Outer Prism Research Articles

Frictional properties of the décollement in the shallow Nankai Trough: Constraints from friction experiments simulating in-situ conditions and implications for the seismogenic zone

Along subduction zone megathrust faults, the transition from smectite to illite with depth occurs at temperatures of ∼150°C and has long been hypothesized to be one of the dominant controls on the updip limit of the seismogenic zone. Despite this, there have been few experimental studies performed at in-situ conditions to test the frictional stability behavior of subduction zone faults across the smectite-illite transition. In this study, we simulated in-situ temperature-pressure-mineral conditions along the shallow plate boundary in the Nankai Trough, and conducted friction experiments to further constrain the role of smectite-illite transition in seismogenesis in subduction zones. We found that the friction coefficient of the simulated sediment increased with the progress of illitization and followed the Reuss average of friction coefficients of the mineral phases. The obtained friction coefficients and the Coulomb wedge model inferred that the pore pressure conditions around the plate boundary may be over-pressured but not as high as lithostatic pressure. Higher friction coefficients of sediment due to diagenetic processes would be required to sustain pore pressure close to lithostatic along the plate boundary. The velocity dependence of friction coefficient (a−b) was always positive (i.e., velocity strengthening, not seismogenic) except for slip rates less than ∼10 μm/s at 171°C where the smectite-illite transition has almost completed. Therefore, shallow slow earthquakes that occur beneath the outer prism in the Nankai Trough cannot be explained only by the frictional instability of unconsolidated sediment. Since the smectite-illite transition mostly completes at ∼30 km landward from the trough axis, the transition from velocity strengthening to velocity weakening observed at 171°C is caused by temperature-dependent frictional behavior of illite-rich material. Hence, the updip limit of the seismogenic zone in subduction zones is mainly controlled by the frictional properties of illite-rich material, not by the smectite-illite transition.

Spatial Patterns in Frictional Behavior of Sediments Along the Kumano Transect in the Nankai Trough

AbstractFault slip activity at subduction zones is governed by sediment frictional properties, which in turn are affected by diagenetic processes. To study the spatial patterns in frictional properties across the Nankai Trough, SW Japan, and their relations to fault slip activity, we used sediment samples (10%–59% clay mineral content) along the Kumano transect covering a large spatial range from the inputs via the outer prism to the inner prism, including the deepest sample ever recovered to date. We performed laboratory friction experiments under in situ effective normal stresses and seawater‐saturated conditions. Our results generally demonstrate that the friction coefficient inversely correlates with the clay mineral content. However, the outer prism sediments show higher friction coefficients than sediments from the other locations for a comparable clay mineral content. All samples show velocity‐weakening behavior at low velocities, but the outer prism and the deep inner prism sediments show velocity strengthening at higher velocities. Based on the experimental results combined with a Coulomb wedge model, we propose that the lowest friction coefficient on the décollement occurs beneath the trenchward portion of the outer prism, whereas the minimum friction coefficient of the prism sediment occurs in the landward portion of the outer prism. In addition, the calculated critical nucleation length for slip instability suggests that the décollement beneath the outer prism area is more frictionally unstable than it is beneath the inner prism. This inference is consistent with the spatial distribution of very‐low‐frequency earthquakes and slow slip events along the shallow Nankai Trough.

Amplification and Tuning of Ground Motion at the Outer Cascadia Accretionary Prism

AbstractGround‐motion amplification in the outer Cascadia subduction zone accretionary prism has been documented previously by comparing earthquake and microseismic vertical ground‐motion and pressure records from an ONC/NEPTUNE Canada cabled observatory site on the outer prism to those from a site on the flank of the Juan de Fuca Ridge. Since then, four additional instruments became operational, and data, now spanning 10 years, include more than 100 large (Mw > 7) distant earthquakes. Well‐tuned response at the outer prism sites is observed in both vertical and horizontal components, with peaks in the spectral ratio of vertical velocity relative to nearby ocean crustal and continental bedrock sites (V/Vref) at 9 s, and in the intra‐site horizontal to vertical spectral ratio horizontal‐to‐vertical spectral ratio at 14 s, both with band widths of ±10% to 15% at half amplitude. The vertical response is consistent with ¼ wavelength compressional wave reinforcement, while the horizontal motion tuning most likely reflects the effects of the low velocity prism sediments on surface‐wave propagation. At the periods of maximum relative motion, outer prism surface‐wave vertical and horizontal accelerations are enhanced by factors of up to 15 and 25, respectively. Similar behaviour is seen in microseismic records from these sites, and, to a lesser extent, in earthquake records from temporarily deployed Cascadia Initiative seismometers along the outer Cascadia prism to the south. Such tuning and amplification must be accounted for when assessing the dynamic behavior of the prism and its basal fault at the time of large local earthquakes.

