AIM second annual meeting September 29-30, 2011 Prague

The computation of the elementary synthetic seismograms

for Isola MT inversions in Dobrá Voda region

using the 3D finite-difference method

AIM second annual meeting

September 29-30, 2011 Prague

in cooperation with industrial partner:

Juraj SEKEREŠ Dagmar SEKEREŠOVÁ

Progseis, Ltd., Trnava, Slovakia

Martin GÁLIS1,2 Lucia FOJTÍKOVÁ1,3 Jiří ZAHRADNÍK4

1 Geophysical Institute, Slovak Academy of Sciences, Bratislava2 Comenius University Bratislava

3 Institute of Rock Structure and Mechanics,Academy of Sciences of the Czech Republic, Prague

4 Charles University Prague

introduction

the area of our interest

used names

Dobra Vodaor

Little Carpathiansor

Male Karpaty

the region withsignificant seismic activity

with respect to Slovak territory (1906 Ms 5.7)

3 methods for moment tensor inversionwere tested and compared

with respect to their accuracy and stability :

FOCMEC Pg and Pn wave polarities

AMT P-wave amplitudes

ISOLA full waveform

introduction

motivation

introduction

motivation

recent earthquakes

national network

MKNET

introduction

ISOLA

first step

synthetic elementary seismogramsare computed for a set of stations

elementary seismogram=

ground motion at the stationdue to one elementary focal mechanism

first step

synthetic elementary seismogramsare computed for a set of stations

elementary seismogram=

ground motion at the stationdue to one elementary focal mechanism

second step

the coefficients of the linear combinationof synthetic elementary seismogramsare find by minimization of the misfit between the real data and synthetics

introduction

ISOLA

introduction

motivation

ISOLA is distributed as a packagewith a programs to computethe elementary seismograms

only for 1D medium

the limitations of 1D mediumwith respect to realistic 3D structures

are obvious

therefore we decided to try to use ISOLAwith synthetics for 3D medium

from model definition to computational model

model by Geofyzika, 1985

extended model


spline interpolation- presence of false details, high-frequency content

2D interpolation



bi-linear interpolation on quadrilaterals- presence of stair-like structures

2D interpolation



bi-linear interpolation on quadrilaterals- presence of stair-like structures

linear interpolation on triangles with the same orientation- generally good results, but still some local sharp corners

2D interpolation


linear interpolation on triangles with adapted orientation

for each quadrilateralthe orientation

with smaller gradient at the triangle’s contactis used for the interpolation

2D interpolation


2D interpolation


linear interpolation on triangles with adapted orientation

we applied 2D interpolation in successive stepsto xy, xz and yz planes

to obtain a 3D grid with spacing 100m

we applied volumetric smoothingto remove the artifacts of linear interpolation

(discontinuous derivatives at the triangle edges)

3D interpolation and smoothing


model visualization

4

2

6

3

5

Vp[km/s]

51 km

31 km

depth = 0 km


4

2

6

3

5

Vp[km/s]

51 km

31 km

depth = 1 km

model visualization


4

2

6

3

5

Vp[km/s]

51 km

31 km

depth = 2 km

model visualization


the model by Geofyzika is definedonly in terms of Vp (P-wave) velocity

for 3D model we used the same Vp / Vs ratio as for 1D model:

Vp / Vs = 1.75

Vp to Vs


-0.50

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

0 1000 2000 3000 4000 5000 6000 7000 8000

ND Cornwell

ND Brocher

Gardner (standard)

1D model

den

sity

[g

.cm

-3]

Vp [m.s-1]

Vp to density


we used FD scheme and program developed in Bratislavamainly by Peter Moczo and Jozef Kristek

finite-difference method

brief info



brief info

in two recent exercises,ESG2006 and E2VP-Cashima project,

participated teams using different numerical methods(finite-difference, finite-element,

spectral element, discontinuous galerkin, pseudo-spectral)



brief info

in two recent exercises,ESG2006 and E2VP-Cashima project,

participated teams using different numerical methods(finite-difference, finite-element,

spectral element, discontinuous galerkin, pseudo-spectral)

these exercises showed thatif our FD scheme was applicable to the problem configuration

(for example if planar free surface was an acceptable approximation)it was the most accurate and the most efficient method

V14

results

0 3 6 9 12 15 18-0.00004

-0.00002

0.00000

0.00002

0.00004-0.00004

-0.00002

0.00000

0.00002

0.00004-0.00004

-0.00002

0.00000

0.00002

0.00004

0 3 6 9 12 15 18

A

E

I

comparison of 1D and 3D synthetic seismogramsSMOL – very close bedrock station

results

Z

NS

EW

0 3 6 9 12 15 18-0.000004

-0.000002

0.000000

0.000002

0.000004

0.000006-0.000004

-0.000002

0.000000

0.000002

0.000004

0.000006-0.000004

-0.000002

0.000000

0.000002

0.000004

0.000006

0 3 6 9 12 15 18

comparison of 1D and 3D synthetic seismogramsHRAD – distant bedrock station

results

Z

NS

EW

0 3 6 9 12 15 18-0.0002

-0.0001

0.0000

0.0001

-0.0002

-0.0001

0.0000

0.0001

-0.0002

-0.0001

0.0000

0.0001

0 3 6 9 12 15 18

results

SPAC – distant station on sediments

comparison of 1D and 3D synthetic seismograms

Z

NS

EW

conclusions

we prepared the 3D computational modelof the Dobra Voda regionfor 3D FD computations

(however the model should be considered as very preliminary)

we applied the 3D FD method to compute3D synthetic elementary seismograms

for ISOLA moment tensor inversion

we tested this procedureon two events, V03 and V14

(the results of the analysis will be presented in next talk)

we are ready to apply this procedureto more events

with the same modelor with improved model once it will be available

thank you for the attention

Documents

AIM second annual meeting September 29-30, 2011 Prague