Variation of proportionality between stress drop and slip, with implications for megathrust earthquakes
LD Seminar Oct. 26, 2022 @USC
Baoning Wu1,2, Christodoulos Kyriakopoulos3, David Oglesby2, Kenny Ryan4
1University of Southern California,
2University of California, Riverside,
3University of Memphis,
4Air Force Research Laboratory,
Copyright by John Wiley & Sons, 1999
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Outline
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Outline
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Elastic shear stress overcomes the “frictional” strength on a fault.
Stress suddenly drops, causing fault to slip, releasing seismic energy.
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Characterizing earthquake stress drop is important for
both understanding earthquake processes
as well as assessing seismic & tsunami hazards.
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A general relation is found:
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(USGS earthquake event page: https://earthquake.usgs.gov/earthquakes/)
2021 M8.2 Alaska EQ
finite fault model
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Considering
We have
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Kanamori and Anderson (1975)
Considering
We have
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Outline
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Kinematic finite fault analysis is now routinely done for large earthquakes. Some more “modern” catalogs are recently available.
Mai and Thingbaijam (2014): “SRCMOD: An Online Database of Finite-Fault Rupture Models”
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Thingbaijam and Mai (2017) megathrust EQs
Magnitude-dependent stress drop for megathrust earthquakes?
Should be constant if right-hand side are constant
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Outline
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Thingbaijam and Mai (2017) megathrust EQs
Should be constant if right-hand side are constant
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Larger earthquake: H/W is smaller
Earth’s surface
Smaller earthquake: H/W is larger
Earth’s surface
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Therefore, the observed magnitude-dependency is also a depth-dependency in a relative sense
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C is known to be depth-dependent: C is smaller when a fault is closer to the Earth’s surface, reflecting a smaller system stiffness
fault
Earth’s surface
fault
Knopoff (1958)
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C is known to be depth-dependent: C is smaller when a fault is closer to the Earth’s surface, reflecting a smaller system stiffness
However, quantitative analyses of the depth-dependence of C is rare. The best reference so far is Parsons et al. (1988), which provides 17 theoretical C values. Only 1 is dipping fault model, and it is in 2D.
A more thorough theoretical investigation of C depth-dependence is needed, before we analyze the observed apparent magnitude dependence.
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BEM with static-crack in an elastic half-space, stress transfer functions of rectangle elements (Okada, 1992)
C depth-dependence is stronger when fault dip angle is smaller, exceeding the general two-fold “rule of thumb”.
A gently dipping fault can have a greater average proximity to the Earth’s surface compared to a steeply dipping fault.
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Conclusions & Implications
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(Wu, Kyriakopoulos, Oglesby, Ryan, GRL in review)
Mai and Beroza (2000)
Thingbaijam and Mai (2017) Fig15
Thingbaijam and Mai (2017)