OOI RFA Cover Sheet. Contact Information: Contact Person: Department: Organization: Address Tel.: Fax: Abstract: (400 words or less)

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1 OOI RFA Cover Sheet LOI Full Addendum Please fill out requested information in all gray boxes Above For Office Use Only Title: Proponent(s): Keywords: (5 or less) Area: Contact Information: Contact Person: Department: Organization: Address Tel.: Fax: Permission to post abstract on ORION Web site: Yes No Abstract: (400 words or less)

2 Please describe below key non-standard measurement technology needed to achieve the proposed scientific objectives: (250 words or less) Site Name Position Water Depth (m) Proposed Sites: Proposed Duration Start Date Deploy (months) Revisits during deployment Site-specific Comments List of Project Participants Suggested Reviewers

3 PRELIMINARY PROPOSAL GEODYNAMICS OF THE CENTRAL AMERICA SUBDUCTION SYSTEM: OBSERVATIONS OF VARIATIONS IN SEISMICTY, STRAIN, AND HYDROTECTONIC EFFECTS Suggested Initial Proponents: Kevin Brown, Andrew Newman, Kirk McKintosh, Sue Bilek, David Chadwell, Nathan Bangs, Glenn Spinelli MONITORING SUBDUCTION ZONE ENVIRONMENTS THROUGH LINKED ORION AND IODP PROGRAMS Introduction The Middle America subduction forearc is a natural laboratory to study a wide variety of geodynamic and hydrologic interactions because there are distinct variations in earthquake behavior, plate coupling, incoming plate topography, and forearc deformation that suggest there are systematic lateral and across-strike changes in the behavior of the subduction thrust. It is also hydrologically very active and potentially linked to the system s geodynamic activity. Subduction zones release ~90% of the world's seismic energy and generate the vast majority of the destructive tsunamis, thus, understanding their dynamics has broad scientific and socio-economic implications. In convergent margin environments, information related to progressive and episodic tectonic strain illuminates the dynamics of slip on the subduction thrust and related plate boundary faults. However, as a result of the general expense of working offshore it is always problematic to achieve a critical mass of the necessary measurements to study the large scale plate boundary dynamics of a subduction zone. We propose to build on existing and ongoing studies in Costa Rica, to allow lower cost with a small number of geodetically and seismically oriented boreholes and associated surface equipment installations, cables, and a powered broad band observatory. Indeed, we contend that the near constant nearby research activity, existing infrastructure, and natural geodynamic advantages of Cost Rica make strategically and scientifically ideal for this type of networked/ multi-instrument/methodology. Geologic Background The convergence rate across the Middle America convergent margin is one of the most rapid at ~9 cm/y. Strain is concentrated near the toe because the margin wedge is primarily composed of igneous Nicoya Complex material overlain by a carapace of porous slope sediments (Fig. 1) (e.g. [McIntosh et al., 1993; Ranero et al., 2000; von Huene et al., 2000]). Mechanically, the Nicoya Complex acts as the backstop to the majority of recent forearc compressional deformation as evidenced by the mostly unbroken and undisrupted nature of the slope sediments and basement unconformity above it (Fig. 1). In front of the leading edge of the basement complex lies eroded material from upslope and fractured Nicoya Complex material that extends from the trench to ~5-7 km landward. Separating the forearc and under thrust materials lies a broad reflector corresponding to the subduction mega-thrust. Other than around basement highs on the incoming plate the décollement zone or megathrust develops through clays (±opal) such as smectite and illite. As temperatures rise down dip, these clays undergo dehydration at depth releasing fluids and causing overpressures with an expected peak production, due to mechanical effects near the 1

4 Carbonate mound and IODP site and Node 2 Carbonate mound and IODP site and Node Depth (km) normally faulted plate slope sediments ODP CORKS 5 te Si 2 te Si 3 te Si km 55 top of basement basement complex plate interface OOST modified from Shipley, McIntosh, Silver, and Stoffa, Journal of Geophysical Research, v. 97, p.4439, Figure 1 2D Seismic cross section showing the overall structure of the Costa Rica subduction system, the projected Nodes and IODP borehole locations, and the locations of the SIO flow/obs instruments that recorded the seismic noise near the toe. toe and chemical effects near the mid forearc, between ~60 to 120 C (i.e. off shore of Nicoya Nicoya) [Saffer, 2003]. The almost complete subduction of the sedimentary section, high convergence rate and high smectite and opal contents make for a large fluid production rate node 2 at depth. Fluids vent along faults producing Buoy carbonate structures across the peninsula Other carbonate mounds node 1 forearc with particular activity in the midtrench forearc region [Bohrmann et al., 2001; Hensen et al., 2004]. The GEOMAR group CORKS have identified over 50 midslope fluid venting structures that range from several 10s m to kms across (Fig. 2). Indeed enrichment in fluids in B, (up to 2 mm), large depletion in Cl (up to 50% related to clay dehydration) and inversely correlated δ18o vs. δd (if we assume the fluids are in equilibrium with dehydrating clays) all suggest we are dealing with temperatures at the depth of the fluid origin > C [Hensen et al., 2004]. In this system, smectite clays and opal are dehydrated beneath the fractured forearc basement at depth of ~12 Figure 2 Shown are the map locations of the two proposed ORION node/ km or more along the subduction thrust in the IODP drill site locations, the existing ODP well locations (ODP CORKS need critical region where ultra-slow earthquakes to be marked), local 2D seismic lines, and the 3D seismic survey region. occur and may initiate. This upward migration 2

