Modelling Pn Wave Speeds Beneath the Central North Island, New Zealand

<p>A new method of modelling Pn-wave speeds is created. The method allows the predominant wavelength features of P-wave speeds in the uppermost mantle to be modelled, as well as estimating values of mantle anisotropy and irregularities in the crust beneath stations, using least-square collocation. A combination of National Network seismometers, local volcanic seismic monitoring networks and temporary deployments are used to collect arrival times from local events, during the period of 1990-2006. The dataset consists of approximately 11200 Pn observations from 3000 local earthquakes at 91 seismograph sites. The resulting model shows distinct variations in uppermost mantle Pn velocities. Velocities of less than 7.5 km/s are found beneath the back-arc extension region of the Central Volcanic Region, and under the Taranaki Volcanic Region, indicating the presence of water and partial melt. The region to the east shows extremely high velocities of 8.3-8.5 km/s, where the P-waves are traveling within the subducting Pacific slab. Slightly lower than normal mantle velocities of 7.8-8.1 km/s are found in the western North Island, suggesting a soft mantle. Pn anisotropy estimates throughout the North Island show predominately trench parallel fast directions, ceasing to nulls in the west. Anisotropy measurements indicate the strain history of the mantle. For the observed upper mantle Pn velocity of 7.3 km/s is one of the lowest seen in the world. Ray-tracing modelling indicate that this region extends to depths of at least 65 km, suggesting an area of elevated heat (700 - 1100 degrees C) at Moho depth. Elevated temperatures can be caused by the presence partial melt (0.4 % to 2.1 % depending on the amount of water present). Beneath the western North Island, the observed slower than normal mantle velocities, indicate a material of lowered shear modulus, susceptible to strain deformation. However, anisotropy estimations in this region, show no significant anisotropy, suggesting that this is a region of young mantle that hasn't had time to take up the signature of deformation. These observations can be explained by a detachment of the mantle lithosphere through a Rayleigh-Taylor instability more than 5 Ma.</p>

Modelling Pn Wave Speeds Beneath the Central North Island, New Zealand

<p>A new method of modelling Pn-wave speeds is created. The method allows the predominant wavelength features of P-wave speeds in the uppermost mantle to be modelled, as well as estimating values of mantle anisotropy and irregularities in the crust beneath stations, using least-square collocation. A combination of National Network seismometers, local volcanic seismic monitoring networks and temporary deployments are used to collect arrival times from local events, during the period of 1990-2006. The dataset consists of approximately 11200 Pn observations from 3000 local earthquakes at 91 seismograph sites. The resulting model shows distinct variations in uppermost mantle Pn velocities. Velocities of less than 7.5 km/s are found beneath the back-arc extension region of the Central Volcanic Region, and under the Taranaki Volcanic Region, indicating the presence of water and partial melt. The region to the east shows extremely high velocities of 8.3-8.5 km/s, where the P-waves are traveling within the subducting Pacific slab. Slightly lower than normal mantle velocities of 7.8-8.1 km/s are found in the western North Island, suggesting a soft mantle. Pn anisotropy estimates throughout the North Island show predominately trench parallel fast directions, ceasing to nulls in the west. Anisotropy measurements indicate the strain history of the mantle. For the observed upper mantle Pn velocity of 7.3 km/s is one of the lowest seen in the world. Ray-tracing modelling indicate that this region extends to depths of at least 65 km, suggesting an area of elevated heat (700 - 1100 degrees C) at Moho depth. Elevated temperatures can be caused by the presence partial melt (0.4 % to 2.1 % depending on the amount of water present). Beneath the western North Island, the observed slower than normal mantle velocities, indicate a material of lowered shear modulus, susceptible to strain deformation. However, anisotropy estimations in this region, show no significant anisotropy, suggesting that this is a region of young mantle that hasn't had time to take up the signature of deformation. These observations can be explained by a detachment of the mantle lithosphere through a Rayleigh-Taylor instability more than 5 Ma.</p>

New thermal constraints for the Anatolian lithosphere from Curie depth point and Pn tomography

