximately east-west, perpendicular to the present plate boundary. The only surface expressions of this boundary are the termination of volcanism and the dome-like uplift of the North Island, which has previously been explained by the presence of a buoyant low-density mantle beneath the northwestern North Island. Elevated temperatures and water content inferred from this study are in agreement with this explanation. The sudden transition displayed in all three data sets, but particularly the crustal thickness step seen in the receiver function, calls for a special explanation. Thermal processes are too diffuse to explain the step and instead a mechanical process is called for. One possibility is that the step was created by convective removal of thickened lithosphere.</p>

scholarly journals Crust and Upper Mantle Inhomogeneities Beneath Western North Island, New Zealand: Evidence from Seismological and Electromagnetic Data.

2021 ◽  
Author(s):  
◽  
Michelle Linda Salmon
Keyword(s):  
Electrical Resistivity ◽  
New Zealand ◽  
Melting Temperature ◽  
Wave Attenuation ◽  
Receiver Function ◽  
Crustal Thickness ◽  
P Wave ◽  
Magnetotelluric Data ◽  
Crust And Upper Mantle ◽  
Southern Limit

<p>Three geophysical techniques have been used to investigate the location and the nature of a large-scale change in crust and uppermost mantle properties below the western North Island of New Zealand. Receiver function analysis reveals a step like change in crustal thickness from ~ 25 km below the northwestern North Island to ≥ 32 km in the southwestern North Island. P-wave attenuation is elevated north of this change in crustal thickness (1000/Qp ≈ 1.9 for α = 0) and is compatible with a wet mantle at near solidus temperatures (T ≈0.97 melting temperature). Attenuation decreases by at least a factor of 2 for the southwestern North Island to values closer to those expected for normal continental lithosphere (1000/Qp ≤ 1 for α = 0). A region of extremely high attenuation (1000/Qp ≈ 5 for α = 0) is observed below the Central Volcanic Region. This value of attenuation is compatible with a wet mantle at temperatures just above melting (T ≈ 1.02 melting temperature). Finally 2D modelling of magnetotelluric data reveals a region of low electrical resistivity (100 Ωm) in the mantle below the region of thinned crust. Like the P-wave attenuation, this region of low resistivity can be explained by a water-saturated mantle at near solidus temperatures (T=0.88-0.97 melting temperature). The changes in crustal thickness, attenuation and electrical resistivity are all coincident with the southern limit of volcanism (~ 39.3°S) at a boundary that runs approximately east-west, perpendicular to the present plate boundary. The only surface expressions of this boundary are the termination of volcanism and the dome-like uplift of the North Island, which has previously been explained by the presence of a buoyant low-density mantle beneath the northwestern North Island. Elevated temperatures and water content inferred from this study are in agreement with this explanation. The sudden transition displayed in all three data sets, but particularly the crustal thickness step seen in the receiver function, calls for a special explanation. Thermal processes are too diffuse to explain the step and instead a mechanical process is called for. One possibility is that the step was created by convective removal of thickened lithosphere.</p>

The CIELO Seismic Experiment

2021 ◽  
Author(s):  
Heather A. Ford ◽  
Maximiliano J. Bezada ◽  
Joseph S. Byrnes ◽  
Andrew Birkey ◽  
Zhao Zhu
Keyword(s):  
Receiver Function ◽  
Function Analysis ◽  
P Wave ◽  
Crust And Upper Mantle ◽  
Waveform Data ◽  
Wyoming Craton ◽  
Sensor Orientation ◽  
Seismic Experiment ◽  
Short Period ◽  
Receiver Function Analysis

Abstract The Crust and lithosphere Investigation of the Easternmost expression of the Laramide Orogeny was a two-year deployment of 24 broadband, compact posthole seismometers in a linear array across the eastern half of the Wyoming craton. The experiment was designed to image the crust and upper mantle of the region to better understand the evolution of the cratonic lithosphere. In this article, we describe the motivation and objectives of the experiment; summarize the station design and installation; provide a detailed accounting of data completeness and quality, including issues related to sensor orientation and ambient noise; and show examples of collected waveform data from a local earthquake, a local mine blast, and a teleseismic event. We observe a range of seasonal variations in the long-period noise on the horizontal components (15–20 dB) at some stations that likely reflect the range of soil types across the experiment. In addition, coal mining in the Powder River basin creates high levels of short-period noise at some stations. Preliminary results from Ps receiver function analysis, shear-wave splitting analysis, and averaged P-wave delay times are also included in this report, as is a brief description of education and outreach activities completed during the experiment.

