ABSTRACT

METHODS

Obtaining Interpretable Receiver Functions to Study Lithospheric Structure

Erin Cunningham1,2, Andrew Frassetto2, Vedran Lekic3

MOTIVATIONRESULTS

CONCLUSIONS

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Figure 5 – Time Domain vs Frequency Domain Deconvolution – Single receiver functions for TA station V42A with 50 Ps and 33 Sp created with iterative time domain deconvolution (Ligorria and Ammon 1999) and frequency domain deconvolution (Bostock 1998).

Figure 6- TA station vs Permanent station- Single receiver functions for TA station U41A with 38 Ps and 27 Sp paths and NM permanent station MPH with 564 Ps and 543 Sp paths.

Figure 7- Automatic vs Manual Picking – Single receiver functions for TA station V42A. Automatic picking (Abt et al. 2009) has 50 Ps and 33 Sp paths while the manual picking has 22 Ps and 19 Sp paths.

Figure 8- Basement geology consistent with consolidated or unconsolidated sediments – Single receiver functions for NM MPH on unconsolidated sediments with 564 Ps and 543 Sp paths and for US CBKS on consolidated sediments with 162 Ps and 72 Sp paths.

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Figure 9- a. Single receiver functions for US CBKS with 162 Ps and 72 Sp paths are calculated with included frequencies of 0.03 – 0.2 (right) and 0.03- 1 (left). b. Normalized negative to positive amplitudes are calculated and graphed for US CBKS, IU RSSD, and IU CCM.

1. Automated vs Manual picking (Figure 7)- automatic picking with loose selection criteria outperforms manual selection data.

2. Iterative Time Domain vs Frequency Domain Deconvolution (Figure 5) – Both preform well for Ps receiver functions, however frequency domain deconvolution yields more robust results for Sp receiver functions.

3. In single Sp receiver functions, the amplitude of the negative phase decreases relative to the positive phase (Figure 9) as higher frequencies are included suggesting that a gradational mid- lithospheric discontinuity exists at approximately 100 km in depth.

4. Thick sediment layers produce multiples which complicate the interpretation of Ps CCP stacks. 5. Sp CCP stacks with TA stations alone (Figure 10) have insufficient data density to sufficiently reduce noise and produce interpretable images.

Receiver functions use converted phases (P to s or S to p) that occur at wavespeed discontinuities to detect impedance contrasts within the Earth. Receiver functions are obtained by deconvolving the parent waveform from the daughted waveform; the convolution can be carried out in the frequency or time domain. CCP stacking is used to reduce noise, as well as visualize and interpret the receiver function results obtained through deconvolution.

Deconvolution CCP stacking Receiver Function

Removing the source and instrument data from the seismogram provides information about the structure

Structure component of seismogram after deconvolution. Provides insight to seismic wavespeed increases and decreases due to layering

Using many single receiver functions to create a 3-D image of the earth

Figure 2- Convolution (*) of components of a seismogram

Figure 3- Common features of a single receiver function

Figure 4- Paths containing receiver function data from many stations (triangles) can be combined and averaged

Conversions of teleseismic compressional (P) to shear (S) waves – and vise versa- across velocity contrasts within the lithosphere can be detected and modeled using the receiver function (RF) approach Thick sediment layers found in the central US, which includes the Reelfoot rift, present a technical challenge in producing clear receiver functions used to analyze lithospheric structure.

We undertake a systematic investigation of the data selection and analysis techniques used in constructing P-to-S and S-to-P receiver functions to determine optimal deconvolution parameters.

We find that varying damping changes the relative phase amplitudes, suggesting that the so-called mid-lithospheric discontinuity may have a gradational character. Also, we find that in thick sediment,, even interpreting crustal thickness fro P to S RFs is difficult; S-to-P RFs are much simpler and easier to interpret, due to their inherent lack of contamination by multiples.

Furthermore, for TA stations even optimal parameter settings yield RFs that, while interpretable, lack the clarity seen in the consolidated sediment and permanent stations. Nevertheless, data from TA stations, supplemented by that from US and NM networks, enabled us to construct S-to-P common conversion point stacks (CCP) that map crustal and lithospheric differences within the area surrounding the Reelfoot rift.

Dense station coverage provided by the EarthScope Transportable Array in the central United States provides an unprecedented opportunity for imaging the crustal and lithospheric structure of relatively understudied region. The receiver function approach has been used to map variations in crustal and lithospheric thickness in the western US with great success. However, unconsolidated sediments characteristic of much of the central US present a technical challenge for both calculating and interpreting receiver functions (Figure 8). As the Transportable Array continues to move into sediment dominated areas, it is important to evaluate how sedimentary cover of significant thickness impacts the overall quality and effectiveness of receiver functions.

Figure 1- TA station coverage – White triangles represent TA stations used in analysis. The total number of TA stations used is 95

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Velocity Decrease

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Abt, D. L., K. M. Fischer, S. W. French, H. A. Ford, H. Yuan, and B. Romanowicz (2010), North American lithospheric discontinuity structure imaged by Ps and Sp receiver functions, J. Geophys. Res., 115, B09301, doi:10.1029/2009JB006914.

Bostock, M. G. (1998), Mantle stratigraphy and evolution of the Slave province, J. Geophys. Res., 103, 21,183–21,200, doi:10.1029/98JB01069.

Ligorria, J. P., and C. J. Ammon (1999), Iterative deconvolution and receiver function estimation, Bull. ‐Seismol. Soc. Am., 89, 1395–1400.

Figure 10- CCP stacks created with: a) Both USArray TA and permanent stations and b)only US ARRAY TA stations.

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