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Results for “Journal of Geophysical Research- Oceans”

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241 records · Page 14Linked to original sources
USGS2012

Tidal triggering of low frequency earthquakes near Parkfield, California: Implications for fault mechanics within the brittle-ductile transition

Studies of nonvolcanic tremor (NVT) have established the significant impact of small stress perturbations on NVT generation. Here we analyze the influence of the solid earth and ocean tides on a catalog of ∼550,000 low frequency earthquakes (LFEs) distributed along a 150 km section of the San Andreas Fault centered at Parkfield. LFE families are identified in the NVT data on the basis of waveform similarity and are thought to represent small, effectively co-located earthquakes occurring on brittle asperities on an otherwise aseismic fault at depths of 16 to 30 km. We calculate the sensitivity of each of these 88 LFE families to the tidally induced right-lateral shear stress (RLSS), fault-normal stress (FNS), and their time derivatives and use the hypocentral locations of each family to map the spatial variability of this sensitivity. LFE occurrence is most strongly modulated by fluctuations in shear stress, with the majority of families demonstrating a correlation with RLSS at the 99% confidence level or above. Producing the observed LFE rate modulation in response to shear stress perturbations requires low effective stress in the LFE source region. There are substantial lateral and vertical variations in tidal shear stress sensitivity, which we interpret to reflect spatial variation in source region properties, such as friction and pore fluid pressure. Additionally, we find that highly episodic, shallow LFE families are generally less correlated with tidal stresses than their deeper, continuously active counterparts. The majority of families have weaker or insignificant correlation with positive (tensile) FNS. Two groups of families demonstrate a stronger correlation with fault-normal tension to the north and with compression to the south of Parkfield. The families that correlate with fault-normal clamping coincide with a releasing right bend in the surface fault trace and the LFE locations, suggesting that the San Andreas remains localized and contiguous down to near the base of the crust. The deep families that have high sensitivity to both shear and tensile normal stress perturbations may be indicative of an increase in effective fault contact area with depth. Synthesizing our observations with those of other LFE-hosting localities will help to develop a comprehensive understanding of transient fault slip below the “seismogenic zone” by providing constraints on parameters in physical models of slow slip and LFEs.

California↗
USGS2012

Earthquakes in the Orozco transform zone: seismicity, source mechanisms, and tectonics

As part of the Rivera Ocean Seismic Experiment, a network of ocean bottom seismometers and hydrophones was deployed in order to determine the seismic characteristics of the Orozco transform fault in the central eastern Pacific. We present hypocentral locations and source mechanisms for 70 earthquakes recorded by this network. All epicenters are within the transform region of the Orozco Fracture Zone and clearly delineate the active plate boundary. About half of the epicenters define a narrow line of activity parallel to the spreading direction and situated along a deep topographic trough that forms the northern boundary of the transform zone (region 1). Most focal depths for these events are very shallow, within 4 km of the seafloor; several well-determined focal depths, however, are as great as 7 km. No shallowing of seismic activity is observed as the rise-transform intersection is approached; to the contrary, the deepest events are within 10 km of the intersection. First motion polarities for most of the earthquakes in region 1 are compatible with right-lateral strike slip faulting along a nearly vertical plane, striking parallel to the spreading direction. Another zone of activity is observed in the central part of the transform (region 2). The apparent horizontal and vertical distribution of activity in this region is more scattered than in the first, and the first motion radiation patterns of these events do not appear to be compatible with any known fault mechanism. Pronounced lateral variations in crustal velocity structure are indicated for the transform region from refraction data and measurements of wave propagation directions. The effect of this lateral heterogeneity on hypocenters and fault plane solutions is evaluated by tracing rays through a three-dimensional velocity grid. While findings for events in region 1 are not significantly affected, in region 2, epicentral mislocations of up to 10 km and azimuthal deflections of up to 45° may result from assuming a laterally homogeneous velocity structure. When corrected for the effects of lateral heterogeneity, the epicenters and fault plane solutions for earthquakes in region 2 are compatible with predominantly normal faulting along a topographic trough trending NW–SE; the focal depths, however, are poorly constrained. These results suggest an en echelon spreading center or leaky transform regime in the central transform region.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

An ocean bottom seismometer study of shallow seismicity near the Mid- America Trench offshore Guatemala

