Transcript Two-station Rayleigh and Love surface wave

Two-station Rayleigh and Love surface wave phase velocities
between stations in Europe
EGU General Assembly 2012
Ö. ÇAKIR, A. ERDURAN, E. KIRKAYA, Y.A. KUTLU, M. ERDURAN
Nevsehir University, Department of Geophysics, 50300, Nevsehir, Turkey ([email protected])
DATA AND METHOD
TOMOGRAPHIC INVERSION
We study the Rayleigh and Love surface wave fundamental mode propagation beneath
Having obtained Rayleigh and Love phase travel times between station pairs for a
the continental Europe and examine inter-station phase velocities employing the two-
given period range, 2D tomographic inversion is performed to estimate variations in
station method for which the source code developed by Herrmann (1987) is utilized. In
surface wave phase velocities over the study area. The Fast Marching Method (FMM) is
the two-station method, the near-station waveform is deconvolved from the far-station
used for the forward prediction step and a subspace inversion scheme is used for the
waveform removing the propagation effects between the source and near-station.This
inversion step. The method is iterative non-linear in that the inversion step assumes
method requires that the near and far stations are aligned with the epicentre on a
local linearity, but repeated applications of FMM and the subspace inversion allows for
common great circle path for which we impose the condition that the azimuthal
non-linear relationship between velocity and traveltime to be accounted for. This
difference of the earthquake to the two-stations and the azimuthal difference between
method has been successfully applied to seismic tomography assuming isotropic
the earthquake to the near-station and the near-station to the far- station are smaller
phase velocities. The source code for the Fast Marching Surface Tomography (FMST)
than 5°. From IRIS and ORFEUS databases, we visually selected 3002 teleseismic,
is provided by N. Rawlinson at the Australian National University and is based on the
moderate-to-large magnitude (i.e. Mw ≥ 5.7) events recorded by 255 broadband
multistage fast marching method (Rawlinson and Sambridge, 2004a, 2004b). The final
European stations with high signal–to-noise ratio within the years 1990-2011.
phase velocity model at 60 s period is shown in Fig. 6 for Love surface waves and in
Corrected for the instrument response, suitable seismogram pairs are analayzed with
Fig. 8 for Rayleigh surface waves. The corresponding travel time residuals before and
the two-station method yielding a collection of phase velocitiy curves in various
after tomography are shown in Figs. 7 and 9 for Love and Rayleigh surface waves,
periods ranges (mainly in the range 25-185 s). Diffraction from lateral heterogeneities,
respectively.
RAYLEIGH
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 EastEuropean 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.
multipathing, interference of Rayleigh and Love waves can alter the dispersion
measurements. The fundamental mode Rayleigh and Love surface wave phase
LOVE
velocity dispersion curves are measured using the MFT tool package in the Computer
Programs in Seismology (Herrmann and Ammon, 2004).
GRA2-SSB station pairs for Rayleigh
KIEV-DRGR station pairs for Love
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.
RAYLEIGH (a)
Fig.1: Rayleigh and Love surface wave MFT results are shown for two different
station pairs.
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.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 twostation paths in case of
RAYLEIGH surface waves.
The other stations in the
continental Europe and the
related areas are shown by
red triangles.
Fig.6: Tomographic phase velocities for the Love surface waves are shown at 60 s
period. The background velocity is assumed as IASP91.
RAYLEIGH (b)
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.
CONCLUSIONS
LOVE (a)
LOVE (b)
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.
ACKNOWLEDGEMENTS
This work is supported by The Scientific and Technological Research Council of Turkey (TUBITAK)
(project number 109Y345).
REFERENCES
Fig.7: Travel time residuals before (a) and after (b) tomography are shown for the Love
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.
phase velocities in Fig. 6. The background (or reference) velocities taken as IASP91 are
generally slower than the tomographic velocities.
Fig.5: The measured Rayleigh and Love phase velocity curves for station PRU are
shown with respect to backazimuth.
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