Christeson, G. L., J. A. Karson, and K. D. McIntosh, Mapping of seismic layer 2A/2B boundary above the sheeted dike unit at intermediate-spreading crust exposed near the Blanco Transform, Geochem., Geophys., Geosyst., (in press), 2010
Funk, J., P. Mann, K. D. McIntosh, and J. Stephens, Cenozoic tectonics of the Nicaraguan depression, Nicaragua, and Median trough, El Salvador, based on seismic-reflection profiling and remote-sensing data, Geol. Soc. Amer. Bull., 121, 1491-1521, 2009, doi:10.1130/B26428.1, 
Lakes Nicaragua and Managua are the two largest lakes in Central America, and they cover a combined area of ~9000 km2 of the presently active Nicaraguan depression and Central America volcanic front. As part of the Subduction Factory focus area of the U.S. National Science “Margins” program, ~1925 km of shallow geophysical data were acquired over Lakes Nicaragua and Managua in May 2006 to establish their late Quaternary structural and stratigraphic history and to better constrain regional models for active tectonics in western Nicaragua, the Gulf of Fonseca, and the Median Trough in El Salvador. In order to investigate regional, upper-crustal deformation resulting from forearc sliver transport and/or slab rollback of the Cocos plate, these new data were integrated with: relocated earthquake epicenters, earthquake focal mechanisms, high-resolution digital topography from the National Aeronautics and Space Administration (NASA) Shuttle Radar Topography Mission (SRTM), published global positioning system (GPS) vectors, onland geologic maps, previous maps of lake bathymetry and bottom sediment types, a previously unpublished regional aeromagnetic data set, and multichannel seismic-reflection profiles from the Gulf of Fonseca and Pacific Ocean. These new data sets have improved bathymetric, bottom sediment, and recent fault maps for both Nicaraguan lakes and can be used as new constraints on the regional geology and tectonics. Three regional structural cross sections across the Nicaraguan depression indicate that the basin is a highly asymmetrical half-graben bounded to the southwest by northeast-dipping, oblique-slip normal faults. Late Oligocene to Holocene extension resulted in footwall uplift along the elevated and folded area of the Nicaraguan Isthmus, and the area of greatest subsidence occurs on the footwall block underlying Lake Nicaragua in the southeast. A similar but younger pattern of footwall uplift adjacent to the down-thrown footwall block is present to the northeast beneath Lake Managua and the Gulf of Fonseca. We interpret this structural pattern as a time-transgressive rift opening, where the oldest extension (late Oligocene–early Miocene) began in the southeast and migrated to the northwest. GPS data indicate that this earlier phase of intra-arc normal rifting is presently being superimposed by arc-parallel, right-lateral shear related to the northwestward transport of the Central America forearc sliver.
Hu, C., P. L. Stoffa, and K. D. McIntosh, First arrival stochastic tomography: Automatic background velocity estimation using beam semblances and VFSA, Geophys. Res. Lett., 35, L23307, 2008, doi:10.1029/2008GL034776,
We present a new tomography method based on the local beam semblance and the very fast simulated annealing (VFSA) global optimization method. The data space is the local beam semblance calculated using local slant stacks for overlapping offset windows, i.e. beam windows, of the original common-shot or common-receiver gathers. On each beam semblance panel, the first coherency peak can be identified with a particular ray parameter, first-arrival traveltime and beam center position. The forward problem can be solved with any ray tracer to find arrivals matching the identified peaks. Our inversion scheme uses VFSA to find the maximum-a-posteriori (MAP) solution and estimates the uncertainty by applying Bayesian analysis of all the sampled models for a specified model parameterization. This integration of automatic local semblance evaluation instead of first-arrival picking and a fast forward modeling method combined with VFSA to determine the optimal model makes our method robust, efficient and accurate.
