Structure Under Mount Rainier, Inferred From Teleseismic Body Waves

1
Structure Under Mount Rainier, Inferred From
Teleseismic Body Waves

• Charles A. Langston
• Journal of Geophysical Research, 1979

2
Introduction

• Earths crust is composed out of different rocks
  with lateral variations
• High-resolution reflecting profiling
• Assume that entire crust and upper mantle behaves
  similarly
• Assume that there are no lateral variations,
  only vertical inhomogeneity

3
Introduction

• This paper examines an Earth model, which cannot
  be explained by only vertical and no lateral
  variations.
• Teleseismic P and S waves were examined to
  determine crustal and upper mantle structure.
• From other studies there is a low velocity zone
  at 45-55 km depth
• This study Define the limits of this LVZ at the
  flank of a Quaternary stratovolcano.

4
Introduction

• From Longmire (LON) data
• P waves exhibit a large tangential component of
  ground motion.
• No homogeneous plane layered Earth model can
  explain these data, because P-SV and SH wave
  systems are decoupled.
• Theory Planar interfaces in Earth model are
  dipping.

5
Introduction

• Used A 3-D ray tracing formalism to find the
  path of rays in the structure.
• Methods All tangential P wave effects will be
  explained by off-azimuth arrivals of primary,
  converted and reverberated waves.
• But - there are diffracted waves
• - there can be other geometries than only
  planar interfaces

6
Contents

• Regional tectonics and setting
• Long-period P wave data instrument calibration
• General method for equalizing the wave form data
  for arbitrary effective source time function
• Theoretical ray amplitudes and synthetic
  seismograms
• Long-period S wave form data
• Discussion and summary
• Criticism

7
Regional setting

• ? Tectonic view over study area (source
  visionlearning.com)
• ? Pacific NW region, with seismic station at LON

8
Regional setting

• Other studies at LON
• Crustal transition between higher mantle P
  velocities and thicker crustal sections (E)
  compared to low P velocities and thin crust (W)
• Eastwards dipping high wave velocity slab
• An eastward thinning low-velocity zone

9
Long-period P wave form data and instrument
calibration

• Back azimuth Azimuth clockwise from the north
  as seen from the receiver to the source.
• For all independent BAZ vector rotation of
  horizontal P wave forms into theoretical ray
  direction.
• Result there was significant tangential motion.

10
Long-period P wave form data and instrument
calibration

• Source mislocation, digitizing error and
  instrument calibration can cause this tangential
  motion.
• Theoretical instrument responses show that the
  differences in wave shapes did not approach any
  of the characteristics of the data.
• Two events with similar vertical component do not
  show a pulse character which was in error for the
  horizontal component.
• ? LON long-period system was well calibrated.
• ? Effect is from Earth structure.

11
P wave Equalization Procedure

• Equalizing the data to compensate for different
  source time functions.
• Math
• Approximation DV(t)I(t)S(t)
• Deconvolution by transforming to frequency domain
• Multiplied by a transformed Gaussian

DV(t)I(t)S(t)EV(t) DR(t)I(t)S(t)ET(t) DT(t)
I(t)S(t)ET(t)
12
P wave Equalization Procedure

• The Deconvolution removes essentially P waves
  except for the first arrivals
• Good consistency in overall shape and timing.
• Thus the particle motion is due to earth
  structure.

13
P wave Equalization Procedure
14
Interpretation of the P Wave Forms

• Ideally Physical model of the Earth structure
  whose wave response matches the observed wave
  forms.
• But - constraints on crustal properties
• - crust is likely to be lateral heterogeneous
• - data are a result of nearly vertical
  propagation of body waves
• Only velocity contrasts and average crustal
  travel times can be resolved.
• Hypothesis-testing approach the tested model
  will be a single dipping interface model.

15
Interpretation of the P Wave Forms

• From models tangential P wave components are
  azimuthally antisymmetric about an azimuth
  perpendicular to the strike of the interface.
• ? SE and NW quadrants are at similar azimuthal
  positions relative to the dip azimuth.
• The direction of dip is indicated by the
  polarities of tangential P and the Ps conversion
  at the interface.

16
Interpretation of the P Wave Forms

• Wave amplitudes give interface velocity contrast
  and dip magnitude. Numerical search and inversion
  procedure done in order to find P and Ps
  amplitudes.
• Results
• High contrasts require small dips and vice versa.
  
• Large contrasts and dips needed to approximate
  observed amplitude.
• Major tectonic feature

17
Interpretation of the P Wave Forms

• Short-period P waves both radial and tangential
  components attain larger overall amplitudes than
  the vertical component.
• Conversion to long-period P waves ? extreme
  frequency dependence due to frequency-dependent
  geometric spreading effects caused by undulations
  in the interface.

18
Interpretation of the P Wave Forms

• There remain problems in explaining some
  first-order effects.
• Evidence there exists a major laterally
  heterogeneous structure under LON, which included
  a high-contrast dipping interface.

19
Long-Period S Wave Forms and Sp conversions

• Investigation of long-period S wave forms to find
  more constraints.
• Prominent Sp precursor was observed.
• Interface at 145 km depth
• Sp has a tangential particle motion.
• For possible interpretations of amplitudes of S
  to Sp a few model calculations where examined.

20
Long-Period S Wave Forms and Sp conversions

• First 3 models underestimate Sp/S ratio.

21
Discussion

• Theory limitations and structure complexity.
• Further information of the dipping structure
  under LON could be obtained from extending the
  techniques used here.
• Deconvolution technique for removing instrument
  and time function effects is useful.

22
Discussion

• Tectonic structure under Mnt. Rainier unclear.
• Interface depth of 15-20 km Interpretations
• Moho has a lot of undulations and varying dips.
• Processes involving volcanism and pluton
  emplacement.

23
Discussion

• Interpretation of long perios Sp-waves is an
  Upper mantle LVZ
• Sp arrivals does not give evidence for an
  interface between 145 and 20 km depth.

24
Summary

• An equalization method was developed to remove
  effects from different source-time functions.
• This was done by deconvolving the Vertical P wave
  component from the radial and applying a Gaussian
  filter
• Tangential P wave forms
• A model of a dipping interface between 15 to 20
  km depth.
• It is suggested that this interface is the Moho

25
Summary

• S waves where used to find more constrains. These
  where not found
• A Sp precursor indicated a conversion at 145 km
  depth.

26
Criticism

• 145Km dipping slab