Effect of side walls on flow around prisms

Two-dimensional laminar flow around three side-by-side rectangular prisms placed between two parallel walls is numerically investigated at a Reynolds number Re = 100. The cross-sectional aspect ratio of each prism is b/h = 3, where b and h are the prism width and height, respectively. The gap spacing (g) between the two adjacent prisms and between the wall and the outer prism is varied from g* (= g/h) = 0.8 to 3.4. The focus is given on the influence of g* on flow structures, Strouhal number (St), and fluid forces. Three distinct flow regimes are identified, namely single vortex street (0.8 ≤ g* < 1.1), twin vortex streets (1.1 < g* < 2.1), and triple vortex streets (2.1 < g* ≤ 3.4). Lift forces on the outer prisms are attractive for single vortex street and repulsive for twin and triple vortex streets. The St being identical for the three prisms exponentially declines from 0.39 to 0.18 with g* increasing from 0.8 to 3.4. Fluctuating lift and time-mean drag also shrink with increasing g*, time-mean drag becoming higher for the middle prism than for the outer prisms. Empirical relations of St and forces are obtained as a function of g*.

Geologic controls on up-dip and along-strike propagation of slip during subduction zone earthquakes from a high-resolution seismic reflection survey across the northern limit of slip during the 2010 Mw 8.8 Maule earthquake, offshore Chile

Abstract A grid of closely spaced, high-resolution multichannel seismic (MCS) reflection profiles was acquired in May 2012 over the outer accretionary prism up dip from the patch of greatest slip during the 2010 Mw 8.8 Maule earthquake (offshore Chile) to complement a natural-source seismic experiment designed to monitor the post-earthquake response of the outer accretionary prism. We describe the MCS data and discuss the implications for the response of the accretionary prism during the earthquake and for the long-term evolution of the margin. The most notable observation from the seismic reflection survey is a rapid north-to-south shift over a short distance from nearly total frontal accretion of the trench sediments to nearly total underthrusting of undeformed trench sediments that occurs near the northern edge of slip in the 2010 earthquake. Integrating our structural observations with other geological and geophysical observations, we conclude that sediment subduction beneath a shallow décollement is associated with propagation of slip to the trench during great earthquakes in this region. The lack of resolvable compressive deformation in the trench sediment along this segment of the margin indicates that the plate boundary here is very weak, which allowed the outer prism to shift seaward during the earthquake, driven by large slip down dip. The abrupt shift from sediment subduction to frontal accretion indicates a stepdown in the plate boundary fault, similar to the stepovers that commonly arrest slip propagation in strike-slip faults. We do not detect any variation along strike in the thickness or reflective character of the trench sediments adjacent to the change in deformation front structure. This change, however, is correlated with variations in the morphology and structure of the accretionary prism that extend as far as 40 km landward of the deformation front. We speculate that forearc structural heterogeneity is the result of subduction of an anomalously shallow or rough portion of plate that interacted with and deformed the overlying plate and is now deeply buried. This study highlights need for three-dimensional structural images to understand the interaction between geology and slip during subduction zone earthquakes.

Seismogenic Zone Structures Revealed by Improved 3‐D Seismic Images in the Nankai Trough off Kumano

AbstractTo reveal the detailed deformation structures related to plate subduction in the Nankai Trough, we applied up‐to‐date technologies to improve our 3‐D seismic images. This region is dominated by a megasplay fault system that consists of a coseismic out‐of‐sequence thrust branching from the plate interface and separating the inner and outer accretionary prism. The 3‐D seismic volume was acquired off Kumano in 2006 as a preliminary site survey as part of the Nankai Trough Seismogenic Zone Experiment. The preprocessed data quality was improved by recovering the broadband responses and by better attenuating multiple reflections and noise. New reflection images were then produced using prestack time migration and prestack depth migration with updated velocity models. The new velocity model suggests the possible existence of a high‐velocity zone just above the megasplay fault, which might indicate petrophysical alteration in the seismogenic zone. The fault geometry with spatial dip angle variation and the overburden velocities are important factors for further estimating the force distribution along the coseismic fault. Deformation structures newly imaged beneath the Kumano Basin and dipping layers above the megasplay fault imply complex thrusting and possible fluid flow paths within the inner prism. Fine‐scale deformation features are clarified in the shallow areas from the outer prism to the transition zone, which are useful for reconstructing the accretionary prism development. A low‐reflectivity zone, including an isolated layered block, may be originally underthrusted sediments that have been remobilized during later strike‐slip faults along the edge of Kumano Basin.