Ivandic, M., I. Grevemeyer, A. Berhorst, E. R. Flueh, and K. D. McIntosh, Impact of bending related faulting on the seismic properties of the incoming oceanic plate offshore of Nicaragua, J. Geophys. Res., 113, B05410, 2008, doi:10.1029/2007JB005291,
A seismic wide-angle and refraction experiment was conducted offshore of Nicaragua in the Middle American Trench to investigate the impact of bending-related normal faulting on the seismic properties of the oceanic lithosphere prior to subduction. On the basis of the reflectivity pattern of multichannel seismic reflection (MCS) data it has been suggested that bending-related faulting facilitates hydration and serpentinization of the incoming oceanic lithosphere. Seismic wide-angle and refraction data were collected along a transect which extends from the outer rise region not yet affected by subduction into the trench northwest of the Nicoya Peninsula, where multibeam bathymetric data show prominent normal faults on the seaward trench slope. A tomographic joint inversion of seismic refraction and wide-angle reflection data yield anomalously low seismic P wave velocities in the crust and uppermost mantle seaward of the trench axis. Crustal velocities are reduced by 0.2–0.5 km s−1 compared to normal mature oceanic crust. Seismic velocities of the uppermost mantle are 7.6–7.8 km s−1 and hence 5–7% lower than the typical velocity of mantle peridotite. These systematic changes in P wave velocity from the outer rise toward the trench axis indicate an evolutionary process in the subducting slab consistent with percolation of seawater through the faulted and fractured lithosphere and serpentinization of mantle peridotites. If hydration is indeed affecting the seismic properties of the mantle, serpentinization might be reaching 12–17% in the uppermost 3–4 km of the mantle, depending on the unknown degree of fracturing and its impact on the elastic properties of the subducting lithosphere.
Ranero, C. R., I. Grevemeyer, H. Sahling, U. Barckhausen, C. Hensen, K. Wallmann, W. Weinrebe, P. Vannucchi, R. E. Von Huene, and K. D. McIntosh, Hydrogeological system of erosional convergent margins and its influence on tectonics and interplate seismogenesis, Geochem., Geophys., Geosyst., 9, Q03S04, 2008, doi:10.1029/2007GC001679,
Fluid distribution in convergent margins is by most accounts closely related to tectonics. This association has been widely studied at accretionary prisms, but at half of the Earth's convergent margins, tectonic erosion grinds down overriding plates, and here fluid distribution and its relation to tectonics remain speculative. Here we present a new conceptual model for the hydrological system of erosional convergent margins. The model is based largely on new data and recently published observations from along the Middle America Trench offshore Nicaragua and Costa Rica, and it is consistent with observations from other erosional margins. The observations indicate that erosional margins possess previously unrecognized distinct hydrogeological systems: Most fluid contained in the sediment pores and liberated by early dehydration reactions drains from the plate boundary through a fractured upper plate to seep at the seafloor across the slope, rather than migrating along the décollement toward the deformation front as described for accretionary prisms. The observations indicate that the relative fluid abundance across the plate-boundary fault zone and fluid migration influence long-term tectonics and the transition from aseismic to seismogenic behavior. The segment of the plate boundary where fluid appears to be more abundant corresponds to the locus of long-term tectonic erosion, where tectonic thinning of the overriding plate causes subsidence and the formation of the continental slope. This correspondence between observations indicates that tectonic erosion is possibly linked to the migration of overpressured fluids into the overriding plate. The presence of overpressured fluids at the plate boundary is compatible with the highest flow rates estimated at slope seeps. The change from aseismic to seismogenic behavior along the plate boundary of the erosional margin begins where the amount of fluid at the fault declines with depth, indicating a control on interplate earthquakes. A previously described similar observation along accreting plate boundaries strongly indicates that fluid abundance exerts a first-order control on interplate seismogenesis at all types of subduction zones. We hypothesize that fluid depletion with depth increases grain-to-grain contact, increasing effective stress on the fault, and modifies fault zone architecture from a thick fault zone to a narrower zone of localized slip.
Christeson, G. L., K. D. McIntosh, and J. A. Karson, Inconsistent correlation of seismic layer 2A and lava layer thickness in oceanic crust, Nature, 445, 418-421, 2007, doi:10.1038/nature05517,
At mid-ocean ridges with fast to intermediate spreading rates, the upper section of oceanic crust is composed of lavas overlying a sheeted dyke complex. These units are formed by dykes intruding into rocks overlying a magma chamber, with lavas erupting at the ocean floor. Seismic reflection data acquired over young oceanic crust commonly image a reflector known as 'layer 2A', which is typically interpreted as defining the geologic boundary between lavas and dykes1, 2, 3. An alternative hypothesis is that the reflector is associated with an alteration boundary within the lava unit4, 5, 6. Many studies have used mapped variability in layer 2A thickness to make inferences regarding the geology of the oceanic crust, including volcanic construction, dyke intrusion and faulting7, 8, 9, 10. However, there has been no link between the geologic and seismological structure of oceanic crust except at a few deep drill holes. Here we show that, although the layer 2A reflector is imaged near the top of the sheeted dyke complex at fast-spreading crust located adjacent to the Hess Deep rift, it is imaged significantly above the sheeted dykes section at intermediate-spreading crust located near the Blanco transform fault. Although the lavas and underlying transition zone thicknesses differ by about a factor of two, the shallow seismic structure is remarkably similar at the two locations. This implies that seismic layer 2A cannot be used reliably to map the boundary between lavas and dykes in young oceanic crust. Instead we argue that the seismic layer 2A reflector corresponds to an alteration boundary that can be located either within the lava section or near the top of the sheeted dyke complex of oceanic crust.