Source mechanism and surface-wave attenuation studies for Tibet earthquake of July 14, 1973

1979 ◽  
Vol 69 (3) ◽  
pp. 737-750
Author(s):  
D. D. Singh ◽  
Harsh K. Gupta
Keyword(s):  
Surface Wave ◽  
Stress Drop ◽  
Wave Attenuation ◽  
Source Parameters ◽  
High Stress ◽  
The Body ◽  
P Wave ◽  
Slip Angle ◽  
Apparent Stress ◽  
Crust And Upper Mantle

abstract Focal mechanism for Tibet earthquake of July 14, 1973 (M = 6.9, mb = 6.0) has been determined using the P-wave first motions, S-wave polarization angles, and surface-wave spectral data. A normal faulting is obtained with a plane having strike N3°W, dip 51°W, and slip angle 81°. The source parameters have been estimated for this event using the body- and surface-wave spectra. The seismic moment, fault length, apparent stress, stress drop, seismic energy release, average dislocation, and fault area are estimated to be 2.96 × 1026 dyne-cm, 27.4 km, 14 bars, 51 bars, 1.4 × 1022 ergs, 157 cm, and 628 km2, respectively. The high stress drop and apparent stress associated with this earthquake indicate that the high stresses are prevailing in this region. The specific quality factor Q is found to vary from 21 to 1162 and 22 to 1110 for Rayleigh and Love waves, respectively. These wide ranges of variation in the attenuation data may be due to the presence of heterogeneity in the crust and upper mantle.

Imaging the Moho and Main Himalayan Thrust beneath the Kumaon Himalaya: constraints from receiver function analysis

2020 ◽  
Vol 224 (2) ◽  
pp. 858-870
Author(s):  
Devajit Hazarika ◽  
Somak Hajra ◽  
Abhishek Kundu ◽  
Meena Bankhwal ◽  
Naresh Kumar ◽  
...  
Keyword(s):  
Poisson’S Ratio ◽  
Receiver Function ◽  
Function Analysis ◽  
Crustal Thickness ◽  
Receiver Functions ◽  
P Wave ◽  
Poisson's Ratio ◽  
Kumaon Himalaya ◽  
Receiver Function Analysis ◽  
Lower Crustal

SUMMARY We analyse P-wave receiver functions across the Kumaon Himalaya and adjoining area to constrain crustal thickness, intracrustal structures and seismic velocity characteristics to address the role of the underlying structure on seismogenesis and geodynamic evolution of the region. The three-component waveforms of teleseismic earthquakes recorded by a seismological network consisting of 18 broad-band seismological stations have been used for receiver function analysis. The common conversion point (CCP) depth migrated receiver function image and shear wave velocity models obtained through inversion show a variation of crustal thickness from ∼38 km in the Indo-Gangetic Plain to ∼42 km near the Vaikrita Thrust. A ramp (∼20°) structure on the Main Himalayan Thrust (MHT) is revealed beneath the Chiplakot Crystalline Belt (CCB) that facilitates the exhumation of the CCB. The geometry of the MHT observed from the receiver function image is consistent with the geometry revealed by a geological balanced cross-section. A cluster of seismicity at shallow to mid-crustal depths is detected near the MHT ramp. The spatial and depth distribution of seismicity pattern beneath the CCB and presence of steep dipping imbricate faults inferred from focal mechanism solutions suggest a Lesser Himalayan Duplex structure in the CCB above the MHT ramp. The study reveals a low-velocity zone (LVZ) with a high Poisson's ratio (σ ∼0.28–0.30) at lower crustal depth beneath the CCB. The high value of Poisson's ratio in the lower crust suggests the presence of fluid/partial melt. The shear heating in the ductile regime and/or decompression and cooling associated with the exhumation of the CCB plausibly created favorable conditions for partial melting in the lower crustal LVZ.

