Over-Arching Themes:

(See Science Plans, page 104)

Understanding the temporal and spatial components of the seismogenic cycle encompassing fast, slow, tsunamigenic, and silent earthquakes:

• What controls the seismic energy release during subduction zone earthquakes?
• What controls the locking patterns on the plate interface and subsequent energy release?
• What controls propagation and slip rates and the distribution of fast, slow, tsunami, and silent earthquakes in time and space?
• What is the nature of temporal changes in strain, fluid pressure, and stress during the seismic cycle?

Methods of Studying Seismogenic Zone Processes:

(See the Science Plan, page 125)

Focus Areas:

General Site Criteria:

• Historic large earthquakes in region
• Much of the seismogenic zone must be imageable by seismic reflection
• Subduction thrust must be drillable near to the trench and updip limit of seismogenic zone
• Data availability and logistical accessibility must favor the site
• Subduction characteristics should optimize scientific return

Primary rationales for Central America (CA) and Nankai:

(See Science Plans, pages 123 through 124)

• Contrasts in seismicity characteristics (magnitude, spatial, temporal)
• Contrasts in plate convergence, incoming sediment amounts and types, heat flow, history
• Well-studied systems and well-characterized seismicity
• Strong links to ODP and the potential for future deep drilling during IODP

Workshops and TEIs:

• TEI: Rheology and Deformation of the Continental Lithosphere, Snowbird, 2000. Publication: Rheology and Deformation of the Lithosphere at Continental Margins, MARGINS Theoretical and Experimental Earth Science Series vol. 1, Columbia University Press, 2004
• Central America SEIZE and SubFac Workshop, Costa Rica 2001
• NanTroSEIZE Workshop, Boulder, 2002
• TEI: The Seismogenic Zone Revisited, Snowbird, 2003. Publication expected 2005 (working title): Interplate Subduction Zone Seismogenesis, MARGINS Theoretical and Experimental Earth Science Series vol. 2, Columbia University Press
• Costa Rica SEIZE Workshop, Keil Germany, 2003

Major Research Activities:

• Seismic reflection/refraction studies:
  • Japanese collection of 2-D MCS in Kii Peninsula Transect
  • Joint Japanese-U.S. 3-D survey in the Muroto Transect
  • U.S. and German MCS/OBS sampling programs offshore Nicaragua and Costa Rica
• Seismometer deployments:
  • Japanese conducted microseismicity programs in Nankai
  • Seismicity observations on and offshore Costa Rica
• Crustal deformation:
  • Continuation of a long-established GPS network on Japan
  • Costa Rica campaign-style GPS deployments
• Heat flow:
  • Regional and focused data collected offshore Costa Rica
  • Regional and focused data collected offshore Nankai
• Experimental work:
  • Smectite and illite bearing gouge at room temperature
  • Frictional properties of Nankai subduction zone mudstones as a function of clay content and mineralogy
• Deformation, fluid transport and consolidation:
  • New efforts largely as part of Ocean Drilling Program Legs 190, 196, 205
  • Sample based studies and experimental and modeling investigations
  • Observations of sea floor fluid flux
• Preparation of IODP proposals: Developed from concepts nurtured at workshops and through MARGINS supported research.  Proposals include:
  • Complex Drilling Proposal for Nankai Trough (overview of drilling plan, ultimately including specific proposals Phases 1-2): 
    • Phase 1: Sampling and Measuring Inputs to the Nankai Seismogenic Zone
    • Phase 2: Mechanical and Hydrologic State of Nankai Mega-Splay Faults: Implications for Seismogenic Faulting and Tsunami Generation 
  • Complex Drilling Proposal for Costa Rica (Overview of drilling plan, Phases 1-2):
    • Phase 1: Constraining and Characterizing the Input in the Seismogenic Zone of the Erosional Central America Convergent Margin
    • Phase 2: Studying the Transition from Stable to Unstable Sliding at Erosional Convergent Plate Boundaries

Intellectual Progress During MARGINS:

• Developed shared community vision of major problems and goals from TEIs and other workshop activities, and helped sharpen IODP deep drilling objectives.
• Conceptualizations:
  • Re-evaluation of historical Japanese seismicity links rupture barriers to structures on the downgoing plate (e.g., Kodaira et al., 2002, GJI 149, 815-835)
  • Out-of-sequence, possibly tsunamigenic, thrusts connect to the plate boundary fault in Nankai
  • Correlation of subduction of Nicaragua seamounts with shallow rupture and tsunami events
• Costa Rica seismicity documents considerable along strike variation, possibly related to thermal properties
• Slow slip events, including a 20 cm event in one year in Nankai, are becoming better known
• Apparent "partial coupling" in Costa Rica might represent temporally alternating locked and creeping segments
• Interseismic microearthquakes do not define the up-dip limit of the seismogenic zone in one well-observed segment in Costa Rica
• Large spatial variations in incoming plate thermal properties:
  • The complex plate history and thermal properties are now well mapped off Costa Rica
  • A high heat flow region has been discovered in the Shikoku Basin (Nankai)
• Compaction or a diagenetic component to the up-dip limit of seismicity in the Nankai Muroto transect is suggested from reflection amplitudes
• The smectite-illite phase transition is not likely responsible for the up-dip seismic-aseismic transition
• High clay contents and regional excess fluid pressures contribute to low resolved shear stresses in the shallow decollement at Nankai
• Detection of increased fluid flow at seafloor is possibly related to transient strain events off Costa Rica

Map (PDF) prepared by the MARGINS Office, showing the available information on where work has been funded to date in the Nankai focus area. (Click map for a larger version with explanatory caption.)