Hu, C., M. K. Sen, P. L. Stoffa, and K. D. McIntosh, Plane wave gaussian beam prestack depth migration, SEG Ann. Meeting, San Antonio, TX, 2200-2204, 2007
McIntosh, K. D., E. A. Silver, I. Ahmed, A. Berhorst, C. R. Ranero, R. K. Kelly, and E. R. Flueh, The Nicaragua convergent margin: Seismic reflection imaging of the source of a tsunami earthquake, Costa Rica, in The Seismogenic Zone of Subduction Thrust Faults, edited by T. Dixon and J. C. Moore, Columbia Univ. Press, New York, 257-287, 2007
Hu, C., K. D. McIntosh, and P. L. Stoffa, Voronoi cell based staggered grid SH wave numerical simulation, SEG Ann. Meeting, New Orleans, LA, 2270-2274, 2006
Hu, C., K. D. McIntosh, H. J. A. Van Avendonk, and P. L. Stoffa, Hybrid ray tracer and amplitude calculation with finite difference, graph theory and ray bending
, SEG Ann. Meeting, New Orleans, LA, 3408-3412, 2006
McIntosh, K. D., Y. Nakamura, T.-K. Wang, R.-C. Shih, A. T. Chen, and C.-S. Liu, Crustal-scale seismic profiles across Taiwan and the western Philippine Sea, Tectonophysics, 401, 23-54, 2005, doi:10.1016/j.tecto.2005.02.015,
We have used combined onshore and offshore wide-angle seismic data sets to model the velocity structure of the Taiwan arc–continent collision along three cross-island transects. Although Taiwan is well known as a collisional orogen, relatively few data have been collected that reveal the deeper structure resulting from this lithospheric-scale process. Our southern transect crosses the Hengchun Peninsula of southernmost Taiwan and demonstrates characteristics of incipient collision. Here, 11-km-thick, transitional crust of the Eurasian plate (EUP) subducts beneath a large, rapidly growing accretionary prism. This prism also overrides the N. Luzon forearc to the east as it grows. Just west of the arc axis there is an abrupt discontinuity in the forearc velocity structure. Because this break is accompanied by intense seismicity, we interpret that the forearc block is being detached from the N. Luzon arc and Philippine Sea plate (PSP) at this point. Our middle transect illustrates the structure of the developing collision. Steep and overturned velocity contours indicate probable large-scale thrust boundaries across the orogen. The leading edge of the coherent PSP appears to extend to beneath the east coast of Taiwan. Deformation of the PSP is largely limited to the remnant N. Luzon arc with no evidence of crustal thickening to the east in the Huatung basin. Our northern transect illustrates slab–continent collision—the continuing collision of the PSP and EUP as the PSP subducts. The collisional contact is below 20 km depths along this transect NE of Hualien. This transect shows elements of the transition from arc–continent collision to Ryukyu arc subduction. Both of our models across the Central Range suggest that the Paleozoic to Mesozoic basement rocks there may have been emplaced as thick, coherent thrust sheets. This suggests a process of partial continental subduction followed by intra-crustal detachment and buoyancy-aided exhumation. Although our models provide previously unknown structural information about the Taiwan orogen, our data do not define the deepest orogenic structure nor the structure of western Taiwan. Additional seismic (active and passive), geologic, and geodynamic modeling work must be done to fully define the structure, the active deformation zones, and the key geodynamic process of the Taiwan arc–continent collision.
