andrew v. newman, jaime a. convers, hermann fritz georgia institute of technology, atlanta, ga, usa...
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Andrew V. Newman, Jaime A. Convers, Hermann FritzAndrew V. Newman, Jaime A. Convers, Hermann FritzGeorgia Institute of Technology, Atlanta, GA, USAGeorgia Institute of Technology, Atlanta, GA, USA
Lujia FengLujia FengEarth Observatory of Singapore, NTU, SingaporeEarth Observatory of Singapore, NTU, Singapore
Ting ChenTing ChenWuhan University, ChinaWuhan University, China
Gavin HayesGavin HayesNational Earthquake Information Center, USGS, USANational Earthquake Information Center, USGS, USA
Yong WeiYong WeiPacific Marine Environmental Lab, NOAA, USAPacific Marine Environmental Lab, NOAA, USA
The Character of Tsunami-genesis in Subduction zone earthquakes, with application to real-time seismic and geodetic warning
• Background on Tsunamigenic and Tsunami Earthquakes
• Identifying Earthquake Energy for real-time earthquake magnitudes and tsunami warnings
• Seismic and geodetic displacement in tsunamigenic earthquakes.
Examples from:– Sumatra (2004, 2005, 2010)
– Solomon Islands (2007, 2010)
– Chile (2010)
– Japan (2011)
• We are working to understand shallow trench rupture, its rupture potential, and imminent warning when it does.
A few key findings:– Still no clear answers for why megathrusts sometimes fail near the trench – The free surface allows for enhanced slip (breaking scaling laws)
– Splay faults are unnecessary
– Most shallow rupture is identifiably slow
Outline:
• Most megathrust events rupture between ~20-50 km depth
• Occasional tsunami earthquakes rupture almost exclusively shallower than 20 km [Polet and Kanamori; 2000].
– These events are problematic because they aren’t usually identified until after the fact
tsunami generation:
are “Earthquakes that create tsunamis much larger than expected given their initial magnitude” –Kanamori [1972].
Rupture in shallow increases tsunami-potential
• free-surface effect causing “unhinged slip” [Satake and Tanioka, 1999]
• Possible rupture on splay fault can enhance vertical displacement [Moore et al., 2008]
• Slowed rupture in Tsunami earthquakes region may enhance slip due to momentum conservation [Newman, unpublished] (e.g. end of a whip)
tsunami generation:
Further correcting for mechanism, depth and distance: Newman and Okal [1998]
E determinations: – All M0=1019 Nm
– 1997 – mid-2010
– Using full rupture duration
Results:– Megathrust ruptures have low
stress drop (reduced E/M0)
- ID new slow events (e.g. NB)
- Middle America Trench is E deficient
Radiated seismic energy
Using P-waves (25°≤ ≤ 80° )
[Convers and Newman, 2011]
Standard Earthquake
M~7.0
Slow-source Tsunami Earthquakemb ~5.8, MS ~7.0, MW~7.7
Energy Deficiency of Tsunami Earthquakes
Poorly consolidated and water rich sediments near the trench reduce rigidity
Normal Vs ~3.0 km/sNear-trench Vs ~.5-1.5 km/s
Shallow slownessGlobal survey of subduction zone events found shallow events have increased rupture duration (reduced rigidity, )
[Bilek & Lay, 1999]
An improved TsE Discriminant• E/M0 method requires a reliable Moment solution
which may take 30 min or more - too late for nearby environments where
• Instead, we aim to ID the slowness of these earthquakes
Theoretical observed rupture duration
Duration
Decay is somewhat dependent on locale
+ o
Energy Scattering
M 5
.5
Time ->
Theoretical Modeling
Total energy at site over time
Cumulative energy at site
Time ->
Theoretical Modeling
Constant Rupture Model gives shape of Energy Growth
X-over overestimates duration
However, Source-Time functions for large earthquakes are not usually box-car functions, and have significant slip decay before cessation.
