7

THREE.DIMENSIONAL VELOCITY STRUCTURES OFVP AND VP/VS AROUND ASWAN AREA, EGYPT

By

Khalil, A E." , El-hady, S. M.*'r' ancl Hoseny. A.""* Ectrllr Science .s De pt., Facultt' o.f Sciettce, Helwart Urtiversit)'

"* National resaarclt Institlte of Astrotntnt ottd Gcoplt':;ics

ABSTRACTDeterminatiort oJ rlrc -1-D Vp antl VpNs velocitl, structures for tlrc northent port o.f Loke Nuscr,

west of tlrc High Datn, Ast,art urea is the rnain task of'present wnrk. Such task is ttc:ltievetl tlrrcuglrtlre inversions ol local tavel time tlota o.f higlt quality recortlecl ort l3 stcttions ol llv Ar'r'cut sci.trnir

network. lnversions urc conducled tlrrough tlffirenr depth slices o/ 0, 1,5,4.3,6.8, 10, 13. t5, l8ancl 22 knt.

The results o.f the 3-D Vp/Vs invcrsions shoyvecl that ntost o.f the seismicii, tends to occur itt the

viciniry oJ zones of lov' Vp/Vs values. Moreover, slrong events tetrcl to occur in Tone s r2f' higlt Vp/V:contrasl. Such obscrvatiort sultltorl.s the ltytotlrcsis of tlrc inlersection tlrcort', in wlticlt sai.srriclll r.r

clusterecl vent near to lhe intersectiotts between the ntajorfaults in tlrc study area.The resuLts ofllrc present vrork help in ntore accurate detenninatirn o.f seisntogettir: lat'ers ilr tltt

sluclj- area. Thus, our rnotlel is usefiil.for nntqt ssisp616gicctl appliccrlions ranging.fi'cttn seisnolt,t:-tortics to nitigation arul preparedttess offuture large sltocks in such higltlt,strategic arta.

INTRODUCTION

Seismic tomography is a tool to deter-mine the subsulf'acc velocity structure us-

ing inversion algorithm. The inversion al-gorithm is non-linear in naLure. The non-linearity stems tiorn thc fact that both theearth model and the hypocentral locationsmust be determined simultaneously. Inmosf cases, the solution to the non-linear'inverse problem is carried out through a

linearization about a reference model.Thus, such solution usually starts by con-structing an initial rnodel (A priori mod-el). The initial model is necessary for thelinearization inversion of travel times torender the expected l'esults.

Evans et al. (1994) provided a suitablesolution to the problem in order to deter-mine the 3-D images of the earth. They

proposcd a method that uses a prio|i vc-locity model (l-D model), travel limcs,and hypocentral locations. A good distribution of local earthquakes is requirc(lthrough the arca under investigation.Also, a good selccl.ion o1'thc grid nodcs is

important and depends on the number ofrays passing through the mode L

In Lhe present study, thc 3-D Vp. 3-DVp/Vs, and also 3-D Vs rnodcls are corn-puted using good quality data. Thc dataare the arrival times picked at high prcci-sion li'orn di-eital wavefbnns with low rc-siduals and -qood ray coverage frorn localearthquakes recorded by the Aswan localtelemetered network.

Data:

In the present study, about 192 evcntsfor Vp inversion and 173 events tbr Vp/

303

304 Klwlil, A. E et al

Vs inversion were selected. The datawere recorded by the Aswan local tele-metered network, which is operated and

maintained by the National Research In-stitute of Astronorny and Geophysics.This seismic network consists of l3 short-period seismometels, with a digital acqui-sition system and a trigger algorithm fbrevent location. Data were sent via a tel-emetry radio link to the acquisition centerat Aswan. All stations are equipped witha vertical seismometcr. except stationsGMR and GRW that are equipped withtriaxial seismometer.

The selected events are characterizedby their well azimuth distr-ibutions (less

than 150 degree) and good locations

32,8'

23.2'32.2" 32.4" 32.6" 32.8"

i

Fig. (l): The location map ol the observed microearthquakes thatstructure Calculations.

covering zr time span tr-om 1988 to 1993with local rnagnitudes ranging from 2 t<> 4and depths ranging from 3 krn to 24 knr.,Fig. (l). The locations werc calculzrtedusing both HYPOTI prograrn (Lee andLahr, 1975) and VELEST pro-eram(Kis-sling 1988).

The 3-D Vp inversion was obtainedusing 2000 P-wave arr-ival times with an

estimated picking accuracy ol' +0.04 s.