Roy, L., M. K. Sen, K. D. McIntosh, P. L. Stoffa, and Y. Nakamura, Joint inversion of first arrival seismic trave- time and gravity data, J. Geophysics Engin., 2, 277-289, 2005, doi:10.1088/1742-2132/2/3/011,
oint interpretation of disparate geophysical datasets can be a powerful tool in deriving meaningful subsurface geologic models. Here we describe a method for joint inversion of first arrival travel-time and gravity data with application to field data from a geologically complex subduction zone. As both tomographic and gravity inversions are prone to non-uniqueness, incorporation of prior information in the model description is crucial to the success of such algorithms. We employ a layer-based model description, in which interfaces (which may also be called iso-velocity lines) are defined by a summation of arc-tangent functions. Arc-tangent functions are highly flexible in mapping smooth interfaces as well as the nearly discontinuous changes in depth of an interface. Within each layer, the velocity is assumed to vary linearly with depth at each surface location. Because of the non-uniqueness of the gravity inversion, we use prior knowledge to relate the velocity to density values. In our application here, the density is related to the velocity using a fourth-order polynomial whose coefficients are assumed to be known. The nonlinear optimization problem is solved by a very fast simulated annealing (VFSA) technique. At each iteration, travel times are generated by the solution of the Eikonal equation while the gravity anomalies are computed using a standard formula. The objective function consists of two parts: one measures the misfit in travel time and the other measures the misfit of gravity anomalies. We applied our technique to field data collected over the Ryukyu subduction zone offshore Taiwan during an ocean bottom seismometer (OBS) experiment (called TAICRUST) conducted in the year 1995. Application to one NS and one EW trending line resulted in acceptable fit of both travel-time and gravity data. The resulting models are helpful in the interpretation of the local geology.
Ranero, C. R., J. P. Morgan, K. D. McIntosh, and C. Reichert, Bendin-related faulting and mantle serpentization at the Middle America trench, Nature, 425, 367-373, 2003, doi:10.1038/nature01961,
The dehydration of subducting oceanic crust and upper mantle has been inferred both to promote the partial melting leading to arc magmatism and to induce intraslab intermediate-depth earthquakes, at depths of 50–300 km. Yet there is still no consensus about how slab hydration occurs or where and how much chemically bound water is stored within the crust and mantle of the incoming plate. Here we document that bending-related faulting of the incoming plate at the Middle America trench creates a pervasive tectonic fabric that cuts across the crust, penetrating deep into the mantle. Faulting is active across the entire ocean trench slope, promoting hydration of the cold crust and upper mantle surrounding these deep active faults. The along-strike length and depth of penetration of these faults are also similar to the dimensions of the rupture area of intermediate-depth earthquakes.
Wang, T.-K., K. D. McIntosh, Y. Nakamura, C.-S. Liu, and H.-W. Chen, Velocity-interface structure of the southwestern Ryukyu subduction zone from EW9509-1 OBS/MCS data, Marine Geophysical Researches, 22, 265-287, 2001, doi:10.1023/A:1014671413264,
A wide-angle seismic survey, combining ocean-bottom seismometers (OBS) and multi-channel seismic (MCS) profiling, was implemented in the southwestern Ryukyu subduction zone during August and September 1995. In this paper, we present the data analysis of eight OBSs and the corresponding MCS line along profile EW9509-1 from this experiment. Seismic data modeling includes identification of refracted and reflected arrivals, initial model building from velocity analysis of the MCS data, and simultaneous and layer-stripping inversions of the OBS and MCS arrivals. The velocity-interface structure constructed along profile EW9509-1 shows that the northward subduction of the Philippine Sea Plate has resulted in a northward thickening of the sediments of the Ryukyu Trench and the Yaeyama accretionary wedge north of the trench. The boundary between the subducting oceanic crust and the overriding continental crust (represented by a velocity contour of 6.75 km/s) and a sudden increase of the subducting angle (from 5 degrees to 25 degrees) are well imaged below the Nanao Basin. Furthermore, velocity undulation and interface variation are found within the upper crust of the Ryukyu Arc. Therefore, the strongest compression due to subduction and a break-off of the slab may have occurred and induced the high seismicity in the forearc region.
Christeson, G. L., K. D. McIntosh, and T. H. Shipley, Seismic attenuation in the Costa Rica margin wedge: Amplitude modeling of ocean bottom hydrophone data, Earth Planet. Sci. Lett., 179, 391-405, 2000, doi:10.1016/S0012-821X(00)00118-7,
Travel-time and amplitude modeling were carried out on data recorded by three ocean bottom seismometers along two strike profiles, located 10–20 km landward of the Middle America Trench offshore Costa Rica. These two profiles bracket a region of rapid change in physical properties and structural styles. The travel-time modeling indicates little structural change along the profiles, and therefore the data are suitable for amplitude modeling with the reflectivity method. For the profile 10 km from the trench, amplitude modeling indicates a margin wedge with velocities of 4–4.4 km/s and Qp=25–50. For the profile 20 km from the trench, the amplitude modeling indicates a margin wedge with velocities of 4.4–4.8 km/s and Qp=50–75. The primary new result of this analysis is the constraint that Qp=25–75 within the margin wedge at both locations. These high attenuation values are consistent with fracturing of the material making up the margin wedge, either ophiolitic Nicoya Complex rocks, dewatered and cemented sediments, or a tectonic mixture of both.
