Table of Contents

1. Executive Summary
2. What is the Seismogenic Zone
3. The Questions Seize will Address
   1. What is the physical Nature of Asperities?
   2. What are the Temporal Relationships Among Stress, Strain and Pore Fluid Composition Throughout the Earthquake Cycle?
   3. What Controls the Updip and Downdip Limits of the Seismogenic Zone of Subduction Thrusts?
   4. What is the Nature of Tsunamigenic Earthquake Zones?
   5. What is the Role of Large thrust Earthquakes in Mass Flux?
4. The Scientific Strategy of SEIZE
   1. The Synergistic, Geographically Focused or "Margins" Approach
   2. Specific Implementation Strategy of SEIZE
      1. Earthquake Seismology
      2. Reflection Seismology
      3. Geodetic Methods
      4. ODP Penetrations
      5. Borehole Observatories
      6. Riser-Type Deep Drilling
      7. Field-Based Observations of Paleoseismogenic Zones
      8. Laboratory Experiments
      9. Modeling
5. The Location of SEIZE Field Sites
   1. Selection Process
   2. Selected Localities for Intensive Focus: Japan and Central America
   3. Application of SEIZE to other Sites Such as Cascadia
   4. Order of Study of Preferred SEIZE Localities
6. The Duration of SEIZE
7. The Cost of SEIZE
8. The Communication of SEIZE Opportunities and Results
9. International Cooperation and SEIZE
10. The Evaluation of SEIZE
11. Appendix

EXECUTIVE SUMMARY

Most of the world's great earthquakes and tsunamis initiate in the "seismogenic zone" a region of thrust faulting in the shallow part of subduction zones. The Seismogenic Zone Experiment (SEIZE) plans to study the relationships among earthquakes, deformation, and fluid flow in this environment. SEIZE will address the following questions: 1) What is the nature of asperities (strong, locked parts) of seismogenic zones? What are the temporal relationships among stress, strain, and fluid composition throughout the earthquake cycle? 3) What controls the up-and downdip limits of the seismogenic zone? 4) What is the nature of tsunamigenic earthquake zone? 5) What is the role of large thrust earthquakes in mass flux of material into (and out of) the subduction system?

SEIZE will proceed by focused investigations combining earthquake seismology, seismic reflection imaging, and geodetic studies in and around a limited number of seismogenic zones. Sampling the incoming material combined with laboratory experiments, modeling and studies of ancient analogues will estimate the nature of the fault rock in the seismogenic zone. Waveform models of the seismic images will predict physical properties of the seismogenic zone. Deep riser drilling will test these models, lead to a better understanding of our questions about the seismogenic zone, and calibrate techniques for monitoring changes in fault zones during the earthquake cycle.

Seismogenic zones selected for focused study are in the Japanese Islands (Nankai Trough and Japan Trench) and Central American (Costa Rica and Nicaragua). Selection of Japan and Central America from fourteen other candidate localities is based on historic earthquake activity, imageability by seismic reflection, geographically accessibility, and suitability for deep riser drilling. Investigations in the selected areas will have direct application to understanding the societally relevant, currently quiescent but paleoseismically active Cascadia seismogenic zone of the Pacific Northwest. The quality of research proposals will determine the order of study of the prime localities.

SEIZE will extend over ten years, culminating in model testing with deep riser drilling. The proposed study of a seismogenic zone will incur US costs of about 10-15 M, exclusive of shiptime and drilling. These funds should leverage substantial international contributions to related studies. A web site listing ongoing programs will hopefully attract other synergistic projects in the same area; the web site will post results before formal publication. Results will also be communicated through international meetings and workshops.

