Beschreibung: We report the results of a two-dimensional tomographic inversion of marine seismic refraction data from an array of ocean-bottom seismographs (OBSs), which produced an image of the crustal structure along the axial valley of the ultraslow spreading Mid-Cayman Spreading Center (MCSC). The seismic velocity model shows variations in the thickness and properties of the young oceanic crust that are consistent with the existence of two magmatic-tectonic segments along the 110 km long spreading center. Seismic wave speeds are consistent with exhumed mantle at the boundary between these two segments, but changes in the vertical gradient of seismic velocity suggest that volcanic crust occupies most of the axial valley seafloor along the seismic transect. The two spreading segments both have a low-velocity zone (LVZ) several kilometers beneath the seafloor, which may indicate the presence of shallow melt. However, the northern segment also has low seismic velocities (3 km/s) in a thick upper crustal layer (1.5–2.0 km), which we interpret as an extrusive volcanic section with high porosity and permeability. This segment hosts the Beebe vent field, the deepest known high-temperature black smoker hydrothermal vent system. In contrast, the southern spreading segment has seismic velocities as high as 4.0 km/s near the seafloor. We suggest that the porosity and permeability of the volcanic crust in the southern segment are much lower, thus limiting deep seawater penetration and hydrothermal recharge. This may explain why no hydrothermal vent system has been found in the southern half of the MCSC.

Beschreibung: Ahyi is a fully submerged arc volcano in the Northern Mariana Islands, northwestern Pacific Ocean. In April and May 2014, the volcano erupted over a period of 15 days. Results from direction-of-arrival calculations show that underwater sound phases associated with the episode were recorded as far as Wake Island, where a hydrophone triplet array is operated as part of the International Monitoring System. After a 3.5-hr-long sequence of hydroacoustic precursory events, explosive volcanic activity occurred in two distinct, several-days-long bursts, accompanied by a notable decrease in low-frequency arrivals that may indicate a shift in signal source parameters. Acoustic resolution of the hydrophone data supersedes broadband networks by almost 1 order of magnitude, successfully identifying seismic events at Ahyi as low as 2.5 mb. Total radiated acoustic energy of the eruption is estimated at 9.7 1013 J, which suggests that submarine volcanic activity contributed significantly to the ocean soundscape.

Beschreibung: Monowai is an active submarine volcanic center in the Kermadec Arc, Southwest Pacific Ocean. During May 2011, it erupted over a period of 5 days, with explosive activity directly linked to the generation of seismoacoustic T phases. We show, using cross-correlation and time-difference-of-arrival techniques, that the eruption is detected as far as Ascension Island, equatorial South Atlantic Ocean, where a bottom moored hydrophone array is operated as part of the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty Organization. Hydroacoustic phases from the volcanic center must therefore have propagated through the Sound Fixing and Ranging channel in the South Pacific and South Atlantic Oceans, a source-receiver distance of ~15,800 km. We believe this to be the furthest documented range of a naturally occurring underwater signal above 1 Hz. Our findings, which are consistent with observations at regional broadband stations and long-range, acoustic parabolic equation modeling, have implications for submarine volcano monitoring.

Beschreibung: We present 2-D seismic velocity models and coincident multichannel seismic reflection images of the overriding plate and the inter-plate boundary of the Nicaragua convergent margin along two wide-angle seismic profiles parallel and normal to the trench acquired in the rupture area of the 1992 tsunami earthquake. The trench-perpendicular profile runs over a seamount subducting under the margin slope, at the location where seismological observations predict large coseismic slip. Along this profile, the igneous basement shows increasing velocity both with depth and away from the trench, reflecting a progressive decrease in upper-plate rock degree of fracturing. Upper mantle-like velocities are obtained at approximate to 10 km depth beneath the fore-arc Sandino basin, indicating a shallow mantle wedge. A mismatch of the inter-plate reflector in the velocity models and along coincident multichannel seismic profiles under the slope is best explained by approximate to 15% velocity anisotropy, probably caused by subvertical open fractures that may be related to fluid paths feeding known seafloor seepage sites. The presence of a shallow, partially serpentinized mantle wedge, and the fracture-related anisotropy are supported by gravity analysis of velocity-derived density models. The downdip limit of inter-plate seismicity occurs near the tip of the inferred mantle wedge, suggesting that seismicity could be controlled by the presence of serpentinite group minerals at the fault gouge. Near the trench, the inferred local increase of normal stress produced by the subducting seamount in the plate boundary may have made this fault segment unstable during earthquake rupture, which could explain its tsunamigenic character.