McIntosh, K. D., and M. K. Sen, Geophysical evidence for dewatering and deformation processes in the ODP Leg 170 area offshore Costa Rica, Earth Planet. Sci. Lett., 178, 125-138, 2000, doi:10.1016/S0012-821X(00)00069-8,
We use a combination of borehole data from Ocean Drilling Program (ODP) Leg 170 and multichannel seismic reflection (MCS) data to quantify thickness changes in underthrust sediments away from the boreholes. Sediments thrust beneath the upper plate at convergent margins may be more rapidly loaded than in any other environment. Depending on the porosity and permeability of the available fluid pathways, these sediments can compact and dewater very rapidly, as observed in this area offshore the Nicoya Peninsula, Costa Rica. Rapid thinning and dewatering was previously interpreted in this area from MCS data, but the lack of velocity data in this deep-water environment caused ambiguity in the estimates of thickness change. We employ a non-linear inversion technique using detailed density data, primarily logs and some laboratory measurements and coincident MCS data to create 1D synthetic seismograms and detailed velocity functions at three ODP drill sites. Because only a small part of one hole was logged with a sonic tool and the laboratory measurements significantly underestimate in situ velocities, these results provide the most accurate estimate of the velocity profiles. We used these velocity functions to depth-migrate seven MCS lines in the vicinity of the trench and lower slope spanning a distance of 9 km along strike. Analysis of the depth-migrated images shows that there is significant variation along strike in how the underthrust section compacts, which appears to be related to the distribution of normal faults on the Cocos Plate. We interpret that preferentially rapid dewatering in the upper part of the underthrust section may lead to deformation below the original decollement and detachments at deeper stratigraphic levels.
McIntosh, K. D., F. E. Akbar, C. Calderon-Macias, P. L. Stoffa, S. Operto, G. L. Christeson, Y. Nakamura, T. H. Shipley, E. R. Flueh, A. U. Stavenhagen, and G. Leandro, Large aperture seismic imaging at a convergent margin: Techniques and results from the Costa Rica seismogenic zone, Marine Geophysical Researches, 21, 451-474, 2000, doi:10.1023/A:1026597927732,
In March and April 1995 a cooperative German, Costa Rican, and United States research team recorded onshore-offshore seismic data sets along the Pacific margin of Costa Rica using the R/V Ewing. Off the Nicoya Peninsula we used a linear array of ocean bottom seismometers and hydrophones (OBS/H) with onshore seismometers extending across much of the isthmus. In the central area we deployed an OBS/H areal array consisting of 30 instruments over a 9 km by 35-km area and had land stations on the Nicoya Peninsula adjacent to this marine array and also extending northeast on the main Costa Rican landmass. Our goal in these experiments was to determine the crustal velocity structure along different portions of this convergent margin and to use the dense instrument deployments to create migrated reflection images of the plate boundary zone and the subducting Cocos Plate. Our specific goal in the central area was to determine whether a subducted seamount is present at the location of the 1990, M 7 earthquake off the Nicoya Peninsula and can thus be linked to its nucleation. Subsequently we have processed the data to improve reflection signals, used the data to calculate crustal velocity models, and developed several wide-aperture migration techniques, based on a Kirchhoff algorithm, to produce reflection images. Along the northern transect we used the ocean bottom data to construct a detailed crustal velocity model, but reflections from the plate boundary and top and bottom of the subducting Cocos plate are difficult to identify and have so far produced poor images. In contrast, the land stations along this same transect recorded clear reflections from the top of the subducting plate or plate boundary, within the seismogenic zone, and we have constructed a clear image from this reflector beneath the Nicoya shelf. Data from the 3-D seismic experiment suffer from high-amplitude, coherent noise (arrivals other than reflections), and we have tried many techniques to enhance the signal to noise ratio of reflected arrivals. Due to the noise, an apparent lack of strong reflections from the plate boundary zone, and probable structural complexity, the resulting 3-D images only poorly resolve the top of the subducting Cocos Plate. The images are not able to provide compelling evidence of whether there is a subducting seamount at the 1990 earthquake hypocenter. Our results do show that OBS surveys are capable of creating images of the plate boundary zone and the subducting plate well into the seismogenic zone if coherent reflections are recorded at 1.8 km instrument spacing (2-D) and 5 km inline by 1 km crossline spacing for 3-D acquisition. However, due to typical high amplitude coherent noise, imaging results may be poorer than expected, especially in unfavorable geologic settings such as our 3-D survey area. More effective noise reduction in acquisition, possibly with the use of vertical hydrophone arrays, and in processing, with advanced multiple removal and possibly depth filtering, is required to achieve the desired detailed images of the seismogenic plate boundary zone.
