P and S receiver functions observed at stations in Europe

INTRODUCTION

We study the teleseismic P and S receiver functions at European seismic

stations. From the IRIS and ORFEUS databeses, we visually select 3002

teleseismic, moderate-to-large magnitude (i.e.Mw ≥ 5.7) events recorded

by 255 broadband European stations with high signal-to-noise ratio within

the years 1990-2011. Corrected for the instrument response, the observed

seismograms are cut to length of 80 s (20 s pre-event, 60 s post-event) in

the case of P receiver functions (PRF) and to a length of 72 s (60 s pre-

event, 12 s post-event) in the case of S receiver function (SRF). We rotate

the station/event reference frame (Z/R/T) into the ray coordinate

reference frame (L/Q/T or P/SV/SH) where the rotation angle is estimated

from a covariance analysis. We use a conventional technique to obtain the

receiver functions (Langston 1979; Vinnik 1977). All the components (L, Q

and T) are deconvolved by the L component (M component) where the

deconvolved Q component (L component) produces PRF (SRF). The

receiver function amplitudes are normalized by the zero-lag time

amplitude of the deconvolved Q component (L component) of PRF (SRF)

(Ammon 1991). The M component obtained by covariance analysis (Farra

and Vinnik 2000) represents S-wave polarization incident beneath the

station. We use a time domain deconvolution approach (Menke 1984;

Sheean et all.1995) to isolate the receiver function. In order to secure the

quality of measurements we suppress multiples from deep interfaces,

remove outliers and average over many measurements. The outliend

analysis procedure yields 1701 L-component SRF and 2103 Q-component

PRF. The azimuthal coverage of both PRF and SRF waveforms binned at

200 backazimuth intervals is appropriate to analyze the observed receiver

functions in terms of both azimuthal and radial anisotrophy beneath most

of the seismic stations.

CONCLUSIONS

The Sp conversions preceding

the direct S phase and also

the crustal multiples are distinctly observed on the waveforms whereas the Ps phases are

partly masked by the crustal multiples. Our solutions indicate that a positive velocity gradient

characterizes the upper crust under stations at Europe. S receiver function observations have

been shown to have significant power for the investigation of upper mantle velocity structure.

We include the S receiver function (SRF) in the inversion procedure, along with the traditional

approach that only utilizes P receiver fuctions (PRF).This new approach is shown to resolve

the underground velocity structure better than the traditional approach, especially at the

upper mantle depths.S receiver functions inherently lack high frequency resolution power due

to strong attenuation through mantle propagation.They are also noisy because of interference

from adverse phases.P receiver functions are of higher frequency, but are susceptible to

complication from interfering crustal multiples.We apply the proposed approach to actual

recordings using the seismic stations located in Europe. Similar to the theorical experiments

the actual data show that S receiver functions are more advantageous at resolving upper

mantle velocity structure. ACKNOWLEDGEMENTSThis work is supported by The Scientific and Technological Research Council of Turkey (TUBITAK) (project number 109Y345).

REFERENCES

Ö. ÇAKIR, A. ERDURAN, E. KIRKAYA, Y. A. KUTLU, M. ERDURANNevsehir University, Department of Geophysics, 50300, Nevsehir, Turkey ( ocakir@nevsehir.edu.tr)

CALCULATION OF RECEIVER FUNCTION

In this section, we explain the parameters used to describe anisotropic

velocity structure and briefly present how to calculate synthetic

seismogrsms and receiver functions of seismic body waves propagating

into a stratified anisotropic medium. The upper mantle and the crust are

assumed to have seismic anisotropy of hexagonal symmetry. The

seismic velocity perturbations arising from weak hexagonal anisotropy

are written as (Backus 1965).

(α2 – α02) / α0

2 = A + B cos 2η + C cos4η for P wave

(β2 – β02)/β0

2 = D + Ecos2η

ρ is the density

for S wave,

Where α and β are P – and S – wave anisotropic velocities,

respectively and η is the angle between the hexagonal-symmetry

axis and the propagation direction of the seismic wave. Parameters with

subscript 0 denote the isotropic reference velocities of P and S waves.

Dimensionless parameters (B, C,and E) denote the anisotropic velocitiy

perturbations, and A and D are the isotropic velocitiy perturbations.