Subseafloor Temperature Variations Influenced by Variations in Bottom Water Temperature and Pressure: New High Resolution Observations and Implications

AbstractA new instrument developed for monitoring acceleration, tilt, and pressure at the ocean floor also measures sediment temperature 1 m below the seafloor. Four deployments have been completed and connected to the Ocean Networks Canada cabled observatory, one on the inner Cascadia accretionary prism, two on the outer prism, and one on the sedimented eastern flank of the Juan de Fuca Ridge. Relative amplitudes and phases of temperature variations measured at the seafloor and in the sediment at periods greater than roughly 1 week constrain the thermal diffusivity of the upper meter of subseafloor sediment to be 4 × 10−7 m2/s. Clear ±0.1‐mK amplitude tidal sediment temperature variations are also resolved. These are too large and regular to be the consequence of downward thermal diffusion from the seafloor and too large to be the consequence of fluid migration driven along the sediment geotherm by poroelastic response to tidal loading. The variations are closely correlated with tidal pressure variations, however, and we infer that these temperature signals reflect adiabatic heating and cooling. The lapse rates inferred from the observations at two of the sites are close to the values for seawater but significantly higher than predicted for a mixture that includes sediment grains. The values observed by both instruments at the outer prism site, located near methane‐bearing‐fluid springs, are particularly high, 20% higher than predicted for a sediment‐seawater mixture. This discrepancy could be reconciled if free gas or methane hydrate were present within the pore volume.

The origin of strata within the inner accretionary prism of Nankai Trough: Evidence from clay mineral assemblages along the NanTroSEIZE transect

AbstractOne of the more prominent architectural elements of the Nankai subduction margin, offshore southwest Japan, is an out‐of‐sequence thrust fault (megasplay) that separates the inner accretionary prism from the outer prism. The inner prism (hanging wall of the megasplay) is dominated by mudstone, which is enigmatic when the sedimentary facies is compared to coeval deposits in the Shikoku Basin (i.e. inputs from the subducting Philippine Sea plate) and to coarser‐grained turbidite sequences from the Quaternary trench wedge. Clay mineral assemblages amplify the mismatches of sedimentary facies. Mudstones from the inner prism are uniformly depleted in smectite, with average bulk values of 23–24 wt%, whereas the Shikoku Basin deposits show progressive decreases in proportions of smectite over time, from averages of 46–48 wt% at 10 Ma to 17–21 wt% at 1 Ma. Plate‐boundary reconstructions for the Philippine Sea region provide one solution to the conundrum. Between 15 Ma and 10 Ma, the Pacific plate subducted near the NanTroSEIZE transect, and a trench‐trench‐trench triple junction migrated to the northeast. Accretion during that period involved sediments that had been deposited on the Pacific plate. Motion of the Philippine Sea plate changed from 10 Ma to 6 Ma, resulting in sinistral slip along the proto‐Nankai Trough. Sediments accreted during that period probably had been deposited near the triple junction, with a hybrid detrital provenance. Renewed subduction of the Philippine Sea plate at 6 Ma led to reorganization of watersheds near the Izu–Honshu collision zone and gradual incision of large submarine canyons on both sides of the colliding Izu arc. Accreted Pliocene mudstones share more of an affinity to the triple junction paleoenvironment than they do to Shikoku Basin. These differences between subducting Shikoku Basin strata and accreted Pacific plate sediments have important implications for interpretations of frictional properties, structural architecture, and diagenetic fluid production.

