Description: A seismic refraction profile recorded along the geologic strike of the Chugach Mountains in southern Alaska shows three upper crustal high-velocity layers (6.9, 7.2, and 7.6 km/s) and a unique pattern of strongly focussed echelon arrivals to a distance of 225 km. The group velocity of the ensemble of echelon arrivals is 6.4 km/s. Modeling of this profile with the reflectivity method reveals that the echelon pattern is due to peg-leg multiples generated from with a low-velocity zone between the second and third upper crustal high-velocity layers. The third high-velocity layer (7.6 km/s) is underlain at 18 km depth by a pronounced low-velocity zone that produces a seismic shadow wherein zone peg-leg multiples are seen as echelon arrivals. The interpretation of these echelon arrivals as multiples supersedes an earlier interpretation which attributed them to successive primary reflections arising from alternating high- and low-velocity layers. Synthetic seismogram modeling indicates that a low-velocity zone with transitional upper and lower boundaries generates peg-leg multiples as effectively as one with sharp boundaries. No PmP or Pn arrivals from the subducting oceanic Moho at 30 km depth beneath the western part of the line are observed on the long-offset (90-225 km) data. This may be due to a lower crustal waveguide whose top is the high-velocity (7.6 km/s) layer and whose base is the Moho. A deep (~54 km) reflector is not affected by the waveguide and has been identified in the data. Although peg-leg multiples have been interpreted on some long-range refraction profiles that sound to upper mantle depths, the Chugach Mountains profile is one of the few crustal refraction profiles where peg-leg multiples are clearly observed. This study indicates that multiple and converted phases may be more important in seismic refraction/wide-angle reflection profiles than previously recognized.

Description: During a seismic reflection survey conducted by the California Consortium for Crustal Studies in the Basin and Range Province west of the Whipple Mountains, SE California, a piggyback experiment was carried out to collect intermediate offset data (12–31 km). These data were obtained by recording the Vibroseis energy with a second, passive recording array, deployed twice at fixed positions at opposite ends of the reflection lines. The reflection midpoints fall into a 3-km-wide and 15-km-long region in Vidal Valley, roughly parallel to a segment of one of the near-vertical reflection profiles. This data set makes three unique contributions to the geophysical study of this region. (1) From forward modeling of the observed travel times using ray-tracing techniques, a shallow layer with velocities ranging from 6.0 to 6.5 km/s was found. This layer dips to the south from 2-km depth near the Whipple Mountains to a depth of 5-km in Rice Valley. These depths correspond closely to the westward projection of the Whipple detachment fault, which is exposed 1 km east of the near-vertical profiles in the Whipple Mountains. (2) On the near-vertical profile, the reflections from the mylonitically deformed lower plate at upper crustal and mid crustal depths are seen to cease underneath a sedimentary basin in Vidal Valley. However, the piggyback data, which undershoot this basin, show that these reflections are continuous beneath the basin. Thus near-surface energy transmission problems were responsible for the apparent lateral termination of the reflections on the near-vertical reflection profile. (3) The areal distribution of the midpoints allows us to construct a quasi-three-dimensional image on perpendicular profiles; at the cross points we determined the true strike and dip of reflecting horizons. This analysis shows that the reflections from the mylonitically deformed lower plate dip to the southwest westward of the Whipple Mountains and dip to the south southward of the Turtle Mountains. The results of this study support the interpretation of crustal reflectivity in the near-vertical reflection profiles to be related to the mid-Tertiary episode of extension which produced the Whipple metamorphic core complex. This association geometrically suggests a more regionally distributed mechanism for crustal thinning as compared with single detachment fault models.

Description: The southern central Chilean margin at the site of the largest historically recorded earthquake in the Valdivia region, in 1960 (Mw = 9.5), is part of the 5000-km-long active subduction system whose geodynamic evolution is controversially debated and poorly understood. Covering the area between 36° and 40°S, the oceanic crust is segmented by prominent fracture zones. The offshore forearc and its onshore continuation show a complex image with segments of varying geophysical character, and several fault systems active during the past 24 m.y. In autumn 2001, the project SPOC was organized to study the Subduction Processes Off Chile, with a focus on the seismogenic coupling zone and the forearc. The acquired seismic data crossing the Chilean subduction system were gathered in a combined offshore-onshore survey and provide new insights into the lithospheric structure and evolution of active margins with insignificant frontal accretion.

Description: Seismic investigations across the convergent Sunda margin off Indonesia provide a detailed image of the crustal architecture of the Sunda plate boundary. The combined analysis and interpretation of wide-angle and reflection seismic data along two coincident profiles across the subduction zone are complemented by additional lines within the forearc domain, which yield some three-dimensional (3-D) constraints on the velocity-depth structure across the margin. A detailed cross section of the subduction zone is presented, which is confirmed by supplementary gravity modeling. The Sunda convergence zone is a prime example of an accretionary margin, where sediment accretion has led to the formation of a massive accretionary prism, with a total width of 〉110 km between the trench and the forearc basin. It is composed of a frontal wedge which documents ongoing accretion and a fossil part behind the present backstop structure which constitutes the outer high. Moderate seismic velocities derived from wide-angle modeling indicate a sedimentary composition of the outer high. The subducting oceanic slab is traced to a depth of almost 30 km underneath the accretionary prism. The adjacent forearc domain is characterized by a pronounced morphological basin which is underlain by a layer of increased seismic velocities and a shallow upper plate Moho at 16 km depth. We speculate that remnant fragments of oceanic crust might be involved in the formation of this oceanic-type crust found at the leading edge of the upper plate beneath the forearc basin.