WHAT IS THE SEISMOGENIC ZONE

Most of the world's great earthquakes are inter-plate underthrusting events in subduction zones (Fig. 1). Although plate tectonics provides the underlying kinematic explanation for these underthrust earthquakes, only a small portion of the plate contact generates earthquakes this portion is the seismogenic zone. Understanding the seismogenic zone provides both fundamental scientific challenges and is of great societal relevance. Accordingly, the Seismogenic Zone Experiment or SEIZE focuses multidisciplinary investigations on such earthquake processes. The most important source of information about the seismogenic zone is obtained from the fault area that ruptures in any one event. The largest events rupture the entire down-dip extent of the seismogenic zone with the along-strike rupture width determining the final size of the event. Large earthquakes are more important than all the smaller events, both from a scientific and societal perspective. About 20 great underthrust events with Mw 8.2 have occurred in the 20th century; the number increases to 42 by counting underthrust events with Mw 8.0. The uneven distribution of these large thrust events (Fig. 1) is one manifestation of the diversity of subduction zones. Many localities are of seismological interest for geophysical and geological investigation (Appendix I: The Seismogenic Zone Experiment (SEIZE) Workshop). Potential SEIZE field sites include areas of great earthquakes and smaller more frequent earthquakes.

A shallowly dipping subduction zone thrust provides a large fault surface that is accessible to study by a combination of drilling and ongoing monitoring using passive and active seismology. These thrusts are part of the subduction conveyor belt. Here we can sample the incoming sediment that undergoes changes in material properties through compaction, lithification, and dehydration reactions during transport to the seismogenic zone. Therefore, the processes that control the partitioning of strain, the flow of fluids, the formation and behavior of faults, and the onset of seismic slip are relatively accessible.

The SEIZE science plan depends on the results of three workshops (Appendix I). Originally an International Lithosphere Program (ILP) workshop on "Dynamics of Lithosphere Convergence" reviewed the progress achieved in recent studies on convergent margins. One result of this workshop was an international research program to study the seismogenic zone of great thrust earthquakes at convergent margins or SEIZE (Fig. 2). A subsequent workshop in June 1997 refined the SEIZE research objectives selected localities for concentrated investigation . A conference on deep riser drilling in July of 1997 endorsed the concept of drilling into the seismogenic zone. Finally In December of 1997, proponents for SEIZE in Central America outlined a specific program of action in that area.

THE QUESTIONS SEIZE WILL ADDRESS

The 1995 ILP workshop report outlines the following research objectives for SEIZE: I. To establish the relationships among earthquakes and: (1) structural geometry, (2) distribution of stress and strain, (3) thermal structure, and (4) nature of fluxes of the fluids and solids in and across a variety of seismogenic zones; and II. To formulate testable quantitative models of how the shallow subduction cycle works, including the complex interactions among the multiple processes. The SEIZE workshop in June 1997 refined these objectives into a number of topical questions:

What is the Physical Nature of Asperities?

Seismologically asperities are areas of higher slip during the earthquakes. They have been interpreted to be "stronger" regions of a fault that have resisted motion during the interseismic period; thus, asperities may be seen by other methods such as geodesy, seismicity, and imaging by seismic reflection. What is the physical nature of asperities? Are they distinct rocks (e.g., basalt vs. sediments)? Are they areas of different frictional properties than non-asperities? Are they independent of materials, for example being controlled by fault geometry, or a fluid-pressure? Are they permanent at least until the next seismic cycle?

Many of the above questions also apply to the nature of the "weaker" seismogenic zone that fills in around the asperities. One extreme view is that the asperities and "weaker" regions are almost identical; perhaps only a subtle variation of frictional characteristics is responsible for their different macroscopic behavior. These questions bear directly on how and where creep occurs within the seismogenic zone. Significant creep occurs in some subduction zones; yet, these regions are seismogenic since small events are scattered throughout. Therefore, the physical nature and effective constitutive law for both "asperities" and the intervening "weaker" regions are key targets of the SEIZE initiative. It would be extremely useful for hazard estimates to know in advance what are the diagnostic signs of an asperity within a subduction zone segment.

What are the Temporal Relationships Among Stress, Strain and Pore Fluid Composition Throughout the Earthquake Cycle?