5 of deep seated fluid should not be unexpected given that fractured ophiolitic basement in the hanging wall will tend to contain dilated fractures propped open at asperities. Repeated subduction of basement topography will further tend to maintain such fracture systems together with any overpressuring related to the high rate of fluid production at depth. Isotopic signatures of fluids in some of these fluid venting structures and in ODP boreholes at the toe appear compatible with an origin deeper in the forearc suggesting there is an up dip flow of fluids nearer the base of the wedge [Kopf et al., 2000; Silver and Leg 170 Scientific Party, 1997]. Superimposed on the system are marked changes in incoming basement topography with a) both relatively smooth sections of plate like that off the Nicoya peninsula, b) rough basement and seamount topography to the south of Nicoya and where the Cocos Ridge is subducting beneath the fore are, and c) heavily trench parallel normally faulted crust off Nicaragua. Plate coupling, earthquake behavior and distribution, and forearc deformation appear to simultaneously change in response to the impact that incoming plate sediment distribution and topography have on the subduction thrust. Both fairly normal fast rupture earthquakes and ultra slow earthquakes have been identified in the system. Geodetic studies also indicate the subduction thrust varies from being fully to only partially coupled (<50%). Partial coupling suggests that over a several year basis a portion of the plate convergence moment is being released in near silent slip events. It is the difference between the behavior of fully and partially coupled subduction responses and slow and fast rupture earthquakes that forms a major component of the observatory studies suggested here. Seismicity and Geodetic Studies Seismicity is highly variable along the extent of the shallow Middle America margin, including the occurrence of a devastating tsunami earthquake off the coast of Nicaragua in Along-strike variability within Costa Rica is also apparent, as there is a transition from smaller magnitude events (M w ~7) between the Osa and Nicoya Peninsulas to larger magnitude events (M w > 7.5) along the Nicoya Peninsula. There are various factors that likely influence this behavior, including variations in the incoming plate topography. The incoming plate offshore Nicoya Peninsula is relatively smooth, whereas the incoming plate to the south is dotted with large seamounts, plateaus, and ridges. One hypothesis links these features (and their sizes) to the slip in large earthquakes, with the seamounts/plateaus limiting the size of the earthquakes in the mid-costa Rica region whereas the smooth incoming plate off Nicoya can allow for more slip and larger earthquakes [e.g. Protti et al., 1995; Bilek et al., 2003]. Ongoing deformation, as measured by land-based GPS geodesy, in the Costa Rica forearc region also appears to be controlled by the topography of the incoming plate. However, geodetic measurements along the ocean floor will improve our understanding of this relationship. Within the Nicoya region itself, there is a long history of damaging earthquakes occurring approximately every 50 years (1853, M?; 1900, M?,;1950, M s 7.7; 1978, M w 7.0; and 1990, M w 7.0). Analysis of the most recent events (1950+) suggests they occurred along the plate interface [Avants et al., 2001], with the 1990 event occurring just south of the Nicoya Peninsula and possibly related to subduction of the Fisher seamount group [Protti et al., 1995]. The earthquakes in 1853, 1900, and 1950 occurred in the central portion of the Nicoya Peninsula, leading some to suggest a ~50 year recurrence interval for large earthquakes in this region; the smaller 1978 event was previously considered to be intraplate. Currently, the margin is an interseismic phase, characterized by small magnitude earthquakes (M w <5), and is storing strain energy to be released in another large earthquake. Recent evidence of an ultra-slow earthquake occurring in the Nicoya region also adds to the spatial and temporal variation in seismic moment release along the margin. This event, observed using Costa Rican and Japanese continuous GPS stations placed over the peninsula, suggests moment release equivalent to an M w 7.0 earthquake over a 4-week period during 2003 [Protti et al., 2004]. Slip from this earthquake appears to have 3

6 occurred along the shallow subduction interface in the area geodetically defined as partially slipping, and seismically defined by frequent small magnitude earthquakes [Protti et al., 2004]. The shallow slip in this event also appears to be different from the much deeper slip estimated along the Cascadia and Japan margins [e.g. Dragert et al., 2001], suggesting a variety of processes or conditions that may lead to these silent slip events. Results from recent experiments to examine the microseismicity along the Nicoya Peninsula are also consistent with along-strike variations in the margin. The CRSEIZE experiment, a joint seismic and geodetic study of the margin, provides high quality relocations of hundreds of small magnitude earthquakes. This experiment suggests a change in the updip limit of seismicity along Nicoya, from 10 km in the south to 20 km in the north, with the boundary located at the change in oceanic crust origin (from warmer CNS crust to colder EPR crust) [Newman et al., 2002]. The observed offset in the updip limit may be related to temperature [Newman et al., 2002; Harris and Wang, 2002] or combinations of temperature and hydrologic/ sediment changes [Spinelli and Saffer, 2004]. All of these observations suggest that the shallow seismogenic zone offshore Costa Rica is a complex system that will require a variety of tools and datasets to understand the interconnections between earthquakes and the subduction environment. GPS geodetic results of deformation across the Nicoya Peninsula, show that much of the deformation in the region can be explained primarily by strong plate locking along the subduction interface updip of the current interplate microseismicity [Norabuena et al., 2004]. These data show that the region along the interface shallower than ~15 km is strongly locked, and may be locked up to the trench. However, onland data are largely insensitive to the offshore patterns of coupling. We present two end member scenarios that illustrate one of the fundamental scientific geodetic issues we will be addressing in the ORION and IODP joint studies. The first model accounts for locking all the way to the trench. The second allows the frontal wedge to release its strain in response to potentially numerous aseismic strain events (to be determined). These events may well be responsible for the hydrotectnic events witnessed in the studies of Brown et al.[in press], see 4 Node 3? Node 2 Node 1 Figure 3 Predicted geodetic deformation across the Costa Rica margin with a) 100% locking (fully coupled) along the lower half and b) entire offshore subduction interface. Both models are entirely consistent with currently available onland geodetic data. Note the dramatic variations in the predicted deformation offshore as compared to onshore regions. These strains will also translate into significant subsurface poroelastic effects (i.e., changes in pore pressure and resulting surface flow).. Models were developed using analytic models for deformation due to slip along a planer fault [Mansinha and Smylie, 1971; Okada, 1992].

