A high-resolution 3D seismic velocity model of the 2010 Mw 8.8 Maule, Chile earthquake rupture zone using land & OBS networks
Knowledge of the spatial distribution of seismic properties within an earthquake rupture zone is essential to our understanding of rupture mechanics. Following the Maule earthquake, there was an international collaborative effort to deploy a dense network of seismic instruments in order to record the aftershock sequence; this means a large dataset is available to perform seismic velocity tomography in the area of the rupture zone. Since most co-seismic slip occurred in the offshore region, it is important to interpret the velocity structure of the marine forearc and the underlying oceanic crust. However, since many aftershocks are located offshore, and thus outside of the land network, both the offshore velocity structure and the location of these aftershocks are inherently poorly resolved. During the period July - December 2010, The National Taiwan Ocean University and The University of Liverpool each deployed ocean-bottom seismometer (OBS) networks in the northern and southern ends of the rupture zone, respectively, comprising a total of 43 stations. We use a catalogue of ~500 seismic events recorded at both land and OBS stations, containing ~60000 hand-picked P- and S-wave travel-times. We use a staggered 1D inversion scheme, which initially incorporates a separate velocity model for the marine forearc in order to form better hypocentral locations for the offshore events. Based on previous estimates for slab geometry, we find that the location of offshore seismicity is more tightly constrained along and above the interface. Taking these improved locations into account, we then re-invert for a new 1D model with station corrections. We present a 3D local earthquake tomography model based on manually-picked arrival times. The incorporation of OBS picks into the inversion elucidates better both the up-dip geometry of the subducting plate and the structure of the marine forearc. Beneath the low vp marine forearc (vp < 6.0 km/s), at depths of 7-15 km, we infer the presence of a high velocity structure (vp > 7.0 km/s); the upper interface of which dips at 10-15°, interpreted as the top of the downgoing oceanic crust. These first-order features are in accordance with results from previous active source studies in the region. We continue to analyse the nature and geometry of velocity anomalies along and around the megathrust, and their relation to rupture models and aftershock distribution.
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A high-resolution 3D seismic velocity model of the 2010 Mw 8.8 Maule, Chile earthquake rupture zone using land & OBS networks
- 1. A high-resolution 3D seismic velocity model of the 2010 Mw8.8 Maule, Chile earthquake rupture zone Using land and OBS networks Stephen Hicks*, Andreas Rietbrock, Isabelle Ryder School of Environmental Sciences, University of Liverpool, UK Chao-Shing Lee National Taiwan Ocean University, Taiwan Matt Miller Universidad de Concepción, Chile *Email: s.hicks@liverpool.ac.uk
- 2. The overriding / underlying question… Can we physically identify asperities and barriers along the megathrust? Megathrust dynamics Material properties
- 3. 6th largest recorded earthquake magnitude 8.8 rupture length 500 km max slip 16m a heterogeneous megathrust? Mainshock hypocentre location: Hayes et al. (2013), GJI Co-seismic slip model: Moreno et al. (2012), EPSL
- 4. Tomographic inversion algorithm: SIMUL2000 (Thurber,1983, JGR) An unprecedented dataset 160 land + 37 OBS stations 670 aftershocks 38,000 P-wave picks 14,000 S-wave picks
- 5. Forearc 2D velocity structure Resolution limits Focal mechanisms from: Agurto et al. (2012), EPSL Hayes et al. (2013), GJI Coastline
- 6. Forearc 2D velocity structure Resolution limits Focal mechanisms from: Agurto et al. (2012), EPSL Hayes et al. (2013), GJI Coastline
- 7. Forearc 2D velocity structure Resolution limits Coastline
- 8. Forearc 2D velocity structure Resolution limits Coastline
- 9. Down-dip segmentation of the megathrust Automatic picks from Rietbrock et al. (2012)
- 10. 3D velocity structure Resolution limits Coastline
- 11. 3D velocity structure Resolution limits Coastline Automatic picks from Rietbrock et al. (2012)
- 12. Shedding light on megathrust dynamics
- 13. Shedding light on megathrust dynamics Pre-seismic locking (90%, 95%): Moreno et al., 2010 Post-seismic Afterslip: Lin et al., 2013 Interface aftershocks: Rietbrock et al., 2012 Co-seismic rupture Nucleation: Hayes et al., 2013 Slip: Moreno et al., 2010 High freq: Kiser & Ishii (2011)
- 14. Shedding light on megathrust dynamics
- 15. Shedding light on megathrust dynamics Post-seismic Afterslip (>1m): Lin et al., 2013 Interface aftershocks: Rietbrock et al., 2012 Pre-seismic locking (90%, 95%): Moreno et al., 2010 Co-seismic rupture Nucleation: Hayes et al., 2013 Slip: Moreno et al., 2010 High freq: Kiser & Ishii (2011)
- 16. Forearc body: composition & origin vp ~ 7.7 km/s; vp/vs ratio ~ 1.8 Positive gravity anomaly Gravity model from Hicks et al. (2012), GRL Observations Composition Ultramafic - weakly serpentinised Christensen & Mooney (1995), JGR; Hacker & Abers (2004), G3 Origin Subducted topographic anomaly? Hicks et al. (2012), GRL
- 17. Forearc body: composition & origin Observations vp ~ 7.7 km/s; vp/vs ratio ~ 1.8 Positive gravity anomaly Composition Ultramafic - weakly serpentinised Christensen & Mooney (1995), JGR; Hacker & Abers (2004), G3 Origin Subducted topographic anomaly? Hicks et al. (2012), GRL Root of Paleozoic granite batholith?
