3-D crustal structure using seismic waves P and S (SimulpPS)

CNRST – MARWANMorocco

Youssef Timoulali

CNRST

timoulali@cnrst.ma

EUMEDGRID-Support / CHAIN/ EPIKH Workshop Rabat 16 June 2011

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There are several different kinds of seismic waves, and they all move in different ways. The two main types of waves are body waves and surface waves..

body waves

Types of Seismic Waves

P wave or primary wave (compressional waves)

S wave or secondary wave

Seismic wave modelisation from source to seismic station

The body wave travel time T from an earthquake i to a seismic station j is expressed using ray theory as a path integral

receiver

source

udsTij

Where u is the slowness field ds is an element of path length(u=1/V où V vitesse locale)

(1) Tijitij

τi : origin time  tij : arrived time (from station)

(2)

Receiver (Location)Observed and arrived times

The source coordinates (x1,x2,x3)Origin timesRay pathSlowness field

Only known

Unknown (Model parameters)

The arrival time tij obs are mesured at a station of network

The calculated arrival times tij cal are determined from equation (1) and (2) using trial hypocenters , origin times and an initial model of the seismic velocity Structure ( the priori information).

Model parameters

rij positif Low velocity rij negatif high velocity

The misfit between observed and predicted (calculated) arrival times are the residuals rij

rij = tij obs –tij cal

The residual can be related to desired perturbations to the Hypocenter and velocity structure parameters by linear approximation

(3)

* To improve the estimation of model parameters (structure and hypocenters) by perturbing them.

* The perturbation is used to minimize some measure of misfit data.

* This generally require an iterative scheme for solution..

Goal of local earthquake tomography

Tomo= section

Graphy = image

Computaion of travel time and ray path

It is essential to compute travel time and ray path in a heterogeneous earth model quicly and accurately.

Two approximately tracing techniques developped by thurber works well when epicentral distance are shorter than 50km:

ART : Approxiate Ray Tracing (Thurber 1983)

CHT : Pseudo-bending method, designed to perturb an ART path closer to the true ray path.

Methodology

Approximate Ray tracing (ART) 

Adopt a brut force approach to finding the minimum time ray path.

A large set of smooth curves connecting the soucre and receiver are constructed using an efficient but somewhat arbitrary scheme

The travel time along each curve is calculated numerically,The estimated true path obtained by computed travel times is presneted by « fastest » curve.

The pseudo-bending (CHT) method

Takes a geometric approach to estimation of the true ray path.

This permits a given ray path estimate to have a curvature which varies along the ray, as well as allowing the path to deviate from a single plan.

Inverse problem

* Optimisation to found the cruste structure

* The inversion alow to construct the near true structure from physiquedata

Type = "Job";JobType = "Normal";Executable = "simulps.exe";StdOutput = "simul.out";StdError = "simul.err";InputSandbox = {"simulps.exe","CNTL","EQKS","MOD","STNS"};OutputSandbox = {"simul.err","output","nodes.out","resol.out","f24","velomod.out","hypo71list","f28","itersum"};Arguments = "-f";ShallowRetryCount = 3;

#Requirements = other.GlueCEUniqueId == "ce1.cnrst.magrid.ma:8443/cream-pbs-eumed"

Job.JDL

Grid computation

3-D computation result

West Alboran Sea crust structure

Total earthquake = 5972Observation total =110176 P obs = 58774, S obs = 51402

PC/WindowsComputation began at 14:47:01Computation finished at 17:43:37

Linux/ GridComputation began at 13-Jun-11 14:01:19Computation finished at 13-Jun-11 14:33:33

Crust velocityCrust velocity perturbation

Profile along longitude 5°W