7 hydrotectonic observations section below). In figure 3, two simple end members models, which both agree with land-based measurements, display wholly different patterns of deformation offshore. The variations in expected deformation in the region along the mid-slope (in the node 2 region) is particularly extreme. This fact alone justifies measuring geodetic parameters in the offshore regions since the potential to rupture shallower than this zone has an increased capability of exciting tsunami activity (discussed below). Because these measurements will define the upper limit of the coupled region, it will be extremely useful for accurately assessing the potential size of future damaging earthquakes. This is an important discovery since globally this portion of the subduction interface does not often rupture during large earthquakes. However, when they do, such as the 1992 Nicaraguan earthquake (immediately north of Nicoya), their rupture is very slow, making their magnitudes difficult to rapidly assess [e.g., Kanamori and Kikuchi, 1993, Newman and Okal, 1998]. Additionally, they are much more likely to create a large tsunami, likely due non-linear forcing and near-trench effects, hence are called tsunami earthquakes [Kanamori, 1972]. Preliminary results suggest that the devastating December 26, 2004 Indonesian earthquake was, at least partially, a tsunami earthquake [Newman and Bilek, 2005]. Obviously, it would be beneficial to be able to globally rapidly determine and assess the potential for future tsunami earthquakes. In order to do so, it is crucial to develop improved seafloor geodetic techniques that can respond to both slow and rapid events. As illustrated in Figure 3, even in Nicoya, where the land is so very close to the trench (one of the closest places globally), land-based geodetic techniques alone are not sufficient to identify whether the shallowest portion of the trench is currently locked (i.e. compare the land based geodetic data between the two models). However, the addition of ocean-bottom geodesy will enable us to uniquely identify the extent of locking across the interface. As well, if the seafloor geodetic network is able to sustain strong shaking from an event immediately below it, it would give real time information about the true slip on the fault which will be useful for authorities to better rapidly determine the tsunami threat. In addition, imaging how the offshore strain field oscillates between stable-sliding and fully locking conditions, will help us to better understand the occurrence and hazard reduction caused by aseismic slip events. There should also be subtle (or obvious) simultaneous indications that are seen within the seismicity data either in terms of micoseismicity or seismic noise of more mysterious and potentially tectono-hydrologic origin (see hydro tectonic section below). In this proposal we plan to accurately assess the distribution of locking along the interface by incorporating the new ocean-floor, subsurface, and land-based geodetic data. We will rapidly invert analytic models for locking along the interface [e.g., Manshina and Smylie, 1971; Okada, 1992, Pollitz, 1998]. Additionally, we will account for surface and interface topography and varying rheology (inferred from seismic tomography [e.g., DeShon et al., 2005]) using more complex forward, and ultimately inverse, 2 and 3D Numerical models. Due to computation time, this more rigorous technique is not adequate for real-time determination of slip after an earthquake, but will be crucial for clarifying the details of locking, which is important for understanding its relationship with microseismicity, heat flow and fluid properties. Margin Hydrogeology There are reasons to suspect that the overall stress state within subduction zones is modulate by there region hydrologic systems. Fluid chemistry and thermal modeling indicate extensive lateral fluid migration from deep sources with the Costa Rica margin. Pore waters recovered from the décollement along the toe of the Costa Rica margin contain solutes that may have been transported up the décollement from at least km landward of the trench [Silver et al., 2000]. For example, the presence of thermogenic hydrocarbons within pore waters sampled near the trench, where in situ temperatures are <5 C, indicates a component of long-distance up-dip fluid 5