ρ = 0.32 α0 + 0.77 (gr/cm3)

d(km) β0(km/

s)

A B C D E 0 0

20.000 3.3600 0.25 0.00 0.03 0.0 0.0 0.06 45.00 45.0

15.000 3.7500 0.25 0.00 0.03 0.0 0.0 0.06 45.00 45.0

42.500 4.4780 0.28 0.00 -0.04 0.0 0.0 -0.08 135.00 -45.0

42.500 4.4930 0.28 0.00 -0.04 0.0 0.0 -0.08 135.00 -45.0

45.500 4.3050 0.28 0.00 -0.04 0.0 0.0 -0.08 135.00 -45.0

45.500 4.3140 0.29 0.00 -0.04 0.0 0.0 -0.08 135.00 -45.0

50.500 4.5660 0.29 0.00 -0.02 0.0 0.0 -0.04 90.00 70.0

50.500 4.6530 0.29 0.00 -0.02 0.0 0.0 -0.04 90.00 70.0

50.500 4.7400 0.29 0.00 0.00 0.0 0.0 0.00 0.00 0.0

30.000 4.8270 0.29 0.00 0.00 0.0 0.0 0.00 0.00 0.0

20.000 4.6570 0.29 0.00 0.00 0.0 0.0 0.00 0.00 0.0

50.000 5.1230 0.29 0.00 0.00 0.0 0.0 0.00 0.00 0.0

50.000 5.2290 0.29 0.00 0.00 0.0 0.0 0.00 0.00 0.0

The layer parameters of the anisotropic model down to 500 km depth are listed in Table 1. d shows layer thickness in km, is Poisson’s ratio and Φ and Ψ are the azimuth and tilt angle of the symmetrry axis. Other parameters are defined on the left column.

Table 1Anisotropic model structure

Figure 1: Theoretical PRF and SRF traces corresponding to the model in Table 1 are shown with respect to backazimuth. Black color traces are anisotropic while red color traces are isotropic obtained with B=E=0.

DATA AND METHOD

The receiver functions can be obtained from the deconvolution of either P

waves (i.e. P receiver functions) or S waves (i.e. S receiver functions). The P

receiver functions primarily emphasize the P-to-S (or Ps) conversions at the

interfaces beneath the station whereas the S receiver functions primarily

emphasize the S-to-P (or Sp) conversions. The receiver functions constrain

the velocity discontinuities and travel times whereas the surface waves

describe the average velocity in the medium, i.e. both data sets

complement each other. The P receiver functions, which are more

frequently utilized, may have certain disadvantages because of the

reverberations (or multiples) that can mask the Ps conversions (e.g.

Wittlinger et al. 2004). This masking effect could be particularly important

for the Ps conversions from the interfaces in the upper mantle (or mantle

lithosphere) arriving almost simultaneously with the crustal multiples. If the

sedimentation beneath the station is significant, then those strong

sedimentary multiples can even mask the Moho Ps conversion.

Figure 2 : Global distrubition of the events used in the P and S receiver function study for station KEV. The 935 events for the P-receiver functions are shown as blue circles and 935 events for the S receiver functions are shown as red circles . The green triangle indicate the location of station KEV . Contours are shown for every 300 distance from the centre of station KEV.

For our receiver function analysis, we use 3002 teleseismic, moderate-to-

large magnitude (i.e.Mw≥ 5.7) events recorded by 255 broadband European

stations within the years 1990-2011. For the P-wave and S-wave receiver

function analysis, epicentral distances ranging from 600 to 1000 were used.

The azimuthal coverage of both PRF and SRF waveforms binned at 200

backazimuth intervals is appropriate to analyze the observed receiver

functions in terms of both azimuthal and radial anisotrophy beneath most

of the seismic stations.

Figure 3: P and S receiver functions are shown. In the upper panels are shown P-wave receiver functions (PRF) of Q and T components at 400 backazimuth for station ARSA. In the lower panels are shown S-wave receiver functions (SRF) of L and T components at 600 backazimuth for station AQU. Various color receiver functions other than green are discarded because of falling outside the error bounds defined by ± standard deviation.

Figure 4: In the upper panels are shown P-wave receiver functions (PRF) of Q and T components at 1800 backazimuth for station ARSA. In the lower panels are shown S-wave receiver functions (SRF) of L and T components at 1200 backazimuth for station AQU.

These and similar RFs are discarded because of uncertainties in the averaging process.

Figure 5: Backazimuth dependent PRF results obtained for station ESK are shown. Red traces show the Q component and blue traces show the T component. The green color traces correspond to the joint inversion of Q-component P receiver functions and Rayleigh fundamental mode phase velocities (e.g. Çakır and Erduran 2011).

Figure 5: Backazimuth dependent SRF results obtained for station ESK are shown. Red traces show the L component and blue traces show the T component. The green color traces correspond to the theoretical L-component SRFs due to the model structure after the joint inversion of P receiver functions and Rayleigh fundamental mode phase velocities.

EGU General Assembly 2012

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α0=β0

α0 ,β0 are the isotropic reference velocities of P and S waves