Map (PDF) prepared by the MARGINS Office, showing the available information on where work has been funded to date in the Costa Rica-Nicaragua focus area. (Click map for a larger version with explanatory caption.)

 

Major Research Gaps:

• Continuous GPS in Costa Rica to monitor foreslip, afterslip, slowslip, and viscoelastic. This is essential to almost all of our theoretical efforts and broad seismogenesis issues
• Offshore seismometer deployments to illuminate larger spatial areas over longer time intervals
• More experimental data are needed on frictional heating along fault contacts
• Improvement of thermal models and temperature estimates along subduction thrusts
• Hydrologic-thermal-chemical modeling to investigate temporal variations and their consequences for faulting and the earthquake cycle
• Further laboratory studies of the physical-chemical processes important to rate-dependent friction, consolidation and metamorphism
• Laboratory data to define material properties, such as poroelasticity and permeability, in order to infer physical meaning of seismic images, and for input to hydrologic and mechanical modeling
• Additional study of paleo-seismogenic zones (including both paleo-decollements and out-of-sequence or “splay” thrusts) in order to guide and complement future drilling:
  • Role of fluid-rock interaction to fault mechanics
• Deep drilling sites need clarification of structural setting and pore-pressure estimates for borehole design (e.g., 3-D seismics)
• IODP direct sampling and monitoring of “seismogenic” capable faults

Ongoing Scientific Problems Motivated by MARGINS Research:

Work to date highlights the longevity of the scientific themes addressed in the science plans, and also sharpens the focus on particular aspects. These include:

What controls the updip limit of the seismogenic zone?

• CA and Japan issue: Do these controls differ for accretionary vs. erosional margins?
• CA issue: Recent data suggest a jump in the depth of the updip limit for the Nicoya Peninsula which corresponds closely to the change from regionally low to regionally high heat flow on the incoming Cocos Plate. Is this finding robust and if so can we quantify the effect of temperature variation on the depth to the updip limit of seismicity?
• CA and Japan issue: Recent lab studies have shown that the smectite-illite transition is not likely to be important in controlling this depth limit. What factors are important (fluid pressure, compaction, opal-quartz transition, quartz precipitation, other diagenetic boundaries) and how can we determine which factor(s) actually apply to a given seismogenic zone?

How does one define the updip limit of the seismogenic zone?

• CA and Japan issue: Do microseismicity and geodesy give the same result?

What controls the seismic energy release during subduction zone earthquakes?

What controls the locking patterns on the plate interface and subsequent energy release?

What controls propagation and slip rates and the distribution of fast, slow, tsunami, and silent earthquakes in time and space?

• CA issue: Does the juxtaposition of a large tsunami earthquake off Nicaragua and a “regular” locked zone of normal earthquakes off the Nicoya Peninsula indicate fundamental differences in the seismogenic mechanism between these regions, or are they a manifestation of temporal changes expected at any seismogenic zone?
• Additional CA issue: Is the variation in seismicity between Nicaragua and Costa Rica related to the variation in normal fault displacement and perhaps sediment trapping on the lower plate?

What is the nature of temporal changes in strain, fluid pressure, and stress during the seismic cycle?

What is the role of slow slip events in releasing the strain energy along seismogenic zones? Are these events episodic?

International Collaborations:

• Japanese MCS, OBS and submersible work in Nankai in regions, and joint U.S.-Japan 3-D studies
• Germans—with a long established program in and around Costa Rica, including multibeam, sampling, MCS, OBS and onshore fieldwork—are often closely linked in collaborative investigations
• Costa Ricans operate seismic networks onshore and are active collaborators of several field programs, including the upcoming large PASSCAL program
• Other collaborations from multiple countries are in the form of intellectual exchanges, as evidenced by participation at the various Workshops and TEIs

Presentations and Publications:

(See “Research” section)

Nuggets:

(Proposal start dates in parentheses)

• Brown (September, 1999): “Benthic Flux Meter Study Across the Costa Rica Margin”
• Screaton (May, 2003): “Modeling Of Coseismic Pore Pressure Changes In Subduction Zones: Implications For Fluid Flow and Planning For Drilling And Long-Term Observatories”
• Brown (May, 2002): “Collaborative Research: Frictional and Mineralogical Properties of Sediments Entering Subduction Zones: Controls on Stress State and Earthquakes”
• van Keken (August, 2001): “Margins: Workshop on Modeling of Subduction Zone Dynamics and Thermal Structure”
• Moore (August, 2003): “Collaborative Research: Seismic Velocity, Compaction, and Pore Pressure in Underthrust Sediments, Nankai Subduction Zone”
• Marone (August, 2000): “Laboratory Frictional Studies Of Fault Gouge: A Test Of Hypotheses For Controls on the Updip Limit of the Seismogenic Zone Along Subduction Thrusts”
• Underwood (September, 2000): “Strike-Parallel Variations in Clay Mineralogy and Their Impact on the Cascadia Seismogenic Zone”
• Stein, Wheat, and Harris (October, 2000): “The Thermal State Of 20-25 Ma Lithosphere Subducting at the Costa Rica Margin, Implications For Hydrogeology, Fluxes, and the Seismogenic Zone”
• McIntosh (January, 2000): “Structure of the Nicaragua/Costa Rica Subduction Zone: A Framework for the Subduction Factory and Seismogenic Zone Initiatives. US/German/Nicaraguan Collaborative Project”
• Schwartz (August, 1999): “Collaborative Research: Imaging the Seismogenic Zone with Geodesy and Seismology: Two Land Ocean Transects Across Costa Rica and the Middle America Trench”
• Dixon (August, 1999): “Imaging the Seismogenic Zone with Geodesy and Seismology: Two Land Ocean Transects Across Costa Rica and the Middle America Trench”

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