The data set for thc 3-D Vp/Vs modelconsists of 850 S-wave an'ival tirnes withpicking accuracy of + 0.10s for thlee-component seismometers and + 0. l2s forvertical seismometers. The earthquakesfbr the VpiVs models were carelully se-

lected to insure a signilicant nurnber of S

arrival times.

33.2"

23.8"

23.6"

23.2"33.2'

have been used in 3D Velocity

^** /rHi".t

o

T H R EE.D I M EN SI ONAL V ELO C ITY STR U CTU R ES

Model caLcuLation:

Tire calculation ot' the 3-D velocitystructures was carried out using SI-MULPSI2 toutine (Evans et. al., 1994).

The algorithm is capahle of producing 3-

D Vp and 3-D VpA/s clustal structure us-

ing the arrival times ol'the Pwave and S-

P travel times from loca[ earthquakes and

also from man made explosions.

The area undcl investiga(ion is repre-sented digitally by glicls. The rnodel di-mensions are 400 krn in the X-direction,400 km in the Y direction, and 28 km inthe Z-direction. The grids are distributedin the X-direction at -200, -30, -20, -10, -5, 0, 5, 10,20, and 200 Km. The distribu-tion of grids is the salne fol the

Y-direction. In the Z-direction the gridsare assigned at 0, 1.5, 4,3, 6,8, 10, 13,

15, 18. 22, and 28 Km. A map view ol'the model is shown in Fig.(2). The coor-dinates of the grid cen[el al'e the latitudc23 34.12- N and the longitucle32"39.00tr'. The horizontal distanceamong the grids is small in the centralparl, where seismicity is more dcnse, trlgive an appr-oximately homogeneous raycoverage within the blocks.

The calculation statts by assigningvelocity values tbr all the nodes of the 3-D grids. These velocity values werc de-

rived fi'om the reference minimum l-Dmodel which has been calculated for the

studied area. The minimum l-D moclelfor the studied area was calculated using

32.2" 1) t' 32.6' :13.?"

23.8' 2i.,t"

x.6' 23.6'

23.4"

23.2', 23.z'.32.7" 32.6" J_1 33.2'

Fig. (2): Ret'erence map showing the location of the 3-D inversion grid: the triangles indicate the seis-mic stations; open circles represent the earthquakes used fbr inversion. The trap shows alsothe location of the fbur cross-sections AAl, BB1, CCI and DD1.

706 Klnlil, A. E et ul.

Velest program (Kissling. 1988). The ve-

locity values at any other poinLs al'e ob-tained by linear intelpolation between the

nodes. The travel tiu.re flom hypocenterto station is calculated hy pseuclo-bending

method (Um ancl Thurber. 1987). Then,the solution is obtained by an itcrativeprocedule, solving l'or hypocenter loca-tion and calculating the velocity of the

rnedium using a darnped least-squares ap-

ploach.

A damping parametel' was added to

the diagonal elements of the separated

medium matrix in orcler to suppress largemodel changes which wourld occur fornear-zero singular values. The dampingparameter for each inversion was chosen

empirically by cvaluatin-c a trade-offcurve of data variance and solution vari-ance, as the damping varies witl.r the nrod-el grid and the data set (Eberhart-Philips,

l 986).

The 3-D Vp wns calculated by fixin-ethe Vp/Vs values as obtained t}om the 3-

D Vp/Vs model and allowing hypocenterrelocation. Also, the 3-D Vp/Vs modelwas inverted by lixing the Vp values as

obtained from the 3-D Vp model and al-lowing hypocenter relocation. The final3-D Vp and Vp/Vs nrodels were calculat-cd by tixing some nodcs o1'Vp ancl Vp/Vsvalues with allowing hypocentel reloca-tion. On the other hand, thc topographyol'the study area has been considered by

applying the station elevation in the inver-sron process.

DISCUSSION

It is important to discuss the reliabilityof the calculated model befbre issuing itspresentation. Thc hest rrcasurc to esti-mate the quality ol'thc invelsion result is

the resolution matrix. Thc resolution

matr-ix describes the distribution ol' intirr-rnation lor each node. such thaL caoh lowis the avera-9ing vector lbr a petrantctcr.

In this study, Ibr thc 3-D Vp ancl Vp/Vsmodels, most ol'the earthqutrke hypocen-[els are presented at depths ran-uing fiom'5 to 20 km. so the resolution is lrigh lorthis depth range. Fig. (3) shows thc ob-

tained resolution (diagonal elernents oIthe resolution matrix) for 4.3, 13 antl 22

km layers fbr both Vp and Vp/Vs.