Obtaining a better understanding of the cycle of stress and strain accumulation and release along subduction plate boundaries is a key objective of SEIZE. Strain monitoring with comprehensive onshore GPS networks and utilization of newly developed sub-sea geodetic technologies can be invaluable for identification of regions and rates of strain accumulation along the plate boundary. Such studies will be essential in defining regions of potentially high seismic hazard and distinguishing these regions from those of aseismic subduction. Moreover, by eventually drilling into potentially seismogenic subduction zones, we will obtain critically important data on the state of stress, pore pressure and the composition of pore fluids. Such data bear directly on the physics of faulting and earthquake nucleation and will provide new insights into the manner by which temporal fluctuations of pore pressure and stress are related in earthquake processes.

In subduction systems, a number of investigations suggest that certain seismogenic zones, and certainly their seaward extension or "décollements," are "weak" faults. Data collected by SEIZE can determine if and why seismogenic zones are weak and will have direct feedback to programs addressing similar questions along the San Andreas fault or other similar crustal faults.

What Controls the Updip and Downdip Limits of the Seismogenic Zone of Subduction Thrusts?

The updip and downdip limits of rupture in great subduction-thrust earthquakes are important factors in seismic and tsunami hazard. The downdip limit determines the landward extent of the seismic source zone, which is important for great earthquake hazard at inland localities. The seaward updip limit is important for tsunami generation. For some earthquakes there is slow rupture of the updip portion of the thrust that generates tsunamis but less prominent seismic waves, i.e., "tsunami earthquakes". Thus the updip portion of the thrust interface, seaward of the high frequency seismic rupture limit is an important part of the study.

A number of physical and compositional explanations have been proposed for the limits of subduction thrusts. It will be important to compare limits based on these proposed explanations to the actual updip and downdip limits of the seismogenic zone, especially for past great earthquakes. It is equally important is to seek observable changes on the subduction thrust at these limits, for example by using multichannel seismic reflection techniques. The updip and downdip limits of the seismogenic zone may be determined from:

• 1) The rupture area of past great earthquakes, from earthquake waveform modeling, from the distribution of aftershocks, from tsunami modeling, and from dislocation modeling of coseismic geodetic data.
  
• 2) The interseismic locked zone determined from dislocation modeling of interseismic geodetic data.
  
• 3) The updip and downdip limit of intermediate and smaller magnitude thrust events. Updip limits appear to range from near the trench to depths of about 10 km. Downdip limits appear to range from less than 10 km for some island arc margins to over 40 km depth for some areas of subduction beneath continents.
  

For the updip seaward limit, initial attention was focused on the boundary between the unconsolidated accretionary prism sediments and crystalline forearc crust. However, in at least some areas, a portion of the seismogenic zone lies beneath the accretionary prism. Thus the updip limit must be controlled by some change in physical properties on the thrust.

hemical-mineralogical changes of current interest include the dehydration and replacement of stable sliding smectite clays to seismogenic illite-chlorite-rich clays at 100-150·C. Another possibility is the transition of shales to slates at about 200-250·C where the dehydration reactions are more complete and the rock strengths are sufficient to support substantial elastic strains. Downdip change in pore pressure is another candidate.

Phenomena possibly controlling down-dip limit include: (a) thermally activated stable sliding above 350·C for crustal composition rocks (with a transition to perhaps 450·C). This transition is observed in laboratory studies and in the maximum depth of earthquakes in continental areas or (b) stable sliding caused by serpentinite in the forearc mantle (if the mantle corner is reached before 350·C). Large amounts of water must be expelled upward into the overlying forearc mantle, so extensive serpentinization is expected. Serpentinite appears to exhibit stable sliding at temperatures below about 500-600·C.