Beschreibung: Extension of the continental lithosphere leads to the formation of rift basins or rifted continental margins if breakup occurs. Seismic investigations have repeatedly shown that conjugate margins have asymmetric tectonic structures and different amount of extension and crustal thinning. Here we compare two coincident wide-angle and multichannel seismic profiles across the northern Tyrrhenian rift system sampling crust that underwent different stages of extension from north to south and from the flanks to the basin center. Tomographic inversion reveals that the crust has thinned homogeneously from ~24 km to ~17 km between the Corsica Margin and the Latium Margin implying a β factor of ~1.3–1.5. On the transect 80 km to the south, the crust thinned from ~24 km beneath Sardinia to a maximum of ~11 km in the eastern region near the Campania Margin (β factor of ~2.2). The increased crustal thinning is accompanied by a zone of reduced velocities in the upper crust that expands progressively toward the southeast. We interpret that the velocity reduction is related to rock fracturing caused by a higher degree of brittle faulting, as observed on multichannel seismic images. Locally, basalt flows are imaged intruding sediment in this zone, and heat flow values locally exceed 100 mW/m2. Velocities within the entire crust range 4.0–6.7 km/s, which are typical for continental rocks and indicate that significant rift-related magmatic underplating may not be present. The characteristics of the pre-tectonic, syn-tectonic and post-tectonic sedimentary units allow us to infer the spatial and temporal evolution of active rifting. In the western part of the southern transect, thick postrift sediments were deposited in half grabens that are bounded by large fault blocks. Fault spacing and block size diminish to the east as crustal thinning increases. Recent tectonic activity is expressed by faults cutting the seafloor in the east, near the mainland of Italy. The two transects show the evolution from the less extended rift in the north with a fairly symmetric conjugate structure to the asymmetric margins farther south. This structural evolution is consistent with W-E rift propagation and southward increasing extension rates.

Beschreibung: We investigate potential relations between variations in seafloor relief and age of the incoming plate and interplate seismicity. Westward from Osa Peninsula in Costa Rica, a major change in the character of the incoming Cocos Plate is displayed by abrupt lateral variations in seafloor depth and thermal structure. Here a Mw 6.4 thrust earthquake was followed by three aftershock clusters in June 2002. Initial relocations indicate that the main shock occurred fairly trenchward of most large earthquakes along the Middle America Trench off central Costa Rica. The earthquake sequence occurred while a temporary network of OBH and land stations ∼80 km to the northwest were deployed. By adding readings from permanent local stations, we obtain uncommon P wave coverage of a large subduction zone earthquake. We relocate this catalog using a nonlinear probabilistic approach within both, a 1-D and a 3-D P wave velocity models. The main shock occurred ∼25 km from the trench and probably along the plate interface at 5–10 km depth. We analyze teleseismic data to further constrain the rupture process of the main shock. The best depth estimates indicate that most of the seismic energy was radiated at shallow depth below the continental slope, supporting the nucleation of the Osa earthquake at ∼6 km depth. The location and depth coincide with the plate boundary imaged in prestack depth-migrated reflection lines shot near the nucleation area. Aftershocks propagated downdip to the area of a 1999 Mw 6.9 sequence and partially overlapped it. The results indicate that underthrusting of the young and buoyant Cocos Ridge has created conditions for interplate seismogenesis shallower and closer to the trench axis than elsewhere along the central Costa Rica margin.

Beschreibung: We present results from a seismic refraction and wide-angle experiment surveying an oceanic core complex on the Mid-Atlantic Ridge at 22°19′N. Oceanic core complexes are settings where petrological sampling found exposed lower crustal and upper mantle rocks, exhumed by asymmetric crustal accretion involving detachment faulting at magmatically starved ridge sections. Tomographic inversion of our seismic data yielded lateral variations of P wave velocity within the upper 3 to 4 km of the lithosphere across the median valley. A joint modeling procedure of seismic P wave travel times and marine gravity field data was used to constrain crustal thickness variations and the structure of the uppermost mantle. A gradual increase of seismic velocities from the median valley to the east is connected to aging of the oceanic crust, while a rapid change of seismic velocities at the western ridge flank indicates profound differences in lithology between conjugated ridge flanks, caused by un-roofing lower crust rocks. Under the core complex crust is approximately 40% thinner than in the median valley and under the conjugated eastern flank. Clear PmP reflections turning under the western ridge flank suggest the creation of a Moho boundary and hence continuous magmatic accretion during core complex formation.