Based on the tonrographic results, the

distribution of Vp and Vp/Vs anomaliesate characterized by marked latelal and

depth variations reflecting structurtrl het-

erogencities. Sharp vclocity contlasts arc

expression o[ horizontal and vertical dis-

continuities allowing delineation ol the

fault geornetry at depth (Philips ancl Mi-chael, 1993). The depth slices for Vp and

Vp/Vs are represented in Fi-uures (4 anc,

5). Relocations ltrr all the events used ir

this study were also conductcd using thc

czrlculated velocity structure.

The results of spatial distrihution ol'

Vp and Vp/Vs values ancl seismicitywith depth are shown through some com-piled vertical cross-sections along pro-

I'iles AAl, BBl, CCI and DDI (FiS. 2).

These sections are shown in Figs. (6 and

7). Thc results ol'these sections conl'irmthe increasing ol' P-wave velocity withdepth with lateral variations in each

layer'. Also, seclions of Vp/Vs pointecl to

the rheology of the studicd area. TI're

low Vp/Vs areas indicate higher rigiditl'crustal rocks of brittle behavior whilc the

increase of Vp/Vs ref'lects arcas ol'more sofien crustal rocks of ductile be-

havior (Seiduzova et al. 198-5).

T H R E E- D I M EN S I O NAL V ELO C ITY STR U CT U R IiS .107

,.,", IN

(A)32..1' 12 5" J2.6' 12.7'.

!2-1'

t>,-C3+ ars'

12.e' a2.9'.

( l))-12.6' 12 7" t2.8" 3f.9'

23.8'

23-7'

23.6'

ar.5'

23A'

x.3'

8.8'

23.1'

23.6'

a5'

ai.{'

8,f,'

23.8"

23-7'

2t.6'

235'

2f,t'

23,8'

23-7'

23-6'

2:l-5"

a14'

*f'

23J'

L1'

Ll-6'

21.5'

2-{'

LAi

Bl'

B-1'

x-6'

8.-i'

a.1'

2f.f,'

12.{' f2 5' .t2-6'

(B)3aJ' 32.5' 32.5' t2.7' 32.0'

31J" J2.5"

32.4' 32.5'

t2-6' 32-7' 32t" -r2.9"

(Il)t2,6' 32.7' !2.8' J2.9'.

BX'

L7,7'

x,6'

aJ'

LlA"

223"

2S.a'

2t.7'

2t-6'

!5'

a.{

233'

mJ' 23.6'

a,7' 23.1'

8.6' 23.6"

2J.s' 25.-r"

L1A' 2f,i'

an3' a!3'

r[,,

\l

3a8' J2,9'

By compiling the tomographic resultsand the distributions ol the relocatedearthquakes, we noted that the ear-th's

crust underneath the studv area could be

divided into two pa,'ts, upper and lower.

t2-4' f2 5

x-8'

B-6'

a3.s'

?-i.{-

13'324- J2,5' 32-6' tZa' 32.8' fr.9' 32.4' 325' rzd' 317" -12.4- 32.0'

kmoloaim

Fig. (3): Resolution values of Vp (A,B andC) and Vp/Vs (D,E and F) fbr three depth slices(4.3,13 and 22km).

23J'

5-6'

E,-S'

8.4'

aa-1'

The upper crust extends tiom the surlhcedown to a depth of 13 km. At the sur-

face slice, the velocity values are gener-

ally very low due to the occurrence ofsedimentary rocks (mainly sandy clay)

.j0(t KlnLiL. A. E ct ol

2-1,6

Cqoa;

=DL-'r- io,

ttt =ZIJJ

^r ici o

h: t,4o

nL-!6 ci,--Uc-

), a Y-_+:

^v,>2 : _u6 ., !

, UEa-f

!=::-r!

.m,aY'JD.)'r

- N,

=\ >. -5

OG>O

'6 aP,a=>.:

O^>6-tnu=

> :lafq>3^-;.tat)ii

l.t.l" .r!.,1" -1:J' .1:.6' J2)'. -12 8' 1:9

.12.1' t:4' J.IJ' Jl 6' -u 7' .t:-8 .12.9

11.6',

especially toward the west, wherc the

thickness ol'the scdintcntury cr)vcr is

higher than the eastcrn part. For the sliceat depth ol 1.5 km thele js an ENE-WSWtrending I'eature o1'low P velocity rlear the

center of the alea. This t'eature may l'e-

flect intersection of the E-W fault system

with the N-S thult system. Such interpre-

tation agrees with Taha (1991).

