SEIZE must focus in a few locations to maximize the essential multidisciplinary interaction and integration. A major goal of the June 1997 SEIZE workshop was to select sites for intensive research. The criteria for selection of localities are as follows: 1) The region must include historic large thrust earthquakes. 2) The subduction thrust must be imageable by seismic reflection techniques over much of the seismogenic zone. 3) The subduction thrust must be drillable, both near its seaward terminus and into the seismogenic zone. 4) The availability of data from previous geological and geophysical surveys and ODP drilling as well as proximity to ports, logistical support, and favorable weather conditions should favor the candidate sites. 5) The geological and geophysical nature of subduction (e.g., convergence rate) is a consideration in site selection.
Earthquake seismologists attending the June 1997 workshop proposed 14 seismologically compelling targets. Consideration of the criteria outlined above reduced the 14 to seven. The report of the June 97 workshop outlines the complete cases for each of the seven candidate localities. After much discussion the workshop participants agreed that SEIZE should be focused in Japan (Nankai Trough and Japan Trench) and Central America (Costa Rica and Nicaragua). Specifically, landward of the Nankai Trough, sediments underthrusting the prism can be traced into the seismogenic zone on existing 2D seismic reflection images; therefore, the material properties of the seismogenic zone are predictable. The seismogenic zone here lies within the planned capability of the OD 21 riser drilling ship. In the Central America region, the Nicoya Peninsula lies over the seismogenic zone and offers an exceptional opportunity for seismic recording and GPS monitoring. The seismogenic zone is located at 10 to 12 km beneath the Nicoya Peninsula, and lesser depths offshore.
The Japanese and Central American seismogenic zones have compelling contrasts and comparisons that behoove their investigation. The Costa Rica margin contrasts well with the Nankai seismogenic zone because the former is non-accretionary and the latter is accretionary. Pelagic sediment dominates underthrusting section of Costa Rica, whereas terrigenous deposits are dominant in Nankai. Costa Rica converges at a high rate whereas Nankai converges at a slow to moderate rate. Costa Rica has a low and Nankai a relatively high thermal gradient. Both the Japan Trench and the Nicaragua Trench have produced tsunamigenic earthquakes, with shallow seismogenic zones. As these tsunamigenic seismogenic zones are within the drilling capability of the JOIDES Resolution, they can be investigated soon. The Central American localities have potential to fulfill goals of MARGINS in crustal recycling and SEIZE. Logistical reasons all support focus of SEIZE in Japan and Central America. In the Japanese Islands the large number of seismic stations both on land and underwater, the extensive GPS network, and an abundance of other available data provides overwhelming scientific investment that a SEIZE program can build on. Both the Japanese and Central American regions have active scientific communities that can develop strong SEIZE efforts. Both are near major ports and easily accessible to study.
Earthquake Seismology: Three methods can potentially characterize the seismogenic zone at subduction zones: 1) seismic tomography; 2) earthquake waveform inversion; and 3) active source imaging and velocity studies. Characterization of the seismogenic zone using earthquake waves as sources is the method that has yielded nearly all we now know about subducted slabs. Unfortunately, the location of shallow earthquake sources at subduction zones, and thus much of characterization, depends on teleseismic arrivals at distant stations and arrivals from stations on land, which are nearly always located on only the landward side of the trench. Such location estimates are likely to have systematic errors associated with them, which are difficult to detect and correct. To properly characterize this zone with earthquake arrivals, sensors are required close to the sources and at a variety of azimuths and distances. This requires that permanent ocean bottom seismic stations be established, some of which should be seaward of the trench axis. Technology exists to establish such stations, which can also double as tsunami detectors and monitors of other geophysical parameters. For many applications, data from these stations should be transmitted to shore in real-time. Seismologists can resolve focal mechanisms from teleseismic waves of earthquakes with a magnitude greater than 5.5 Ms (magnitude from surface waves). Broadband waveform inversion of earthquakes with magnitudes greater than 7 Ms and inversion of tsunamis recorded at tide gauges can resolve information from source processes. Measurements should also in-clude the recording of S-wave transmission through the seismogenic zone, including S-wave splitting, to estimate fracture orientation and to monitor changes in state of stress.