Ranero, C. R., R. E. Von Huene, E. R. Flueh, M. Duarte, D. Baca, and K. D. McIntosh, A cross section of the convergent Pacific margin of Nicaragua, Tectonics, 19, 335-357, 2000, doi:10.1029/1999TC900045,
Prestack depth migration of multichannel seismic reflection lines across the Pacific margin of Nicaragua has yielded an accurate depth image to about a 9-km depth from the deep ocean basin to the coast. The margin contains the Sandino forearc basin, probably underlain by oceanic igneous basement and fronted by a small prism accreted at the seaward end of the continental basement. Seismic stratigraphy and drill hole information indicate that sediment has been accumulating since Late Cretaceous. The margin configuration formed between late Cretaceous and Paleocene time and has endured since that time. Uplift of the outer high and slope was probably coeval with subsidence of a deep basin beneath the shelf. From middle-late Eocene time to Oligocene time, the outer high was a barrier to sediment transport. A similar Late Cretaceous to Oligocene tectonic history has been described for the Guatemalan and Costa Rican segments of the Pacific margin. We speculate that the structure of the Pacific forearc basin formed by subduction initiation at the edge of the Caribbean igneous province. Since late Oligocene time, margin-wide subsidence occurs in the Nicaraguan margin, perhaps related to subduction erosion of the upper plate. Coeval steep reverse and normal faulting along local structures in the forearc basin might occur by transpression along margin-parallel strike-slip faults. These faults have been active since the early development of the basin, but the greatest rate of vertical displacement along them was in early - middle Miocene time, probably related to a plate kinematic reorganization involving the collision of Central and South America.
Silver, E. A., M. Kastner, A. T. Fisher, J. D. Morris, K. D. McIntosh, and D. M. Saffer, Fluid flow paths in the Middle America trench and Costa Rica margin, Geology, 28, 679-682, 2000, doi:10.1130/0091-7613(2000)28<679:FFPITM>2.0.CO;2,
The hydrology of the subducting plate and its dewatering behavior through the shallow subduction zone is linked to the structure and deformation of the forearc prism, the nature of the seismogenic zone, the composition of seawater for selected elements, and the composition of the residual slab subducted to depths of magma generation at the volcanic arc. Two locally independent systems of fluid flow govern the transport of heat and chemistry through the Costa Rica subduction complex, a dominantly nonaccretionary subduction zone. One fluid system is the margin wedge, décollement, and underthrust sediment section. Fluid sources include local sediment compaction and mineral dehydration at depth. A second flow system occurs in basement, beneath the sedimentary sequence on the incoming plate. This region is characterized by extremely low conductive heat flow, and the sediment overlying basement has pore-water geochemistry similar to that of seawater. Flow nearly parallel to the trench could be directed by permeability associated with faults and driven by a combination of differential heating and earthquake strain cycling.