Seismic P-wave receiver function modelling of Archean cratonic crust: A global perspective

2020 ◽  
Author(s):  
Poulami Roy ◽  
Kajaljyoti Borah
Keyword(s):  
Continental Crust ◽  
Receiver Function ◽  
Function Analysis ◽  
Crustal Thickness ◽  
Global Perspective ◽  
P Wave ◽  
Evolution Process ◽  
S Wave ◽  
Economic Significance ◽  
Formation And Evolution

&lt;p&gt;Cratons are representative of the oldest cores of continental crusts. Study of cratons is important &amp;#160;as they preserve the pristine nature of continental crusts as well as they have economic significance as a major source of the world's mineral deposits. The crustal thickness, crustal composition, structure and physical properties of crust-mantle transition (the Moho) are the key parameters for understanding the formation and evolution of continental crust. The ratio of &amp;#160;seismic P-wave and S-wave velocity (Vp/Vs) is used as a parameter to understand the petrologic nature of the Earth's crust. Using these parameters, we address the crustal properties of all Archean cratons. The teleseismic P-wave receiver function analysis reveals that all the Eoarchean (4-3.6 Ga) cratons (Superior, North Atlantic Craton, North China Craton, Yilgarn, Zimbabwe, Kaapvaal) have crustal thickness ranges between 34-42 km and Vp/Vs ratio 1.68-1.79, the Paleoarchean (3.6-3.2 Ga) cratons (Baltic shield, Pilbara, Tanzania, Grunehogna) have 29-52 km crustal thickness and Vp/Vs ratio 1.7-1.85, the Mesoarchean (3.2-2.8 Ga) cratons (Sao Francisco, Guapore, Yangtze, Antananarivo) have 36-53 km thickness and Vp/Vs ratio 1.7-1.9, and Neoarchean (2.8-2.5 Ga) cratons (Guiana, Anabar, Gawler, Napier, Tarim) have 36-59 km thickness and Vp/Vs ratio 1.64-1.95. The nature of crust-mantle transition is overall sharp and flat. &amp;#160;We also found that the crusts which are stabilized earlier, are thinner compared to the later stabilized crusts. Our findings are well-correlated with the craton evolution process predicted by Durrheim and Mooney (1994), where older crusts are thin due to delamination process and relatively younger crusts are thick due to basaltic underplating. Our result of higher Vp/Vs ratio in the relatively younger crusts corroborates with the mafic nature of the crust whereas the older crusts are felsic-intermediate resulting lower Vp/Vs ratio. Our study is unique as it includes most of the global cratons and suggests a global model of continental crust formation and evolution process.&lt;/p&gt;

Structure of the crust and upper mantle beneath the Bransfield Strait (Antarctica) using P-wave Receiver Functions

2020 ◽  
Author(s):  
Joan Antoni Parera-Portell ◽  
Flor de Lis Mancilla ◽  
José Morales ◽  
Javier Almendros
Keyword(s):  
Upper Mantle ◽  
Crustal Thickness ◽  
Receiver Functions ◽  
P Wave ◽  
The South ◽  
Crust And Upper Mantle ◽  
Bransfield Strait ◽  
The Antarctic ◽  
Oceanic Slab ◽  
The Phoenix

&lt;p&gt;&lt;span&gt;The Bransfield Strait is a tectonically active region located between the South Shetland archipelago (SSI) and the Antarctic Peninsula (AP), characterised by the presence of an incipient back-arc spreading ridge driven by on-going slab rollback of the Phoenix plate under the Antarctic and Shetland plates. Twelve broad-band seismic stations deployed in the region are used to obtain P-wave receiver functions from teleseismic earthquakes to improve the current understanding of the crust and upper mantle structures. This includes the depth and spatial variability of the Moho discontinuity, the average crustal Vp/Vs ratio and the thickness of the Mantle Transition Zone (MTZ). Results reveal a highly variable crustal thickness in the South Shetland block, ranging from ~30 km near the SW and NE ends of the South Shetland Trench to ~15 km in the central Bransfield Basin (Deception Island), where the highest Vp&lt;/span&gt; &lt;span&gt;&amp;#8260; Vs ratios in the region are &lt;/span&gt;&lt;span&gt;reached&lt;/span&gt;&lt;span&gt; (&gt; 2). In contrast, the AP displays typical and homogeneous continental crust characteristics with an average crustal thickness of ~34 km and Vp/Vs ~1.77. A low velocity zone (LVZ) is identified under all stations suggesting partial melting in the upper mantle beneath the lithosphere, which is widespread throughout the region and not only confined to the mantle wedge above the subducted Phoenix oceanic slab. There is evidence of magmatic underplating under the SSB in accordance with the LVZ together with the active volcanism and the high crustal Vp/Vs ratio in the area. The Phoenix oceanic slab is inferred to subduct steeply, as the MTZ appears already thickened under the AP. &lt;/span&gt;&lt;/p&gt;

scholarly journals Low-frequency earthquakes reveal punctuated slow slip on the deep extent of the Alpine Fault, New Zealand