Description: A seismic wide-angle section offshore Costa Rica is presented across the boundary between oceanic crust generated at the East Pacific rise (EPR) and at the Galápagos spreading center (GSC) as indicated by magnetic anomalies. This suture, where the Farallon plate broke up ∼23 Ma ago, is marked by pronounced velocity variations throughout the crust including a low-velocity body in the lower crust. This body is well constrained by refracted waves above the inversion zone and by strong PmP reflections from its lower boundary. The distinctness of this body and the local gravity field point to an igneous intrusion rather than serpentinized rock. Typical oceanic crust is found adjacent to the suture zone.

Description: The Cascadia convergent margin is a first-order research target to study the impact of rapid sedimentation processes on the mechanics of frontal subduction zone accretion. The near-trench part of the accretionary prism offshore Washington is affected by strongly increased glacial age sedimentation and fan formation that led to an outstanding Quaternary growth rate with landward vergent thrust faulting that is rarely observed elsewhere in accretionary wedges. Multichannel seismic reflection data acquired on the ORWELL project allows us to study the structure and dynamics of the atypical frontal accretion processes. We performed a kinematical and mechanical analysis of the frontal accretion structures, and developed a dynamic Coulomb-wedge model for the landward-verging backthrust formation. Backthrusting results from heterogeneous diffuse strain accumulation in the mechanically heterogeneous Cascadia basin sediment succession entering the subduction zone, and strain partitioning along a midlevel detachment that is activated by gravitational loading caused by rapid glacial age sedimentation. These complex deformation processes cause the passive “upward” delamination of the upper turbidite beds from the basal pelagic carbonate section similar to triangle-zone formation and passive backthrust wedging in foreland thrust belts caused by rapid burial beneath syntectonic sediment deposits. The deformation mechanism at the tectonic front of the Cascadia margin is an immediate response to the strongly increased late Pleistocene sediment flux rather than to atypical physical boundary conditions as generally thought.

Description: We report measurements made with an ocean bottom array which was operated for 10 days on the Mid-Atlantic Ridge just south of the 5°S transform fault/fracture zone. A total of 148 locatable earthquakes with magnitudes ∼0.5–2.8 were recorded; seismic activity appears to be concentrated within the western half of the median valley. The median valley seismic zone is bounded in along-axis direction by the transform fault to the north and the tip of the axial volcanic ridge to the south. A few scattered events occurred within the inside corner high, on the transform fault, and in the western sidewall close to the segment center. Earthquakes reach a maximum depth of 8 km below the median valley floor and appear to be predominantly in the mantle, although a few crustal earthquakes also occurred. The presence of earthquakes in the mantle indicates that it is not strongly serpentinized. We infer the median valley seismic activity to primarily arise from normal faulting.

Description: The shallow seismogenic portion of subduction zones generates damaging large and great earthquakes. This study provides structural constraints on the seismogenic zone of the Middle America Trench offshore central Costa Rica and insights into the physical and mechanical characteristics controlling seismogenesis. We have located ~300 events that occurred following the MW 6.9, 20 August 1999, Quepos, Costa Rica, underthrusting earthquake using a three-dimensional velocity model and arrival time data recorded by a temporary local network of land and ocean bottom seismometers. We use aftershock locations to define the geometry and characteristics of the seismogenic zone in this region. These events define a plane dipping at 19° that marks the interface between the Cocos Plate and the Panama Block. The majority of aftershocks occur below 10 km and above 30 km depth below sea level, corresponding to 30–35 km and 95 km from the trench axis, respectively. Relative event relocation produces a seismicity pattern similar to that obtained using absolute locations, increasing confidence in the geometry of the seismogenic zone. The aftershock locations spatially correlate with the downdip extension of the oceanic Quepos Plateau and reflect the structure of the main shock rupture asperity. This strengthens an earlier argument that the 1999 Quepos earthquake ruptured specific bathymetric highs on the downgoing plate. We believe that subduction of this highly disrupted seafloor has established a set of conditions which presently limit the seismogenic zone to be between 10 and 35 km below sea level.

Description: New seismic and geodetic data from Costa Rica provide insight into seismogenic zone processes in Central America, where the Cocos and Caribbean plates converge. Seismic data are from combined land and ocean bottom deployments in the Nicoya peninsula in northern Costa Rica and near the Osa peninsula in southern Costa Rica. In Nicoya, inversion of GPS data suggests two locked patches centered at 14 ± 2 and 39 ± 6 km depth. Interplate microseismicity is concentrated in the more freely slipping intermediate zone, suggesting that small interseismic earthquakes may not accurately outline the updip limit of the seismogenic zone, the rupture zone for future large earthquakes, at least over the short (∼1 year) observation period. We also estimate northwest motion of a coastal “sliver block” at 8 ± 3 mm/yr, probably related to oblique convergence. In the Osa region to the south, convergence is orthogonal to the trench. Cocos-Caribbean relative motion is partitioned here, with ∼8 cm/yr on the Cocos-Panama block boundary (including a component of permanent shortening across the Fila Costeña fold and thrust belt) and ∼1 cm/yr on the Panama block–Caribbean boundary. The GPS data suggest that the Cocos plate–Panama block boundary is completely locked from ∼10–50 km depth. This large locked zone, as well as associated forearc and back-arc deformation, may be related to subduction of the shallow Cocos Ridge and/or younger lithosphere compared to Nicoya, with consequent higher coupling and compressive stress in the direction of plate convergence.