Christeson, G. L., K. D. McIntosh, T. H. Shipley, E. R. Flueh, and H. Goedde, Structure of the Costa Rica convergent margin, offshore Nicoya Peninsula, J. Geophys. Res., 104, 25443-25468, 1999,
We present the results of a seismic refraction survey conducted offshore Costa Rica near the Nicoya Peninsula. A dip profile and three strike profiles were carried out over 22 ocean bottom hydrophones and seismographs and were also recorded by land receivers. These data are used to construct a crustal structure model of the convergent margin from 20 km seaward of the Middle America Trench onto the Nicoya Peninsula. The best constrained portion of our model is the velocity at the top of the margin wedge immediately below the slope apron. Velocities increase from 3.5 to 4.2 to 4.6 km/s at distances of 10, 20, and 30–50 km landward of the trench. These velocities are higher than observed within margin wedges at other well-studied convergent margins but lower than the velocities within the adjacent Nicoya Complex, which are ∼5.5 km/s at similar depths below the surface. We interpret the margin wedge velocities as indicating that material similar to the Nicoya Complex extends seaward to near the lower slope but that fracturing, alteration, or accretion processes have lowered the velocity of the margin wedge with respect to the Nicoya Complex. The seismic refraction data cannot constrain the exact thickness or velocity of a possible low-velocity zone (LVZ) overlying the subducting plate; however, geologically reasonable structures are only produced with a LVZ <400 m thick. Velocities in the upper part of oceanic layer 2 are ∼3.5–4.0 km/s within the subducting slab. These velocities are unusually low for oceanic crust of this age and may correlate with a proposed highly permeable zone at the top of the subducting crust. The top of the subducted slab is well resolved, and deepens from 5 km depth at the trench to 15–16 km depth at the Nicoya Peninsula coastline. The dip angle of the subducting plate increases from 6° to 13° at a distance of ∼30 km from the trench. Interplate seismicity appears to become common ∼55 km from the trench where the plate boundary is at ∼14 km depth.
Zelt, C. A., A. M. Hojka, E. R. Flueh, and K. D. McIntosh, 3D simultaneous seismic refraction and reflection tomography of wide-angle data from the central Chilean margin, Geophys. Res. Lett., 26, 2577-2580, 1999, doi:10.1029/1999GL900545,
We present an application of three‐dimensional (3D) simultaneous seismic refraction and reflection tomography for velocity and interface structure. The inversion technique and method for developing the starting model are specifically designed for relatively sparse wide‐angle data acquired across strongly‐varying structure. The data were recorded in a region of seamount subduction on the Chilean margin and consist of seven receivers and ten intersecting airgun profiles over a 90×90 km area providing constraint to 25 km depth. The tomographic method and the final model are assessed through a comparison with the large‐scale geologic features of the margin and a resolution test. The 3D model shows the Valparaiso forearc basin, the accretionary wedge, the subducting plate, and possibly a subducted seamount. Our results show the potential of relatively sparse 3D wide‐angle data.
McIntosh, K. D., and Y. Nakamura, Crustal structure beneath the Nanao forearc basin from TAICRUST MCS/OBS line 14, TAO: Terrestrial, Atmospheric and Oceanic Sciences, 9, 345-362, 1998
Nakamura, Y., K. D. McIntosh, and A. T. Chen, Preliminary results of a large offset seismic survey west of Hengchun Peninsula, Southern Taiwan, TAO: Terrestrial, Atmospheric and Oceanic Sciences, 9, 395-408, 1998
Stavenhagen, A. U., E. R. Flueh, C. R. Ranero, K. D. McIntosh, T. H. Shipley, G. Leandro, A. Schulze, and J. J. Danobeitia, Seismic wide-angle investigations in Costa Rica: A crustal velocity model from the Pacific to the Caribbean coast, Zbl. Geol. Palaont. Teil. I, H.3-6, 393-408, 1998
Christeson, G. L., Y. Nakamura, K. D. McIntosh, and P. L. Stoffa, Effect of shot interval on ocean bottom seismograph and hydrophone data, Geophys. Res. Lett., 23, 3783-3786, 1996,
Data collected by 18 ocean bottom receivers for a seismic line shot at both 50‐m (∼24 s shot interval) and 125‐m (∼58 s shot interval) shot spacing provide a direct field comparison of the effect of shot interval on marine wide‐angle seismic data. Our results indicate that both shot spacings produce high‐quality refraction data in shallow water (<1000 m) on hydrophone and vertical channel data. In deeper water, the data quality of the 50‐m line is adequate for the vertical channel, but it is often poor at large offsets for the hydrophone channel in comparison to the 125‐m shot spacing data. A theoretical model to explain these observations provides further information useful for designing an experiment using ocean‐bottom receivers.
