Infrasound workshop 897

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Infrasonic Observations of Earthquakes J. Paul
Mutschlecner and R. W. Whitaker Los Alamos
National Laboratory 2003 Infrasound Technology
Workshop October 27-29, 2003 JaJolla, California
Opinions are those of the authors not of the US
government
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Overview

• Review infrasound signals from earthquakes
• Understand infrasound signals with respect to
  earthquake characteristics
• Provide estimate for earthquake infrasound
  signals
• Discuss future directions - modeling and
  earthquake parameterization

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Infrasound Earthquake Data Set

• Number of earthquake events 30
• Number of signals 47
• Number of stations recording 4
• 1983 - 2002
• 13 multi-station detections
• Analysis is ongoing

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Local Distribution Map
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0.5-3.0 Hz data
2/22/02 M 5.7 32.38 N 115.35W Channel data and
Infra_Tool Az Dev 1.7o
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Data Set Characteristics
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Data Set Characteristics
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Data Set Characteristics
ltDurgt 14.0 min s 9.4
ltfgt 0.84 Hz s 0.49
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Duration (in min) vs Mb
Examination of several other parameters Shows
no well defined trends Possible indication of
range dependence
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Long Signal Duration for EQ

• Source (EQ) duration
• But most Earthquakes have motion over only a few
  seconds to a minute (aftershocks may contribute
  for larger Mb
• Atmospheric propagation
• Usual competition between pulse stretching and
  dissipation
• HE signals of similar size have shorter
  durations
• Source extension by seismic surface waves
• Relation to magnitude supports this concept
• Known cases of extended sources (eg Coalinga,
  1983, and Northridge, 1994)
• But for longest durations source must be 100s km
  in diameter and durations are not always
  symmetric about epicenter signal

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Fmax vs Reported Depth
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Amplitude Scaling for Distance and Stratospheric
Wind

• Where
• Vd - wind component directed, source to array
• Vz - zonal component of stratospheric (50 km)
  wind
• Vm - meridional component of stratospheric wind
• q- azimuth to source
• Distance and wind parameters are empirically
  derived
• and b 1.45 and k 0.018

See An Empirical Study of Infrasound
Propagation, 1999, J. Paul Mutschlecner, Rodney
W. Whitaker and Lawrence H. Auer, LA-13620-MS,
Los Alamos National Lboratory
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Scaled Amplitude vs Mb(all events)

• R2 0.591
• 0.419
• b 0.454

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Peak Vertical Velocity (at 1 km) and ML40 Events

• R2 0.595
• 0.230
• b 0.558

From Empirical Analysis of Near-Source Ground
Motion, 1980, A. F. Shakal and D. L. Bernreuter,
NUREG/CR-2095 (UCRL-53028)
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Sources of Uncertainty
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Statistical Simulations
denote random variations
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Results of Statistical Simulations
50 simulations Monte Carlo approach
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Summary

• Relations determined
• Log(Asc) vs Mb
• Duration vs Mb
• Frequency vs depth
• Uncertainty sources identified and match to
  observations
• Better earthquake source parameterization needed
• Source modeling being established

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Back Ups
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Sources of Uncertainty
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Frequency of Max Energy vs Mb
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Close-in Ground Motion Velocities for 10/15/79
EQ, Imperial Valley (6.6)
From Empirical Analysis of Near-Source Ground
Motion, 1980, A. F. Shakal and D. L. Bernreuter,
NUREG/CR-2095 (UCRL-53028)
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Scaled Amplitude for Best Wind Data

• R2 0.691
• 0.405
• b 0.556