By extending seismic recording arrays offshore we can monitor the build-up of stress in the oceanic crust as the earthquake cycle progresses. In regions up-plate from an asperity, this stress buildup has caused intraplate focal mechanisms to take on a stronger compressional component than would otherwise be present. Although extraction of focal mechanisms using OBS data has been complicated by uncertain performance of horizontal components, focal mechanisms from intraplate earthquakes can be recovered using 3-4 instruments. Where earthquakes are monitored regionally with modern broadband instrumentation, source processes are routinely being determined to for events with magnitudes as small as 3.5. Although this can be done from shore to some extent, in a subduction zone, 3-component OBSs would greatly extend capabilities.
Recent observations in California have revealed the presence of seismic waves controlled by a low-velocity layer of fault gouge in a strike-slip fault zone. This waveguide supports dispersive wave propagation in the same fashion as does a low velocity crust overlying mantle. Very effective excitation of the waveguide occurs since the source is located within the waveguide. Simple modeling as a single layer between two half-spaces has allowed extraction of fault zone thickness and the shear velocity of the infilling material. In the California example, fault thicknesses of 120-170 meters and shear velocities of 0.7-0.85 km/sec have been observed from interface waves. Lower- resolution body-wave studies yield 1-2 km wide zones with shear velocities of 2-3 km/s and Vp/Vs ratios of 2-2.3.
We can expect similar physics to govern subduction fault zones. Broad-band seismometers located on islands have observed low-frequency guided waves traveling up slabs. On land, the trapped waves were recognized by their phase velocity, so use of this phenomenon will require a linear array of OBSs in the trench, and, as necessary, enough land and sea seismic stations to provide usable locations. Depending on the distribution of sources and receivers, the potential for two-dimensional tomography exists. If asperities (strong regions) have a velocity structure that is different from regions that are freely slipping (or nearly so), they should be imageable by two dimensional tomography, depending on the source-receiver distribution.
Reflection Seismology: Imaging of the seismogenic zone at depths of 10-20 km in subduction zones will require new experimental designs. In its simplest form, the imaging must define the top of the down-going slab and structures within the base of the overriding plate, from the deformation front, landward through the seismogenic zone. These will help define the geometry of the subduction zone, possible asperities, and erosion and accretion at the base of the overriding plate, and properties of the fault zone. We must be able to observe seamounts and thrust packages at vertical scales of ~500 m and lateral extent of ~1 km at depths of 10-20 km.
Seismic sources must be large to penetrate to the needed depths, yet contain a broad-band spectrum of energy to preserve resolution and allow waveform inversions of the seismic reflections. Seafloor swathmapping provides 3D information that greatly constrains interpretations and helps locate seismic lines in areas of minimal out-of-plane effects. Multichannel seismic (MCS) reflection methods, particularly 3D acquisition and processing can provide high-quality images of the décollement and structures above and below. Although there is always a desire for higher resolution and deeper penetration, depth is limited by attenuation and source strength, and resolution by frequency content of the source. The reflection and refraction techniques become more powerful when combined than when applied separately. Closely spaced ocean bottom seismographs/ hydrophones (OBS/H) along a modern normal incidence reflection line can extend structure to depth and can provide velocity data to aid processing. These data will also provide background velocities to combine with reflection waveform analysis. The few examples of such combined data suggest we can image to the depths where great earthquakes nucleate.