The relative importance of these potential controls is presently unproven. To assess these parameters thoroughly and rigorously, we need to understand what controls variations in temperature, pore pressure and mineralogy on the subduction thrust, and how these interrelated influences determine the stability field of the thrust plane. These variables include the thickness of insulating sediment on the incoming crust, the age of the subducting oceanic plate, the convergence rate, the thrust dip profile, the porosity and permeability and fluid flux and migration paths along the décollement, and the radioactive heat generation and thermal structure of the overlying forearc.

What is the Nature of Tsunamigenic Earthquake Zones?

In most subduction zone segments, the seismogenic zone where earthquakes nucleate does not extend all the way to the trench axis. There is a narrow strip of aseismic plate boundary from the trench down to a depth of a few kilometers, where seismic moment release is rare. When great and large earthquakes rupture the seismogenic zone, they generate tsunamis that can be quite hazardous. Empirical and theoretical relations have established a connection between the size of underthrust earthquakes and their tsunamis. Locally, an underthrust event generates a tsunami much larger than expected. There are still several competing hypotheses for this anomalous behavior, one of which is that these earthquakes have their slip concentrated at very shallow depths. For a few events, tsunamis were generated by slip in the shallowest aseismic region. For example, in the 1992 Nicaragua earthquake, slip was concentrated near the trench while the aftershocks were scattered through the more typical seismogenic zone. Therefore, the upper "aseismic" zone is capable of being seismogenic, at least in some places and some of the time. Thus, a good target of investigation for SEIZE would be rupture zones of tsunamigenic earthquakes such as the 1992 Nicaragua event or the 1896 Sanriku event off Japan. For hazard estimates it would be extremely useful to know the structural/tectonic characteristics of a subduction segment capable of a tsunamigenic earthquake, such as convergence rate, sediment supply, structure of the top of the oceanic plate, accretion rate, and upper plate structure and stratigraphy.

What is the Role of Large Thrust Earthquakes in Mass Flux?

A fundamental process of subduction zones is the transfer of material from one plate to the other. In the upper part of subduction zones, including the seismogenic zone, this can take the form of the addition of material from the subducting plate to the base of the overriding plate by underplating, with consequent uplift. Alternatively, removal of material from the base of the overriding plate by a number of processes leads to tectonic erosion, manifested by subsidence. We do not know whether major thrust earthquakes are part of the mechanism of either of these processes or whether they arise purely from slip between the two plates with no transfer of material. If the latter is true, then the regions of the slip surfaces of the earthquakes would be zones of no transfer of material. The association of areas of rupture with regions of the forearc known to exhibit underplating or tectonic erosion, however, suggest that large thrust earthquakes are involved in either one or perhaps both of these processes. This issue can be resolved by comparing: 1) seismic reflection images of the basal detachment, 2) the earthquake or microearthquake-determined locations of the detachment, and 3) changes in shape of the seafloor above the zone of underplating or tectonic erosion.

THE SCIENTIFIC STRATEGY OF SEIZE

The Synergistic, Geographically Focused or "Margins" Approach

SEIZE is a component of the MARGINS program. The MARGINS approach concentrates resources on areas targeted for intense, multidisciplinary research. In these focus areas, interaction between researchers involved in field data collection, numerical simulation and laboratory analysis results in unparalleled synergism. SEIZE intends to investigate active systems as a whole, viewing a seismogenic zone not so much as a "geological" entity, but more as a complex physical, chemical and biological system, subject to a variety of influences. The processes that fundamentally govern the evolution of the seismogenic zone are lithospheric deformation, mass fluxes, sedimentation, and fluid flow. The goal of SEIZE is to focus a coordinated, interdisciplinary investigation of these processes.

Specific Implementation Strategy of SEIZE

The Seismogenic Zone Experiment must consist of several integrated components designed to characterize the key features of a specific seismogenic zone. Typical field components would consist of earthquake seismology, seismic reflection imaging and refraction velocity studies, shallow and deep drilling, long-term monitoring and geodetic measurement, and on-land studies of an exhumed seismogenic zone. Evaluation and even design of the field components will require experimental and modeling studies.