An evaporite‐bearing accretionary complex in the northern front of the Betic‐Rif orogen

AbstractThe Guadalquivir Accretionary Complex forms a largely oblique prism at the northern edge of the Betic‐Rif orogen, where Miocene sediments plus allochthonous evaporite‐bearing units were accreted during the displacement of the Alborán Domain toward the west. Traditional interpretations end the tectonic structuring of the Betic Cordillera at the present topographic front, beyond which gravitational and/or diapiric processes would predominate. However, this study shows pervasive tectonic deformation in the outer prism with coherent oblique shortening kinematics, which is achieved through an alternation of roughly N‐S arcuate thrust systems connected by E‐W transfer fault zones. These structures accord well with the geophysical models that propose westward rollback subduction. The main stage of tectonic activity occurred in the early‐middle Miocene, but deformation lasted until the Quaternary with the same kinematics. Evaporite rocks played a leading role in the deformation as evidenced by the suite of ductile structures in gypsum distributed throughout the area. S‐ and L‐ gypsum tectonites, scaly clay fabrics, and brittle fabrics coexist and consistently indicate westward motion (top to 290°), with subordinate N‐S contraction almost perpendicular to the transfer zones. This work reveals ductile tectonic fabrics in gypsum as a valuable tool to elucidate the structure and deformational history of complex tectonic mélanges involving evaporites above the décollement level of accretionary wedges.

Deformation of the Nankai Trough inner accretionary prism: The role of inherited structures

AbstractAccretionary prisms commonly grow seaward, with the strata of the inner prism consisting of older, previously accreted outer prism rocks overlain by thick fore‐arc basin strata. We focus on the Nankai Trough inner accretionary prism using three‐dimensional (3‐D) seismic data and logging data from the Integrated Ocean Drilling Program (IODP). We update the 3‐D seismic volume using well velocity data to better constrain deeper horizons. Interpretation of these horizons reveals multiple folds with axial surfaces that strike near parallel to modern outer prism thrust faults, and we interpret that these folds formed as a result of thrust faulting. Reactivation of one inner prism thrust fault continued until at least ∼0.44 Ma, after the modern fore‐arc basin formed, indicating that the inner prism had continued deformation until that time. Structural restorations of these folded seismic horizons demonstrate that ∼580 m of slip occurred on this steeply dipping reactivated thrust after fore‐arc basin formation. Structural interpretation and analysis of logging‐while‐drilling data, including borehole images, in the deep inner prism revealed intense deformation of a generally homogenous lithology characterized by bedding that dips steeply (60°–90°), intersected by faults and fractures that have a range of dips and densities. Our study of the deep Kumano Basin provides new insights into the structure of the inner prism and reveals that although the inner prism has partially preserved inherited outer prism structures, these older folds and faults are steeply rotated and cut by multiple fracture populations during subsequent deformation.

What 2‐D multichannel seismic and multibeam bathymetric data tell us about the North Sumatra wedge structure and coseismic response

AbstractRecent large earthquakes have prompted studies to reevaluate seismicity and rupture propagation behavior along the world's major subduction margins. Our study area covers the entire fore arc from northwest of northern Sumatra to west of Simeulue Island, the southern portion of the 2004 tsunamigenic earthquake rupture zone. The accretionary prism width is up to ~180 km, water depths between ~4.5 km near the Sunda Trench and &lt;1 km on fore‐arc high. The wedge consists of a steep outer slope (5–12°), a plateau ~100–120 km wide with anticlinal folds spaced 2–15 km apart, and a steep inner slope adjacent to the Aceh Basin. Analysis of seismic profiles and bathymetry reveal three main structural zones consistent along‐strike, from the trench landward: (1) predominantly landward vergent folds, (2) mixed vergent folds, and (3) predominantly seaward vergent folds. This paper uses those zones to propose a geometry of an underlying rigid backstop. This backstop is seaward dipping and extends from under the Aceh Basin to beneath the mixed vergence zone. A dynamic backstop possibly exists seaward of the rigid backstop and is responsible for the steep slope of the outer prism. Indurated accreted sediments form the landward vergence zone. Along with the possible dynamic backstop beneath the outer wedge, and the rigid backstop in the inner wedge, all behave as a solid block coseismically. This block allows great earthquake rupture to propagate farther seaward toward the Sunda Trench, with resultant hazardous tsunamigenic potential.