Five ocean bottom seismometers recorded seismicity near the Mid-America Trench offshore Guatemala for 27 days in 1979. The array was emplaced in the lower slope region, just above the topographic trench, in the area investigated during Deep Sea Drilling Project legs 67 and 84. Approximately 170 events were recorded by three or more seismometers, and almost half were located with statistical hypocentral errors of less than 10 km. Most epicenters were located immediately landward of the trench axis, and many were further confined to a zone northwest of the array. In terms of depth, most events were located within the subducting Cocos plate rather than in the overlying plate or at the plate-plate boundary. Their apparent concentration in the lower crust and upper mantle may suggest that the upper crust does not have the strength to support earthquake-generating stresses. The data permit construction of a magnitude-duration scale, calibrated with m b magnitudes for events located by the World-Wide Standard Seismograph Network (WWSSN) and recorded by our array and by the network recording foreshocks and aftershocks of the 1979 Petatlan earthquake. Most magnitudes ranged between 3.0 and 4.0 m b , and the threshold magnitude of locatable events was about 2.8 m b . Two distinct composite focal mechanisms were determined. One appears to indicate high-angle reverse faulting in the subducting plate, in a plane parallel to trench axis strike. The other, constructed for some earthquakes in the zone northwest of the array, seems to show normal faulting along possible fault planes oriented quasi-perpendicular to the trench axis. The normal faulting is consistent with the segmentation of the Cocos plate that has been proposed from land evidence. Such segmentation might be evidenced offshore by normal faulting along planes subperpendicular to trench strike. Alternatively, the seismicity zone and associated normal faulting mechanism may be the subsurface expression of the tectonics responsible for the San Jose Canyon, a prominent submarine canyon located farther upslope. Finally, projection of our seismicity sample and of well-located WWSSN events from 1954 to 1980 onto a plane perpendicular to the trench axis shows a distinct gap between the shallow seismicity located by our array, and the deeper Wadati-Benioff zone seismicity located by the WWSSN. We tentatively ascribe this gap to inadequate sampling, but we suggest that it requires further investigation.

Journal of Geophysical Research Solid Earth↗
USGS2012

Origin of Hawaiian tholeiite: A metasomatic model

Two voluminous magma types generated in the mantle underlying the Pacific plate are mid‐ocean ridge tholeiite (MORB) erupted at the East Pacific Rise spreading center and Hawaiian tholeiite (HT) erupted above the Hawaiian hot spot or melting anomaly. MORB has low initial 87 Sr/ 86 Sr ratios and low amounts of all incompatible trace elements including rare earths; chondrite‐normalized patterns are depleted in light rare earths. HT, by contrast, has higher initial 87 Sr/ 86 Sr and higher amounts of incompatible trace elements; chondrite‐nor‐malized patterns are enriched in the middle and light rare earths. HT is generally poorer in CaO and Al 2 O 3 and much richer in total iron and TiO 2 compared with MORB having the same MgO content. Primary magma compositions for the two volcanic systems are calculated in Fe‐Mg equilibrium with residual olivine (Fo 92 ). MORB is generated by partial melting of a trace element depleted Iherzolite source leaving a residual assemblage dominated by olivine and orthopyroxene. The percentage of partial melting for a primary magma containing 15% MgO is calculated to be 35–42% in a source mantle having a heavy rare earth content of 3×chondrite and 33–35% MgO. HT, represented by Kilauea tholeiite, is generated by partial melting of a mixture of unmelted and residual mantle for MORB which has been modified by metasomatic addition of a nephelinitic fluid, amphibole, and minor amounts of apatite and Fe‐bearing phases such as sulfide and magnetite/ilmenite. This model yields a picritic magma in equilbrium with magnesian dunite at high (>40%) degrees of partial melting. The source also has 35% MgO before partial melting. Melting in both systems in polyvariant and not controlled by lower‐temperature invariant equilibria. The low‐velocity zone is considered to be the source of metasomatic fluids that are driven upward into the lowermost lithosphere in response to a thermal plume. Picritic primary magmas are produced by shear melting, localized in the zone of thinned and metasomatized lithosphere beneath Hawaii. Melt extraction is rapid and episodic at intervals of months to decades; magma is not stored in the mantle but passes upward to a plexus of storage reservoirs located 2–6 km beneath the surface of Kilauea. Kilauea primary magmas fractionate olivine during upward transport to reach bulk compositions of 13–14% MgO in storage. Different magma batches erupted to the surface, distinguished by different major and minor element compositons compared at similar MgO content, represent combinations of differing degrees of metasomatic enrichment, differing degrees of partial melting, and some effects of premelting mantle heterogeneity.

