Direction Of Plate Convergence Research Articles

Localized extensional tectonics in an overall reverse-faulting regime, Northeast Japan

In Northeast Japan, it has been recognized that trench-normal compressional stresses, aligned in the approximate direction of plate convergence, tend to dominate stress fields over a broad region. However, a particularly notable event was the shallow, normal-faulting earthquake swarms with a T-axis oriented in the E–W or NW–SE directions that occurred immediately after the 2011 Tohoku-Oki earthquake near the Pacific coast in the Southeast Tohoku district. The stress tensor inversion represents the pre-Tohoku-Oki earthquake stress field in this area as a normal-faulting stress regime with the minimum principal horizontal stress oriented in a roughly NW–SE direction. Additionally, the stress regime varies with depth from normal faulting at shallow depths (<15 km) to thrust faulting at greater depths. Seismic tomography and magnetotelluric soundings defined a geophysical anomaly with low seismic velocity and low resistivity clearly visible beneath the swarm activity, strongly supporting the existence of an interconnected network with fluid-filled porosity. The upper boundary of the conductor is in good agreement with an extensional–compressional stress transition zone. A plausible explanation for these drastic changes in the stress regime is upward flexure of the upper crust due to partly anelastic deformation in the weakened lower crust. Additionally, remarkable upwarping and localized extensional tectonics during the late Pleistocene reflect the long-term rheological heterogeneities in the crust beneath the seismic source region.

Differences and similarities in the Cocos–North America and Cocos–Caribbean convergence, as revealed by seismic moment tensors

We investigate the differences and similarities in Cocos–North America and Cocos–Caribbean convergence, reflected by seismicity and seismic moment. We use well-located hypocenters of earthquakes in the convergence margin, as well as within the subducted slab. We sort these data by number of events in the two converging margins, and by their magnitudes. We also use this database to determine an improved geometric model of the subducted slab. We find a shallow-dipping subducting Cocos plate underneath North America and a steeper dip slab under the Caribbean plate. The transition between them appears to be smooth.Centroid Moment tensor solutions indicate that almost all of the thrust-faulting earthquakes along Cocos–North America take place at shallow depths. Normal-faulting events along this margin only take place to depths of 100 km. Thrust- and normal-faulting events take place at all depths along the Cocos–Caribbean margin. Cummulative scalar seismic moment for shallow, thrust-faulting events, is larger along Cocos–North America.T axes of intermediate-depth, normal- and thrust-faulting events show that the subducted Cocos plate is in maximum tension along the direction of maximum dip. Azimuth of earthquake slip vectors for shallow events along the Cocos–North America margin agree well with the direction of plate convergence. They do not agree along the Cocos–Caribbean margin; instead, agreement is found with Cocos–North America relative plate motion.Compensated Linear Vector Dipole (CLVD) ratio, which measures how different a seismic source is from a pure double-couple, along both margins is inversely proportional to scalar seismic moment, indicating that for larger magnitudes rupture is closer to a double-couple mechanism than at smaller moments.

Deformation of erosive and accretive forearcs during subduction of migrating and non‐migrating aseismic ridges: Results from 3‐D finite element models and application to the Central American, Peruvian, and Ryukyu margins

AbstractSubduction of aseismic oceanic ridges causes considerable uplift and deformation of the upper plate and may lead, for example, to the indentation of the forearc, the formation of marine terraces, or distinct fault patterns in the upper plate. Depending on the orientation of the ridge relative to the plate convergence direction, the ridge may either be stationary or migrate along the margin. Here we use three‐dimensional numerical models to investigate the tectonic evolution of forearcs affected by ridge subduction. In different experiments, we distinguish between migrating/non‐migrating ridges and accretive/erosive margins, respectively. Our results reveal that displacement and strain fields above migrating and non‐migrating ridges are asymmetric with respect to the ridge axis unless both ridge and plate convergence direction are perpendicular to the trench. As the asymmetric deformation pattern shifts along the margin through time, uplift caused by the underthrusting ridge is followed by subsidence when the ridge crest passed by, and regions initially experiencing shortening may subsequently undergo extension and vice versa. If the forearc comprises an accretionary prism, the ridge‐induced reentrant is larger than those in models with erosive forearcs and strain localizes in the frontal part of the wedge. Additional models with a setup adjusted to the Cocos and Gagua Ridges provide constraints on the onset of their subduction at the Central American and Ryukyu margins at ~2 Ma and 1 Ma, respectively. Displacement and strain fields from a model for the Nazca Ridge collision zone show good agreement with geological data from marine terraces and Quaternary faulting.