- 18. Forearc body: composition & origin Observations vp ~ 7.7 km/s; vp/vs ratio ~ 1.8 Positive gravity anomaly Composition Ultramafic - weakly serpentinised Christensen & Mooney (1995), JGR; Hacker & Abers (2004), G3 Origin Subducted topographic anomaly? Hicks et al. (2012), GRL Root of Paleozoic granite batholith? Mantle upwelling during Triassic extension Vásquez et al. (2011), J. Geol.
- 19. Implications for Maule seismogenesis 1 2 3 Ultramafic bodies beneath coast inhibit rupture propagation Low vp gradient and lower vp/vs values associated with high slip High vp/vs controls up-dip limit of seismogenesis 4 Afterslip may be compositionally-driven
- 20. Can we physically identify asperities and barriers along the megathrust? Up-dip barrier: Fluid-saturated sediments Down-dip barrier: Long-lived ultramafic bodies in crust
- 21. 3D velocity structure Resolution limits Coastline
- 22. A segmented seismogenic zone!
Editor's Notes
- Chilean subduction zone responsible for some of the world’s largest earthquakes Seismic images of entire rupture zone => show how compositional variations can have influence on ruptures Apply to Maule 2010 – best recorded Focus on results not method. Previous talks based more on active source, I use passive, local earthquake tomography method.
- Simple framework – what are asperities and barriers? Subducting or overriding… How can we physically identify? – traveltime tomography
- 6th largest Nucelation then 500km long Rupture between trench and coastline 2 asperities => heterogeneous TRANSITION: Well understood in terms of megathrust dynamics. But how can we do the seismic imaging?
- Most aftershocks & slip offshore Aftershock selection criteria 2) Run through inversion technique for Vp and Vp/Vs ratio 3) Produce best images of a megathrust earthquake rupture zone yet. TRANSITION: Now lets have a look at these images. Plotted on section located perpendicular to trench.
- 1) Can define plate interface
- 1) Can now examine other areas of model
- Orientation, main features Interface, marine forearc, CD, mantle. Accretionary wedge – overpressured sediments. Interface not well defined => heterogeneity So, here is our 2D P-wave velocity model, which is plotted on a section oriented perpendicular to the trench. Red colours represent higher velocities, whereas blue colours represent the lower P-wave velocities. The white circles represent earthquakes relocated in this velocity model. The triangle represents the location of the coastline. So, now you see relative to a 1D model, we are now really starting to pick out some more complex structure that you’d expect in a subduction zone. This dipping interface with velocities of just over 7km/s seems to represent the plate interface and is well defined the band of seismicity. The low velocity zone between the trench and coastline seems to represent the forearc basin, and there’s another basin west of the volcanic arc, and beneath this, we have a portion of continental mantle. It is clear that the megathrust interface isn’t particularly clearly defined beneath the coastline, and this may suggest we may have some heterogeneity in this region.
- Can image subducting slab – hydrated oceanic crust Marine forearc elevated vp/vs – overpressured fluids No evidence of hydrated mantle Dry forearc crust TRANSITION: Now let’s make some interpretations based on this megathrust geometry and velocities
- TRANSITION: Next I’m going to now plot the full 2010 aftershock sequence relocated in this new velocity model. Interface defined in similar way Most features remain the same. But, anomalies present above interface beneath coast. Most prominent in sections A and C.
- TRANSITION: Next I’m going to now plot the full 2010 aftershock sequence relocated in this new velocity model. Interface defined in similar way Most features remain the same. But, anomalies present above interface beneath coast. Most prominent in sections A and C.
- Greatest locking with lower velocities Nucleation point of mainshock up-dip of central anomaly Low slip with anomalies High frequency radiaition avoids anomalies. TRANSITION: in a similar way, now let’s have a look at the Vp/Vs.
- Greatest co-seismic slip occurs in narrow region of slightly reduced vp/vs. Afterslip (mostly aseismic) at higher Vp/Vs down-dip areas. TRANSITION: Now let’s think about the composision and origin of our P-wave velocity anomalies we are observing
- Complicated plot, but will take you through this. Histogram of relocated earthquakes, velocities plotted along interface. Vp – 6.5 – 7.5km/s, vp/vs~1.85 in most seismically active region. Take-home point: strong link between composition and seismogenesis TRANSITION: Made some basic interpretations, but let’s take this further by going into 3D
- Compare to experimental and numerical compositional velocity models. Gravity model above forearc!!!
- 1) Coastal ranges composed of granitic batholith, but no obivous correlation and hard velocitieis corresponding to residual magmatic intrusion.
- Small triassic igneous gabbroic outcrops along coast Related to deep source, extensional period. Slab window?
- TRANSITION: In more general terms, we can suggest a new model for the seismogenic zone along central Chile.
- Locking – asperity / barrier may change over time.
- 1) Similar features for 2D but worth noting Vp/Vs of anomalies is around 1.80. TRANSITION: Now let’s plot the velocities along the interface and relate this to fault dynamics.
- Seismogenic zone can be subdivided into 3 distinct areas. Transititon: Again more generally, lets have another look at the question I posed at the start of the talk.