Upper mantle velocities determined from local observations of deep earthquakes

1973 ◽  
Vol 63 (3) ◽  
pp. 807-817
Author(s):  
Warwick D. Smith
Keyword(s):  
Travel Time ◽  
Upper Mantle ◽  
Regional Studies ◽  
Local Velocity ◽  
Low Velocity ◽  
Low Velocity Zone ◽  
Velocity Models ◽  
Deep Earthquakes ◽  
Actual Travel Time ◽  
Velocity Zone

abstract A method for determining upper mantle velocities is presented. Observations of deep earthquakes at small epicentral distances enable standard travel-time tables to be modified for regional studies. This is done by calculating the ratio of the actual travel time to that given in the tables, for a range of focal depths. Local velocity models may then be determined. The Jeffreys-Bullen travel times have been modified to suit the New Zealand region. Strong evidence is presented for velocities higher than the Jeffreys-Bullen values at depths less than about 160 km, and below that a low-velocity zone extending to a depth of at least 300 km.

scholarly journals Long-period P waveform modeling of upper mantle phases in the West Mediterranean basin

1994 ◽  
Vol 37 (1) ◽  
Author(s):  
N. A. Pino
Keyword(s):  
Upper Mantle ◽  
Velocity Model ◽  
Low Velocity ◽  
Waveform Modeling ◽  
Low Velocity Zone ◽  
Velocity Models ◽  
Long Period ◽  
Crustal Earthquakes ◽  
Lower Amplitude ◽  
Velocity Zone

Long-period P waveforms of some Italian crustal earthquakes recorded at the WWSSN stations located in the Iberian Peninsula have been modeled to derive a 1D upper matle compressional velocity model. A technique based on the Cagniard-de Hoop method has been used to compute synthetic seismograms. Waveforms have first been computed for published velocity models referred to different tectonic provinces and compared with the data. A model that strongly improves the fits to the data is then presented. The proposed model, called WMP, is characterized by a 100 km thick lid overlaying a low velocity zone, a 1% velocity discontinuity located at 313 km depth, that is required to fit a lower amplitude phase, and an abrupt increase in the velocity gradient starting from 370 km. This latter is preferred to the sharp discontinuity located at about 400 km that is present in various models obtained for upper mantle structure with analogous techniques. Within the lid and the low-velocity zone, WMP displays features that are typical of old ocean structures like the Northwest Atlantic Ocean.

Review: Receiver function studies in the southern Canadian Cordillera

1995 ◽  
Vol 32 (10) ◽  
pp. 1514-1519 ◽  
Author(s):  
John F. Cassidy
Keyword(s):  
British Columbia ◽  
Upper Mantle ◽  
Receiver Function ◽  
Vancouver Island ◽  
Moho Depth ◽  
Canadian Cordillera ◽  
Low Velocity ◽  
Strait Of Georgia ◽  
Low Velocity Zone ◽  
Velocity Zone

Receiver function analysis has proven to be a powerful, yet inexpensive tool for estimating the S-wave velocity structure of the crust and upper mantle beneath three-component seismograph stations in the southern Canadian Cordillera. Receiver function studies using a portable broadband seismograph array across southwestern British Columbia provided site-specific estimates for the location of the subducting Juan de Fuca plate. The oceanic crust was imaged at 47−53 km beneath central Vancouver Island, and 60–65 km beneath the Strait of Georgia. Further, these studies revealed a prominent low-velocity zone (VS = −1.0 km/s) that coincides with the E reflectors imaged ~5–10 km above the subducting plate on Lithoprobe reflection lines. The E low-velocity zone was shown to extend into the upper mantle beneath the Strait of Georgia and the British Columbia mainland, to depths of 50–60 km. Combining the receiver function and refraction models revealed a high Poisson's ratio (0.27–0.38) for this feature. The continental Moho was estimated at 36 km beneath the Strait of Georgia, and a crustal low-velocity zone associated with the Lithoprobe C reflectors beneath Vancouver Island was interpreted to extend eastward, near the base of the continental crust, to the British Columbia mainland. Analysis of data from the recently deployed Canadian National Seismograph Network demonstrates the variations in crustal thickness and complexity across the southern Canadian Cordillera, with the Moho depth varying from 35 km in the Coast Mountains, to 33 km near Penticton, to 50 km near the Rocky Mountain deformation front.