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 include 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 co- or post-seismic 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 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 pore-water chemistry anomalies. At the very least, these records will establish the "steady-state" hydrologic conditions in various parts of the formations that host seismogenic zones, including the faults themselves. They may also define precursor, co-seismic, or post-seismic 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 long-term 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.

THE LOCATION OF SEIZE FIELD SITES

Selection Process

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.

Selected Localities for Intensive Focus: Japan and Central America

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 and Central America:

(Nankai Trough and Japan Trench; Figs. 3, 4 & 5)

(Costa Rica and Nicaragua: Fig. 6)

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.

 

Application of SEIZE to Other Sites Such as Cascadia

The criteria for site selection articulated above exclude, to greater or lesser degrees, many thrust-seismogenic subduction zones that are for many reasons very worthy of study. In many cases, this is a simple consequence of either the inability of existing tools and technologies to image or access the seismogenic parts of the thrust rupture events and hence the lack of important constraints on where and how rupture occurs.

Understanding rupture potential and mechanism at these other sites is both societally and scientifically important. Thus one of the most important goals of an observational effort like SEIZE will be to extend the understanding of the fundamental mechanisms of seismic rupture beyond prime sites. To accomplish this, we must ensure that the major results can be generalized. We must also carefully consider the variables that control the seismogenic behavior and ascertain which are critical. Measurements of these critical variables at other sites will provide the most useful extension of SEIZE results.

Cascadia is a good example of a subduction zone with substantial seismic hazard that may benefit from application of SEIZE results (Fig. 7). The Nankai Trough is similar to Cascadia in many respects, excepting the latter does not have historical earthquakes. Lessons learned in the Nankai trough may permit association the material properties of the seismogenic zone with particular temperature, pressure and lithological conditions. This should lead to an understanding of how those conditions manifest themselves in geophysical and geological observations such as seismic reflectivity, deformation style, heat flow, etc. For example, a detailed quantitative comparison of the décollement reflection in Nankai and Cascadia, once a ground-truthed model for the Nankai reflection is available, will provide considerable insight into the seismogenic potential of Cascadia. If these geophysical and geological characteristics of the seismogenic zone in Nankai can be adequately determined, they can be used to identify location and area of potentially seismogenic portions of the Cascadia margin.

 

Order of Study of Preferred SEIZE Localities

Our goal is to conduct full SEIZE programs as outlined above and scheduled in Table 1 in Japan and Central America, and with applications to other margins. The major and most expensive scientific activities probably will not be simultaneous in both regions. Proposal excellence will determine which localities receive initial focus. Development of a large program in one locality will hopefully draw related investigations to realize the full synergism of SEIZE.

THE DURATION OF SEIZE

An integrated analysis of the seismogenic zone requires at least a ten-year program. The ordering of certain elements of the program are undeniable, for example, extensive surveys (Table 1, item 2) are required prior to deep drilling (Table 1, item 5). Other aspects of the program are completely interwoven with results from one area potentially triggering further studies in another. Thus this time sequencing is a preliminary model that would be subject to revision as the program progresses.

The first 5-6 years of SEIZE will focus on developing geological and geophysical background (Table 1, items 1-4) for the candidate seismogenic zone. Monitoring of seismicity, strain, and fluid flow, whether at the surface or in boreholes is required for the full duration of SEIZE to understand precursors of earthquakes (Table 1, items 1 & 5). Monitoring of earthquakes and strain will include inversion of earthquakes for slip and geodetic studies. Modeling will be necessary throughout, to guide the data acquisition and to evaluate results. During the final three years of the program riser drilling and subsequent data analysis will ultimately test predictions of the nature of the seismogenic zone

Table 1. Schedule for Implementation of Research: Monitoring earthquakes and strain (1) would define the seismogenic zone and would locate targets for imaging (2). ODP drilling will determine the nature of incoming material near the deformation front (4). Images (waveforms) of the seismogenic zone plus experiments (6) on the incoming material will lead to model predictions (7) on the nature of the materials in the seismogenic zone that can be ultimately tested by deep drilling (8). Locating the deep drilling site will require extensive site surveys, including seismic reflection imaging (2).

