Waveform response to the morphology of 2-D subducted slabs

Zhou, Hua‐wei; Chen, Genmeng

doi:10.1111/j.1365-246x.1995.tb05729.x

Cited by 4 publications

(4 citation statements)

References 22 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…It has long been observed that sharp structures have profound effects on waveforms, and that such waveform complexity can be used as an independent constraint on slab structure. There have been a few attempts to use 2‐D and 3‐D forward modeling to investigate the effects of slab geometry and heterogeneity on waveforms [ Zhou and Chen , 1995; Vidale , 1987; Igel et al , 2002]. Some preliminary results based upon broadband sparse data coverage were obtained for slabs in other subduction zones, such as the slabs beneath North America [ Vidale and Garcia‐Gonzalez , 1988] and Kuril‐Kamchatka [ Cormier , 1989].…”

Section: Introductionmentioning

confidence: 99%

Waveform modeling of the slab beneath Japan

et al. 2007

View full text Add to dashboard Cite

[1] The tomographic P wave model for the Japan subduction zone derived by Zhao et al. (1994) has two very striking features: a slab about 90 km thick with P wave velocities 3-6% higher than the surrounding mantle and a mantle wedge with À6% low-velocity anomalies. We study three-component seismograms from more than 600 Hi-net stations produced by two earthquakes which occurred in the downgoing Pacific Plate at depths greater than 400 km. We simulate body wave propagation in the three-dimensional (3-D) P wave model using 2-D finite difference (FDM) and 3-D spectral element (SEM) methods. As measured by cross correlation between synthetics and data, the P wave model typically explains about half of the traveltime anomaly and some of the waveform complexity but fails to predict the extended SH wave train. In this study we take advantage of the densely distributed Hi-net stations and use 2-D FDM modeling to simulate the P-SV and SH waveforms. Our 2-D model suggests that a thin, elongated low-velocity zone exists atop the slab, extending down to a depth of 300 km with an S wave velocity reduction of 14% if a thickness of 20 km is assumed. Further, 3-D SEM simulations confirm that this model explains a strong secondary arrival which cannot easily be imaged with standard tomographic techniques. The low-velocity layer could explain the relatively weak coupling associated with most subduction zones at shallow depths (<50 km), generally involving abundant volcanic activity and silent earthquakes, and it may also help to further our understanding of the water-related phase transition of ultramafic rocks, and the nature of seismicity at intermediate depths ($70-300 km).Citation: Chen, M., J. Tromp, D. Helmberger, and H. Kanamori (2007), Waveform modeling of the slab beneath Japan,

show abstract

Section: Introductionmentioning

confidence: 99%

Waveform modeling of the slab beneath Japan

et al. 2007

View full text Add to dashboard Cite

show abstract

“…The A-A cross-section shows the S (upper) and ScS (lower) ray paths and depths at which they pierce the other transects (black vertical lines). Zhou & Chen (1995) showed that the waveform effects because of a slab having a 2 per cent increase in V S compared to the surrounding mantle (as is imaged in tomography models) are very difficult to detect compared to a slab with a 10 per cent V S contrast. Cross-sections B-B through E-E show the piercing location of S (cross) and ScS (circle).…”

Section: Discussionmentioning

confidence: 99%

“…However, the strength of velocity heterogeneity in TXBW is probably underestimated because of the smoothing imparted in the inversion process. Zhou & Chen (1995) showed that the waveform effects because of a slab having a 2 per cent increase in V S compared to the surrounding mantle (as is imaged in tomography models) are very difficult to detect compared to a slab with a 10 per cent V S contrast. Future work should consider realistic 3-D subducted slab geometries with stronger velocity contrasts.…”

Section: Discussionmentioning

confidence: 99%

Differential t* measurements via instantaneous frequency matching: observations of lower mantle shear attenuation heterogeneity beneath western Central America

