Mingling Tomography with Waveform Modeling

A depleted, destablized continental lithosphere near the Rio Grande Rift [Song and Helmberger, 2007],

Upper Mantle Structure beneath USArray derived from Waveform Complexity [Sun and Helmberger, 2010],

Juan de Fuca Subduction Zone from a mixture of Tomography and Waveform Modeling [Chu and Helmberger, 2010].

Tomographic model of Western US

P tomographic model from Burdick et al., 2009

Most of the features are gone at depth greater than 300 km. A lot of observed waveform complexities are produced by relatively shallow mantle structures.

Waveform complexity along Rio Grande Rift zone

[Song and Helmberger, 2007]

Azimuthal dependence of multi-pathing

SENW

The preferred strong multi-pathing for the event from the south suggests that the anomalies in the upper mantle dip to the south [Sun, 2009].

Strong multi-pathing, large ∆LR

No multi-pathing, small ∆LR

Waveform complexity along Rio Grande Rift zone

To fit this group of waveform data, enhancing the existing tomographic model (fast region inflated by a factor of 3 with slow region by 2) is required [Song and Helmberger, 2007].

If the S velocity anomaly is only half as suggested by S-wave tomography, we would expect a substantial underestimate of its temperature anomaly.

Using waveform modeling in constraining the velocity structure (P and S) is crusical to understand the origin of these anomalies.

MPD on Southwestern US

Instead of a 2D transect of La Ristra array, 3D structures are expected near Rio Grande rift and Colorado Plaetau [Sun and Helmberger, 2010].

In-plane vs. out-plane multi-pathing

Both multi-pathing are related to the sharp edges either in-plane or out-plane.

In-plane multi-pathing: records show more order along distance.

Out-plane multi-pathing: records show more order along azimuth.

Most data show the combination.

[Sun and Helmberger, 2010]

Summary

Juan de Fuca Slab modeled with a 60km layer with P and S velocities of 8.2 and 4.5km/s with penetration to 250km into a background tomography image by Schmandt and Humphreys(2010) where the reference model has been changed to mT7 containing a LVZ .

A lower Slab beneath Northern Nevada is required in the Transition Zone with enlarged P and S velocities by 3 and 7% which appears compatible with Van der Lee and Nolet (1997) and receiver functions by Cao and Levander(2010)

Summary

MPD is a useful tool and complementary to the

tomographic model for mapping out the sharp

features.

The dominant sharp structures beneath western

US dip to the southeast direction.

Amplitudes from the waveform data are good

indications of sharp features.

2D modeling of the data

6%

3%-2.5%

Outline

Tomographic models and waveform complexity

A new tool, Multi-pathing detector (MPD), to detect the sharp features

Examples of Western US upper-mantle structure