2::

l2 {" -t!-r l:6' l:.7 17A' ll I'

At depths of 6 krn, 8 krn, l0 knr. 13

krn, 15 krn and l8 km, an E-W ancl N-Selongated anomalics are found Thescelongatcd anomalies are chat'actcrizccl byhigher values ol Vp (r'an-eing l'l'oln 6.2 to6.6 km/s) compared to the surroundingareas. These anomalies could be inler-preted as either shallow dike intrusions(Taha. 1997) ot due to uplifiing ol thcbasement rocks (Issawi, 1978).

km0102010

1.2,-r' l2 J' lzJ -rl i' tt-7" Jt,N' t2 9',

THREE-DIMENSIONAL VELOCI, )' sTR i.tc ! t xt'J .t09

;O

_c

ocZ

E-'+

Oc

aCa

aOE

=1E

rril

N

=^- t.,I

!..oll

oc)E'

';_=x

flJ-

;.

Ed=

Ilclrtir e yrlocit]., changc ill ']i,

R

within the very-low Vp/Vs zoncs and acl-

justs to thc hi-eher \/p,Ts arcas

For the lower crust (13 28 krn). lowVp/Vs values were generally obscrvecl

around the more tl'actuled zones indicat-ing a more brittle behavior of rocks Thchigher Vp and Vp/Vs vzrlues wcle oh-served around the boundalics of l,he faulttrend. The largest carthclutikc occulrins

Fot the results ol' 3-D Vp/Vs tomo-graphic in the upper crust, it is known thatthe mean-valuc o1'Vp/Vs ratio is the mostimportant parameter to investigate therheclogic pattern. The near'-surtace highVp/Vs values within the uppcr zone (li'omsea level down to l.-5 krn) may representthe presence ol water-saturatecl cracks.For depths in ran-sc ol' 4.5- l0 km,rnicroearthquake activity wns occurred

3t0 KhaLil, A. E et al.

23 t"

2r 6'

rJ 5'

/+'

ouc-o!

oU.=

i(l aEooo

-^o

=oi,6t _7(,.-!

=0"-a@9

.,n_

/9-> -o

l7r1 .; o

nO

l(NO}

401r,)h

= F-=-X

)&-= oa+,q_

.dae

-=>5:ln

E]J

G-

12!'

on November l4th l98l of local magni-tude 5.5 (Woodward-Clyde Consultants,

1985) was located at the high contrast ofvery-low and high Vp/Vs values (Fig.5),

This finding means the lespective event

took place close to the brittle-ductile tran-

srtlon zone.

In the upper- crust, tlre seismicity is

concentlated in the eastern side near the

32i_ ll 6' 377- 11 8'

12.7' lt s" 12 c'

,17

rl"

intersection of Khor El-Ramla fault withthe extension of Kalabsha l'ault. On the

other hand, in the lower crust, the seis-

micity is concentrated in the western par'1

near the intersection of Kalabsha and

Abu Dirwa taults. This supports thc hy-pothesis of the intersection theory (Tal-

wani, and Rajendran, 199 l) where locali-zation of stresses tends to occur near the

intersections of major faults.

ll s'

ll 7-

21

km0r3

T H R EE.DI M ENS IONAL V ELOC ITY STRU CT U RES

Vpr\,s

31;:l:i?!f,!

t=H

-U l=

I.

km. These values might,hc related to the

intersection betwecn structural faults ol'the interested area. It could be enrpha-sized that the most ol' seismicity are oc-curred in the zones of hi-eh contrast be-

tween the low and high Vp/Vs values.The clistribution of carrtlrquakes c()n-strained with the tonroglaphic resilltssupports the hypothcsis ol' thc int-crscc-tion thcoly. Most ol'thc scismicit-v wus

dLCO

cZ:E

lO

E

cc

-2!s

caE

!I

N-!

=ri9rrO-a!2o-s.!;=9F

=&f-'

:>l);-

cll .

l;

fi

, CONCLUSION

The overall Vp and Vp/Vs patterns re-flect the complex geologic structure fea-

tures fbr the investigatcd area. In thepresent work, hi-eh Vp/Vs values were as-

sociated with the parts of low rigidity.Through the 3-D Vp/Vs structure, the lowVp/Vs values were observed at the differ-ent slices fbl depths in range of 1.5-18

E-I v1, 1i.