The best way to obtain high-quality images is by using 3D seismic reflection, particularly with enhanced processing such as 3D dip-moveout and 3D prestack migration. These techniques require high quality data as well as high-performance computing capability. Use of a high capacity, broad source, a ~6 km streamer, and OBS(H) at perhaps 500 m spacing would likely be necessary for adequate images. With extensive pre-stack processing, the 6 km streamer will provide adequate images of shallow structure, although velocity information will be limited. Where the structures above the seismogenic zone are more complex (probably the more common case), first order corrections for the overlying structure are essential. If the shallow structure is not properly accounted for, reflection amplitudes and waveforms of deeper events will be severely distorted. Short of a full 3D program, a swath 3D approach could correct for some of the structural complexities. A high capacity broad-bandwidth source, densely spaced OBHs along a dip line, and a multiple-streamer ship shooting a series of parallel lines (the number and spacing would have to be determined from modeling) would produce exceptional observations.
The use of multi-OBS/H enables us to get fine images of the seismogenic zone. Recent experiments suggest that, OBS/Hs spaced along a 2D line every 500 m with the combination of tomography might give a reasonable 2D image. This image will still suffer from the 3D effects. Such densely-spaced OBS/H provide the information to develop a proper velocity field for the entire margin. This is essential to the full characterization of the margin and is an important method to improve locations of microearthquakes.
Geodetic Methods: A fundamental measure of slip on the seismogenic zone is the deformation of the surface of the overriding plate from the trench to the backarc. Measurement of the surface deformation predicts, through appropriate models, maps of locked and slipping portions of the seismogenic zone. The geodetic measurements must be able to measure deformation rates that may approach a few cm/yr in both horizontal and vertical dimensions over 100-200 km range from the trench. Traditional methods such as leveling only measure the vertical component and must be carried out over long distances to tie into a stable plate interior. GPS is currently the premier method for determining 3D displacements in a global reference frame. Simple models of elastic, interseismic strain at seismogenic zones feature rapid subsidence nearest the front of it diminishing in rate inland and crossing over to uplift roughly above the deepest extent of the locked zone. The horizontal expressions of such elastic strain models predict a smoother transition, with the near trench portion of the overriding plate moving mostly with the downgoing plate velocity and decreasing towards the stable plate interior. The vertical component of motion can be highly diagnostic of the dip of the seismogenic zone; to be most effective, measurements must be made to ~100 km of the trench to define the down-dip extent and within a few tens of km to define the updip extent of the seismogenic zone. In the case of land-based GPS, choosing a location where the coastline extends as close to the trench as possible is a great advantage towards “imaging” the locked and slipping portions of the seismogenic zone. In the marine environment, underwater sound transmission can tie seafloor reference points to sea surface platforms whose positions are simultaneously determined with GPS. Results from initial tests imply that uncertainties in velocity vector estimation should be 5 mm/yr or less. Besides standard GPS campaigns carried out at year-scale separation, any geodetic monitoring of the seismogenic zone requires the incorporation of continuously operating GPS receivers both to more quickly recover the quasi-steady-state interseismic deformation and to provide the potential to measure any transient strains related to coor postseismic deformation.
ODP Penetrations: Although depth-limited, ODP penetrations must be an integral part of SEIZE. Subduction zones are conveyor belts, moving materials from the surface through the seismogenic zone to great depth. Therefore, ODP-style penetrations of about a km can sample the materials that ultimately become the fault rock of the seismogenic zone. A SEIZE program will require a series of holes to characterize the incoming sediments and rocks, and their associated fluids. It will be essential to characterize important geologic properties in three dimensions, so drilling strategies will have to expand beyond the typical 2-D transects.
The décollement zone is the shallow, seaward manifestation of the seismogenic zone megathrust. Fluids sampled from some décollement zones may have migrated from the seismogenic zone. Therefore, sampling and ultimately instrumentation of this structure, both down-dip and along strike, provides access to the pulse of the seismogenic zone.
In addition to sampling the incoming material and monitoring, relatively shallow ODP penetrations can opportunistically provide information on deeper levels of subduction zones related to the seismogenic zone. For example drilling into diapirs can sample material brought up from great depths, and constrain the pressure-temperature conditions in the forearc. Deeply sourced fluids sampled at shallow depths in monolithologic forearcs may provide unique information about processes at depth. Drilling into out-of-sequence thrusts in areas of slope erosion can access deeper levels of faults than normal accessible by ODP.