2021 ◽  
Author(s):  
Calum Chamberlain ◽  
D Shelly ◽  
John Townend ◽  
Timothy Stern
Keyword(s):  
New Zealand ◽  
Wave Attenuation ◽  
Low Frequency ◽  
P Wave ◽  
Discrete Events ◽  
Exponential Relationship ◽  
Magnitude Distribution ◽  
Alpine Fault ◽  
Seismic Reflectivity ◽  
Frequency Magnitude Distribution

We present the first evidence of low-frequency earthquakes (LFEs) associated with the deep extension of the transpressional Alpine Fault beneath the central Southern Alps of New Zealand. Our database comprises a temporally continuous 36 month-long catalog of 8760 LFEs within 14 families. To generate this catalog, we first identify 14 primary template LFEs within known periods of seismic tremor and use these templates to detect similar events in an iterative stacking and cross-correlation routine. The hypocentres of 12 of the 14 LFE families lie within 10 km of the inferred location of the Alpine Fault at depths of approximately 20-30 km, in a zone of high P-wave attenuation, low P-wave speeds, and high seismic reflectivity. The LFE catalog consists of persistent, discrete events punctuated by swarm-like bursts of activity associated with previously and newly identified tremor periods. The magnitudes of the LFEs range between ML - 0.8 and ML 1.8, with an average of M L 0.5. We find that the frequency-magnitude distribution of the LFE catalog both as a whole and within individual families is not consistent with a power law, but that individual families' frequency-amplitude distributions approximate an exponential relationship, suggestive of a characteristic length-scale of failure. We interpret this LFE activity to represent quasi-continuous slip on the deep extent of the Alpine Fault, with LFEs highlighting asperities within an otherwise steadily creeping region of the fault. © 2014. American Geophysical Union. All Rights Reserved.

scholarly journals Low-frequency earthquakes reveal punctuated slow slip on the deep extent of the Alpine Fault, New Zealand

2021 ◽  
Author(s):  
Calum Chamberlain ◽  
D Shelly ◽  
John Townend ◽  
Timothy Stern
Keyword(s):  
New Zealand ◽  
Wave Attenuation ◽  
Low Frequency ◽  
P Wave ◽  
Discrete Events ◽  
Exponential Relationship ◽  
Magnitude Distribution ◽  
Alpine Fault ◽  
Seismic Reflectivity ◽  
Frequency Magnitude Distribution

We present the first evidence of low-frequency earthquakes (LFEs) associated with the deep extension of the transpressional Alpine Fault beneath the central Southern Alps of New Zealand. Our database comprises a temporally continuous 36 month-long catalog of 8760 LFEs within 14 families. To generate this catalog, we first identify 14 primary template LFEs within known periods of seismic tremor and use these templates to detect similar events in an iterative stacking and cross-correlation routine. The hypocentres of 12 of the 14 LFE families lie within 10 km of the inferred location of the Alpine Fault at depths of approximately 20-30 km, in a zone of high P-wave attenuation, low P-wave speeds, and high seismic reflectivity. The LFE catalog consists of persistent, discrete events punctuated by swarm-like bursts of activity associated with previously and newly identified tremor periods. The magnitudes of the LFEs range between ML - 0.8 and ML 1.8, with an average of M L 0.5. We find that the frequency-magnitude distribution of the LFE catalog both as a whole and within individual families is not consistent with a power law, but that individual families' frequency-amplitude distributions approximate an exponential relationship, suggestive of a characteristic length-scale of failure. We interpret this LFE activity to represent quasi-continuous slip on the deep extent of the Alpine Fault, with LFEs highlighting asperities within an otherwise steadily creeping region of the fault. © 2014. American Geophysical Union. All Rights Reserved.

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