&lt;p&gt;&lt;span&gt;Continental deformation can be &lt;/span&gt;&lt;span&gt;described in two end-member approaches: &lt;/span&gt;&lt;span&gt;&lt;em&gt;block&lt;/em&gt;&lt;/span&gt;&lt;span&gt; (or microplate) and &lt;/span&gt;&lt;span&gt;&lt;em&gt;continuum &lt;/em&gt;&lt;/span&gt;&lt;span&gt;models&lt;/span&gt;&lt;span&gt;. The first considers a strong lithosphere with deformation localized in fault zones. For &lt;/span&gt;&lt;span&gt;t&lt;/span&gt;&lt;span&gt;he latter, however,&lt;/span&gt;&lt;span&gt; the lithosphere is weak&lt;/span&gt;&lt;span&gt; and deforms as a thin viscous sheet. The Anatolia &amp;#8211; Aegean domain represents both continuum and plate-like deformation. Furthermore, &lt;/span&gt;&lt;span&gt;r&lt;/span&gt;ecent modeling studies suggest a dynamic support mechanism of the Anatolian plateaus, with dynamic topography estimates ranging from 1 to 3 km for various crustal models and geodynamic scenarios, although the gravity and crustal thickness data support predominant Airy isostasy. The solution to both intricacies relies on the thermal structure of the crust and the lithosphere. Available thermal considerations stem from either the uppermost mantle velocity structure or thermal modeling with assumptions on radiogenic heat production and boundary conditions. Yet, homogeneous and independent constraints on the lithospheric structure are scarce. We aim to contribute to this knowledge gap by providing Curie Point Depths (CPDs), which corresponds to the depth at which rock-forming minerals lose their magnetization at the Curie temperature, ~580 &lt;sup&gt;o&lt;/sup&gt;C.&lt;br&gt;&lt;br&gt;R&lt;span&gt;esolution of deep magnetic sources requires spectral methods with large windows, which reduce the CPD resolution. Moving &amp; overlapping smaller windows have been used in order to increase the resolution, but these introduce spectral leakage and bias. &lt;/span&gt;In previous studies, subjective wavenumber ranges of the magnetic anomaly spectra were used, often combined with wrong scaling factors between map units and the equations. This resulted in generally erroneous CPD estimates. Furthermore, CPD uncertainties have often been unquantified for the study area. &lt;span&gt;We use a wavelet transform method, which overcomes the artifacts due to segmentation of magnetic signal to finite windows, results in higher spatial resolution as well as enabling uncertainty estimation. &lt;/span&gt;We used as large an area as possible for constraining the edge effects away from the study area. The resultant CPD map spatially correlates well with low Pn velocity areas, locations of volcanoes, and thermal springs.&lt;/p&gt;

Seismic velocity and anisotropy of the uppermost mantle beneath Madagascar from Pn tomography

SUMMARY The lithosphere of Madagascar records a long series of tectonic processes. Structures initially inherited from the Pan-African Orogeny are overprinted by a series of extensional tectonic and magmatic events that began with the breakup of Gondwana and continued through to the present. Here, we present a Pn-tomography study in which Pn traveltimes are inverted to investigate the lateral variation of the seismic velocity and anisotropy within the uppermost mantle beneath Madagascar. Results show that the Pn velocities within the uppermost mantle vary by ±0.30 km s–1 about a mean of 8.10 km s–1. Low-Pn-velocity zones (&lt;8.00 km s–1) are observed beneath the Cenozoic alkaline volcanic provinces in the northern and central regions. They correspond to thermally perturbed zones, where temperatures are estimated to be elevated by ∼100–300 K. Moderately low Pn velocities are found near the southern volcanic province and along an E–W belt in central Madagascar. This belt is located at the edge of a broader low S-velocity anomaly in the mantle imaged in a recent surface wave tomographic study. High-Pn-velocity zones (&gt;8.20 km s–1) coincide with stable and less seismically active regions. The pattern of Pn anisotropy is very complex, with small-scale variations in both the amplitude and the fast-axis direction, and generally reflects the complicated tectonic history of Madagascar. Pn anisotropy and shear wave (SKS) splitting measurements show good correlations in the southern parts of Madagascar, indicating coherency in the vertical distribution of lithospheric deformation along Pan-African shear zone as well as coupling between the crust and mantle when the shear zones were active. In most other regions, discrepancies between Pn anisotropy and SKS measurements suggest that the seismic anisotropy in the uppermost mantle beneath Madagascar differs from the vertically integrated upper mantle anisotropy, implying a present-day vertical partitioning of the deformation. Pn anisotropy directions lack the coherent pattern expected for an incipient plate boundary within Madagascar proposed in some kinematic models of the region.

