Generation of Converted Love Waves in Irregular Crustal andUpper Mantle StmctWes
MitsuruYoshidall Fortran Plan Office6124‑7 Yutaka‑chou, Kasukabe‑Shi, Saitama‑Ken, 344‑0066 Japan

1 Introduction≠･azimuthal angleof inC･dence

seismic surface waves, which are excited by earthquakes or
nuclear explosion test, are propagated alongthe Earth'S surfaceof the oceanand continent.When the surface waves travel
laterally inl10mOgeneOTS regions,the conversion &om Rayleighwaves to Love waves IS likely to occur. h the present study wenumeriCally modelthe mountain root structure of the Tien Sham,which is located at the westem part of Lop Nor nucleartest sitein China. Seismic long十period Rayleighwaves are assumed tobe propagated obliquely inthe motmtain root structure･ The

tl: irreguLar front inthetransverse dareCtion

propagation of Rayleighwaves acrossthe irregular crustaland
upper mantleincluding double low velocityzones beneath血emountainroot is simulated bythe use of 3‑D finite difference
method l1, 2]･ The generation of Love waves copverted &om

Rayleighwaves can be observed throughtime series and wave
motion fields̲ The eigenfunctions of converted Love waves areinvestigated onthe basis of normal modetheory･2

MountainrootstructureandRayleighwave

The distance between sites 5 and 15 15 250 km.

beneathsite lO'. However,these pattem is not found in the Vcomponent･ The oblique displacement distributions of the W
BLnd U components are extended in die upper mantle near die
low velocityzone (LVZ4) at depths of l1215 ‑162･5 km･ They

can be caused by reflections of Rayleighwaves travelling
towardsthe wave'Sincomng direction. Parts of scattered waves

propagation

are transmitted in inner parts of the Earthandmight be

The oblique incidence of plane Rayleighwaves on theirregular

Fig I. A bird's eye Ⅵew of the oblique mcldence ofRayleigh waves

aont (tl) of the mountain root structure isschematically shownin Fig･ 1･ Wavesare comlng &omthe le氏(site 5) and are outgoing to theright･ Waves travel towards site
15 (め ‑0 deg.) and 15'(4･=60 deg･) forModels T and LB･ The

model LB has a double low velocity zone 0,VZ) beneaththemountain root structure while the Model T does not include aLVZ. Physical parameters of velocities of compressional(Vp)and shear (Vs) waves,and density(p ) for the eastern andwestern regions of die Tien Sham are used･When Rayleighwaves travel for the azimuthalangle of incidence of め ‑0 deg･,
血ere is no surface wave interconversion l3]. Conversion丘omRayleighwaves to Love waves increases with the increase of theazimuthalangle of incidence l4, 5]･ Scattered waves for 4'=60

deg.are shown in Fig･ 2･ Rayleighwaves of the W and U
componentsare not so different between Models T and LB,

converted to compressionalwaves (P waves) and shear waves (Swaves) [6, 7]･ hthe crust and upper mantle between yte lO'and15', wherethe renection is not slgnificantthe positive (solidline) and negative (dotted line) displacements of the W and U
componentsarealtemately distributed inthe inner parts as wellas on the grotmd surface･The displacements of converted Lovewaves ofthe V

component, as well asthose of Rayleighwaves of the Wand U
components, decrease graduallywiththe increase of depthinthe
crust and upper mantle (Fig. 3). However,the maxima of theamplitudes of the V‑Component are not always located at theground surface. The maximum amplitude of ‑207 (xl0‑5) cm inthe V‑component, denoted in the depthof about 30 km inthe

right side of site lO', impliesthatthe amplitude pattem isformed inthe process of conversion from Rayleighto Lovewaves beneaththe mountainroot structure.

while bve waves of the V componentare significantly excited
at site 5 for Model LB, The amplitudes of the V componentare
also greater at site 15'for Model LBthan for Model Tl

3

Wave motion field of the displacement

4Eigenfunctions of Rayleigh waves and converted

Love waves

Forthe oblique incidence of Rayleighwaves on themountainroot structuresthe conversion Bom Rayleighwaves toThe displacement‑depthprofiles of the W, U, and V
components for Model LB are showninFig･ 3, in whichthe
Claps time of the time series given in Fig. 2 is 270 secI For bothdie W and U components the displacement distributions
obliquely contoured downwardsinthe

right direction areobserved in a depth range from 150 kmbeneathsite 5 to 300 km

Love waves is likely to occur owing tOthe slope of Moho
discontinuity. The existence of a double LVZ in the crust andthe mantle sb･engthensthe scattering of the verticaland radial
Components, which leads tothe generation of Love waves
whose particle displacement consists of the transverse

component･ For both Models T and LBthe eonversionfrom

Rayleighwaves to Love wares is maximum near at periods ofEIGENFUNCTION and ELGENVALUE
2520‑25 s l4]･ The eigenftmct10nS Of RayleighWチveS and Lovewavesare showninFig. 4,along withthe elgenValues (C).

慧podisepnlta;:Rm,e:thicTiCsbS:霊三霊慧iaTthew;V.eusndOsfiihaece,慧close tothat of Love waves ylL.The stress function of Rayleighwaves of血e W component y2R is relatively similar to血at ofLove yaves y2L ･ The wave number (k) of converted Lovewaves IS Smallerthanthat of Rayleighwaves;the wavelength( 1 ) of converted Love. waves is longerthanthat of Rayleighwaves. The phase veloclty(C) of converted Love waves is faster
thanthat of Rayleigh waves.
￠ニ600‑(T)･･‑･(LB)

ln U00000O020∩ ))n )C100002001O∩ )O1a0O2OnU
ハU0 0 00･(sIoLx)LuUraPn三dLuV

二二‑/4000
2000

Fig 4. Elgen如lCtiorlS OfRayleighwaves (ylR, y2R, y,R, y4R )and Lovewaves (ylL, y2L) foraperiod of20 S Adotdash line denotes the depth
37 5 km of the Moho discontmulty. Eigenvalues areindlCated by phase0veloclties (6)

‑2000
‑4000
090

180

270360450Tlme (SeC)
Flg. 2, Time series of scattered waves of the vertical(Ⅴ), radial N), andtransverse Cv) components at slte 5 forangle oflnCidence of60 dog

5CoJlClusion

Particle motion of Rayleighwaves consists ofthedisplacements of the vertical (W)and radial (U) Components･For Rayleighwave propagation in irregular crustaland upper
mantle structures, a part of Rayleighwave energy converts toLove wave energy,the displacement of Love waves beingtheb･ansverse component (Ⅵ. The generation of converted Lovewaves due toanobliqueincidence of Rayleighwaveswithan
azimuthalangle of 60 deg. onthefront of the mountain rootstructure has been simulated bythe use of 3‑D丘nite
difference method and the wave motion field of

thedisplacement of converted Love waves has been depicted aswell asthose of incident Rayleighwaves. It is suggested &om
the eigen如ICtion analysis thatthe displacement‑depthcurve of

converted Love waves is close tothat of incident Rayleighwaves of the vertical component atperiods of 20‑25S.
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Fig. 3. Wave motion fields forthe vertical(W), radial (切,andtransverse (Ⅴ) Components at an Claps time Of270 S

GeoselenCeUnion Meeting