an array analysis of seismic surface waves
DESCRIPTION
An array analysis of seismic surface waves. James Gaherty and Ge Jin LDEO Columbia University. Thoughts and Overview. Surface-waves from earthquake sources provide powerful tool for probing upper mantle structure beneath arrays Good depth resolution - PowerPoint PPT PresentationTRANSCRIPT
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An array analysis of seismic surface waves
James Gaherty and Ge JinLDEO Columbia University
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Thoughts and Overview• Surface-waves from earthquake sources provide powerful tool for
probing upper mantle structure beneath arrays– Good depth resolution– Constrain both absolute and relative velocity – Sensitive to anisotropy and attenuation
• Energetic and coherent wavefield amenable to array analysis– Longest wavelength: outer aperture of array– Shortest wavelength: ~ interstation spacing
• Challenges associated with:– dispersive character– propagation complexity (wavefield heterogeneity)
• Examples: – USArray Transportable Array– Small regional PASSCAL arrays
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Problem: Near-receiver imaging using surface waves
•Traditional approach measures travel time or velocities from source to receiver•Mostly sensitive to source-receiver path•Desired information contained in interstation variability•Nearby waveforms very similar•Exploit using multichannel crosscorrelation
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Problem: Near-receiver imaging using surface waves
Approach1. Automatic GSDF Method– Multi-channel cross correlation to extract frequency-
dependent relative phase and amplitude variations 2. Phase gradiometry– Invert phase variations for 2D variations in dynamic phase
velocity -- Eikonal tomography3. Amplitude Correction– Utilize amplitude variations to correct estimate true
structural phase velocity from dynamic phase velocity – Helmholtz tomography
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Automatic GSDF Method
• Similarity – reduce measurement uncertainty
• Minimal cycle skipping• Multichannel –
measurement redundancy
Real Waveform
Real WaveformFrom nearby
Stations
CrossCorrelation
Narrow-BandFilter
WaveletFitting
Phase DelayDifference
Group DelayDifference
Amplitude
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Processing Example: Original Waveforms
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Processing Example: Cross-Correlation Waveforms
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Processing Example: Wavelet Fitting
Real Data
Fitting Wavelet
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Redundant Time Difference Measurement
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Phase Velocity Inversion
EikonalTomography
Phase difference Between Stations
Apparent Phase Velocity
EventStacking
Averaged Apparent Phase
Velocity
AmplitudeCorrection
Structure Phase Velocity
EventStacking
Averaged Phase Velocity
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Phase GradiometryTravel Time SurfaceApparent Phase Velocity
Eikonal TomographyLin et al.,2009
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Eikonal TomographyFrom Phase Difference to Phase Velocity
Observations:
Modeled as:
Invert for slowness variations S(x,y) with a penalty function
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Eikonal Tomography
Event: 200806171742Period: 60s
2
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Focusing Effect
Propagation Direction Anomaly Amplitude
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Amplitude Correction of Phase Velocity
Friederich et al. 2000
Real Corrected Uncorrected
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Single Event 1
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Single Event 2
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Multi-Event Average
http://www.LDEO.columbia.edu/~ge.jin
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Small PASSCAL Array
32 Seconds
Rayleigh
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Small PASSCAL Array
50 Seconds
Rayleigh
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Thoughts on Array Design for Upper Mantle Imaging
• Surface waves provide critical constraints on upper-mantle structure
• Period range of interest 20-200 s – wavelengths of 80-800 km – maybe don’t need all of this, but the bigger the better
• Even spatial coverage in 2D for wavefield analysis• Interstation spacing likely less critical than other (body-
wave) needs? Oversampling is good however.• Broadband is important!• Common instruments (or at least well calibrated) –
need accurate instrument response for cross-correlation and amplitude analyses