Disputable Disputable nnon-Don-DCC ccomponents of omponents of sseveral everal
sstrong trong eearthquakes arthquakes
Petra AdamovPetra Adamováá
Jan ŠílenýJan Šílený
MotivationMotivation
For some strong events a large non-DC component is reported by agencies
This component is often questionable (large events, tectonic origin) it can be false due to source finiteness (strong event is modeled as point source)
Introduction to second degree Introduction to second degree momentsmoments
Standard moment tensorPoint source: couple of planes
a ‘dot’
Introduction to second degree Introduction to second degree momentsmoments
“Finite source” parameters from point source approximation
• Detailed modeling of slip on the fault plane:
- costly
- often data not available (near the fault)
• 2nd degree moments are advantageous alternative
geometry of the source duration of the source process average slip on the fault spatial and temporal centroid rupture velocity vector
Inside the ‘dot’
TheoryTheory
Moments till degree 2, Doornbos (1982)Moments till degree 2, Doornbos (1982)Temporal centroid
Spatial centroid
Source ellipsoidRupture propagation
Source process duration
Zero degree moment tensor (standard MT)Zero degree moment tensor (standard MT)
Inverse scheme: full waveform inversionInverse scheme: full waveform inversion
exclusion of the non-physical solutions inversion is faster
Standard MT
Estimation of second degree moments
Applications: Location of earthquakesApplications: Location of earthquakes
Standard focal mechanismsStandard focal mechanismszero degree moments
Jack-knife test - Izmit and KobeJack-knife test - Izmit and Kobe
Kobe – 2Kobe – 2ndnd degree moments degree moments
Ide et al. (1996)10 km
Example of seismogram fitExample of seismogram fit
Black: observed seismograms
Red: synthetic seismograms
Izmit – 2Izmit – 2ndnd degree moments degree moments
Clevede et al. (2004)
Synthetic experiment: Estimate of Synthetic experiment: Estimate of error due to noise for the Izmit error due to noise for the Izmit
earthquakeearthquake5 samples of random noise of 20% and 30% of maximum amplitude superimposed on the records
Level 20%: stable solution, small deviation,
for source ellipsoid orientation – max. deviation 5 degrees
Level 30%: unstable solution, large deviation
for source ellipsoid orientation – max. deviation 17 degrees
Black ellipsoid: extreme deviation with 20 % noise
Blue ellipsoid: extreme deviation with 30 % noise
Focal mechanismsFocal mechanisms
Standard MT
Standard MT without second degree effects
• computing standard MT
• estimation of 2nd degree moments
• removing 2nd degree moments from the data
• computing new standard MT
ConclusionsConclusions• Second degree moments bring additional information above standard MT: source ellipsoid, duration of the source process, average rupture velocity
• Neglect of source finiteness, which remains in the data even after an essential low-pass filtering may originate spurious non-DC components in the mechanism.
• The procedure suggested reduces spurious non-DC components substantially