Hawaii↗
USGS2012

The mechanics and three-dimensional internal structure of active magmatic systems: Kilauea volcano, Hawaii

Interpretation of abundant seismic data suggests that Kilauea's primary conduit within the upper mantle is concentrically zoned to about 34-km depth. This zoned structure is inferred to contain a central core region of relatively higher permeability, surrounded by numerous dikes that are in intermittent hydraulic communication with each other and with the central core. During periods of relatively high magma transport, the entire cross section of the conduit is utilized. During periods of relatively low to moderate transport, however, only the central core is active. As the conduit penetrates the oceanic crust and enters the volcanic shield, it simultaneously supplies the deeper sections of the rift zones (6-to 10-km depth) and the roots of the summit reservoir with picritic magma. The rift zones at depth are inferred to be almost wholly molten and to possess a high degree of fluid continuity from Heiheiahulu in the East Rift Zone, 45 km westward through the roots of the summit magma reservoir, and well into the Southwest Rift Zone. Higher in the shield, the subcaldera magma reservoir and the shallow rift zones occupy the 2-to 4-km depth interval. Summit-differentiated olivine tholeiite (ρ ≈ 2.62 g cm −3 ) is periodically injected laterally along a horizon of neutral buoyancy within the rift zones, where the density of the magma is just balanced by the in situ density of the shield (Ryan, 1987 a , b ). Deep rift zone intrusions push seaward the deep tectonic blocks of the volcano's south flank. Shallow rift intrusions build a sheeted dike complex, inferred to be in isostatic equilibrium with the higher-density deep rift cores below. General finite element analyses are presented for the deformation and stress fields surrounding such dikes in the horizontal and vertical planes. The dike tip in two and three dimensions is surrounded by a tubular core of tensile (σ 1 , σ 2 ) and shear stress (τ max ). The displacement field is characterized by counterrotating cells on either side of the dike tip which, in vertical orientation, produce the characteristic subsidence above the dike complex, with uplift on either side, forming a ridge-trough-ridge structure. A finite element model of Kilauea's shield computes the displacement fields and principal stress (σ 1 ) distributions resulting from intrusive activity on each or both of the rift zones. Within the summit region, tensile stress lobes produced by the three-dimensional upward extension of the intrusions superpose constructively to produce calderawide regimes of tensile stress, conducive to caldera development. Parametric studies of (1) intrusion in the East Rift Zone only, (2) intrusion in the Southwest Rift Zone only, and (3) intrusion in both rift zones demonstrate their unique kinematic contributions. For case 1, the caldera undergoes a counterclockwise rotation (torque up state) conducive to the development of rightstepping en echelon eruptive fissures, as exemplified by the August 14, 1971, eruption. For case 2, the caldera undergoes a clockwise rotation (torque down state) conducive to the development of left-stepping eruptive fissures, as occurred during the December 31, 1974, eruption. For case 3, the caldera substructure is driven due southward, producing the southward migration of the upper portions of the summit magma reservoir.

Journal of Geophysical Research Solid Earth↗
USGS2012

Evolution of deep structure along the trans-Alaska crustal transect, Chugach Mountains and Copper River Basin, southern Alaska

One of the most important results of the Trans-Alaska Crustal Transect investigations is the discovery that more than one third of the North American plate in southern Alaska (Chugach Mountains and Copper River basin) consists of tectonically underplated oceanic lithosphere. In southern Alaska, exposed accreted tectonostratigraphic terranes, include, from south to north along the transect, the Prince William, Chugach, Peninsular, and Wrangellia terranes. Chief results from seismic refraction data include the following: (1) A sequence of layers of low and high velocity (5.7–7.8 km/s), more than 10 km thick, dips gently northward, with its top at the surface in the southernmost Chugach terrane and at more than 20-km depth beneath the southern Peninsular terrane. A high-velocity layer at the top of this sequence corresponds to metabasalt in the southernmost Chugach terrane. This sequence is interpreted to be tectonically underplated fragments of the Kula plate and its sedimentary overburden. (2) An intermediate-velocity layer (6.35–6.5 km/s) at 9-km depth beneath both the northern Chugach and southern Peninsular terranes appears to extend without offset across the deep projection of the suture between these two terranes, the Border Ranges fault system. (3) A crustal “root” between depths of 19 and 57 km beneath the northern Peninsular and Wrangellia terranes appears to extend without offset across the deep projection of the suture between these two terranes, the West Fork fault system. This crustal root also appears to abut the tectonically underplated sequence to the south. Unfortunately, in results 2 and 3 the apparent crosscutting and abutting relationships can not be unambiguously resolved. Three scenarios, A, B, and C, have been offered to explain the evolution of this structure. Scenarios A and B differ in interpretation of the depth extent of the northern Chugach and southern Peninsular terranes. In scenario A these terranes extend to only 9-km depth, where they rest on an unknown middle crust, interpreted as a deeper part of the Peninsular terrane. In this scenario, the northern Chugach and southern Peninsular terrenes moved landward as a tectonic wedge, during tectonic underplating in the latest Cretaceous or early Tertiary, and uplifted the upper crust of the Peninsular terrane to the north. In scenario B, the northern Chugach and southern Peninsular terranes extend to 20-km depth. In this scenario, they were compressed but not detached and displaced during accretion. Scenarios A, B, and C differ in interpretation of the deep crustal root that appears to abut the tectonically underplated sequence. Since the Peninsular and Wrangellia terranes were at or below sea level prior to the mid-Cretaceous, the root was probably emplaced in mid-Cretaceous or younger times. In scenario A the root is lower crust of North America that moved southward during tectonic underplating of the Kula plate. In scenario B the root is, in part, tectonically underplated rocks similar to the Kula plate sequence to the south. In scenario C the root is, in part, magmatically underplated rocks.