The upper mantle of southern British Columbia

1977 ◽  
Vol 14 (5) ◽  
pp. 1100-1115 ◽  
Author(s):  
A. J. Wickens
Keyword(s):  
Upper Mantle ◽  
Coastal Region ◽  
Vancouver Island ◽  
Love Waves ◽  
Low Velocity ◽  
Low Velocity Zone ◽  
Rayleigh And Love Waves ◽  
Generalized Matrix ◽  
Inversion Techniques ◽  
Velocity Zone

A concentration of temporary and permanent long-period stations has been used to record Rayleigh and Love waves over a region bounded by Vancouver Island in the west and a line approximately 400 km to the east. Phase-velocity information for both Rayleigh and Love waves has been calculated and inverted to provide estimates of models along the profiles. Generalized matrix inversion techniques have been employed to set confidence limits on the models. No significant upper-mantle low-velocity zone was detected under Vancouver Island or the adjacent coastal region. To the east a shallow upper-mantle low-velocity zone dipping to the northeast was required to fit the data. The transition from crust to mantle was sharper and more prominent to the northeast than to the southwest.

scholarly journals The April 29, 1965, Puget Sound earthquake and the crustal and upper mantle structure of western Washington

1977 ◽  
Vol 67 (3) ◽  
pp. 693-711 ◽  
Author(s):  
Charles A. Langston ◽  
David E. Blum
Keyword(s):  
Upper Mantle ◽  
Source Parameters ◽  
Puget Sound ◽  
P Wave ◽  
Low Velocity ◽  
Crust And Upper Mantle ◽  
Low Velocity Zone ◽  
Crustal Model ◽  
Short Period ◽  
Velocity Zone

abstract Simultaneous modeling of source parameters and local layered earth structure for the April 29, 1965, Puget Sound earthquake was done using both ray and layer matrix formulations for point dislocations imbedded in layered media. The source parameters obtained are: dip 70° to the east, strike 344°, rake −75°, 63 km depth, average moment of 1.4 ± 0.6 × 1026 dyne-cm, and a triangular time function with a rise time of 0.5 sec and falloff of 2.5 sec. An upper mantle and crustal model for southern Puget Sound was determined from inferred reflections from interfaces above the source. The main features of the model include a distinct 15-km-thick low-velocity zone with a 2.5-km/sec P-wave-velocity contrast lower boundary situated at approximately 56-km depth. Ray calculations which allow for sources in dipping structure indicate that the inferred high contrast value can trade off significantly with interface dip provided the structure dips eastward. The effective crustal model is less than 15 km thick with a substantial sediment section near the surface. A stacking technique using the instantaneous amplitude of the analytic signal is developed for interpreting short-period teleseismic observations. The inferred reflection from the base of the low-velocity zone is recovered from short-period P and S waves. An apparent attenuation is also observed for pP from comparisons between the short- and long-period data sets. This correlates with the local surface structure of Puget Sound and yields an effective Q of approximately 65 for the crust and upper mantle.

