What is characteristic about a characteristic earthquake Implications
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, v 1. 0 (RIP). Even, M = Mmax Time Barrier Z Asperity Log(N) X M Mmax
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. Even, M = Mmax Time Barrier Z Asperity Log(N) X M Mmax
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. Even, M = Mmax Time Barrier Z Asperity Asperit y Log(N) X M Mmax
b-value as a stress-meter: Lab Goebel et al. , 2013 • 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. © 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. 1002/2014 GL 062863
Imaging b-values: Start 1997 Wiemer and Wyss, JGR, 1997 Color scale! 26. 08. 2014 Seite 8
3 B 10 Cumulative Number Parkfield revisited in 2004: b=1. 35+/-0. 1 2 10 1 10 A b=0. 47+/-0. 03 0 10 B 1 2 3 Magnitude 4 5 A : 1966 M 6 Beben Schorlemmer et al. , JGR, 2004
Parkfield until 2011 • The b-values increased slightly for just a few months. Tormann et al. , 2013, a, b Seite 10
Parkfield Implications • At Parkfield: An asperity setup for an M 6 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 all the M 5. 4, 5. 6. , 5. 8? So when an event reaches a certain size … it carries on? Barrier Bar rier Z Asperity
A systematic survey in CA Tormann et al. , 2014 Seite 13
A systematic survey in CA • 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
Imaging in subduction zones Tormann et al. , Nature Geoscience (2015)
First order: Tectonic footprint D Tormann et al. , Nature Geoscience (2015) A B C
A footprint +/-stable in time M 9 slip contours 1998 – 2003 2004 – 2011 Seite 17
Three suggestions • No indication of strong segmentation as seen at Parkfield • Area of lowest b +/area of future rupture. • Decrease prior to the mainshock? M 9 slip contours Parkfield 2004 – 2011
Until the M 9 event March 11 2004 – 2011 3 months post Seite 19
Time-series within the 10 m slip contour • Rapid recovery of avalues AND the bvalues! • Very short when compared to the average recurrence of M 9 (~500 years) • Decrease in b prior to M 9 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
M 8 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. , M 8) • 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
2004 – 2011 3 months post Maybe most surprising: Rapid recovery! Months 49 2013 – 2014 Seite 25
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 • 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? Even, M = Mmax Time Barrier Z Asperity Log(N) X M Mmax
Thank you! 26. 08. 2014 Seite 30
Completeness problems? No. 2004 – 2011 3 months post 2013 – 2014 Seite 31
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