Borehole Observatories: SEIZE will benefit greatly from emplacement of permanent observatories including seafloor seismic and fluid flow, observatories and borehole monitoring devices. Technology for construction of such observatories at subduction zones exists, and can be accomplished with an electro-optical cable to provide power to experiments and communications to shore. Although initially expensive, savings in ship time, and the constant availability of real-time data make emplacement of observatories practical where cable lengths are relatively short.
Instrumented, hydraulically-sealed boreholes (CORKs) provide a real-time record of sub-surface transient events manifested by temperature, pressure, and porewater chemistry anomalies. At the very least, these records will establish the “steadystate” hydrologic conditions in various parts of the formations that host seismogenic zones, including the faults themselves. They may also define precursor, coseismic, or postseismic signals related to seismic events, since it is almost certain that hydrologic signals are sensitive to changes in stress, ground motion, and fault-zone slip. During SEIZE, it will be essential to correlate CORK data with synoptic OBS or borehole seismometer results. In addition, other downhole sensors (which may require emplacement or periodic replacement) can be incorporated with a wireline-deployable CORK. These complementary sensors might include hydrophones, geophones, tiltmeters, strain gauges, or chemical sensors. Hydraulic access through the CORK accommodates a continuous osmotic fluid sampler or periodic borehole fluid extraction for time-series determinations of pore water chemistry.
Active hydrogeologic tests, conducted by submersible or ROV through the hydraulic port on the CORKs, provide in-situ determinations of formation transmissivity/ permeability and storativity. The duration of these tests can be extended to minimize effects of drilling and maximize the radius of investigation. Furthermore, the in-hole tidal signal variations can constrain the mechanical/hydrologic properties of the tested interval.
The existing CORKs seal the borehole as a single volume and allow conditions to be monitored in a single interval of open hole or perforated casing only. Monitoring and testing of multiple intervals (which require sophisticated casing strings and drillstring packers) is necessary if the variations with depth of the fluid regime is to be delineated in a single hole.
Riser-Type Deep Drilling: Drilling into a seismogenic zone or relevant deep objectives that are inaccessible by the current capability of JOIDES Resolution is one of the major goals of the SEIZE. Proposed Japanese riser drill ship (OD21 drilling vessel) provides an opportunity to achieve this goal. Experience gained through DSDP/ODP drilling indicates that convergent margin borehole conditions are generally quite hostile. Overpressured pore fluid, swelling clay, and stress-induced hole collapse often cause unstable hole conditions. Such instability has hindered core recovery, wireline logging, and longterm measurements. Deployment of a drilling-mud circulation system (riser system) can overcome such obstacles, especially in deep holes.
Current OD21 specifications call for implementation of a riser in two phases, initially at a 2500-2800 m length and later 4000 m length. The drill string will be 12000 m in length. A blowout prevention system at the seafloor will control hydrocarbon risk. The Conference on Cooperative Ocean Riser Drilling (CONCORD) set drilling into the seismogenic zone as the first priority of an international deep drilling program (OD21). The first phase of OD21 (2003-2008) would target a hole starting at about 2500-2800 m water depth with a 6000 m penetration to the seismogenic zone. To best locate optimal sites for an extraordinary scientific program like SEIZE, it is essential to conduct site survey and preparatory experiments, including conventional ODP drilling.