A dipping Moho and crustal low-velocity zone from Pn arrivals at The Geysers-Clear Lake, California

1982 ◽  
Vol 72 (5) ◽  
pp. 1551-1566
Author(s):  
Peter M. Shearer ◽  
David H. Oppenheimer
Keyword(s):  
Upper Mantle ◽  
Azimuthal Velocity ◽  
Apparent Velocity ◽  
Clear Lake ◽  
Low Velocity ◽  
Magma Body ◽  
Low Velocity Zone ◽  
Velocity Anisotropy ◽  
The Geysers ◽  
Velocity Zone

abstract Relative Pn arrival times across an array of stations at The Geysers-Clear Lake region in northern California indicates that the upper mantle velocity is 8.0 km/sec, and that the Moho dips 5.3° at 57° to the northeast in this area, reflecting thickening of the crust toward the continent. These results agree with regional trends in the Bouguer gravity field. The travel-time residuals with respect to the dipping model suggest a crustal low-velocity zone beneath Mt. Hannah, consistent with reported teleseismic delays, and the low-velocity zone is interpreted as representing partial melt. No delays are observed in The Geysers, precluding the existence of an extensive magma body beneath the steam production area. Azimuthal variations in apparent velocity may reflect upper mantle azimuthal velocity anisotropy, but such an interpretation is uncertain due to the limited azimuthal distribution of earthquakes.

A linear gradient crustal model for south Hawaii

1981 ◽  
Vol 71 (5) ◽  
pp. 1503-1510
Author(s):  
Fred W. Klein
Keyword(s):  
Travel Time ◽  
Focal Mechanism ◽  
Low Velocity ◽  
Linear Gradient ◽  
Low Velocity Zone ◽  
Focal Mechanism Solutions ◽  
Local Earthquakes ◽  
Velocity Gradients ◽  
Crustal Model ◽  
Velocity Zone

abstract A new crustal model with linear velocity gradients within layers does as good a job of locating earthquakes on south Hawaii as any model yet published. Incorporating linear gradients means the model can be simpler and free of artificial velocity discontinuities. Using travel-time residuals from local earthquakes and consistency of focal mechanism solutions as tests, it is seen that a low-velocity zone at the base of the crust is not required.

V. Petrogenesis and discussion

1971 ◽  
Vol 268 (1192) ◽  
pp. 707-725 ◽  
Keyword(s):  
Partial Melting ◽  
Upper Mantle ◽  
Experimental Studies ◽  
Low Velocity ◽  
Low Velocity Zone ◽  
Magma Genesis ◽  
Element Abundances ◽  
Incompatible Elements ◽  
Basaltic Magmas ◽  
Velocity Zone

Basaltic magmas are formed by partial melting of a source rock of peridotitic composition (pyrolite) under upper mantle conditions. Experimental studies of the mineralogy of pyrolite and the melting relations of various basaltic magmas under high-pressure conditions are integrated in an attempt to present an internally consistent model of source composition, derived liquid compositions and residual mantle compositions. The role of a small (0.1 %) content of water in the upper mantle is treated in some detail. The presence of the low velocity zone in the upper mantle is attributed to a small (< 5 %) degree of melting of pyrolite containing approximately 0.1% water. The small liquid fraction present in the low-velocity zone is highly undersaturated olivine nephelinite or olivine melilite nephelinite. Other magma types of direct upper mantle derivation ranging from olivine trachybasalt to olivine melilitite and to tholeiitic picrite are assigned to a genetic grid expressing the depth (pressure) of magma segregation, the degree of partial melting of the source pyrolite, the water content and approximate temperature of the magma. While this genetic model can account for variations in major element abundances and normative mineralogy among basalts, there are variations in abundances of the incompatible elements, particularly K, Rb, Ba, and the rare earths, which are inconsistent with a model invoking a constant source composition for all mantle-derived basalts. Additional factors producing source inhomogeneity, particularly in incompatible element abundances, include the possibility of two-stage melting and of chemical zoning within the low-velocity zone. It is suggested that vertical migration of a fluid or incipient melt phase, enriched in H 2 O, CO 2 and incompatible elements, occurs within the low-velocity zone. The evolution of continental and oceanic rift systems and of the Hawaiian volcanic province is discussed in relation to the depths and conditions of magma genesis derived from the models of magma genesis.

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