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

2021 ◽  
Author(s):  
◽  
Anya Mira Seward
Keyword(s):  
Elevated Temperatures ◽  
Least Square ◽  
P Wave ◽  
Moho Depth ◽  
Strain History ◽  
Uppermost Mantle ◽  
Wave Speeds ◽  
Partial Melt ◽  
Volcanic Region ◽  
Pn Velocity

<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>

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

2021 ◽  
Author(s):  
◽  
Anya Mira Seward
Keyword(s):  
Elevated Temperatures ◽  
Least Square ◽  
P Wave ◽  
Moho Depth ◽  
Strain History ◽  
Uppermost Mantle ◽  
Wave Speeds ◽  
Partial Melt ◽  
Volcanic Region ◽  
Pn Velocity

<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>

scholarly journals Tomographic image of the crust and uppermost mantle of the Ionian and Aegean regions

1997 ◽  
Vol 40 (1) ◽  
Author(s):  
B. Alessandrini ◽  
L. Beranzoli ◽  
G. Drakatos ◽  
C. Falcone ◽  
G. Karantonis ◽  
...  
Keyword(s):  
Wave Velocity ◽  
Mediterranean Area ◽  
P Wave ◽  
Moho Depth ◽  
Uppermost Mantle ◽  
Low Velocity ◽  
Central Mediterranean ◽  
Data Set ◽  
Velocity Anomaly ◽  
P Wave Velocity

We present a tomographic view of the crust and uppermost mantle beneath the Central Mediterranean area obtained from P-wave arrival times of regional earthquakes selected from the ISC bulletin. The P-wave velocity anomalies are obtained using Thurber's algorithm that jointly relocates earthquakes and computes velocity adjustments with respect to a starting model. A specific algorithm has been applied to achieve a distribution of epicentres as even as possible. A data set of 1009 events and 49072 Pg and Pn phases was selected. We find a low velocity belt in the crust, evident in the map view at 25 km of depth, beneath the Hellenic arc. A low velocity anomaly extends at 40 km of depth under the Aegean back arc basin. High velocities are present at Moho depth beneath the Ionian sea close to the Calabrian and Aegean arcs. The tomographic images suggest a close relationship between P-wave velocity pattern and the subduction systems of the studied area.

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

2019 ◽  
Author(s):  
◽  
Rayan Yassminh
Keyword(s):  
United States ◽  
New England ◽  
Velocity Ratio ◽  
Site Amplification ◽  
P Wave ◽  
Seismic Structure ◽  
Eastern United States ◽  
Uppermost Mantle ◽  
Pn Velocity ◽  
Mantle Temperature

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

Delineation of a low‐velocity body under the Roosevelt Hot Springs geothermal area, Utah, using teleseismic P‐wave data

Geophysics ◽  
1981 ◽  
Vol 46 (10) ◽  
pp. 1456-1466 ◽  
Author(s):  
Russell Robinson ◽  
H. M. Iyer
Keyword(s):  
Hot Springs ◽  
Velocity Structure ◽  
P Wave ◽  
Wave Form ◽  
Geothermal Area ◽  
Uppermost Mantle ◽  
Low Velocity ◽  
Near Surface ◽  
High Attenuation ◽  
Partial Melt

To assess the nature of the heat source associated with the Roosevelt Hot Springs geothermal area, we have investigated the P‐wave velocity structure of the crust and uppermost mantle in the vicinity of the Mineral Mountains, southwest Utah, a region of late Cenozoic rhyolitic and basaltic volcanic activity. A roughly square (30 × 30 km) array of 15 seismographs, centered on the mountains, was operated for a period of 46 days, during which 72 teleseismic events were recorded with sufficient quality for calculation of P‐wave traveltime residuals. Relative residuals, using the array average for each event as reference, show a clear pattern of azimuthal variation of up to 0.3 sec. This pattern implies the existence of a localized region of relatively low‐velocity material extending up from the upper mantle to depths of about 5 km under the Mineral Mountains. A three‐dimensional (3-D) inversion of the data confirms this conclusion and yields a model featuring a region of low velocity (5 to 7 percent less than the surrounding rock) centered under the geothermal area and extending from about 5-km depth down into the uppermost mantle. The near‐surface velocities obtained in the inversion clearly reveal the structure of the region, part of the Basin and Range province. An azimuthally changing pattern of wave‐form distortion, restricted to the central Mineral Mountains, indicates the presence of a small but intensely anomalous region of low velocity and high attenuation at depths of about 15 km. Although we cannot rule out an explanation for the low velocity purely in terms of compositional changes, in view of the geothermal and volcanic manifestations found in the region we prefer an explanation in terms of abnormally high temperature and a small fraction of partial melt. A partial melt model implies a much greater heat reservoir than does a model involving only circulation along deep fault zones.