8 migration. Geochemical analyses of fluids collected from the frontal décollement [Kimura et al., 1997] indicate that key reactions occurred at temperatures of at least C but less than 150 C [Silver et al., 2000; Chan and Kastner, 2000]. This inferred temperature window overlaps the temperatures for both opal and smectite dehydration. Thermal models for the Nicoya subduction zone predict temperatures of 100 C and 150 C are found along the plate interface at 40 to 75 km and 60 to 100 km downdip from the trench respectively [Harris and Wang, 2002]. Fluid sources are porewaters entrained with subducting sediment and fluids produced from mineral diagenetic reactions. The sediment on the subducting Cocos Plate is typically ~350 m thick [Shipboard Scientific Party, 1997; Spinelli and Underwood, 2004]. Most (~99%) of the incoming sediment column off Nicoya Peninsula is subducted [Saito and Goldberg, 2001]. The hemipelagic sediment (upper ~150 m) is 10 wt-% opal and 60 wt-% smectite, on average [Spinelli and Underwood, 2004]. As initially deposited, the sediment contains more than 70% water by volume. Distributed over the entire sediment column, 96% of that water is within pore spaces, 3.6% is in smectite, and 0.4% is in opal. Once subduction begins, rapid loading consolidates sediment within the first ~5 km from the trench [Shipley et al., 1990; Kimura et al., 1997; McIntosh and Sen, 2000; Saffer et al., 2000; Saito and Goldberg, 2001] expelling large volumes of pore fluid. Hydrous minerals contain most of the fluid within subducted sediment beyond ~20 km into the subduction zone [Spinelli and Underwood, 2004]. Opal diagenesis in the underthrust sediment likely occurs km into the portion of the subduction zone where the proposed observatories will be located. Most of the smectite-to-illite reaction occurs km into the subduction zone [Spinelli and Saffer, 2004]. Fluid released from opal and smectite diagenesis may 1) provide a source for mid-slope mud volcanoes / fluid venting sites and 2) affect fluid pressures and effective stresses along the plate boundary fault. Past work off CR show that in some locations the up ward flow rates are sufficiently rapid to locally perturb shallow thermal gradients in the seep systems in the mid slope regions by 0.02 to 0.08 C (Zuleger et al., 1996). Fluid pressure within the subduction zone is modulated by the distribution of fluid sources (from sediment compaction and dehydration reactions) and permeability. Rapid porosity loss and low permeability within the clay-rich sediment on the Costa Rica margin sustain pore-fluid pressures above hydrostatic and cause underthrust sediment in the shallow portion of the subduction zone to be underconsolidated [Saffer et al., 2000]. Farther into the subduction zone, large diagenetic fluid sources likely generate excess fluid pressures. Once opal and smectite dehydration are exhausted, elevated fluid pressures should dissipate [Spinelli et al., submitted]. The resulting down-dip increase of effective stress may, in turn, affect the degree of locking on the plate interface or the up dip limit of seismicity. Hydrotectonic Observations: Indications of Multiple Silent Slip Events off Nicoya There are strong indications that the offshore regions off Nicoya are only partially coupled. Hydrotectonic events were first reported for the shallow subduction system for regions off Costa Rica subduction system (Brown et al., 2005). Off Costa Rica in 2000, we measured anomalously rapid flow events that coincided simultaneously with bursts of seismic noise originating from the forearc. We recorded these events during a six-month-long deployment of osmotically driven fluid flow meters distributed across the Costa Rica (Pacific) convergent margin. The anomalous flow signals occur through the surface of the wedge near the intersection of the frontal sedimentary wedge and basement (Fig. 1). We were able to correlate the flow signals on three sets of instruments spaced 15 km apart, thus measuring simultaneous flow events across at least 30 km of the margin along the strike of the trench. The forearc in this region is predominantly composed of fractured Nicoya ophiolitic basement with a carapace of slope sediments and a small frontal wedge of sediments [Hinz et al., 1996, McIntosh et al., 1993]. 6

9 Flow rate (mm/d) Site 5 Site 3 Site 2 Site 5, RMS Days since 1/1/ RMS Noise (residual/mean) Figure 4 (A) The osmotic flow meter data from three sites. The three events are marked by enhanced in and out flow and several peaks that match between instruments. The flow is measured by inserting a chamber into the sea bed and then channeling the flow through the instrument. Within the instrument a chemical tracer is injected at a constant rate with an osmotic pump into the fluid stream and then sampled at points above and below the injection point with the fluid being drawn into long coils. The serial record of the tracer dilution can then be determined in the coils to calculate the flow rate. The Site 5 instrument had a broken down flow pump and did not record this polarity. The flow did however appear to reverse (no tracer at upper sampling point) at the same time the other instruments experienced events and the final large accelerated out event was faithfully recorded. B) The normalized RMS noise at site 5 is presented as a measure of changing background noise amplitude over time. In order to account for individual instrument site and calibration effects we present normalized RMS (RMS norm ) data for amplitude comparison, where RMS norm =(RMS-RMS ave )/RMS ave where RMS represents the data and RMS ave is the averaged signal at an individual site. The noise and flow peaks correlate suggesting a direct temporal relationship between them. Three similar accelerated flow events were seen over a 6 month period (Fig. 4a). Individual flow events displace between ±3 mm to 5 mm through the seabed and initiate and terminate generally within a few days of each other (the relative temporal error bars of the current flow instrumentation is 1 to 2 days in duration for this deployment). The anomalous flow has several sub-peaks (two or three peaks) distributed in groupings over a few week period. The direction of the flow varies between instruments but accelerations of inflow and outflow occur at the same time (i.e. within the timing error bar for the osmotic flow meters). Episodes of high amplitude seismic noise on a collocated OBS at Site 5 correlate with the three periods of accelerated fluid flow (Fig. 4b). Background noise amplitude was computed as the average root mean square (RMS) value of the vertical channel signal over consecutive 12-hour periods. Ground motion amplification effects due to this instrument s location on soft sediment, which tends to experience large displacements for a given stress oscillation imposed by a seismic wave train, is believed to be responsible for producing the high amplitude seismic (RMS) signal at this site. This is confirmed by local earthquake magnitudes calculated at Site 5 that are consistently 1-2 orders higher than at other stations. The occurrence of three flow events coincident with three periods of elevated seismic "noise" (Fig. 4) argues strongly that these phenomena share the same fundamental driving force. We hypothesize in Brown et al., that the simplest interpretation is that the flow and seismic noise result from progressive and slow (perhaps several 100s m/day) passage of an aseismic rupture dislocation and associated poroelastic stress field (i.e. [Rudnicki and Hsu, 1988, Ge and Stover, 2000]) under the flow meter locations, propagating from a region down dip of the instrument location along the décollement zone (Fig. 5). The flow instruments respond to the shallow vertical diffuse porous flow signal generated by the strain field in the slope sediments and the fractured Nicoya basement while the noise is generated in deep fracture systems (potentially in both the Nicoya basement and underlying oceanic crust) through which flow is being accelerated around the rupture tip (Fig. 5). These pressure gradients increasingly intensify and accelerate fracture flow as the dislocation approaches the instruments and then drop as they propagate past. We suggest the large amplitude seismic noise is associated with momentum and fluid velocity changes generated at fracture constriction points [Julian, 1994] similar to the increasingly noisy and complex vibrations associated with the "knocking of pipes" as faucets are turned on. 7

