# Lecture 7. Few points about earthquakes Some basic facts and questions Great Chilean earthquake /Valdivia earthquake / of 1960 (Mw=9.5) and recent Tahoku.

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Lecture 7. Few points about earthquakes Some basic facts and questions Great Chilean earthquake /Valdivia earthquake / of 1960 (Mw=9.5) and recent Tahoku earthquake (Mw=9.0) of 2011 Megathrust earthquakes and structure of the upper plate Perspective: Cross-scale dynamic models Application: “GPS Shield” concept for TEWS Outline

The cause of larger earthquakes is the plate tectonics and most of them happen at plate boundaries About 80% of relative plate motion on continental boundaries is accommodated in rapid earthquakes With few exceptions, earthquakes do not generally occur at regular intervals in time or space. Some basic facts

The shear strain change associated with large earthquakes (i.e. coseismic strain drop) is of the order of 10 -5 – 10 -4. This corresponds to a change in shear stress (i.e. static stress drop) of about 1–10 MPa. The repeat times of major earthquakes at a given place are about 100–1000 years on plate boundaries, and 1000–10 000 years within plates. Some basic facts

Ideal Real

Some basic facts Deformation modes rare The magnitude–frequency relationship (the Gutenberg–Richter relation) log N(M) = a − bM, b is about 1

Why some plate boundaries glide past each other smoothly, while others are punctuated by catastrophic failures? Why do some earthquakes stop after only a few hundred meters while others continue rupturing for a thousand kilometers? How do nearby earthquakes interact? Why are earthquakes sometimes triggered by other large earthquakes thousands of kilometers away? Some basic questions

Chile earthquake (2010, Mw=8.8)

Valdivia earthquake (1960) Slip distribution

Region of Valdivia earthquake (1960) GPS data

Tohoku earthquake, 2011

Japan, 2011, Fit of the co-seismic GPS data Japan, 2011, Inverted Slip, m Tohoku Great Earthquake, 2011 (Mw=9.0) Hoechner et al. in prep Tsunami based on source from GPS data inversion Model versus DART buoys data

Fuller et al., Geology, 2006 Song and Simons, Science, 2003 Wells et al.,JGR, 2003

Megathrust earthquakes and structure of the upper plate Song and Simons, Science, 2003 Wells et al.,JGR, 2003

Song and Simons, Science, 2003 Correlation of large slip regions (asperities) with the negative gravity anomalies (sedimentary basins) Possible explanation (not convincing)

Fuller et al., Geology, 2006 Better explanation

Fuller et al., Geology, 2006 Better explanation However, this model should be recalculated including mantle lithosphere

Zones of seismicity Perspectives: Cross-scale dynamic models

Full set of equations mass momentum energy Elastic deformation is included in our geological- time-scale (mln years) Andes model

Final effective viscosity if

Frictional instabilities governed by static-kinetic friction Stress Slip Time The static-kinetic (or slip- weakening) friction: stress slip Lc static friction kinetic friction experiment Constitutive law Ohnaka (2003)

Frictional instabilities governed by rate- and state-dependent friction were: V and  are sliding speed and contact state, respectively. a, b and  are non-dimensional empirical parameters. D c is a characteristic sliding distance. The * stands for a reference value. Dieterich-Ruina friction: At steady state:

How b-a changes with depth ? Scholz (1998) and references therein Note the smallness of b-a.

The depth dependence of b-a may explain the seismicity depth distribution Scholz (1998) and references therein

The Central Andes model 35 Ma 18 Ma Trench roll-back 0 Ma South American drift Sobolev and Babeyko, Geology, 2005 The central Andes model at geological time-scale Friction  = 0.05 Delaminating lithosphere

30 We can continue calculation at seismic-cycle time-scale (years)

Friction down 0.0315 0.0285

Friction up 0.0285 0.0315

Friction down 0.0315 0.0285

Dynamic relaxation: Modified FLAC = LAPEX (Babeyko et al, 2002) For geological-time-scale models ρ iner >> ρ (ρ is real density). By taking ρ iner = ρ we can model seismic waves

Rupture and seismic waves modeled with the Andes thermomechanical model Movy file waves.avi

Conclusions Large earthquake is still poorly understood phenomenon Observed correlation with the structure of the upper plate (not subducting plate) is surprising and intriguing The best (till now) explanation is stability of the wedge (Fuller at al, 2005), but thier model needs update Interesting perspective is a cross-scale modeling allowing simulation of seismic cycle or even rupture propagation in the same model that explains geological-time-scale processes

Application: “GPS shield” concept for Tsunami Early Warning

Max. wave heights for «southern» fault Bengkulu 37

Max. wave heights for «northern» fault Bengkulu 38

Epicenter and magnitude are the same. Not the same with tsunami impact in Bengkulu. 39

40 Bathymetry chart across the trench AA’ A Siberut Trench Padang Bathymetry off Padang: An important player

41

42

43 The two scenarios are easily to distinguish by their GPS fingerprints Patch 1 Patch 2

Japan, 2011, Fit of the co-seismic GPS data Japan, 2011, Inverted Slip, m Tohoku Great Earthquake, 2011 (Mw=9.0) Hoechner et al. in prep Tsunami based on source from GPS data inversion Model versus DART buoys data

Japan, 2011, Fit of the co-seismic GPS data Japan, 2011, Inverted Slip, m Tohoku Great Earthquake, 2011 (Mw=9.0)

47 Concept of the “GPS-Shield” for Indonesia: Configuration and resolution Earthquake magnitude Location of maximum uplift Sobolev et al., 2006, EOS Sobolev et al., 2007, JGR

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