Ford¹,

Garnero²,

Thorne³

2012

Geophysical Journal International

View full text Add to dashboard Cite

SUMMARY We infer shear attenuation in the lower mantle by using the method of instantaneous frequency matching to calculate differential t* between core‐reflected ScS and direct S (δt*ScS‐S). The instantaneous frequency at the envelope peak of a seismic phase is related to the average Fourier spectral frequency of that phase. To estimate δt*ScS‐S for a given trace, we first calculate the instantaneous frequency at the envelope peak of S and ScS. The trace is then attenuated through convolution with a suite of t* operators until the instantaneous frequency at the envelope peak of the seismic phase with the initially larger instantaneous frequency matches the value of the smaller instantaneous frequency from the initial calculation. The differential t* operator required to accomplish the match is then δt*ScS‐S. We also calculate δt*ScS‐S from the slope of the spectral ratio of windowed ScS and S. Both the spectral ratio and instantaneous frequency methods produce consistent results for high signal‐to‐noise ratio synthetic waveforms with S and ScS well separated in time, and where there are no other interfering phases. The instantaneous frequency method gives more stable results for low signal‐to‐noise ratio waveforms, and where S and/or ScS are affected by other interfering seismic phases. The instantaneous frequency matching method is applied to broadband data from South American earthquakes recorded in California that sample the lower mantle beneath Central America and the Cocos plate. δt*ScS‐S ranges from approximately –4 to 2 s, but are predominately negative, suggesting S is more attenuated than ScS for these data. We estimate the possibly contaminating effects of 3‐D velocity heterogeneity on δt*ScS‐S through analysis of synthetic seismograms computed for a cross‐section through a tomographically derived model of global shear wave heterogeneity, using an axisymmetric finite difference algorithm. Synthetics for path geometries of our data predict a δt*ScS‐S of ∼0.2 s. We investigate the effect of seismic anisotropy by comparing δt*ScS‐S before and after a subset of the data were corrected using splitting parameters obtained by linearizing the particle motion of the S and ScS phases. The rms error of the residuals between the corrected and uncorrected δt*ScS‐S is ∼0.2 s. Neither of these efforts, however, match the large negative observed δt*ScS‐S values, suggesting the mid‐mantle beneath western Central America is in fact much more attenuating than the lowermost mantle below it, or S may be broadened by out‐of‐plane propagation effects, involving the remains of the Farallon plate containing stronger velocity heterogeneity than is imaged by seismic tomography.

show abstract

“…As pointed out by Vidale (1987) and Cormier (1989), source-side slab diffraction/multipathing causes waveform distortion recorded teleseismically, such as S and ScS, and also causes focusing and defocusing at teleseismic distances (Vidale 1987;Cormier 1989;Weber 1990;Sekiguchi 1992;Perrot et al 1994;Zhou & Chen 1995). Such waveform effects and amplitude perturbation due to the high-velocity slab have often been used to constrain the geometry and depth of the slab near the source side.…”

Section: F I N I T E D I F F E R E N C E M O D E L L I N Gmentioning

confidence: 99%

Validating tomographic model with broad-band waveform modelling: an example from the LA RISTRA transect in the southwestern United States

Song¹,

Helmberger²

2007

Geophysical Journal International

View full text Add to dashboard Cite

S U M M A R YTraveltime tomographic models of the LA RISTRA transect produce excellent waveform fits if we amplify the damped images. We observe systematic waveform distortions across the western edge of the Great Plains from South American events, starting about 300 km east of the centre of the Rio Grande Rift. The amplitude decreases by more than 50 per cent within array stations spanning less than 200 km while the pulse width increases by more than a factor of 2. This feature is not observed for the data arriving from the northwest. While the S-wave tomographic image shows a fast slab-like feature dipping to the southeast beneath the western edge of the Great Plains, synthetics generated from this model do not reproduce the waveform characteristics. However, once we modify the tomographic image by amplifying the velocity contrast between the slab and adjoining mantle by a factor of 2-3, the synthetics produce observed amplitude decay and pulse broadening. In addition to the traveltime delay, amplitude variation due to wave phenomena such as slab diffraction, focusing and defocusing provide much tighter constraints on the geometry of the fast anomaly and its amplitude and sharpness as demonstrated by a forward sensitivity test and snapshots of the seismic wavefield. Our preferred model locates the slab 200 km east of the Rio Grande Rift dipping 70 • -75 • to the southeast, extending to a depth near 600 km with a thickness of 120 km and a velocity of about 4 per cent fast. In short, adding waveform and amplitude components to regional tomographic studies can help validate and establish structural geometry, sharpness and velocity contrast.

show abstract

Waveform response to the morphology of 2-D subducted slabs

Cited by 4 publications

References 22 publications

Waveform modeling of the slab beneath Japan

Waveform modeling of the slab beneath Japan

Differential t* measurements via instantaneous frequency matching: observations of lower mantle shear attenuation heterogeneity beneath western Central America

Validating tomographic model with broad-band waveform modelling: an example from the LA RISTRA transect in the southwestern United States

Contact Info

Product

Resources

About