THE COST OF SEIZE

Our initial and essential step in understanding the seismogenic zone will involve U.S. costs of about 10 to 15 million dollars, exclusive of ship time and drilling costs. This estimate encompasses efforts in both the Japanese and Central American areas, but only one major deep riser drilling program. U.S. investment will leverage related investigations supported by scientific funding agencies of a number of nations. A program less ambitious than outlined above will provide valuable information on the selected seismogenic zones; however, the synergism of a comprehensive study should yield a greater benefit per dollar invested than the more limited approach.

THE COMMUNICATION OF SEIZE OPPORTUNITIES AND RESULTS

A SEIZE Web site is maintained as part of the MARGINS Office (currently at the University of Hawaii). The SEIZE Web site will provide the following: (1) information concerning upcoming field expeditions and experiments, (2) a data base and/or pathways to access data recently acquired by SEIZE, and (3) a news bulletin board to foster communication across the different disciplines in the SEIZE community. By making information concerning upcoming cruises and field experiments available in a timely fashion, other researchers can capitalize on these opportunities and secure funds to participate in these projects or design piggy-back projects. In addition, we envision that recently acquired SEIZE data and/or pathways to access the data will be available on the Web site soon after acquisition. This time frame will vary for different data types. Existing data acquired in the SEIZE natural laboratory will also be compiled, cataloged, and entered into the data base. Rapid dissemination of data and new ideas will help focus the community, which, in turn, will lead to a more interdisciplinary approach toward studying the seismogenic zone. For example, the occurrence and distribution of a large earthquake within the study region should be documented and made available over a time scale of hours to all participants; preliminary images of the seismogenic décollement zone should be released as they become available. Data availability, together with the news bulletin board, will improve communications between observationalists, experimentalists, and theoreticians. This enhanced communication will allow rapid determination of what are the critical observations and experiments necessary for constraining models of the seismogenic zone. Additionally, first order model predictions can be immediately tested. Such an iterative approach between modeling and data analysis is the necessary first step towards developing realistic quantitative models of the seismogenic zone. In addition to the Website, international meetings, workshops, and publications will promptly communicate the results of SEIZE.

INTERNATIONAL COOPERATION AND SEIZE

International SEIZE coordination will be done by the international steering committee chartered out of the Inter Margins office. We envision the following international cooperative elements to facilitate the long-term scientific goals of SEIZE: 1) International cooperation in scientific planning and program design or major components of SEIZE. 2) Assistance in international collaboration in scientific program activities. 3) Coordination in technology development. 4) Close cooperation with ODP and IODP to achieve drilling objectives. 5) Data sharing and data analysis coordination. A data handling system or center should be established to promote international planning and exchange of data, perhaps in addition to the web site envisioned at the US MARGINS Office.

THE EVALUATION OF SEIZE

We anticipate that SEIZE will not be block funded but progress through a series of peer reviewed proposals/grants. This funding mechanism provides continual evaluation that functions every time a proposal is submitted and reviewed.

APPENDIX I: LIST OF WORKSHOP REPORTS RELEVANT TO SEIZE

(Reports are listed in chronological order)

Uyeda and Hasagawa, 1996, The Seismogenic Zone Experiment (SEIZE): A Proposal for an Integrated Study, in the Dynamics of Lithosphere Convergence: International Lithosphere Program Workshop Report, p. 3-8

The Seismogenic Zone Experiment (SEIZE) Workshop, Waikoloa, Hawaii, June 3-6, 1997 (http://www.soest.hawaii.edu/moore/seize/report.html)

Larsen, H. C. and Kushiro, Ikuo, 1997, Conference on Cooperative Ocean Riser Drilling 116 p.

Silver, E., 1998, The Costa Rica-Nicaragua Seismogenic Zone (CRiNiSEIZE): Workshop Report