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.

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.

P–Coda Evidence for a Layer of Anomalous Velocity in the Crust Beneath Leduc, Alberta

1972 ◽  
Vol 9 (7) ◽  
pp. 845-856 ◽  
Author(s):  
P. G. Somerville ◽  
R. M. Ellis
Keyword(s):  
Synthetic Seismograms ◽  
Precambrian Basement ◽  
Upper Crust ◽  
Spectral Ratios ◽  
Low Velocity ◽  
Crustal Model ◽  
Short Period ◽  
Large Velocity ◽  
Theoretical Results ◽  
Velocity Zone

Previous seismic studies of crustal structure using short-period P-coda recorded in the vicinity of Leduc in central Alberta have indicated that serious discrepancies exist between the experimental observations and those based on a horizontally layered model of the crust in both the time and frequency domains.Using vertical-radial spectral ratios and synthetic seismograms, a modified crustal model has been derived which gives better agreement between experimental and theoretical results. This model involves the insertion of a layer several kilometers thick having large velocity contrast with respect to the surrounding media at the base of the Precambrian basement (12 km deep). The new crustal model is discussed in the light of evidence for a low velocity zone in the upper crust in certain continental regions.

A seismic refraction survey along the southern Rocky Mountain Trench, Canada

1975 ◽  
Vol 65 (1) ◽  
pp. 37-54 ◽  
Author(s):  
G. T. Bennett ◽  
R. M. Clowes ◽  
R. M. Ellis
Keyword(s):  
Seismic Refraction ◽  
Rocky Mountain ◽  
P Wave ◽  
Open Pit ◽  
Low Velocity ◽  
Low Velocity Zone ◽  
Fault Structure ◽  
Record Section ◽  
Southern Rocky Mountain ◽  
Velocity Zone

abstract An unreversed seismic refraction profile has been recorded in the southern Rocky Mountain Trench from 50°N to 53°N. Using blasts from two open-pit coal mines, 44 recordings were obtained over a distance of 540 km. These were combined into a record section in which instrument and shot variations were included to show amplitude variations along the profile. Interpretation involved Weichert-Herglotz integration of p-delta curves to obtain a velocity-depth structure and the calculation of synthetic seismograms for comparison with the record section. Refractors with apparent P-wave velocities of 6.5 to 6.6 km/sec and 8.22±0.04 km/sec are interpreted as the surface of the Precambrian basement and the Mohorovičić discontinuity, respectively. A prominent travel-time delay associated with the 6.5 km/sec branch is interpreted in two possible ways. One explanation is the existence of a crustal low-velocity zone beginning 3 km beneath the basement, depth of 6.5 km, and having a depth extent of 9 to 15 km with associated velocities of 5.5 to 6.1 km/sec, respectively. The second interpretation proposes a high-angle crustal fault near Radium. The resultant model has an up-fault structure with depth to basement of 6.5 km and depth to the M-discontinuity of 51 km and a down-fault structure with corresponding values of 12.1 and 58 km. On the basis of gravity and magnetic trends, the fault strikes northeasterly. In either interpretation, a velocity gradient is present in the lower crustal section and the thickness of the crust is in excess of 50 km. Analysis of larger amplitude arrivals shortly after the Pn phase is consistent with the interpretation of a low-velocity zone, 8 km beneath the M-discontinuity and approximately 7 km thick.

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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