The structure and fault activity of the Makran accretionary prism

The Makran Subduction Zone has the highest incoming sediment thickness (up to 7.5 km) of any subduction zone. These sediments have formed a wide accretionary prism (∼400 km). Seismicity in the Makran is generally low; however the margin experienced an Mw 8.1 earthquake in 1945 which generated a significant regional tsunami. Seismic reflection data and swath bathymetry data from offshore Pakistan are used to analyze the structure and fault activity of the outer accretionary prism. The outer prism has a simple structure of seaward verging imbricate thrust faults, many continuous for over 100 km along strike. Fault activity is analyzed using basin stratigraphy and fault geometry, revealing a frontal continuously active zone, a central intermittently active zone, and a landward inactive zone. Over 75% of the faults in the seaward ∼70 km of the prism show evidence for recent activity. The décollement occurs within the lower sediment section, but steps onto the top‐basement surface in regions of elevated basement topography. Fault spacing (6 km) and taper (4.5°) are comparable to other margins such as S. Hikurangi, Cascadia and Nankai, suggesting that high sediment input is not leading to an unusual prism structure. The décollement is unreflective, which is unexpected considering other prism characteristics predicting a weak surface, and may suggest a potentially stronger décollement than previously predicted. This study provides a significant advance in our understanding of the structure of an end‐member convergent margin and demonstrates that systematic analyses of accretionary prism structure can help to elucidate subduction zone dynamics with ultimate relevance to seismogenic potential.

Evidence for episodic aseismic slip across the subduction seismogenic zone off Costa Rica: CORK borehole pressure observations at the subduction prism toe

Slow slip events, or “silent” earthquakes, may relieve a significant amount of stress at many subduction plate boundaries, both downdip of the limit of seismogenesis, and within the seismogenic zone itself in cases where seismic energy release accounts for only a fraction of the plate tectonic displacement rate (Schwartz and Rokosky, 2007). Slow slip has been identified in several instances downdip of the landward limit of the seismogenic zone and is often accompanied by seismic tremor or low-frequency earthquake activity along and above the plate interface (referred to as “episodic tremor and slip”, or ETS). Little is known, however, about the spatial distribution and history of slip between great earthquakes along the seismogenic thrust interface itself which lies mostly offshore. In this article we present formation pressure transients observed in two deep-sea boreholes near the toe of the subduction prism off Nicoya Peninsula, Costa Rica, which followed ETS events observed on shore by 1–2 weeks. The signatures of the transients are consistent with local slip on the shallow part of the thrust interface, with the underthrusting plate experiencing relaxation and the outer prism experiencing contraction. The delay between the tremor activity and the pressure transients observed c. 100 km seaward at the prism toe suggests either slow propagation across the seismogenic zone or delayed deformation at the outer part of the prism triggered by the slip beneath Nicoya. Such slip may serve generally to relieve stress at subduction zones, but also to increase stress in parts of the plate boundary where interseismic slip does not occur.

A discrete episode of seismic and aseismic deformation of the Nankai trough subduction zone accretionary prism and incoming Philippine Sea plate

New insights into episodic deformation at the Nankai trough subduction zone are provided by data from two Ocean Drilling Program borehole hydrologic observatories drilled into the Philippine Sea plate (Site 1173) and the seaward part of the Nankai accretionary prism off southwestern Japan (Site 808), and from an array of high-sensitivity borehole accelerometer and velocity seismometers on Shikoku Island (Hi-net). Fluid pressures monitored at multiple levels in each of the offshore boreholes document steady secular trends that indicate contraction of the crust and sedimentary section at the incoming plate site, and relaxation of the accretionary prism toe. The rate of strain (ca. 10− 6 yr− 1) inferred from the rates of pressure rise at Site 1173 (up to 5 kPa yr− 1) is similar to that which would be produced by plate convergence if convergent elastic strain were distributed over a region a few tens of km wide. Opposing these trends are transient pressure anomalies observed in late June/early July 2003 that indicate an episode of rapid relaxation of the incoming plate and contraction of the outer prism. Concurrent with these transients, a swarm of very-low-frequency earthquakes occurred farther landward in the prism above the estimated seaward limit of the currently locked seismogenic portion of the subduction thrust. Reverse-fault mechanisms determined for one event of this swarm and several of another also indicate transient contraction of the prism. We suggest that both the earthquakes and the pressure transients are the consequences of an aseismic slip dislocation that initiated on about June 26, 2003, at or near the up-dip limit of the locked portion of the subduction thrust, and propagated seaward over the course of about 10 days to the accretionary prism toe along the decollement separating the prism and underthrust section. Deformational events like this may serve incrementally to relieve stress locally along the subduction thrust and to load neighboring areas. In addition to demonstrating that the prism is far from inactive during interseismic intervals, the observations may also provide a small-amplitude analog for strain and hydrologic response at the time of great subduction earthquakes.