Journal of Geophysical Research↗
USGS2009

Is the track of the Yellowstone hotspot driven by a deep mantle plume? - Review of volcanism, faulting, and uplift in light of new data

Geophysical imaging of a tilted mantle plume extending at least 500 km beneath the Yellowstone caldera provides compelling support for a plume origin of the entire Yellowstone hotspot track back to its inception at 17 Ma with eruptions of flood basalts and rhyolite. The widespread volcanism, combined with a large volume of buoyant asthenosphere, supports a plume head as an initial phase. Estimates of the diameter of the plume head suggest it completely spanned the upper mantle and was fed from sources beneath the transition zone, We consider a mantle–plume depth to at least 1,000 km to best explain the large scale of features associated with the hotspot track. The Columbia River–Steens flood basalts form a northward-migrating succession consistent with the outward spreading of a plume head beneath the lithosphere. The northern part of the inferred plume head spread (pancaked) upward beneath Mesozoic oceanic crust to produce flood basalts, whereas basalt melt from the southern part intercepted and melted Paleozoic and older crust to produce rhyolite from 17 to 14 Ma. The plume head overlapped the craton margin as defined by strontium isotopes; westward motion of the North American plate has likely “scraped off” the head from the plume tail. Flood basalt chemistries are explained by delamination of the lithosphere where the plume head intersected this cratonic margin. Before reaching the lithosphere, the rising plume head apparently intercepted the east-dipping Juan de Fuca slab and was deflected ~ 250 km to the west; the plume head eventually broke through the slab, leaving an abruptly truncated slab. Westward deflection of the plume head can explain the anomalously rapid hotspot movement of 62 km/m.y. from 17 to 10 Ma, compared to the rate of ~ 25 km/m.y. from 10 to 2 Ma. A plume head-to-tail transition occurred in the 14-to-10-Ma interval in the central Snake River Plain and was characterized by frequent (every 200–300 ka for about 2 m.y. from 12.7 to 10.5 Ma) “large volume (> 7000 km 3 )”, and high temperature rhyolitic eruptions (> 1000 °C) along a ~ 200–km-wide east–west band. The broad transition area required a heat source of comparable area. Differing characteristics of the volcanic fields here may in part be due to variations in crustal composition but also may reflect development in differing parts of an evolving plume where the older fields may reflect the eruption from several volcanic centers located above very large and extensive rhyolitic magma chamber(s) over the detached plume head while the younger fields may signal the arrival of the plume tail intercepting and melting the lithosphere and generating a more focused rhyolitic magma chamber. The three youngest volcanic fields of the hotspot track started with large ignimbrite eruptions at 10.21, 6.62, and 2.05 Ma. They indicate hotspot migration N55° E at ~ 25 km/m.y. compatible in direction and velocity with the North American Plate motion. The Yellowstone Crescent of High Terrain (YCHT) flares outward ahead of the volcanic progression in a pattern similar to a bow-wave, and thus favors a sub-lithospheric driver. Estimates of YCHT-uplift rates are between 0.1 and 0.4 mm/yr. Drainage divides have migrated northeastward with the hotspot. The Continental Divide and a radial drainage pattern now centers on the hotspot. The largest geoid anomaly in the conterminous U.S. is also centered on Yellowstone and, consistent with uplift above a mantle plume. Bands of late Cenozoic faulting extend south and west from Yellowstone. These bands are subdivided into belts based both on recency of offset and range-front height. Fault history within these belts suggests the following pattern: Belt I — starting activity but little accumulated offset; Belt II — peak activity with high total offset and activity younger than 14 ka; Belt III — waning activity with large offset and activity younger than 140 ka; and Belt IV — apparently dead on substantial range fronts (south side of the eastern Snake River Plain only). These belts of fault activity have migrated northeast in tandem with the adjacent hotspot volcanism. On the southern arm of the YCHT, fault activity occurs on the inner, western slope consistent with driving by gravitational potential energy, whereas faulting has not started on the eastern, outer, more compressional slope. Range fronts increase in height and steepness northeastward along the southern-fault band. Both the belts of faulting and the YCHT are asymmetrical across the volcanic hotspot track, flaring out 1.6 times more on the south than the north side. This and the southeast tilt of the Yellowstone plume may reflect southeast flow of the upper mantle.

Wyoming↗