Geology and geomorphology of Masol paleonto-archeological site, Late Pliocene, Chandigarh, Siwalik Frontal Range, NW India

The Masol paleonto-archeological site is located in the Siwalik Frontal Range in the north of Chandigarh (Punjab, NW India). Many fossils and stone tools can be observed in the colluviums that overlap the present topography constituted by Late Pliocene continental sediments. The Masol paleonto-archeological site is located in the center of a trenched anticline compatible with the direction of plate convergence between India and Eurasia. Erosion processes are very actives and efficient in the area. Around 80m of vertical erosion occurred in the Patiali Rao valley and regressive erosion incised the Siwalik Hills for ∼12km. At least two levels of fluvial terraces are visible in the Patiali Rao valley and laterally apart the Pichhli River. River bank erosion, gullies, collapses, cavities, regressive erosion, landslides and in situ disaggregation have been observed and are responsible of the significant excavation of the anticline. Substrate composed of sand, sandstone and silt is very erodible in case of heavy rain. Slope destabilizations by seasonal monsoon are responsible of a large part of the colluviums overlapping the present slopes. Some colluviums could be due also to in situ disaggregation of sandstone formations. Due to the very active erosion and to their position on the topography, we believe that these colluviums are very recent.

Shear-Wave Velocity Structure in the Vicinity of the 2012 Mw 7.8 Haida Gwaii Earthquake from Receiver Function Inversion

Abstract The thrust mechanism of the 2012 M w 7.8 Haida Gwaii earthquake suggests convergence across the transpressive Pacific–North America plate boundary in the region is accommodated by underthrusting, with important consequences for seismic‐ and tsunami‐hazard analysis. This article investigates the crustal structure and extent of subduction beneath Haida Gwaii by nonlinear inversion of receiver function data processed from teleseismic recordings at five land‐based seismograph stations. Three of these stations were deployed since the 2012 earthquake to extend coverage to the southeast and have not been analyzed previously. The inversions provide estimates of the shear‐wave velocity structure beneath much of Moresby Island. Results indicate a positive velocity contrast at approximately 18–26 km depth, interpreted as a shallow continental Moho. A 12–17 km thick low shear‐wave velocity zone is also identified, which increases in depth from ∼25 to 42 km along the direction of plate convergence, which is interpreted as subducting oceanic material. These results provide the first evidence that the subducting oceanic plate extends beneath the entirety of Moresby Island.

Nonvolcanic tremor locations and mechanisms in Guerrero, Mexico, from energy‐based and particle motion polarization analysis

AbstractWe introduce the Tremor Energy and Polarization (TREP) method, which jointly determines the source location and focal mechanism of sustained nonvolcanic tremor (NVT) signals. The method minimizes a compound cost function by means of a grid search over a three‐dimensional hypocentral lattice. Inverted metrics are derived from three NVT observables: (1) the energy spatial distribution, (2) the energy spatial derivatives, and (3) the azimuthal direction of the particle motion polarization ellipsoid. To assess the tremor sources, TREP assumes double‐couple point dislocations with frequency‐dependent quality factors (Q) in a layered medium. Performance and resolution of the method is thoroughly assessed via synthetic inversion tests with random noise, where the “observed” data correspond to an NVT‐like finite difference (FD) model we introduce. The FD tremor source is composed of hundreds of quasi‐dynamic penny‐shaped cracks governed by a time‐weakening friction law. In agreement with previous works, epicentral locations of 26 NVTs in Guerrero are separated in two main groups, one between 200 and 230 km from the trench, and another at about 170 km. However, unlike earlier investigations, most NVT hypocenters concentrate at 43 km depth near the plate interface and have subparallel rake angles to the Cocos plate convergence direction. These locations have uncertainties of ~5 km in the three components and are consistent with independent results for low‐frequency earthquakes in the region, supporting their common origin related to slip transients in the plate interface. Our results also suggest the occurrence of NVT sources within the slab, ~5 km below the interface.

Seismic velocity structure and anisotropy of the Alaska subduction zone based on surface wave tomography

AbstractSouthcentral Alaska is a complex tectonic region that transitions from subduction of Pacific crust to flat slab subduction—and collision—of overthickened Yakutat crust. Because much of the Yakutat crust has been subducted, seismic imaging is needed in order to understand the crustal and upper mantle structural framework for this active tectonic setting. Here we use teleseismic Rayleigh waves to image large‐scale variations in shear wave structure. Our imaging technique employs a two‐plane wave representation with finite frequency sensitivity kernels. Our 3‐D isotropic model reveals several features: the subducting Pacific/Yakutat slab, slow wave speeds characterizing the onshore Yakutat collision zone, slow wave speeds of the Wrangell subduction zone, and a deep tomographic contrast at the eastern edge of the Pacific/Yakutat slab. We produce anisotropic phase velocity maps that exhibit variations in the fast direction of azimuthal anisotropy. These maps show the dominance of the Yakutat slab on the observed pattern of anisotropy. West of the Yakutat slab the fast directions are approximately aligned with the plate convergence direction. In the region of the Yakutat slab the pattern is more complicated. Along the margins of the slab the fast directions are roughly parallel to the margins. We identify notable differences and similarities with published SKS splitting measurements. Integrative modeling using 3‐D anisotropy models and different seismic measurements will be needed in order to establish a detailed 3‐D anisotropic velocity model for Alaska. This study provides a large‐scale starting point for such an effort.