_t t2 Klrulil. A. E cr ul

\ ) n <r scc

v;-

il

(J.,

:2

{.i

Ll.,

66F

6e

'a

-r

=cl!-

a)Labo-

E-

=qeazC==a.J,.D

o)

=-€EC.

Co.=

--..JCA-,L I:

-a

>?=v-eC

=..'=?

,' =,-,==z--)>

,L -a>=:941.

tL

orril trr rjrrhll '

o.2-

.=.,.

rrtl rtr r1td.r11

T ]] IT II E, D I I,I 1,, N S I O N A L V II LO C ITY ST R U C T t J R E S -l /-l

\rp\rrl;-=g=_:€EtGi;

=--') ",-=a))'_ a))

oE=r

:.)

Uf,

=OOC

U,-

=r,

1) ,-

'ao-oO=cL-a

-C1,5ao>i

co

'?-a"Dq=ta_

'r' =C-

2>

')=O ,_

;.f--

bjjn

=!aLu) Lt tlcJ(

.-'O-

)"i

.a

LIL\ ilr Lil(liC

3t4 Khalil, A. E et ul.

clustered close to the intersestions be-

tween the ma.jor taults. The result of thecurrent study is useful fbr future works inthe region concerning with seismotecton-ics and seismic hazard evaluation.

ACKNOWLEDGEMENTS

We wish to express our deepest grati-tude to Mohammad C. Al-Ibiary, Prof'es-sor of Seismology, Faculty o[' Science,Helwan University and Ezz El-Din Mo-hamed Ibrahim. Prolbssor of Seismology,National Research Institute of Astronomyand Geophysics (NRIAC) fbr their kindencouragement and valuable discussionduring the progress of the whole wolk.

REFERENCES

Ebbrhart-P hilip.r, D., I 986. Three-di rnensional

velocity structure in Northerrr CaliforniaCoast Ranges frorn inversion ol'Localearthquake amival tirnes: B.S.S.A., vol.

76. pp 1025-1052.

Eberhart-Phililts, D., oncl Michacl, A. .1., 1993.

Three-dimensional velocity structure,

seismicity and tault structure in the Park-

tield region, central Calitbrnia, j. Geo-

phys. Res., vol. 98,'137-758.

Evans, J. R., Philips, D. E., and Thurber, C.,'1994. User's Manual tbr Simulpsl2 forImaging Vp and Vp/Vs, USGS-OFR-431.

Gharib, A. A., 1992. The Crust and the Uppermantle Structure in the Lake Aswan from

Seismic Waves Generated by Earthquakes

and Explosions: Ph.D. Thesis, Fac. Sci.,

Cairo Univ., Egypt.

lssawi, 8., 1978. Ceology of Nubia, West

area, Westem Desert, Egypt: Annals Geol.

Survey ofEgypt, vol. 3 :327-31)2.

.Kisslirtg, 8.. l9ti8. Geotorno-eraplry with

Local Eartlrquake Data, Reviews ol' Ge-

ophys., vol., 26, No. 4. pp. 659-698.

Lee, W. it. K., o,rd kthr, J. c., lc)75. A corn-

puter Program foryletermining hypocen-

ter, magnitude, and tirst motion pattern ol'

local ea(hquakes, U.S. Geol. Survey

open li le Rep., 75-3 I I .

Tahu, Y. S., 1997. Evaluation of the Crustal

Structure Settin-e of Aswan area. Ph.D.

Thesis, Faculty of Sciencc, Cairo Univer-

slty.

Seiduzova, S.S., Surov, V.P., Matasotct,

1.M., Arabev, K.A., Suchkova, S.S., Ri-

abova, 5.D., and N.N. Yankovaskttt'n

1985: Special f'eatures of deep structurcs

of earth's crust and seismic hazard lor

the territory of eastern Uzbekistan . Papcr

presented at the 3rd international sym-

posium on the analysis of seismicity

and seismic risk. Liblice, Czechoslovakia,

Jtult'e 17-22 .

Talwani, P. and K. Rajenclran 1991. Sonte

seismological and geometric t'eatures ol'

intraplate plate earthquakes; Tectono-

phys. vol.l86, pp. l9-41.

Um, J., and Thurber. C., 1987. A fast algo-

rithrn tbr three-dimensional seismic ray

tracing (abst.): Eos Trans. AGU, vol. 67,

p.304

Woodward-Clyde Consultants, 1985; Earth-

quake activity and stability evaluation tirr

the Aswan High Dam; Unpublishecl re-

porti High and Aswan Dam Authority,

Ministry of lrrigation, Egypt.