Parkfield Relocations

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Parkfield Relocations
Streaks, multiplets and holes High Resolution
spatio-temporal behavior of Parkfield
Seismicity F. Waldhauser, W. L. Ellsworth, D. P.
Schaff and A. Cole GRL, September
2004 Streaks Sub-horizontal alignments of eqs
along fault plane Multiplets recurrent eqs,
same size, same place Holes No eqs in
particular regions
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Motivation

• Parkfield, transition between creeping and
  locked segment of SAF.
• 1000s eqs recorded since 1969 by NCSN.
• Borehole records reveal repeating earthquakes,
  same size, same place.
• Simple location methods JHD do not give
  resolution required.
• Relocation - image with strongly organized eqs
  in time and space.

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Relocation Method
NCSN
HypoDD
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Sources of location errors
Velocity structure uncertainties
Strategies Custom 1-D models Master event
relocation Joint hypocenter-velocity
inversions
Phase picking uncertainties
Strategies Careful repicking (not
effective) Waveform cross-correlation (often
very effective)
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Double-Difference Example Hayward Fault,
California
Note fault streak
from Waldhauser Ellsworth (2000)
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Relative event location using waveform
cross-correlation
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Cross-correlation of pair of similar events
P-waves
S-waves
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Data

• Digital waveforms 1984 - 2002 (5269 - 4505
  relocated).
• Bootstrap for errors few meters (cross-corr)
  hundreds (picks).
• Residuals RMS from 90 ms 3 - 20 ms 1ms 5
  meters
• Large events phase picks (nucleation point)
• Small events cross-correlation position of
  center of moment release

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Major Features
Width seismically active fault is smaller. Width
of a few hundred meters. Spatial clustering
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Major Features
Big Streak Highly clustered
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Big Streak

• Patches nearly vertical.
• Offset few 100 meters
• Multiple faults SE
• Single fault plane NW
• Multiplets with nearly identical waveforms
• Rupture same patch, same size

• Possible geometric, or frictional controls
• Complexity different rock strata intersecting
  fault plane

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Major Features
Locked zone Deeper earthquakes. 5 - 12 km 2
linear features, NW plunge.
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Deep structure
Short horizontal streaks Repeating earthquakes
rare Eqs occupy isolated areas, few events share
fault patch.
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Deep structure
Hole 1966 Earthquake hypocenter. Bounded by
upper and lower streaks Eqs occupy isolated
areas, few events share fault patch.
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Deep structure
3 moderate earthquakes, nucleated in deep streak,
propagated to hole Large stress change near
hypocenter (streak). 93/11 most impulsive. This
events release most stress, average is low,
rupture is over small area.
Aftershocks is Mgt4.6 concentrate in deeper
streak. Aftershocks closely related to mainshock
rupture (92/10, 93/4) 94/12 nucleates in no
preexisting cluster, aftershock in cluster 3 km
away.
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Major Features
General structure, features remain if pre 84
events are used. Big Streak is robust over many
cycles.
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Slip rates
Trilateration and GPS data from 1991- 1998. The
1966 Hole, transition between creeping-locked
zones. 12 km streak - transition deep creep -
shallower locked part (1966) Near Gold Hill,
between 5 - 10 km seems locked. Bounded by upper
and lower seismic streaks?
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EQ Physics Outline
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EQ Physics Outline
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EQ Physics Outline