S. Bilek, pers. Comm.
Method rapidly ID’s Tsunami Earthquakes
Discriminant: C=E/Tr3 (∝ E/M0)
[Newman and Convers, in revision]
For TsE, evaluating the ratio of energy to rupture duration (Tr) yields potentially more robust information given that TsE are:
- Low E (~10x) - Long Tr (~ 3x)
For events larger than 7 only TsE have E/Tr3 < 5e7
Particularly useful because both can be evaluated from just the P-wave energy growth.
Method rapidly ID’s Tsunami EarthquakesDiscriminant: C=E/Tr3 (∝ E/M0)
Real-time: http://geophysics.eas.gatech.edu/RTerg
Routinely and automatically calculate E and TR for all earthquakes with initial magnitude ≥ 5.5 (early 2009 to present)
Automatic webpage is generated and results are disseminated to the PI, students, USGS, and Pacific Tsunami Warning Center. (Feeds through email and text messages)
1st results often within 10 minutes of rupture, with automatic updates following.
Stand-alone version is now operational at the PTWC (GT system is research mode).
Earliest tsunami waves take 30 m or more…thus the methods can supply ample warning!
2010 Mw 7.8 Mentawai Earthquake• >400 fatalities (from tsunami)• many survivors thought M6• initial reports of 5-9 m runup • possible history of TsE here
with 1907 M7.6 recently identified [Kanamori et al., 2010]
Summary:• Event was a “Tsunami EQ” (TsE)
• Large tsunami given magnitude
• Shallow rupture• Slow TsE
• Low E/M0
• Long duration• Tsunami excitation can be
estimated from FFM w/ rupture velocity correction
Newman et al., GRL [2011]
Real-time detection1: Nucleation + 8:13 [m:s]2: Nucleation + 10:52 [m:s]3: Nucleation + 15:45 [m:s]4: Nucleation + 21:15 [m:s]5: Nucleation + 26:55 [m:s]
Newman et al., GRL 2011
Finite fault solution: 2010 Sumatran EQ
P, SH, Rayleigh and Love Waves modeled
Solution on 11.6° dip- max slip near 2 m updip of nucleation point- rupture duration, TR=125 s- M0=6x1020 Nm (MW=7.75)
Newman et al., GRL 2011
Slip scaling by VR
Shear velocity:
Upper/Lower plate params.
Slip
Rigidity
Density (constant)
Shear wave velocity
Rupture velocity ( ~0.8)
Upper plate slip:
For constant M0
Slip scaling by VR
€
Du = D lβ l
1.25VR
⎛
⎝ ⎜
⎞
⎠ ⎟
2
€
2 = μ ρ
Shear velocity:
€
u, l
D
μ
ρ
β
VR
Upper/Lower plate params.
Slip
Rigidity
Density (constant)
Shear wave velocity
Rupture velocity ( ~0.8)
Upper plate slip:
€
Duμ u = D lμ l
For constant M0
Tsunami models
Original Finite-Fault
Unscaled FFM max local runup < 3 m
Scaled FFM max local runup +10 m
Open-ocean wave heights well predicted well fit in period and timing but over-predict amplitude
Scaled solution
Tsunami models
Ocean-bottom pressure sensor is along highly attenuated path (along trench and near trend of strike)
Other inversion results:
P, SH waveform inversion of Lay et al., 2011
Case study: Solomon IslandsSolomon Islands earthquakes as a subduction end-
member
– Rapid convergence of very young crust allows normal megathrust seismogenesis in the very shallow subduction environment
– Near-trench land ideal for shallow seismogenic zone geodetic measurements
1 April 2007 MW 8.1 Solomon Islands megathrust earthquake and tsunami
1 April 2007 MW 8.1 Solomon Islands Earthquake
Taylor et al., Nat. Geosc. 2008Uniform 5m slip model with steep geometry (=38°)
1 April 2007 MW 8.1 Solomon Islands Earthquake
Data from Fritz and Kalligeris 2008
Flow-depth less affected by localized “splash-up”
1 April 2007 Mw 8.1 Solomon Islands Earthquake
Data: Coral uplift, coastal subsidence( Fritz & Kalligaris, 2008; Taylor et al., 2008)
All data collected between 2 weeks and 1 month after event
Chen et al. GRL, 2009
Model Deformation:• We apply the analytic method of describing elastic
deformation do to displacement along a planar dislocation [Okada, 1992]
• Adapt method for a plane of discrete dislocation sub-fields (to model distributed rather than uniform slip)
Trade-off between smoothness and fitSmoothing minimizes ‘roughness’ of the slip surface [e.g. Harris & Segall, JGR, 1987]
S=slip
Identify a subjective trade-off between improved model fit and overly smoothed result (non-unique solution).
