Post on 28-Dec-2015
transcript
Teleseismic Location
• find direction of signals based on Array algorithms• backtrace ray paths through the earth• simplifications: flat earth, plane waves• usually high or reasonable waveform similarity
Epicentre Location using Arrays
estimated ray path
EstimatedSource
ReceiverArray
Angle of Incidence, Slowness
P
global velocity model
Problem: inaccuracy due to deviations from velocity model at the receiverSolution: array calibration (empirical corrections to direction)
Principle of Array Analysis
incomingplane wave
S1 S2S3 earth’ssurface
recording stations
time
S1
S2
S3
resulting seismograms
t2 t1 t3
for a given station geometry: t1, t
2, t
3 (observed) → plane wave (azimuth and slowness) → t
1', t
2', t
3' (theo)
for appropriate configuration
incomingplane wave
S1 S2S3 earth’ssurface
recording stations
t1, t
2,..., t
n (observed) → plane wave → t
1', t
2',..., t
n' (theo)
(t1, t
2, ... , t
n) ≈ (t
1', t
2', ... , t
n' )
aperture too large / frequencies too high
incomingplane wave
S1 S2S3 earth’ssurface
recording stations
t1, t
2,..., t
n (observed) → plane wave → t
1', t
2',..., t
n' (theo)
(t1, t
2, ... , t
n) ≠ (t
1', t
2', ... , t
n' )
highveloc.
lowveloc.
problem with small arrays
incomingplane wave
S1 S2S3 earth’ssurface
recording stations
estimated ray path
EstimatedSource
ReceiverArray
Angle of Incidence, Slowness
P
global velocity model
Two ways of determining the plane wave
time
S1
S2
S3
resulting seismograms
t2 t1 t3
a) measure t1,t2,t
3 directly and invert for slowness,azimuth
b) try many plane waves systematically,inversely apply (t
1',t
2',t
3') delays and sum:
assume plane wave with slowness and azimuth,compute theoretical delays (t
1',t
2',t
3') and apply,
in most cases it looks like this:
if you come close the true values of slownessand azimuth you will get aligen signalsand constructive summation:
comparesummationamplitudes
FK diagram
30°
60°
120°
150°210°
240°
300°
330°
4
8
12
slo
wn
ess azimuth
constructive summation(correct t
1', t
2', t
3')
destructive summation(wrong t
1', t
2', t
3')
Example: FK analysis, GRF arrayEvent S. XinJiang, 25-Jul-2007, mb 4.6
30°
60°
120°
150°210°
240°
300°
330°
4
8
12
slo
wn
ess azimuth
Tradeoff: location accuracy and coherency
Frequency
Array aperture
no coherency
no array features
lowresolution
good array features
location possible,
low coherency
Arrays in Germany4°
6°
6°
8°
8°
10°
10°
12°
12°
14°
14° 16°
46°
48° 48°
50° 50°
52° 52°
54° 54°
4km
100km
1000km
GERES: aperture ~4kmfrequencies: 1 - 50 Hz
GRF: aperture ~100kmfrequencies: 0.1 – 5 Hz
GRSN: aperture ~1000kmfrequencies: 0.01 – 0.5 Hz
Array aperture
no coherency
no array features
lowresolution
good array features
location possible,
low coherency
0.05 501
GRSN
GRF
GERES
10°
10°
12°
4km
100km
1000km
Frequency (Hz)
Resolution of German Arrays
Benefits of Array Data Processing
• Improvement of signal/noise ratio• Determination of slowness and azimuth• Phase identification• Location of remote events• Rupture tracking
Phase Identification
P wave
PP wave Source
Earth’s SurfaceReceiver
Angle of Incidence -> Slowness
P
PP