1. Evan Solomon School of Oceanography University of Washington
Continuous Seafloor Observations to Understand the Nature and Dynamics of Fluid Flow through Seismic Cycles
Evan Solomon
School of Oceanography
University of Washington

2. General Review of Subduction Zone Hydrogeology
Saffer and Tobin (2011)
Fluid-rock reactions control frictional properties of faults though:
Diagenesis and mineral precipitation
Fault zone alteration
Pore fluid pressure variations control effective stress and develop through:
Disequilibrium compaction
Mineral dehydration reactions
Kastner et al. (2014)
Pore fluid migration is also an important control on pore pressure evolution along the plate boundary

3. Pore Pressure Evolution – Costa Rica Example
Depth (km)
Saffer and Wallace (2015)
Saffer and Tobin (2011)
Transition in reflectivity at shelf break not tied to smectite-illite transition
Suggest upper plate structure, permeability, and fluid flow important for controlling plate boundary pore pressure distribution
Bangs et al. (2015)

4. Questions
What is the background fluid flow distribution and how does it relate to inferred pore pressure distribution from seismic attributes and models?
What is the nature of fluid flow and its relationship to fault slip (i.e. how do flow rates vary through SSE and seismic cycles)?
There are very few systematic surveys of fluid flow across subduction zones, and there are few long-term observations of fluid flow rates both at and outside of fault zones

5. Approaches
Fluids are “messengers from the deep” and inform us about fluid-rock reactions and fluid sources (i.e. compaction, mineral dehydration)
Requires holistic approach with fluid flow rate and composition observations from the deformation front to the shelf.
Requires other observations – complements and expands upon seismic attributes, borehole observations, onshore cGPS, and seafloor APGs
Explain that surface and IODP coring are important for constraining background flow distribution. IODP only one point or a few. Requires expansion from surface heat flow and piston cores. Coring data has been instrumental in showing long-range transport of fluids. No time to go into this.
Ideal Suite of Observations
1. Coring and heat flow transects informed by seismic data
2. IODP coring and CORK observatories (P, T, fluid flow, chem)
3. Continuous seafloor fluid flow meter deployments

6. Seafloor Fluid Flow Meters
Seafloor fluid flow meters provide long-term, continuous records of fluid flow rates and chemistry for periods of weeks to years at relatively high temporal resolution
Fabrication and deployment of these instruments is relatively cheap compared to other types of offshore measurements
Since fabrication and deployment are relatively economical, flow meters can be deployed in an array both along and across strike
They are sensitive to small displacement of fluid, and can be used to monitor both long-range transport of fluid and the local flow response to volumetric strain (i.e. during fault slip)

7. Seafloor Fluid Flow Meters
Flow meters can be used to continuously monitor in fluid flow rates in faults as well as local flow response to volumetric strain
= compressibility
s’ = effective stress
V = volume
’ =  - Ppf
Figure from Brown et al., 2005, EPSL
Given a typical resolvable flow rate of 5 × 10-5 cm/d, can record a flow response to a minimum stress rate change of ~1-10 Pa/d

8. Seafloor Fluid Flow Meters
Dd = net vertical fluid displacement in response to subsurface volumetric strain
= porosity
b = aquifer thickness
Figure from Brown et al., 2005, EPSL
Due to the high hydraulic impedance of typical prism sediments, rapid changes in flow rate are due to deformation of the upper few meters of sediment below the seafloor instrument

9. Seafloor Fluid Flow Meters
There are currently two fluid flow meters in use that are capable of measuring low fluid flow rates in marine sediments, both are based on a tracer injection and sampling driven by osmotic pumps
CAT meter – Tryon, SIO
Can resolve a flow in response to a stress rate change of ~ Pa/d in a m3 unit
Measures 1-D with delayed fluid chemistry

10. Seafloor Fluid Flow Meters
Mosquito – Solomon, UW
Can resolve a flow in response to a stress rate change of ~ Pa/d in a m3 unit
Measures 3-D fluid flow field with instantaneous fluid chemistry
No effects of bioturbation or seafloor constriction

11. Results from CAT Meter Deployment Offshore Costa Rica
Described in Brown et al., 2005, EPSL; LaBonte et al., JGR; and Tryon, 2009 Geology
Flow rates are into the sediment
Simple numerical model suggests events produced by net extensional mstrain of ~ in top 10 m at the outer rise.

12. Results from CAT Meter Deployment Offshore Costa Rica
Described in Brown et al., 2005, EPSL; LaBonte et al., JGR; and Tryon, 2009 Geology
Fully coupled poroelastic simulations indicate origin at the toe at a depth <4km, bilateral propagation at 0.5 km/day

13. Events Recorded ~2 Years Later in CORKs at Costa Rica
1st event
May 2003
2nd event
October 2003
From Davis and Villinger, 2006, EPSL; Solomon et al., 2009, EPSL

14. Questions That Can be Addressed with an Integrated Geodetic and Hydrologic Observatory
Does fluid flow at the seafloor reflect variations in pore pressure inferred from seismic attributes and modeling studies?
How do fluid flow rates and composition vary during SSE and seismic cycles and what do these variations indicate about the relationship between pore pressure, fault slip, and fluid flow?
What geologic processes generate fluid overpressures?
- combination of flow rate and fluid composition monitoring
4. What is the role of fault slip in margin-wide dewatering?
As an example, in reference to the SSE cycle. During SSE fault zone permeability increases enhancing fluid flow rates and in so, leads to a decrease in pore pressure. In the inter-SSE period, permeability decreases, flow rates return to background levels, and pore pressure rises until next SSE.

15. What a Subduction Zone Hydrogeologic Observatory Might Look Like
Mosquito
CAT meter
CORK
CORK