Disputable non-DC components of several strong earthquakes Petra Adamová Jan Šílený

Motivation  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 moments Standard moment tensor Point source: couple of planes  a ‘dot’

Introduction to second degree moments “Finite source” parameters from point source approximation Detailed modeling of slip on the fault plane: - costly - often data not available (near the fault) 2 nd 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’

Theory Moments till degree 2, Doornbos (1982) Temporal centroid Spatial centroid Source ellipsoid Rupture propagation Source process duration Zero degree moment tensor (standard MT)

Inverse scheme: full waveform inversion exclusion of the non-physical solutions  inversion is faster Standard MT Estimation of second degree moments

Applications: Location of earthquakes

Standard focal mechanisms zero degree moments

Jack-knife test - Izmit and Kobe

Kobe – 2 nd degree moments Ide et al. (1996) 10 km

Example of seismogram fit Black: observed seismograms Red: synthetic seismograms

Izmit – 2 nd degree moments Clevede et al. (2004)

Synthetic experiment: Estimate of error due to noise for the Izmit earthquake 5 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 mechanisms Standard MT Standard MT without second degree effects computing standard MT estimation of 2 nd degree moments removing 2 nd degree moments from the data computing new standard MT

Conclusions 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