Field-Based Observations of Paleoseismogenic Zones: Field studies of onland analogues can provide critical information about rock properties and alteration products over the ranges of P-T conditions relevant to the seismogenic zone (~125°-400°C). On land observations, sampling and associated lab measurements will feed into conceptual models of the seismogenic zone that can be initially tested by seismic reflection techniques, and ultimately by drilling. Drilling results may be extended and better understood through firm knowledge of ancient analogues. Paleoseismogenic zones will be studied with the disciplines of structural geology, metamorphic petrology, geochemistry, and geochronology. Particular attention should be focused on structural packages that may represent the paleo-décollement and on out-of-sequence faults that display large amounts of vertical displacement of the paleotemperature structure. Analyses should focus on contrasts among hanging wall, footwall and associated shear zones. These contrasts may be defined by differences in deformation fabrics, vein mineral paragenesis, stable-isotope composition of vein minerals, fluid inclusion microthermometry, vein density and orientation, alteration of organic matter, and phyllosilicate diagenesis. Determination of the thickness of paleoseismogenic zones will provide constraints on waveform models from seismic reflection data. Timing of faulting can be established using such methods as fission track geochronology.
Laboratory Experiments: A laboratory-based program of controlled experiments is critical to the success of SEIZE to link the various indirect measurements to in situ conditions of the seismogenic zone. The composition, temperature, stress, and mechanical state of the rocks and fluids of the seismogenic zone will be inferred from remotely-sensed data, such as measurements of surface heat flux, seismic velocity and reflectivity, fluid fluxes, and geochemical signatures. The relationships among these proxies remain insufficiently known to extrapolate chemical and physical data collected at shallow levels to infer conditions existing at seismogenic depths. SEIZE must therefore include a comprehensive program of laboratory experiments documenting relevant physical-chemical processes and elastic and material properties, at in situ temperature, stress, and fluid pressure. These experiments should involve sediments and laboratory-generated analogs, altered oceanic basement, serpentinites and their exhumed equivalents, representative of the décollement zone and underthrust sequences. The experimental data will provide important input to hydrologic and mechanical modeling efforts, which will in turn help focus experimental investigations.
Laboratory experiments should address at least the following fundamental processes and rock features: 1) Steady-state fluid-rock reactions and their kinetics, partition coefficients, and isotopic fractionation factors. 2) Thermally and physically activated mineral dehydration reactions and their impact on rheological boundaries. 3) The changes in relationships among seismic velocity (Vp and Vs), attenuation, density, fluid content and composition, and stress during compaction, diagenesis, metamorphism, and deformation, necessary to infer the physical meaning of seismic images and wave propagation. 4) The linkage of chemical and physical processes to changes in porosity, permeability, stress, and rheology, crucial to a complete understanding of the temporal and spatial changes in seismogenic behavior and interplate coupling (e.g., velocity strengthening/weakening relationships, seismic/aseismic stress release).
Modeling: Because access to the seismogenic zone is limited, numerical models will be essential for integrating the field observations and laboratory results. Initially the models will be important for guiding data collection needs and laboratory experiments. New observations and parameter values will refine the existing models and guide further sampling and experiments. For example existing tectonic models are often constrained only by onshore geodetic data. The addition of strain and tilt data from offshore observatories will improve our ability to use these models to understand the seismic deformation cycle. Another example concerns the need for refining existing models of fluid pressure. New laboratory results and drill core observations of the average composition of the oceanic crust will constrain the mass of fluids and rate of release over the seismogenic zone. As our level of knowledge about the important processes grows, it is anticipated that new models will be required that account for multiple coupled processes. These would include, for example, the coupling of pore pressure, stress, and temperature, or coupled fluid flow, chemical reactions, and transport. Moreover, some existing models will need to be extended from two to three dimensions to account for variations along strike of important controlling processes. Simulations will be required on a range of scales from the borehole to the entire subduction zone. Models of borehole hydrologic data provide needed input to larger scale hydrologic models of the entire margin. Smaller scale process models, involving such aspects as rupture dynamics or sediment consolidation, provide insights into the important controls on larger scale observations. The ultimate goal is to have models that test hypotheses about the nature and extent of the seismogenic zone. Models on such a large scale necessarily require many simplifications compared to the natural system. The insights needed to determine which simplifications are possible come from comparing smaller scale models with observations.