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