Exploring the underlying mechanism of LURR theory

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Exploring the underlying mechanism of LURR theory

• Can Yin
• Supervisor Prof. Peter Mora
• QUAKES, Department of Earth Sciences
• University of Queensland

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Load/Unload Response Ratio promising but
controversial

• As a phenomenon, high LURR prior to major
  earthquake has been found in many earthquakes by
  retrospective examination. It is also observed in
  several real time predictions.
• Since accelerating seismic moment release is also
  observed in many earthquakes, it is attractive to
  find a common underlying mechanism for these two
  phenomena. If they share a common root, does the
  critical region size in AMR apply to LURR?

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Definition of LURR
LURR is originally defined in differential form,
making it a state variable depending only on the
state of the system at that moment. In current
implementation, we define the loading duration
instead of loading increment itself as P and
introduce the integral form of response rate as
follow Accordingly, LURR is defined as If
the averaging time is long enough, T is almost
equal to T-.
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However, the sacrifice of compromise to the
integral definition of LURR is it now involves
many other factors, whose relations with LURR are
yet to be found. The new LURR is related to the
history of system evolution and thus dependent of
many extrinsic parameters, such as magnitude
range of selected earthquakes, the spatial range
of earthquakes used, fault parameters, etc. For
example, when defining effective Coulomb Failure
Stress for criteria of load/unload, we simply
define it either as that on the fault plane of
ensuing main shock or that on an optimal fault
plane determined by the tectonic stress field. In
the former case, not all the earthquakes occurred
on the main fault plane while in the latter
case, it is proved that the tectonic stress field
changes direction prior and after a major
earthquake. In either case, the definition of
Coulomb Effective Stress is kind of ambiguous.
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Re-examine critical region sizes by LURR

• We examine our study on last 5 major earthquakes
  in California with magnitudes greater than 6.5
  between 32 N and 40N latitude since 1980. The
  catalog used in this study is the Council of the
  National Seismic System (CNSS) Worldwide
  Earthquake Catalog, which is accessible via the
  World Wide Web at the Northern California
  Earthquake Data Center (http//quake.geo.berkeley.
  edu/cnss). According to the relation between
  critical region radius and ensuing mainshock
  magnitude in AMR, radii of the critical region of
  all 5 earthquakes are larger than 100km.
• As in Australia, due to limited data, we only
  examined the last two well-recorded earthquakes
  with magnitude greater than 5.0.

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1983.05.02 Coalinga Earthquake (M6.7)
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1987.11.24 Superstition Hills (M6.6)
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1989.10.18 Loma Prieta (M7.0)
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1992.06.28 Landers Earthquake (M7.3)
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1994.01.17 Northridge Earthquake (M6.7)
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1997.03.05 Burra, South Australia. M5.0
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Burakin, West Australia (a series of Mgt5.0
earthquakes since late 2001 till early 2002 )
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Discussion Note All plots here are plotted with
incorporation of disturbance of random process,
which is expressed by Y90, i.e. LURR Y/Y90.
LURRgt1.0 means it can be considered as
abnormality at 90 confidence level. LURR0
indicates that data is insufficient to make
significant calculation (the minimum sample size
is 20 events in a group).And upper limit of
magnitude of earthquake taken into calculation is
4.0. From the figures above, it is obvious that
smaller the region, the more prominent the LURR
abnormality will be. How to explain it? Two
possible answers (1) Either the method used in
AMR to correspond the region size where LURR has
peak value to critical region size does not apply
to LURR or (2) there is simply no critical
region. More close to the future epicenter, more
critical. This is a very intuitive explanation.
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• Look at the epicenters of these earthquakes. If
  the seismogenic time of an earthquake is decades
  long, given the proximity of time and space
  between two earthquakes, say Landers(7) and
  Northridge(8), how could you distinguish which
  precursory events belongs to which major
  earthquake? If they are not distinguishable, the
  critical regions of each earthquake will be
  intermingled. Then how large is each region?

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Re-orientation of small faults as a possible
mechanism of current LURR implementation

• Some researchers found that the tectonic stress
  field rotate before and after the major
  earthquakes in California.
• In order to find a clue for the relation between
  stress rotation and LURR abnormality, we
  calculate LURR under different fault planes,
  which is uniquely determined by tectonic pressure
  axis (P-axis). Below are some examples of LURR
  change with optimal P axis direction. P axis
  change from 90 deg(West) clockwise to 0 (North)
  until 90 deg(East).

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Loma Prieta, CA
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Northridge, CA
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Burakin, West AUS
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Discussion

• LURR abnormality pattern in California is
  obviously different from that in Australia. In
  Northridge, for example, LURR has peaks in many
  directions at different time. In Burakin,
  however, LURR has peaks only in direction along
  North-South. It is likely that the tectonic
  stress field fixes in this direction for a long
  time. This may be the result of the relatively
  stable tectonic stress field in Australian plate.
• When the crust is far away from criticality,
  small faults tend to be randomly orientated, LURR
  calculated for any fixed direction will surely
  fluctuate around unity. But when the crust is
  close to failure, the number of fractures along a
  particular direction determined by the built-up
  tectonic stress will get dominant, causing
  calculated LURR along this direction high up.
  Such rotation mechanism partly solves the dilemma
  brought by the ambiguous definition of Coulomb
  Effective Stress in current implementation of
  LURR.

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Summary

• The method to identify critical region size in
  AMR doesnt apply to LURR. Failure to find better
  power-law curve fitting at smaller region in AMR
  may be due to observational limit rather than
  physical meaning, because there is not enough
  seismic events to obtain a statistically
  significant curve-fit within smaller region.
• LURR may not share a common underlying mechanism
  with AMR, but may be caused by critical
  sensitivity which is reached by the phase up of
  previously randomly oriented small fractures
  re-orientating towards a certain direction
  determined by tectonic stress prior to main
  shock, resulting in higher LURR in this
  direction. Such re-orientation may be a kind of
  underlying mechanisms of LURR phenomenon for
  current implementation.