Transcript body waves - Unesco Magrid

CNRST – MARWAN
Morocco
3-D crustal structure using seismic
waves P and S (SimulpPS)
Youssef Timoulali
CNRST
[email protected]
EUMEDGRID-Support / CHAIN/ EPIKH Workshop Rabat 16 June 2011
Types of Seismic Waves
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
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
Tij 
 uds
(1)
tij  i  Tij
(2)
source
Where u is the slowness field
ds is an element of path length
(u=1/V où V vitesse locale)
τi : origin time
tij : arrived time (from station)
Model parameters
Receiver (Location)
Observed and arrived times
Only known
The source coordinates (x1,x2,x3)
Origin times
Ray path
Slowness field
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).
The misfit between observed and predicted (calculated) arrival times
are the residuals rij
rij = tij obs –tij cal
rij positif Low velocity
rij negatif high velocity
The residual can be related to desired perturbations to the
Hypocenter and velocity structure parameters by linear
approximation
(3)
Goal of local earthquake tomography
Tomo= section
Graphy = image
* 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..
Methodology
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.
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 physique
data
Grid computation
Job.JDL
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-pbseumed"
3-D computation result
West Alboran Sea crust structure
Total earthquake = 5972
Observation total =110176
P obs = 58774, S obs = 51402
PC/Windows
Computation began at 14:47:01
Computation finished at 17:43:37
Linux/ Grid
Computation began at 13-Jun-11
14:01:19
Computation finished at 13-Jun-11
14:33:33
Crust velocity perturbation
Profile along longitude 5°W
Crust velocity