scholarly journals Crustal density structure of the northwestern Iranian Plateau

2019 ◽  
Vol 56 (12) ◽  
pp. 1347-1365 ◽  
Author(s):  
Vahid Teknik ◽  
Abdolreza Ghods ◽  
Hans Thybo ◽  
Irina M. Artemieva
Keyword(s):  
High Density ◽  
Central Iran ◽  
P Wave ◽  
Arabian Plate ◽  
Scale Model ◽  
Moho Depth ◽  
Caspian Basin ◽  
The South ◽  
South Caspian Basin ◽  
Iranian Plateau

We present a new 2D crustal-scale model of the northwestern Iranian plateau based on gravity–magnetic modeling along the 500 km long China–Iran Geological and Geophysical Survey in the Iranian plateau (CIGSIP) seismic profile across major tectonic provinces of Iran from the Arabian plate into the South Caspian Basin (SCB). The seismic P-wave receiver function (RF) model along the profile is used to constrain major crustal boundaries in the density model. Our 2D crustal model shows significant variation in the sedimentary thickness, Moho depth, and the depth and extent of intra-crustal interfaces. The Main Recent Fault (MRF) between the Arabian crust and the overriding central Iran crust dips at approximately 13° towards the northeast to a depth of about 40 km. The geometry of the MRF suggests about 150 km of underthrusting of the Arabian plate beneath central Iran. Our results indicate the presence of a high-density lower crustal layer beneath Zagros. We identify a new crustal-scale suture beneath the Tarom valley between the South Caspian Basin crust and Central Iran and the Alborz. This suture is associated with sharp variation in Moho depth, topography, and magnetic anomalies, and is underlain by a 20 km thick high-density crustal root at 35–55 km depth. The high-density lower crust in Alborz and Zagros may be related to partial eclogitization of crustal roots below about 40 km depth. The gravity and magnetic models indicate a highly extended continental crust for the SCB crust along the profile. Low observed magnetic susceptibility of the Kermanshah ophiolites likely indicates that the ophiolite rocks only form a thin layer that has been thrust over the sedimentary cover.

Lithospheric architecture of the Ligurian Basin from seismic travel time tomography

2021 ◽  
Author(s):  
Anke Dannowski ◽  
Heidrun Kopp ◽  
Ingo Grevemeyer ◽  
Grazia Caielli ◽  
Roberto de Franco ◽  
...  
Keyword(s):  
Seismic Data ◽  
Velocity Model ◽  
P Wave ◽  
Central Basin ◽  
Uppermost Mantle ◽  
Mantle Boundary ◽  
Oceanic Spreading ◽  
Refraction Seismic ◽  
Back Arc ◽  
Ligurian Basin

&lt;p&gt;The Ligurian Basin is located north-west of Corsica at the transition from the western Alpine orogen to the Apennine system. The Back-arc basin was generated by the southeast retreat of the Apennines-Calabrian subduction zone. The opening took place from late Oligocene to Miocene. While the extension led to extreme continental thinning little is known about the style of back-arc rifting. Today, seismicity indicates the closure of this back-arc basin. In the basin, earthquake clusters occur in the lower crust and uppermost mantle and are related to re-activated, inverted, normal faults created during rifting.&lt;/p&gt;&lt;p&gt;To shed light on the present day crustal and lithospheric architecture of the Ligurian Basin, active seismic data have been recorded on short period ocean bottom seismometers in the framework of SPP2017 4D-MB, the German component of AlpArray. An amphibious refraction seismic profile was shot across the Ligurian Basin in an E-W direction from the Gulf of Lion to Corsica. The profile comprises 35 OBS and three land stations at Corsica to give a complete image of the continental thinning including the necking zone.&lt;/p&gt;&lt;p&gt;The majority of the refraction seismic data show mantle phases with offsets up to 70 km. The arrivals of seismic phases were picked and used to generate a 2-D P-wave velocity model. The results show a crust-mantle boundary in the central basin at ~12 km depth below sea surface. The P-wave velocities in the crust reach 6.6 km/s at the base. The uppermost mantle shows velocities &gt;7.8 km/s. The crust-mantle boundary becomes shallower from ~18 km to ~12 km depth within 30 km from Corsica towards the basin centre. The velocity model does not reveal an axial valley as expected for oceanic spreading. Further, it is difficult to interpret the seismic data whether the continental lithosphere was thinned until the mantle was exposed to the seafloor. However, an extremely thinned continental crust indicates a long lasting rifting process that possibly did not initiate oceanic spreading before the opening of the Ligurian Basin stopped. The distribution of earthquakes and their fault plane solutions, projected along our seismic velocity model, is in-line with the counter-clockwise opening of the Ligurian Basin.&lt;/p&gt;