10 Figure 5 A conceptual model that relates, a) Momentum changes caused by changes in flow velocity cause coupled reverberation of the fractue walls the flow at the surface, and seismic noise, to the passage of a rupture dislocation along the décollement. The stress field around the rupture tip generates both net increases or decreases in total stress that intensify as it approaches. These stresses impose a related pressure field on fluids within fracture systems in the crystalline Nicoya and subducting oceanic basement and within the pores of the sediments. This causes flow though the porous networks and, depending on the sign of the stress changes, in and outflow at the surface. The increasing stress/pressure in the fracture networks ultimately accelerate flow within the fracture systems to initiate sustained vibration of the fracture walls. It is this vibration that causes the seismic noise. This mechanism Julian, 1994 Outer rise extension Strain release events recorded here Instruments Sediment wedge Extension generates extension and vertical hydrofractures in the sub-surface Strain or fludization wavelet Aseismic strain waves and episodic slip Strain induced fracture flow and harmonic tremor at rupture tip may explain seismic tremors that have been increasingly observed within subduction zones and suggests we can see deeper into the earthquake process by remote observation than we could ever have imagined previously. Tremor-like seismic noise has been observed in a few subduction systems just below the seismogenic region [Rogers and Dragert, 2003, Obara, 2002] and has been attributed to fluid generated through dehydration processes in the subducted oceanic crust. Here we infer that such processes should be intrinsic to the whole subduction system including the, potentially temporally variable, but generally well seismically coupled regions that can generate destructive earthquakes and the freely slipping regions up-dip of the seismogenic zone. We say this because the ultra slow earthquake that initiated off of the southern Nicoya region (ref ) suggests that even coupled regions (over several year time scales) may undergo small episodic movements occasionally. Why Instrument Middle America? In this proposal we plan to emplace a network of observation wells in the mid-forearc regions of Middle America that will allow us to examine the forearc behavior off the Nicoya Peninsula of Costa Rica. Several features make this region a prime site for studying plate dynamic questions regarding the interactions of earthquakes and fluid flow, and perhaps forearc fluid chemistry and related processes at depth. CR off Nicoya is also already exceedingly well set up for a Moderate Sized Observatory effort for a wide variety of reasons. We outline these below but would like to emphasize that we expect that work here will return significant results in a limited timeframe because: A) there is already significant infrastructure in place, B) it has a fast convergent rate, high levels of tectonic activity including both earthquakes (on decadal scales) and nearly silent slip events (perhaps even on annual time scales), and overall the geology is relatively simple. A) Existing and ongoing related onshore and offshore efforts: There are existing borehole observatories on land (on the Nicoya Peninsula) and offshore (at the toe of the wedge on the incoming plate), and more are planned.. The proposed hole in the mid- forearc region will provide a connection across the forearc between the observations at the toe and those on the peninsula. Currently, a US and German funded ESF project will put in continuous GPS, borehole strain, and seismic observatories on land in the Nicoya area. These borehole observatories will extend across a portion of the variably 8

11 coupled subduction plate boundary. They will also track strain and noise events onshore and near offshore above the seismogenic zone to near the down dip end. It does appear, however, that the partially locked region extends somewhere offshore of the peninsula. We point to an event recorded in 2003 in Japanese continuous GPS data on Nicoya that reveal a large ultra-slow earthquakes with an M w 7 that last several weeks. This event initiated somewhere off shore of the peninsula, and propagated down dip. Offshore there are existing corked ODP borehole observatories at the toe of the wedge. Two sites are located on the incoming plate at the toe of the prism. Pore pressure is already being monitored at these sites and there is every indication that anomalous pore pressure excursions have been seen at these sites that we suggest might relate to creep events (Kastner, personal communication). There is already considerable site survey infrastructure both offshore and onshore in the proposed region. For example, there is already an extensive existing seismic reflection survey (both 2 and 3D, Figure 1.) network both across the Nicoya forearc and regionally all along this Middle America region [McIntosh et al., 1993] These data are processed, already mostly published, and in large part exist on the IODP site survey data bank as part of IODP drilling programs in this region. There are other extensive surface surveys and geophysical data sets (TOBI, camera tow, Alvin, ROV investigations, heat flow, coring, CTD work) available on the faulting, landslide, and seepage distributions that will allow the proper sitting of related surface monitoring networks and the IODP boreholes. Indeed, this is one of the best-mapped margins around the globe. There is already an extensive seismological data base (for summary of seismicity studies see seismological section above) B) Geologic/Geodynamic Advantages: One of the more geodynamically significant aspects of this region is that the plate boundary system is apparently only partially coupled based on good on-land initial results (the ultra slow earthquake being one such indicator) and offshore we witness repeated. While there are locked patches (at least on the scale of 3-5 years), overall the system has 50% coupling. We presume, and plan to test, that over 3-5 year periods variable amounts of slippage occurs over a good portion of the subduction fault offshore. If so then there will be consistently several and perhaps even many slip movements to resolve and study over a short time period. It is these events that we wish to study in terms of their dynamics and their relationship to any seismic noise and overall seismicity patterns. Additional geologic/geographic reasons for this region: 1. The Middle America (MA) region has a fast convergence rate of ~9 cmyr -1. Thus, offshore geodetic systems (such as vertical strain and horizontal strain measurements), will work well even with repeat accuracies of a few cm. In this setting estimates of differential plate convergence across the offshore portions of the subduction zone will be well above the uncertainties in a relatively short time period (3-5 years). These measurements will reveal the cross subduction system coupling patterns (i.e. up and down dip coupling limits). 2. The expected extreme strain variations around the up dip limit will also allow truly effective semi-continuous and continuous geodetic surveys to be developed and then transported to other systems globally. 3. The presence of the Nicoya Peninsula means the submerged forearc region is relatively narrow (~60 km). Thus, the submerged target area from the coast to the trench can be studied with a limited number of boreholes and surface installations. 4. It is known to be highly hydro-tectonically active: We have evidence for coupling between seismic noise and fluid flow events at this margin (Brown et al., 2005). Three events were recorded in a 6-month period 9

