What is characteristic about a characteristic earthquake? Implications from multi-scale studies of the relative earthquake size distribution

Stefan Wiemer (ETH)Thessa Tormann (ETH)Bogdan Enescu (Univ. of Tsukuba) Jochen Woessner (ETH, now RMS)

Potsdam, June 16 2015

What is a ‚Characteristic Earthquake‘?

An earthquake that is typical („characteristic“) for fault (region). In time? In magnitude? In not following Gutenberg-Richter? • First-order characteristic: An major event repeat regularly with the same

magnitude, re-rupturing the same ‘asperities’ and avoiding/stopping at ‘barriers’. Gap hypothesis, v1.0 (RIP).

X

Z

Asperity

Barrier

Barrier

Even, M = Mmax

MmaxLo

g(N)

M

Time

What is a ‚Characteristic Earthquake‘?

An earthquake that is typical („characteristic“) for fault (region). • Other meanings: Characteristic in magnitude in the sense that it is

“much” larger than the next largest one” (Characteristic versus GR). Recurrence can still be random in time with a mean recurrence governed by energy conservation.

X

Z

Asperity

Barrier

Barrier

Even, M = Mmax

MmaxLo

g(N)

M

Time

What is a ‚Characteristic Earthquake‘?

Second possibility: Limited somewhat in maximum size by geometrical constrains, but random in time and maybe, but not necessarily following a GR. And maybe sometimes a rupture can jump an asperity for an even bigger one. A much more complex Earth.

X

Z

Asperity

Barrier

Barrier

Even, M = Mmax

MmaxLo

g(N)

M

Time

Asperity

b-value as a stress-meter: Lab

• Laboratory experiments since the 1960‘s have shown that the b value in the size distribution of acoustic emission events decreases linearly with differential stress.

Goebel et al., 2013

© Thomas Goebel, USC

b-values as a stress-meter: Earth

• First-order differential stress gradients (depth, faulting styles) imprint on b.

• Decrease of b-values with depth: Spada et al. (2013), first indication of the brittle-ductile transition zone.

• Schorlemmer et al. (2005): Faulting styles

Goebel et al., 2013

b-values as a stress-meter

• Using a simple frictional strength model for stresses in the continental lithosphere combined with earthquake b values measured as a function of depth, Chris Scholz proposed in 2015 what we in 2013 did not dared to do:

b=1.23 - 0.0012 * (σ1 - σ3)• Of course, to be used with great

caution!

C. Scholz, On the stress dependence of the earthquake, Geophysical Research Letters, 10.1002/2014GL062863

Imaging b-values: Start 1997

26.08.2014 Seite 8Color scale!Wiemer and Wyss, JGR, 1997

1 2 3 4 5Magnitude

b=0.47+/-0.03 A

B

100

101

102

103

Cum

ulat

ive

Num

ber

b=1.35+/-0.1

: 1966 M6 Beben

B A

Schorlemmer et al. , JGR, 2004

Parkfield revisited in 2004:

Seite 10Tormann et al., 2013, a, b

Parkfield until 2011

• The b-values increased slightly for just a few months.

Parkfield Implications

• At Parkfield: An asperity setup for an M6 class “characteristic” event stable throughout the seismic cycles.

• Mainshocks are somewhat characteristic in magnitude, because the asperity is bounded to the North and South were rupture often will die out.

• Events are largely random in recurrence time, the stress level in the asperity remains high, the asperity is ready again for a new event, which will come on average every 20-35 years.

Seite 11

Parkfield Characteristic Earthquake?

• Characteristic in size (unless it jumps to be an 1857 type).

• Stationary Poissonian in time (?)• Likely not fully GR – where are all the M5.4, 5.6.,

5.8? So when an event reaches a certain size … it carries on?

Z

Asperity

Barrier

Barrier Asperity

A systematic survey in CA

Seite 13Tormann et al., 2014

• A complex picture. Few well established low b-value patches (i.e., asperities), some well established high b areas (‘barriers’).

• Problem: Non liner size distributions.

• Many areas where imaging is not possible due to few data.

Seite 14

A systematic survey in CA

Imaging in subduction zones

Tormann et al., Nature Geoscience (2015)

First order: Tectonic footprint

Tormann et al., Nature Geoscience (2015)

CA

D

B

A footprint +/-stable in time

Seite 17

1998 – 2003 2004 – 2011

M9 slip contours

Three suggestions

2004 – 2011

M9 slip contours

• No indication of strong segmentation as seen at Parkfield

• Area of lowest b +/- area of future rupture.

• Decrease prior to the mainshock?

Parkfield

Until the M9 event March 11 2011

Seite 19

2004 – 2011 3 months post

Time-series within the 10 m slip contour

• Rapid recovery of a-values AND the b-values!

• Very short when compared to the average recurrence of M9 (~500 years)

• Decrease in b prior to M9 before depends somewhat on choice of area/Mc.

26.08.2014 Seite 20

b-value difference map

• Increase in parts of the mainshock slip area, near largest slip patch.

Seite 21

M8 relative rupture potential change

• Change in a-value and the change in b-value determine the change in the probability of a large event (e.g., M8)

• Stress transfer to the areas outside of the 10 m slip contour.

• Obvious in a way, predicted by models, but here you can SEE it.

26.08.2014 Seite 22

Observation 1

Areas of lowest b-value before the mainshock slip the most

Consistent with the hypothesis that they are the most stressed areas.

Seite 23

Observation 2

Areas with the highest slip show the largest increase in b

Consistent with the hypothesis that they are the most stressed areas, hence can and want to release the most stress.

Seite 24

Maybe most surprising: Rapid recovery!

Seite 25

3 months post

2004 – 2011

Months 4-9 2013 – 2014

Big changes in b after the mainshock (purple), then rapid recovery (cyan)

Nearly recovered Back to where we were before?

• Parkfield all over again!

Hazard implications

• No segmentation. Who knows which of the asperities will rupture in the next one.

• No safe period. Enough energy/stress in the system even after a ‚big one‘ -? Stationary Poissonian process for hazard works best (but we may not may the interface itself).

• Highest hazard just outside the largest slip areas.

• Watch the north towards Hokkaido: lowest and lowering b-values. 2013 – 2014

Characteristic earthquake implications

X

Z

Asperity

Barrier

Barrier

Even, M = Mmax

MmaxLo

g(N)

M

Time

Asperity

• Variability is the key. Some asperities, some barriers, often the possibility to interact, sometimes a ‘shadow’, sometime ready for the next one.

• The good news: Maybe we can image the relative stress distributions in some case and use them as prior information in earthquake simulators?

Thank you!

26.08.2014 Seite 30

Completeness problems? No.

Seite 31

2013 – 20142004 – 2011 3 months post