Reconnaissance seismic refraction-reflection surveys in northwestern New Mexico

1984 ◽  
Vol 74 (4) ◽  
pp. 1263-1274
Author(s):  
Lawrence H. Jaksha ◽  
David H. Evans
Keyword(s):  
New Mexico ◽  
Wave Velocity ◽  
Lower Crust ◽  
Seismic Waves ◽  
Seismic Refraction ◽  
Velocity Model ◽  
P Wave ◽  
Uppermost Mantle ◽  
P Wave Velocity ◽  
Crustal Thinning

Abstract A velocity model of the crust in northwestern New Mexico has been constructed from an interpretation of direct, refracted, and reflected seismic waves. The model suggests a sedimentary section about 3 km thick with an average P-wave velocity of 3.6 km/sec. The crystalline upper crust is 28 km thick and has a P-wave velocity of 6.1 km/sec. The lower crust below the Conrad discontinuity has an average P-wave velocity of about 7.0 km/sec and a thickness near 17 km. Some evidence suggests that velocity in both the upper and lower crust increases with depth. The P-wave velocity in the uppermost mantle is 7.95 ± 0.15 km/sec. The total crustal thickness near Farmington, New Mexico, is about 48 km (datum = 1.6 km above sea level), and there is evidence for crustal thinning to the southeast.

Constraints on the composition of the crust and uppermost mantle in northwestern Canada: Vp/Vs variations along Lithoprobe's SNorCLE transect

2005 ◽  
Vol 42 (6) ◽  
pp. 1205-1222 ◽  
Author(s):  
Gabriela Fernández-Viejo ◽  
Ron M Clowes ◽  
J Kim Welford
Keyword(s):  
Wave Velocity ◽  
Upper Mantle ◽  
Laboratory Data ◽  
High Ratio ◽  
P Wave ◽  
S Wave ◽  
Uppermost Mantle ◽  
Crust And Upper Mantle ◽  
Slave Craton ◽  
Crustal Composition

Shear-wave seismic data recorded along four profiles during the SNoRE 97 (1997 Slave – Northern Cordillera Refraction Experiment) refraction – wide-angle reflection experiment in northwestern Canada are analyzed to provide S-wave velocity (Vs) models. These are combined with previous P-wave velocity (Vp) models to produce cross sections of the ratio Vp/Vs for the crust and upper mantle. The Vp/Vs values are related to rock types through comparisons with published laboratory data. The Slave craton has low Vp/Vs values of 1.68–1.72, indicating a predominantly silicic crustal composition. Higher values (1.78) for the Great Bear and eastern Hottah domains of the Wopmay orogen imply a more mafic than average crustal composition. In the western Hottah and Fort Simpson arc, values of Vp/Vs drop to ∼1.69. These low values continue westward for 700 km into the Foreland and Omineca belts of the Cordillera, providing support for the interpretation from coincident seismic reflection studies that much of the crust from east of the Cordilleran deformation front to the Stikinia terrane of the Intermontane Belt consists of quartzose metasedimentary rocks. Stikinia shows values of 1.78–1.73, consistent with its derivation as a volcanic arc terrane. Upper mantle velocity and ratio values beneath the Slave craton indicate an ultramafic peridotitic composition. In the Wopmay orogen, the presence of low Vp/Vs ratios beneath the Hottah – Fort Simpson transition indicates the presence of pyroxenite in the upper mantle. Across the northern Cordillera, low Vp values and a moderate-to-high ratio in the uppermost mantle are consistent with the region's high heat flow and the possible presence of partial melt.

scholarly journals A simple P-wave polarization analysis: its application to earthquake location

1994 ◽  
Vol 37 (5) ◽  
Author(s):  
B. Alessandrini ◽  
M. Cattaneo ◽  
M. Demartin ◽  
M. Gasperini ◽  
V. Lanza
Keyword(s):  
Ground Motion ◽  
Seismic Wave ◽  
Heterogeneous Media ◽  
Seismic Wave Propagation ◽  
Least Square ◽  
P Wave ◽  
Motion Direction ◽  
Arrival Times ◽  
Hypocentral Location ◽  
Ray Path

We present a method for hypocentral location which takes into account all three components of ground motion and not only the vertical one, as it is usually done by standard least-square techniques applied to arrival times. Assuming that P-wave particle motion direction corresponds to the propagation direction of the seismic wave, we carried out a simple statistical analysis of ground motion amplitudes, carefully using three-component records. We obtained the azimuth and the emersion angle of the seismic ray, which, added to Pg and Sg arrival times, allowed us to find reliable hypocentral coordinates of some local events by means of a ray-tracing technique. We compared our locations to those obtained using a least-square technique: our polarization method's dependence on the accuracy of the model used (on the contrary, the least-square technique proved to be quite stable with respect to changes in the model's velocity parameters) led us to conclude that polarization data provide coherent information on the true ray-path and can be successfully used for both location procedures and seismic wave propagation studizs in strongly heterogeneous media.

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