12 in 2000 suggesting a high density of episodic slip events are occurring at least within certain regions of the system. In this same region we now know we are dealing with a new class of recently resolved large ultra slow earthquakes such as the M w ~7 event that occurred over a few week period in 2003 [Protti et al., 2004]. Very little is understood about these classes of ultra-slow events. 5. The structure and permanent strain distribution across the margin is relatively simple. The mid slope region has only minor extensional/thrust faulting and apart from any long-term subsidence related to proposed subduction erosion in this region (ref..) it seems the strain relating to repeated slippage on the basal décollement does not result in significant permanent strain across much of the mid slope region. This is constrained because the slope sediments and unconformity surface that lies above the Nicoya basement (Fig. 2) and minor brittle thrust structures that disrupt it have only minimal displacements of tens of m. Permanent surface strain (shortening) is concentrated within ~7 km of the toe of the wedge and may be largely limited to slippage on the frontal thrust itself. Therefore it should be possible to relate surface strain to subsurface displacements and fluid pressure variations via a relatively simple poroelastic model constrained by a limited number of geodetic observations. 6. There are indications in both surface mapping and coring studies that deep fluids are seeping to the surface along structures in the forearc. This suggest that at least around the minor structures that fracture the Nicoya basement there is significant permeability and a hydrologic system that is proposed to connect to seismogenic depths. We hope to evaluate such structures as proxy monitoring wells utilizing flux meters (Brown et al., 2005). However, these indications of fluid seepage mean we will be evaluating the permeability structure of the basement around the wells in detail so we expect to be conducting basic hydrologic borehole test as part of any observatory emplacement. Given that they emit a variety of volatiles (dissolved methane, H 2 S, and probably He), surface seepage regions also act as synergetic foci for a variety multidisciplinary studies including studies focusing on volatile fluxes in subduction environments and biogeochemical active systems and microbiology. There are also reasons to expect that pulsed aqueous outputs, and resultant changes in geochemical fluxes and biological activity, will also modulated by the same hydrotectonic processes (poroelastic effects) that drive sub surface pore pressure changes and seismic noise. 7. The climate is mild most months of the year and the site is close to Panama (easy access for ships). Scientific Objectives There are number of interdisciplinary themes that can be address by a combination of both borehole observatories and surface measurements of seismicity and strain. However, because we intend to emplace observation wells within the crystalline Nicoya ophiolitic basement we also will be able to core, date, and unravel subsidence patterns in the forearc. The following are some of the principal scientific objectives we will be addressing with this proposal: 1. What are the interrelationships between both larger and micro earthquakes, tremor-like noise and both seismic and non-seismic (but not necessarily silent) slip events? Are nearly silent slip events associated with enhanced or reduced background seismicity patterns? Are they implicated in the pre-rupture build up of instabilities proceeding larger events? Do they occur at higher frequencies after larger local seismic rupture events? Do nearly silent slip events account for the 50% coupling on this margin. Can we observe broader regional scale triggering of events by stress transference or other dynamic mechanisms? Can we observe the tremor-like noise in the mid slope region utilizing ODP observatory borehole? Is the noise related to ultra-slow episodic slip at depth on the subduction thrust? Can we track rupture events as they migrate across the wedge do they 10

13 migrate up dip, down dip, or along strike and if so how fast do they propagate at different levels? 2. Where is the up dip limit of the coupled subduction fault? Is there such a thing as an up dip limit in a partially coupled subduction zone or do locked and unlocked regions move around or have temporally variable coupling properties. Only by undertaking geodetic measurements in the central forearc regions can such issues be resolved. 3. How generally hydrologically active are the structures in the forearc 2 and 3 D, is the Nicoya basement a generally permeable and hydrologically active terrain? What is the pore pressure distribution in the terrain (is it overpressure and if so by how much and how does this impact the stress state on the subduction thrust)? Do deep fluids get channeled to the surface from seismogenic depths through this basement and how steady state is the fluid flow? Related to this are some basic geochemical questions. Is there evidence that the compositions of the deep fluids and gasses originating from the seeps vary with time and are such variations tectonically modulated (i.e. Do we have distinct pre, syn-, and post rupture compositional variations). Do the compositions of the seeps also systematically vary across the forearc in response to changing temperatures and digenetic reactions at depths approaching those of the seismogenic zone. We could also address related Biogeochemical questions concerning the interrelationships of biogeochemistry of fluxes and geochemical cycles. Technical Objectives: 1) Can we utilize the quite borehole seismic, pore pressure, and tilt observatories to observe the progressive onset of unusual seismic noise and low frequency seismic events that signal slow/rapid slippage of a partially coupled plate boundary? Can we tie in slip events as identified by surface geodetic methods (onland and offshore), to tilt variations in the borehole and pulsations of pore pressure in the porous media around the boreholes? The borehole observatories in the Nicoya basement have several important technical advantages and objectives. Not only will they provide subsurface measurement of pore pressure and tilt but they will provide the opportunity to study low level seismic signals. A quite borehole environment is required because of the high levels of ambient oceanographic noise normally present in oceanographic environments and in OBS data. Studies have shown that only by placing seismometers in bore holes can we ensure to insulated the sensors from ambient ocean noise and enhance our ability to study what may be very subtle signal originating from the deeper subduction interface. We plan also to investigate the possibility of cementing volumetric strain meters into the Nicoya basement. Such measurements require stiff wall rock rheologies, similar to that of the instruments. 2) Can we devise effective modeling and networking strategies that will interlink seabed, subsurface, and land observations. We are just initiating preliminary modeling efforts (Figure 3) to study the elastic effects for the types of deformation events we are proposing are occurring on this margin. This models will simultaneously predict, the expect tilt, surface deformation patterns, the poroelasticly inducted portions of flow, and pore pressures delineated in various seized events on the subduction thrust. 3) How best can we link and power the bore hole and surface observations of flow and strain in order to build up an effective relatively low-cost plate boundary monitoring system that will give us an integrated data set in perhaps real time via either buoy satellite uplink) and/or short cable based nodes. This is a longer-term objective that is best addressed in the context of the proposed ORION program. IODP BOREHOLES AND THE GENERAL ORION OBSERVATRORY PLAN We intend to directly harness the seismogenic and tsunamogenic objectives of the ORION and IODP 11