Beschreibung: Active ridge propagation frequently occurs along spreading ridges and profoundly affects ridge crest segmentation over time. The mechanisms controlling ridge propagation, however, are poorly understood. At the slow spreading Mid-Atlantic Ridge at 21.5°N a seismic refraction and wide-angle reflection profile surveyed the crustal structure along a segment controlled by rapid ridge propagation. Tomographic traveltime inversion of seismic data suggests that the crustal structure along the ridge axis is controlled by melt supply; thus, crust is thickest, 8 km, at the domed segment center and decreases in thickness toward both segment ends. However, thicker crust is formed in the direction of ridge propagation, suggesting that melt is preferentially transferred toward the propagating ridge tip. Further, while seismic layer 2 remains constant along axis, seismic layer 3 shows profound changes in thickness, governing variations in total crustal thickness. This feature supports mantle upwelling at the segment center. Thus, fluid basaltic melt is redistributed easily laterally, while more viscose gabbroic melt tends to crystallize and accrete nearer to the locus of melt supply. The onset of propagation seems to have coincided with the formation of thicker crust, suggesting that propagation initiation might be due to changes in the melt supply. After a rapid initiation a continuous process of propagation was established. The propagation rate seems to be controlled by the amount of magma that reaches the segment ends. The strength of upwelling may govern the evolution of ridge segments and hence ultimately controls the propagation length.

Beschreibung: We developed thermal models for the Chile subduction zone along two profiles at 38.2°S and 42°S within the rupture area of the 1960 M = 9.5 Valdivia earthquake and south of the 2010 M = 8.8 Maule earthquake. The age difference of the subducting Nazca Plate has a major impact on the thermal regime, being much younger and hotter in the south. Seafloor heat flow observations confirm this difference but also indicate that in the southern area, heat advection at the outer rise cools the incoming plate. Heat flow values derived from the depth of gas hydrate bottom-simulating reflectors are in general agreement with probe and borehole measurements. The positions where the plate interface reaches temperatures of 100–150°C and 350–450°C differ between the two profiles. If these temperatures control the updip and downdip limits of the interplate seismogenic zone, the seismogenic zone widens and shifts landward to greater depths from south to north. Observed microseismicity, however, seems to fade at temperatures much lower than 350–450°C. This discrepancy can be explained in three alternative ways: (1) deformation in a thick subduction channel controls the seismic/aseismic transition; (2) microseismicity recorded over a limited time period does not represent the rupture depth of large interface earthquakes; or (3) the serpentinized mantle wedge controls the downdip limit.

Beschreibung: Water transported by slabs into the mantle at subduction zones plays key roles in tectonics, magmatism, fluid and volatiles fluxes, and most likely in the chemical evolution of the Earth's oceans and mantle. Yet, incorporation of water into oceanic plates before subduction is a poorly understood process. Several studies suggest that plates may acquire most water at subduction trenches because the ocean crust and uppermost mantle there are intensely faulted caused by bending and/or slab pull, and display anomalously low seismic velocities. The low velocities are interpreted to arise from a combination of fluid-filled fractures associated to normal faulting and mineral transformation by hydration. Mantle hydration by transformation of nominally dry peridotite to water-rich serpentinite could potentially create the largest fluid reservoir in slabs and is therefore the most relevant for the transport of water in the deep mantle. The depth of fracturing by normal-fault earthquakes is usually not well constrained, but could potentially create deep percolation paths for water that might hydrate up to tens of kilometers into the mantle, restrained only by serpentine stability. Yet, interpretation of deep intraplate mineral alteration remains speculative because active-source seismic experiments have sampled only the uppermost few kilometers of mantle, leaving the depth-extent of anomalous velocities and their relation to faulting unconstrained. Here we use a joint inversion of active-source seismic data, and both local and regional earthquakes to map the three dimensional distribution of anomalous velocities under a seismic network deployed at the trench seafloor. We found that anomalous velocities are restrained to the depth of normal-fault micro-earthquake activity recorded in the network, and are considerably shallower than either the rupture depth of teleseismic, normal-fault earthquakes, or the limit of serpentine stability. Extensional micro-earthquakes indicate that each fault in the region slips every 2–3 months which may facilitate regular water percolation. Deeper, teleseismic earthquakes are comparatively infrequent, and possibly do not cause significant fracturing that remains open long enough to promote alteration detectable with our seismic study. Our results show that the stability field of serpentine does not constrain the depth of potential mantle hydration.