McIntosh, K. D., and E. A. Silver, Using 3D seismic reflection data to find fluid seeps from the Costa Rica accretionary prism, Geophys. Res. Lett., 23, 895-898, 1996,
Submersible dives planned using 2D and 3D seismic reflection data off Costa Rica successfully found numerous sites of fluid expulsion predominantly along surface fault scarps. These data were used because more typical data, such as deep‐towed side‐scan sonar, bottom photography, and heat flow, were unavailable. Because fluids and fluid pressure distribution profoundly affect the way an accretionary prism deforms, detecting fluid pathways and recovering fluids is a key to its deformation processes. Detailed seismic interpretation of the shallow subsurface identified apparently active faults which are most likely to act as fluid pathways. Due to a quasi inverse relationship between surface dip and reflection amplitude, the amplitude of the seafloor reflection, displayed in map‐view, shows the orientation and extent of surface scarps, many of which appear related to subsurface faults. Because the seafloor amplitude response depends on seafloor morphology as well as near‐surface physical properties, removing the effect of the surface dip, calculated directly from picked surface structure, leaves a residual that may be related to variation in physical properties. Although the search for fluid seeps by submersible dives over the Costa Rica prism was sparse and irregular, the seeps that have been discovered are all on structural highs of the apron/prism boundary.
Silver, E. A., J. Galewski, and K. D. McIntosh, Variation in structure, style, and driving mechanism of adjoining segments of the North Panama deformed belt, in Geologic and Tectonic Development of the Caribbean Plate Boundary in Southern Central America, edited by Paul Mann, Geol. Soc. Amer. Spec. Paper, 295, 225-233, 1995
McIntosh, K. D., E. A. Silver, and T. H. Shipley, Evidence and mechanisms for forearc extension at the accretionary Costa Rica convergent margin, Tectonics, 12, 1380-1392, 1993,
Seismic reflection data across the upper trench slope off the Nicoya Peninsula, Costa Rica, reveal a wide zone of nearly trench-parallel normal faults. Although work in the last decade has shown that normal faults are present at many convergent margins, most examples (e.g., Japan, Peru-Chile, and Guatemala) have been associated with margins experiencing subduction erosion or non-accretion. In contrast, extension in the Costa Rica study area apparently is coeval with frontal accretion and underplating. The normal faults across the Costa Rica forearc are striking in seismic section due to the well-layered, 2-km-thick upper slope apron. Fault plane reflections and reflector terminations show that the faults extend through the sedimentary apron and apparently into the underlying accretionary prism, indicating a deep-seated deformation process. The zone of extension is from the midslope area to within 10 km of the shelf edge, a minimum width of about 20 km; the estimated extension across the zone is at least 1.5 to 3 km. Within the apron section, spacing between the faults is generally 200–500 m, and nominal fault dip is 20°–40° and predominantly landward. Activity on the normal faults appears to have occurred over a significant period of time based on increased displacement with depth and on fault-controlled sedimentary thickening. At least some of the faults may be presently active; shallow reflectors and possibly the seafloor are displaced by faulting. Contemporary sediment accretion is documented by the same seismic reflection profiles showing offscraping and underplating near the toe of the wedge and out-of-sequence thrusting primarily below the midslope area. The consistent landward normal fault dip may be influenced by structural anisotropy in the prism and possible extensional reactivation of earlier thrust faults associated with accretion processes. With the available data it is not possible to conclusively determine the cause of the stress field leading to the upper prism and apron extension. However, the three most likely causes are underplating, changes in basal shear stress, or a brief episode of subduction erosion.
Shipley, T. H., K. D. McIntosh, E. A. Silver, and P. L. Stoffa, Three-dimensional seismic imaging of the Costa Rica accretionary prism: Structural diversity in a small volume of the lower slope, J. Geophys. Res., 97, 4439-4459, 1992,
Conventional two-dimensional seismic reflection investigations have been generally relied upon to provide images of large to medium scale structural features in accretionary prisms. We undertook a three-dimensional seismic reflection survey of a small part of a prism arcward of the Middle America Trench off Costa Rica to more correctly image structure and to use the improved structural information to examine the processes of accretion. This survey reveals small features, with dimensions of hundreds of meters, while also defining features thousands of meters in lateral extent, both of which were underappreciated in conventional two-dimensional data from the same area. We have imaged active off scraping at the trench and both duplexing and out-of-sequence faulting a few kilometers arcward of the trench. Fault spacing and reflector geometry vary dramatically over a space of several hundred meters. Some of these variations are related to visible changes in morphology of the underlying oceanic basement, but others are not so easily documented. Fault surface reflections define an architecture which may control gross fluid motion through the prism. This architecture is apparently formed by duplexing and out-of-sequence faulting and has been maintained by periodic motion on some of the out-of-sequence faults. The slope sediment apron records multiple phases of deformation. Abundant small offset reverse faults break the seafloor and indicate recent shortening of a broad region of the underlying prism. A primary result of this survey is appreciation of the structural diversity across a small width of an accretionary prism.