14 programs with regards to building the progenitor to an integrated 3D geodetic, seismological, and hydrotectonic offshore array (Fig. 6). We will utilize both surface and subsurface instrument arrays. We plan a modest start with only two proposed nodes at which we will have a single instrumented borehole and surface instruments on and around large areas of nearby seepage. Such an array will naturally engender in its building and subsequent developments a greater understanding of multiple cross-disciplinary processes such as the interaction of dynamic processes along plate boundaries and hydrotectonic processes. There are direct out links to subduction fluxes (geochemical mass balances) and biogeochemical processes at these same nodes and, thus, there is out reach to an even wider grouping of scientists. Last but by far not least is the broader impact of subduction zone seismogenic hazard mitigation. Why Develop an Integrated Observatory?: In order to achieve a full 3D understating of the strain and tectono-hydrological field across the forearc we plan to simultaneously measure multiple components of the system. Based on our initial modeling studies of the expected strain field across the updip limit we have to measure vertical, horizontal, and tilt components of the strain field (Fig. 3). The volumetric/poroelastic components can also studied by evaluating the subsurface pore pressure, volumetric strain, and resultant fluid displacements through the surface of the forearc. To achieve a totality of understanding all these components have to be linked directly through the application of systematic fully comprehensive numerical models. This will allow for the longer term scientific understanding of how such systems evolve through time (i.e. the full seismic cycle). For rapid response and hazard mitigation simple instantaneous trigger methods and subsequent data distribution networks need to be developed for an effective immediate warning system to be implemented. Several fully integrated studies are the initially the only way to understand how to achieve such sophisticated objectives in different hazard environments. Ultimately by understanding the full complexity of such systems we plan to boil down and package different hazard mitigation methodologies to suite the range different tectonic environments in a cost effective manner. Borehole and Drilling Plan Borehole observatory The Seismogenic Arrayy Previous IODP Holes The borehole drilling program will achieve our vertical geodetic dimension, our seismological, and a component of our hydrotectonic (pore pressure) objectives. In order to achieve these objectives we plan to undertake the following drilling plan. Four sites (two principal and two alternates) would be cored m into basement in the mid slope regions of the fore arc (Fig. 7). These two drill sites will be approximately km deep (sub 12 CTD array seeps Buoy seeps Marine GPS Centroid Land continuous GPS stations Multiport node Bore hole observatory, seismometer, tilt, pore pressure, temperature, volumetric strain VSM Pressure gauges, flow meters,temp probes, ADCPs, other seep instrumentation Figure 6 Schematic representation of the overall expected observatory architecture showing the 3D distribution of the various instrument packages, buoy, and 2 ORION nodes.

15 seafloor, Fig. 7 Seismic crosssection) and are in 2-3 km water depth (Fig. 1). These two sites are approximately 8-10 km apart and are situated close to areas where there is natural surface seepage. Off Nicoya these would be placed approximately equidistantly across the fore arc between the existing ODP boreholes at the toe and the onshore instillations planned on the peninsula. The primary objective would be geodynamic observatory studies. They would encompass both the aseismic and expected partial coupled region offshore of the Nicoya peninsula. All sites would be first cored to basement. Past drilling indicates hole instability in the slope sediments requires such sites be cased to full depth. Standard, core based, pore water, and heat flow analyses would also be conducted to ascertain thermal and geochemical/hydrologic conditions as well as basic physical property examination of porosity, permeability and state of consolidation of the Two-Way Travel Time (s) Two-Way Travel Time (s) D Seismic Line 60 Top of Oceanic Crust slope sediments. RAB would be considered potential highly useful data for studying fracture patterns in the deeper section and particularly the Nicoya basement or any intersected fault zones. Subsequently, in phase two relatively simple CORKs would be installed in the cased holes. The expected maximum depths of the CORK installations would be between km, within the range of a riser-less ship, although the deeper sites at <2 km water depth might be best achieved with riser drilling. The basement penetration would ensure better coupling between the instruments and the formation as well as allowing us to examine the permeability and pore pressure responses of the basement terrain. The borehole instrumentation we plan to install within the boreholes include the following a) tilt meters (min. resolution ~0.1 μradians) at three/four levels, a broadband observatory seismometer at the base of the hole, pore 13 Distance from trench (km) Mud volcano D Seismic Line 120 Top of Oceanic Crust Distance from trench (km) Mud volcano

