Coincident seismic-wave velocity and reflectivity properties of the lower crust beneath the Appalachian Front, west of Newfoundland

1991 ◽  
Vol 28 (1) ◽  
pp. 94-101 ◽  
Author(s):  
François Marillier ◽  
Mike Dentith ◽  
Karin Michel ◽  
Ian Reid ◽  
Brian Roberts ◽  
...  
Keyword(s):  
Velocity Structure ◽  
Seismic Velocity ◽  
Seismic Refraction ◽  
Ultramafic Rocks ◽  
Seismic Reflection Profile ◽  
Crustal Extension ◽  
Seismic Wave Velocity ◽  
Seismic Velocity Structure ◽  
Iapetus Ocean ◽  
Deep Seismic Reflection Profile

We have determined the seismic velocity structure of the crust in the vicinity of the Appalachian deformation front off western Newfoundland and the adjacent Gulf of St Lawrence. These measurements were made from two perpendicular wide-angle seismic refraction profiles, one of which is collinear with a previously recorded deep seismic reflection profile. The Grenville foreland crust, about 45 km thick, is characterized by velocities of 6.35 km/s in its upper part and 6.7 km/s in its lower part. Close to the coast of Newfoundland, a deep crustal reflective wedge is bounded by a northwest-dipping reflector and by the crust–mantle boundary, which is at only 39 km depth beneath the wedge. In the wedge, velocities of 7.2–7.3 km/s may indicate the presence of mafic and ultramafic rocks. We speculate that several processes could have caused the high velocities and the high reflectivity. The most attractive is perhaps crustal extension with consequent underplating during the formation of the Iapetus Ocean or during later reactivation by Carboniferous strike-slip movements.

Hybrid shortest path and ray bending method for traveltime and raypath calculations

Geophysics ◽  
2001 ◽  
Vol 66 (2) ◽  
pp. 648-653 ◽  
Author(s):  
Harm J. A. Van Avendonk ◽  
Alistair J. Harding ◽  
John A. Orcutt ◽  
W. Steven Holbrook
Keyword(s):  
Ray Tracing ◽  
Shortest Path ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
Seismic Refraction ◽  
Computation Time ◽  
Hybrid Scheme ◽  
Seismic Velocity Structure ◽  
Order Of Magnitude ◽  
Ray Bending

The shortest path method (SPM) is a robust ray‐tracing technique that is particularly useful in 3-D tomographic studies because the method is well suited for a strongly heterogeneous seismic velocity structure. We test the accuracy of its traveltime calculations with a seismic velocity structure for which the nearly exact solution is easily found by conventional ray shooting. The errors in the 3-D SPM solution are strongly dependent on the choice of search directions in the “forward star,” and these errors appear to accumulate with traveled distance. We investigate whether these traveltime errors can be removed most efficiently by an SPM calculation on a finer grid or by additional ray bending. Testing the hybrid scheme on a realistic ray‐tracing example, we find that in an efficient mix ray bending and SPM account for roughly equal amounts of computation time. The hybrid method proves to be an order of magnitude more efficient than SPM without ray bending in our example. We advocate the hybrid ray‐tracing technique, which offers an efficient approach to find raypaths and traveltimes for large seismic refraction studies with high accuracy.

scholarly journals Seismic velocity structure at the southern termination of the 2016 Kumamoto Earthquake rupture, Japan

2020 ◽  
Vol 72 (1) ◽  
Author(s):  
Yasuhira Aoyagi ◽  
Haruo Kimura ◽  
Kazuo Mizoguchi
Keyword(s):  
Fault Zone ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
Earthquake Rupture ◽  
2016 Kumamoto Earthquake ◽  
Seismic Velocity Structure ◽  
Kumamoto Earthquake ◽  
Seismogenic Layer ◽  
The 2016 Kumamoto Earthquake ◽  
Tectonic Line

Abstract The earthquake rupture termination mechanism and size of the ruptured area are crucial parameters for earthquake magnitude estimations and seismic hazard assessments. The 2016 Mw 7.0 Kumamoto Earthquake, central Kyushu, Japan, ruptured a 34-km-long area along previously recognized active faults, eastern part of the Futagawa fault zone and northernmost part of the Hinagu fault zone. Many researchers have suggested that a magma chamber under Aso Volcano terminated the eastward rupture. However, the termination mechanism of the southward rupture has remained unclear. Here, we conduct a local seismic tomographic inversion using a dense temporary seismic network to detail the seismic velocity structure around the southern termination of the rupture. The compressional-wave velocity (Vp) results and compressional- to shear-wave velocity (Vp/Vs) structure indicate several E–W- and ENE–WSW-trending zonal anomalies in the upper to middle crust. These zonal anomalies may reflect regional geological structures that follow the same trends as the Oita–Kumamoto Tectonic Line and Usuki–Yatsushiro Tectonic Line. While the 2016 Kumamoto Earthquake rupture mainly propagated through a low-Vp/Vs area (1.62–1.74) along the Hinagu fault zone, the southern termination of the earthquake at the focal depth of the mainshock is adjacent to a 3-km-diameter high-Vp/Vs body. There is a rapid 5-km step in the depth of the seismogenic layer across the E–W-trending velocity boundary between the low- and high-Vp/Vs areas that corresponds well with the Rokkoku Tectonic Line; this geological boundary is the likely cause of the dislocation of the seismogenic layer because it is intruded by serpentinite veins. A possible factor in the southern rupture termination of the 2016 Kumamoto Earthquake is the existence of a high-Vp/Vs body in the direction of southern rupture propagation. The provided details of this inhomogeneous barrier, which are inferred from the seismic velocity structures, may improve future seismic hazard assessments for a complex fault system composed of multiple segments.

Seismic velocity structure and gravity anomalies: a comparison

1987 ◽  
Vol 140 (1) ◽  
pp. 115-120
Author(s):  
Yoshibumi Tomoda ◽  
Hiromi Fujimoto
Keyword(s):  
Velocity Structure ◽  
Seismic Velocity ◽  
Gravity Anomalies ◽  
Seismic Velocity Structure

What Is Fling Step? Its Theory, Simulation Method, and Applications to Strong Ground Motion near Surface Fault Ruptures

2021 ◽  
Author(s):  
Yoshiaki Hisada ◽  
Shinya Tanaka
Keyword(s):  
Velocity Structure ◽  
Seismic Velocity ◽  
Strong Motion ◽  
Theoretical Method ◽  
Simulation Method ◽  
Near Surface ◽  
Seismic Velocity Structure ◽  
Strong Motion Records ◽  
Near Fault ◽  
Fling Step

ABSTRACT We present the theory of the fling step and a theoretical method for simulating accurately the near-fault strong motions, and apply it to reproduce various strong-motion records near surface faults. Theoretically, the fling step is the contribution of the static Green’s function in the representation theorem (Hisada and Bielak, 2003), and we show that this theory holds for any seismic velocity structure. We first demonstrate the validity of this theory using theoretical solutions of a circular fault model in a homogeneous full-space. Next, we apply the theory to layered half-spaces, present a theoretical method based on the wavenumber integration method, and introduce various techniques to simulate the near-fault ground motions including fling steps with high accuracy. Finally, we demonstrate the effectiveness of the method by reproducing various strong-motion records near surface fault ruptures and discuss the characteristics of near-fault strong motions including the fling step and the forward directivity pulse. We made all of the software and data used in this article available on the internet.

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