Regional site response and uppermost mantle seismic structure of central and eastern united states

This dissertation examines seismological data from regional earthquake sources in order to examine the seismological character of the crust and uppermost mantle in central and eastern United States. Firstly, site amplification of regional highfrequency Lg seismic phases is estimate ed using a Reverse-Two Station (RTS) method. RTS results show topography and sediment thickness are likely to affect amplification and both factors likely frequency-dependent. There is a negative correlation between the RTS-measured amplification and shallow shear-wave velocity. It appears that both regional topography (i.e., long-wavelength topography) and deeper subsurface seismic structures (basins and sediments) have a large impact on site amplification. Subsequently, Pn and Sn travel time tomography is used to estimate the upper most mantle P-wave (Pn) velocity, S-wave (Sn) velocity, and the velocity ratio (VPn/VSn). In addition to velocity, effective attenuation of Sn phase (Q[superscript -1]sn) is also measured. The result shows regions of high velocity such as southern Georgia, eastern South Carolina and NMSZ and low Q[subscript Sn] values. The V[subscript Pn]/V[subscript Sn] ratio shows values higher than the average in regions such as the Mississippi Embayment, New England, and south Appalachian. V[subscript Pn]/V[subscript Sn] ratios are lower than the average in regions such as northwestern CEUS, South Georgia and eastern Texas. We estimated the uppermost mantle temperature by applying a constrained grid-search algorithm includes the observed V[subscript Sn], V[subscript Pn] and Q[subscript Sn] with the calculated velocities of specific compositional models. The uppermost mantle temperature result, [about]300-500C, beneath the northern mid-continent, and the highest temperature, 1100 C, beneath New England

Kinematics of the outer halo of M 87 as mapped by planetary nebulae

Aims. We present a kinematic study of a sample of 298 planetary nebulas (PNs) in the outer halo of the central Virgo galaxy M 87 (NGC 4486). The line-of-sight velocities of these PNs are used to identify subcomponents, to measure the angular momentum content of the main M 87 halo, and to constrain the orbital distribution of the stars at these large radii. Methods. We use Gaussian mixture modelling to statistically separate distinct velocity components and identify the M 87 smooth halo component, its unrelaxed substructures, and the intra-cluster (IC) PNs. We compute probability weighted velocity and velocity dispersion maps for the smooth halo, and its specific angular momentum profile (λR) and velocity dispersion profile. Results. The classification of the PNs into smooth halo and ICPNs is supported by their different PN luminosity functions. Based on a Kolmogorov–Smirnov (K–S) test, we conclude that the ICPN line-of-sight velocity distribution (LOSVD) is consistent with the LOSVD of the galaxies in Virgo subcluster A. The surface density profile of the ICPNS at 100 kpc radii has a shallow logarithmic slope, −αICL ≃ −0.8, dominating the light at the largest radii. Previous B − V colour and resolved star metallicity data indicate masses for the ICPN progenitor galaxies of a few ×108 M⊙. The angular momentum-related λR profile for the smooth halo remains below 0.1, in the slow rotator regime, out to 135 kpc average ellipse radius (170 kpc major axis distance). Combining the PN velocity dispersion measurements for the M 87 halo with literature data in the central 15 kpc, we obtain a complete velocity dispersion profile out to Ravg = 135 kpc. The σhalo profile decreases from the central 400 km s−1 to about 270 km s−1 at 2–10 kpc, then rises again to ≃300 ± 50 km s−1 at 50–70 kpc, to finally decrease sharply to σhalo ∼ 100 km s−1 at Ravg = 135 kpc. The steeply decreasing outer σhalo profile and the surface density profile of the smooth halo can be reconciled with the circular velocity curve inferred from assuming hydrostatic equilibrium for the hot X-ray gas. Because this rises to νc,X ∼ km s−1 at 200 kpc, the orbit distribution of the smooth M 87 halo is required to change strongly from approximately isotropic within Ravg ∼ 60 kpc to very radially anisotropic at the largest distances probed. Conclusions. The extended LOSVD of the PNs in the M 87 halo allows the identification of several subcomponents: the ICPNs, the “crown” accretion event, and the smooth M 87 halo. In galaxies like M 87, the presence of these subcomponents needs to be taken into account to avoid systematic biases in estimating the total enclosed mass. The dynamical structure inferred from the velocity dispersion profile indicates that the smooth halo of M 87 steepens beyond Ravg = 60 kpc and becomes strongly radially anisotropic, and that the velocity dispersion profile is consistent with the X-ray circular velocity curve at these radii without non-thermal pressure effects.