Testing with new stress minimization
[ Feng, Newman, and Chen, 2009] Chen et al., GRL, 2009
Coseismic Slip Distribution Comparison
Strain minimizationRoughness minimization
Model: = 29°, free-surface at trench; Maximum slip (100 m)
Feng, Newman, & Chen, 2009
Maximum slip stabilizes near 30 m
Solomon 2007 event results:
Interface slip Surface uplift
Two large patches of slip occur in the very shallow trench, though not a tsunami earthquake because tsunami affects were expected.
End-member: where subduction of very young crust allows for normal rupture in shallow region.
Chen et al., GRL, 2009
3 Jan 2010 Mw 7.1 Solomon Islands Earthquake and tsunami
News reports of tsunami devastating villages (strange for an M 7)
Trans-oceanic Tsunami identified by DART Buoys
NSF- RAPID funding for: postseismic geodetic tsunami surveys
(event occurred on Sunday; project funded on Friday; left on Saturday)
(Courtesy NOAA-PMEL)
X
Ndoro Point (7m runup)
Retavo Village (7.5 m runup)
Comparable tsunami inundationto 2007 Mw 8.1 earthquake
No postseismic deformation
Subsidence Everywhere
Subsidence along coast requires very large slip seaward (updip)
Predicted tsunami heights
Energy from SI event
- Not energy deficient (doesn’t look like other tsunami earthquakes)
Possibly not slow (fast rupture due to upward propagation of normal seismogenic interface)
or
May have been slow, but with very large slip
~33 s duration suggests 1-1.5 km/s rupture
a normal TsE, rupturing ¼ of its normal length?
• Most megathrust events rupture between ~20-50 km depth• Tsunami earthquakes rupture shallower than 20 km
Total megathrust rupture?:
are “Earthquakes that create tsunamis much larger than expected given their initial magnitude” –Kanamori [1972].
• Possible to rupture both? Difficult to ID since seismic models of interface slip are imprecise and little land exists near trench for geodetic study
Near-trench rupture
Sumatra 2004: total megathrust rupture.Sumatra 2005: normal megathrust rupture.
Chile 2010 : normal megathrust rupture.
Japan 2011: total megathrust rupture, similar to Sumatra 2004
E/M0
Energy rate
2.6e14 W
• Energy Rate for Tohoku 2011 event deficient• Rupture slower than Sumatra 05, or Chile 2010• Similar to 2004 event (perhaps smaller TsE area),
NEIC Finite-Fault ModelDmax ~30 m
=10.5°
FFM: Model 26Dmax ~50 m
= 12.36°
FFM: Model 26
GEOnet GPS processed by Caltech/JPL
GPS: Coseismic DisplacementsData are from model Aria 0.3 from CalTech/JPL Aria team (processed at 5:40 and 5:55 UTC, based on 5 min solutions)
Maximum displacements: 5.2 m horizontal, -1.1 m vertical
FFM: Model 26
GEOnet GPS processed by Caltech/JPL
Real-time PMEL source model
Source: NOAA/PMEL/Center for Tsunami Research Team
Real-time fault plane solutions are found using pre-determined Green’s functionsto estimate the far-field tsunami excitation
Real-time PMEL source model
Tsunami-inverted slip planes are used to predict seafloor displacement.
