Two-station Rayleigh and Love surface wave phase velocities between stations in Europe

DATA AND METHODWe study the Rayleigh and Love surface wave fundamental mode propagation

beneath the continental Europe and examine inter-station phase velocities

employing the two-station method for which the source code developed by

Herrmann (1987) is utilized. In the two-station method, the near-station waveform

is deconvolved from the far-station waveform removing the propagation effects

between the source and near-station.This method requires that the near and far

stations are aligned with the epicentre on a common great circle path for which

we impose the condition that the azimuthal difference of the earthquake to the

two-stations and the azimuthal difference between the earthquake to the near-

station and the near-station to the far- station are smaller than 5°. From IRIS and

ORFEUS databases, we visually selected 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, suitable seismogram pairs are analayzed with the two-

station method yielding a collection of phase velocitiy curves in various periods

ranges (mainly in the range 25-185 s). Diffraction from lateral heterogeneities,

multipathing, interference of Rayleigh and Love waves can alter the dispersion

measurements. The fundamental mode Rayleigh and Love surface wave phase

velocity dispersion curves are measured using the MFT tool package in the

Computer Programs in Seismology (Herrmann and Ammon, 2004).

CONCLUSIONS

ACKNOWLEDGEMENTSThis work is supported by The Scientific and Technological Research Council of Turkey (TUBITAK)

(project number 109Y345).

REFERENCESHermann. R.,(1987), Computer programs in seismology. Tech. Rep. St. Louis University, St. Louis,

Missouri.Herrman. R.B., and C.J. Ammon (2004), Surface waves, receiver functions and crustal structure.

Computer Programs in Seismology, version 3.30, Saint Louis University.Kennett B.L.N., and Engdahl E.R., (1991),Travel times for global earthquake lacation and phase

association, Geophsy. J. Int., 105, 429-465.Rawlinson. N., and M. Sambridge (2004a), Wavefront evolution in strongly heterogeneous layered

mediausing the fast marching method, Geophys. J. Int. 156, 631-647.

Rawlinson. N., and M. Sambridge (2004b), Multiple reflection and transmission phases in complex layeredmedia using a multistage fast marching method, Geophysics 69, 1338-1350.

Snoke. J.A.,(2009),Traveltime Tables for iasp91 and ak135, Seismological Research Letters, 80(2),206 262.

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

EGU General Assembly 2012

In order to secure the qulity of measurements we select only smooth portions of

the phase velocity curves, remove outliers and average over many

measurements. We finally discard these average phase velocity curves suspected

of sufferring from phase wrapping errors by comparing them with a reference

Earth model (i.e.IASP91 by Kennett and Engdahl 1991). The outlined analysis

procedure yields 5109 Rayleigh and 4146 Love individual phase velocity curves.

The azimuthal coverage of the respective two-station paths is proper to analyze

the observed dispersion curves in terms of both azimuthal and radial anisotropy

beneath the study region.

Fig.2: Average Rayleigh phase velocity curves for the BFO-MOA and OJC-GIMEL stations pairs are shown on the left. Average Love phase velocity curves are shown on the right for the station pairs of JAVC-SSB and WIT-PAB.

Fig.1: Rayleigh and Love surface wave MFT results are shown for two different station pairs.

Fig.3: Average Rayleigh and Love phase velocity curves for some selected station pairs as indicated on top of each panel are shown. These and similar average phase velocity curves are discarded from the database due to incertainties in the averaging process. In the upper panel, the observed phase velocities are grouped in double and in the lower panel, the measurement quality quickly decreases with higher periods.

TOMOGRAPHIC INVERSIONHaving obtained Rayleigh and Love phase travel times between station pairs for a

given period range, 2D tomographic inversion is performed to estimate variations

in surface wave phase velocities over the study area. The Fast Marching Method

(FMM) is used for the forward prediction step and a subspace inversion scheme is

used for the inversion step. The method is iterative non-linear in that the inversion

step assumes local linearity, but repeated applications of FMM and the subspace

inversion allows for non-linear relationship between velocity and traveltime to be

accounted for. This method has been successfully applied to seismic tomography

assuming isotropic phase velocities. The source code for the Fast Marching Surface

Tomography (FMST) is provided by N. Rawlinson at the Australian National

University and is based on the multistage fast marching method (Rawlinson and

Sambridge, 2004a, 2004b). The final phase velocity model at 60 s period is shown

in Fig. 6 for Love surface waves and in Fig. 8 for Rayleigh surface waves. The

corresponding travel time residuals before and after tomography are shown in

Figs. 7 and 9 for Love and Rayleigh surface waves, respectively.

Fig.4: Some two-station great circle paths are shown for both Rayleigh (lower panel) and Love (upper panel) surface waves. Station PRU shown by a green triangle in the middle has 46 two-station paths in case of LOVE surface waves and 57 two-station paths in case of RAYLEIGH surface waves. The other stations in the continental Europe and the related areas are shown by red triangles.

Fig.5: The measured Rayleigh and Love phase velocity curves for station PRU are shown with respect to backazimuth.

LOVE

RAYLEIGH

LOVE (a)

RAYLEIGH (a) RAYLEIGH (b)

LOVE (b)

GRA2-SSB station pairs for Rayleigh

KIEV-DRGR station pairs for Love

Fig.6: Tomographic phase velocities for the Love surface waves are shown at 60 s

period. The background velocity is assumed as IASP91.

Fig.7: Travel time residuals before (a) and after (b) tomography are shown for the

Love phase velocities in Fig. 6. The background (or reference) velocities taken as

IASP91 are generally slower than the tomographic velocities.

Fig.8: Tomographic phase velocities for the

Rayleigh surface waves are shown at 60 s period.

The background velocity is assumed as IASP91.

The Trans-European suture zone separating the

East-European Craton from the complex tectonics

of the west Europe is cleary visible on the

tomogrpahic image. The eastern velocities are

approximately 5% faster than the background

IASP91 model while the western velocities are

approximately 5% slower. This observation is also

true for the Love surface waves in Fig. 6, but not

as distinct as in the case of Rayleigh surface

waves.

Fig.9: Travel time residuals before (a) and after (b) tomography are shown for the Rayleigh phase

velocities in Fig. 8. The background (or reference) velocities taken as IASP91 are generally faster than

the tomographic velocities.

Surface wave tomographic results show higher velocities in the east Europe while lower velocities in the generality of west Europe. Tomographic results obtained for short and long period surface waves show similar images as in Figs. 6 and 8, although for long periods raypath coverage gets relatively weaker. We obtained tomographic images considering surface wave propagation independent of azimuth, i.e. no azimuthal anisotropy is considered. However, we do not rule out the azimuthal anisotropy as we will also pursue this kind of anisotropy in our database. The current results indicate clear Rayleigh-Love discrepancy in the observed phase velocity curves, which may be explained by radial anisotropy or transverse isotropy with vertical symmetry axis. We eventually intent to jointly interpret the surface waves (Rayleigh and Love), receiver functions (P and S) and SKS splitting data in terms of a common anisotropic structure beneath a seismic station.