16 pressure measurements at the base and perhaps a secondary level. Strain meters and accelerometers would also being considered. Given their stiffness the volumetric strain instruments should be emplaced within the Nicoya basement. We would also have a limited temperature array to get an undisturbed thermal gradient. This could be increased in complexity if we see any hydrologic activity along the structures we plan to drill through. The tilt meters may also indicate if slippage occurs on the minor thrusts we plan to drill through. There are potentially several bore hole monitoring methods available, we refer to the IODP CORKS here because they are the current IODP mainstay. Should simpler subsurface monitoring strategies become available they will be utilized but we would like to penetrate basement to achieve good instrument coupling and to ascertain if is has significant permeability. We propose currently either simple CORKs or CORK-II installations. Multiple levels may be necessary if we encounter a hydrologically active structure within the section. Below the cemented casing, the CORK-II allows the possibility of multiple screened intervals, separated by packers. If desirable, a single-interval CORK could also be designed similar to the simple early ones. We expect many of the holes will need to be cased to depth and we aim to make use of a number of these cased holes for later observatory studies (see below). These latter single level CORKs are potentially deployable in preexisting cased holes and so we could simply reoccupy the original cased wells reducing the cost significantly. As a general strategy for the hydrologic observatories, we foresee several sequential phases to the monitoring. Following drilling, the initial 1 to 1.5 years would be devoted to equilibration and obtaining background parameters, with minimal disturbance. The following 1-1/2 years could allow for active (but non-destructive) experiments/ perturbations such as hydrologic tests, active water sampling, and geophysical experiments. Subsequent years would prioritize passive monitoring with emphasis on time series measurements of pressure and temperature, and seismologic/geodetic. Possible additional sampling could include geochemical sampling (but only if we see evidence that this is logical such as if we intersect a hydrologically active fault). Other experiments/measurements (e.g., Osmo samplers, point diffusion hydrogeologic tests) would be conducted if they do not disturb the longterm pressure, seismological, and temperature measurements. We also see secondary geodynamic objectives being accomplished from core-based studies of the paleo-water depths as reviled by foraminifer s assemblages and by dating any unconformities within the slope sediments and certainly at the Nicoya basement slope basin contact. The mid slope site locations will also allow the subsidence history and any time transgressive ages of the unconformity on the top of the Nicoya basement to be studied. These objectives would fill out the picture of potential long-term subduction erosion patterns across this margin. Surface Monitoring Program The surface-monitoring program will include the following minimum elements to measure hydrotectonic activity, vertical strain, and horizontal strain: 1) Precision bottom pressure sensors to study both the oceanographic effects and changes in the height of the seabed. These instruments will achieve our vertical strain component objectives. To be most effective these require both bottom pressure sensors and water column measurements. The Vertical Strain Meter (VSM) is an instrument that is being proposed this August to NSF (Brown, SIO) that will measure vertical displacement of the seabed over long time periods to a resolution of about 1 cm/yr (perhaps several mm/y) utilizing precision pore pressure measurements. The VSM is based on CTD water column measurements and a pair of Digiquartz transducers that, if tuned well, can resolve signals that are up to 10-8 of the full-scale signal. Two issues have to be confronted: Researchers using these same transducers to monitor for tsunamis have found that the drift is fairly linear after an initial break-in period of about a month, however, each transducer 14

17 has its own characteristic drift. If we measure the drift accurately and it can be predicted we can subtract it from the signal. The two transducers are utilized to account for this effect. Oceanographic noise can be an important impediments to these types of measurements tides, storm surge, currents, internal waves, oceanographic fronts, and atmospheric weather/climate all cause pressure signals that will be recorded on the bottom pressure sensors. The largest of these are the tides and internal waves but since they are periodic and we are looking for relatively long-term vertical movement they should be easy to filter out. Annual climatic cycles and longer term and non-periodic effects such as oceanographic fronts and meso-scale eddies, and changes in mixing layer depth are far more problematic. Monitoring the water column above the instrument with several CTDs (perhaps 4 to 5) is the best solution attached to the buoy. These effects can then be directly filtered from the pressure record. CTDs measure temperature and salinity (and therefore density) are relatively low cost off the shelf instrumentation. Atmospheric pressure fluctuations should also be monitored (land stations and on the buoy). 2) Electronic flow meters and osmotically driven instruments (Fig. 8): These measurements will achieve our surficial hydrotectonic component objectives. The Optical Tracer Injection System (OTIS), an electronic data logging flow meter developed at Scripps [LaBonte and Brown, in prep], is designed for high-resolution measurements in higher flow rate regimes, m/yr, and is currently being utilized for real time data acquisition via a WHOI Buoy at the Nootka-Cascadia triple junction. These instrument are intended to look for transient hydrotectonic events related to changes in surficial fluid expulsion rates. OTIS results from recent Monterey deployments showed sensitivity from rates of 0.05 m/y to > 50 m/y. The Figure 8 Seafloor photograph showing an example microobservatory that could site at the end of a distributary cable that records, vertical flow through the seabed, temperature, (pressure in the ORION version), bottom water currents (ADCP), and bottom water currents. Subsidiary sensors could include attached heat flow probes. This instrument also records serial fluid samples that will allow long term changes in aqueous and gas chemistries to be recovered. An ORION version would require detachable coil systems to be developed. temporal resolution of minutes was also demonstrated through the clear tidal overprint recorded in both the flow-data and a temperature sensor that was contained in the sample chamber (Fig. 8). The expelled fluids were ~5 C above ambient, consisted with the elevated flow rates at this site (Fig. 8b) and exhibited the same tidal signal. We plan to also make temperature probe measurements at the seep sites to get co located signals. Note: The OTIS flow meters can also equipped with a osmotic samplers allowing simultaneous sampling of both aqueous chemistry, and dissolved Nobel gasses. 3) GPS and acoustic measurements: These measurements will achieve our horizontal strain component objectives. There are two potential ways of utilizing this instrumentation. Campaign mode GPS-A: During the initial stage of the project we propose to use the campaign mode approach that holds a ship at the center of the transponder array for several tens of hours to collect the necessary data. We plan to outfit the ROV/HOV support vessel with the necessary GPS and hydrophone system. As ROV operations are 15