77 -vertical ‘measurements’ are subset at 0.5°x0.5° grid
Solutions weighted 3:1 to GPS components ( 3 components x 545 station = 1635 values)
Joint GPS-tsunamimeter inversion
Model: Comb3
Dmax ~50 mUzmax ~8 m
GPS: Coseismic DisplacementsData are from model Aria 0.3 from CalTech/JPL Aria team (processed at 5:40 and 5:55 UTC, based on 5 min solutions)
Maximum displacements: 5.2 m horizontal, -1.1 m vertical
GPS: Coseismic DisplacementsModel is solution developed from joint GPS-’tsunami’ inversion
GPS: Coseismic DisplacementsData and Model
GPS: Coseismic DisplacementsResiduals are shown to scale from before
RMS - H= 0.028 m RMS -V= 0.039 m
N=545 stations (bounded by plotted region)
Joint inversion results:
(Newman, Nature 2011)(Sato et al., Science, 2011)
Summary (Japan Trench):Tohoku 2011 earthquake was a Total Megathrust rupture
Joint GPS-tsunami, and teleseismic FFM models agreeDmax~50 m, in 2 lobes very near trench
adjacent to Sanriku 1896 tsunami earthquake
Massive slip in TsE regionrequired for observed tsunami
Summary:• Improvements in seismic tsunami warning for giant and slow tsunami
EQs:
• TsE from Mentawai, Java (2x), Nicaragua, Peru (not Solomons)
• Giant EQs in Japan, Sumatra and Chile
• In the Solomon Islands:
• Shallow slip is the norm, and tsunamis are frequent
• up to 30 m in 2007 Mw 8.1
• up to 7.6 m in 2010 Mw 7.1 (TsE, but possibly not slow)
• Some giant earthquake exhibit total megathrust rupture, enhancing their tsunamigenesis
• Evaluating tsunami hazards going forward…
Seafloor geodesy for shallow locking studies:
Onland geodesy cannot constrain offshore deformation
http://www.kaiho.mlit.go.jp/info/kouhou/h23/k20110406/k110406-2.pdf
Sato et al., Science, 2011
Geodetic Survey Division Office Marine Navigation and Oceanographic Department
Dense seafloor geodetic networks can:
• Image locking building for shallow rupture– (need long-term stable measurements)
• Provide direct warning of rapid seafloor displacement– Rather than focus on seismicity which is a symptom of slip,
geodetic tools can directly observe deformation driving tsunami.
– (Need cable or buoyed systems to relay data)
• This will cost $$ (or €€, ¢¢,¥¥, ££)– If instrument costs go down to $50k, can densly instrument
the Japan trench for ~$20M (EarthScope PBO is ~$100M)– About 0.01% of the projected cleanup cost for Japan.
DART – Real-time ocean bottom pressure sensors
measuring the height of the water column (long wave-length) above them
NOAA and PTWC
50 m seafloor displacement
http://www.jamstec.go.jp/j/about/press_release/20110428/
From repeat seafloor bathymetry
Near trench locking off of Peru
In 1996 a destructive tsunami earthquake occurred near trench about 100 km north
Large Subduction Earthquakes Moment Magnitude ∝ log10 (Area x Slip)
Slip by magnitude:1+ m in M710+ m in M8
Recurrence controlled by: Convergence rate (1-10 cm/yr)
and locking efficiency 5 -100%
Area increased by angle/thick crust/reduced T.
~20 km
Tsunami Generation:
Tsunami amplitude is controlled by vertical seafloor displacement
Impact on land controlled by:• distance from source• “fetch” (lateral extent) of seafloor displacement• path (focusing/ blocking features)• coastal amplification features (e.g. harbors)
• Timing controlled by distance/depth of water column (a few min to 10+ hours)
Very Large and Great Earthquakes:
Subduction zones:
•> 90 % of all great earthquakes (M8+)
•All giant earthquakes (M9+) •Most explosive volcanism (not discussed today)
Heavy population density in coastal environment
Newman, Peng, & Hoyos (in revision)
Sea floor pressure sensors