Changes in the stress field after the 2008 M7.2 Iwate‐Miyagi Nairiku earthquake in northeastern Japan

AbstractIn order to investigate changes in the stress field following the 2008 M7.2 Iwate‐Miyagi Nairiku earthquake in NE Japan, we determined many focal mechanisms within the focal area, using data from both the dense aftershock observation network deployed just after the main shock and other temporary and routinely operated stations in the surrounding area. Applying stress tensor inversions to these focal mechanism data, we estimated the detailed spatial distribution of the principal stress orientations in this event's source area. Estimated orientations of the maximum principal stress (σ1) axes differ significantly before and after the main shock. Before the main shock, the σ1 axis is oriented WNW‐ESE across the entire studied area, which is parallel to the plate convergence direction. After the main shock, however, the orientation of the σ1 axis changed significantly and showed remarkable spatial variation. In the northern and southern parts of the aftershock area, located relatively far from the main shock large slip area, the σ1 axes are oriented WNW‐ESE, similar to those before the main shock. In contrast, to the east and west of the central part of the aftershock area, the σ1 axes are oriented N‐S and NE‐SW, respectively. This characteristic spatial pattern of σ1 axis orientations is approximately consistent with that of static stress change associated with the main shock rupture, which strongly suggests that the spatially heterogeneous stress after the main shock is caused by such static stress change. The differential stress magnitude before the main shock is estimated to be less than 10–30 MPa, suggesting that the main shock faults are weak.

A family of repeating low-frequency earthquakes at the downdip edge of tremor and slip

We analyze an isolated low-frequency earthquake (LFE) family located at the downdip edge of the main episodic tremor and slip (ETS) zone beneath western Washington State. The 9000 individual LFEs from this repeating family cluster into 198 swarms that recur roughly every week. Cumulative LFE seismic moment for each swarm correlates strongly with the time until the next swarm, suggesting that these LFE swarms are time predictable. Precise double-difference relative locations for 700 individual LFEs within this family show a distribution that is approximately 2 km long and 500 m wide, elongated parallel to the relative plate convergence direction. The distribution of locations (<300 m vertical spread) lies within a few hundred meters of two different plate interface models and has a similar dip. Peak-to-peak LFE S wave amplitudes range from 0.2 to 18 nm. Individual LFEs exhibit a trend of increasing magnitude during swarms, with smaller events at the beginning and the largest events toward the end. The largest LFEs cluster in a small area (300 m radius) coincident with maximum LFE density. We propose that the less-concentrated smaller LFEs act to unlock this patch core, allowing it to fully rupture in the largest LFEs, usually toward the end of a swarm. We interpret the patch responsible for producing these LFEs as a subducted seamount on the downgoing Juan de Fuca (JdF) plate. LFE locking efficiency (slip estimated during 5 years from summing LFE seismic moment divided by plate-rate-determined slip) is at most 20% and is highly concentrated in two ∼50 m radius locations in the larger patch core. Estimated individual LFE stress drops range from 1 to 20 kPa, but could also be significantly larger.

Seismic activity beneath the Nankai trough revealed by DONET ocean-bottom observations

We conducted a detailed investigation of seismic activity from January 2011 to February 2013 along the Nankai trough off the Kii Peninsula, central Japan, by using data obtained from the DONET ocean-bottom observation network. The hypocenters are mostly within the subducting Philippine Sea (PHS) plate, although a few are along the plate boundary or in the sedimentary wedge below the Kumano forearc basin. The seismic activity can be separated into events above and below 20 km depth, which corresponds approximately to the Moho. The hypocenter distributions are distinctly different for these groups. The seismic activity in the oceanic crust can be further separated into three clusters. Most of the seismic activity recorded in our data represents aftershocks of the 2004 off the Kii Peninsula earthquakes (M JMA = 7.1, 7.4, and 6.5), which occurred in the PHS plate. The hypocenter distribution in the oceanic crust correlates well with the location of the Paleo-Zenisu ridge, which is formed by a chain of seamounts that is subducting beneath the forearc basin. The hypocenters in the uppermost mantle are aligned on a plane dipping to the southeast, consistent with the existence of a thrust fault cutting through the lithosphere of the oceanic plate. The focal mechanisms of the earthquakes show that the axis of compressive stress in the PHS plate is oriented N–S, almost perpendicular to the direction of plate convergence, indicating a complex tectonic regime in this region. These results suggest that intraplate shortening may be occurring in the subducting oceanic plate.

Strain decoupling across the décollement in the region of large slip during the 2011 Tohoku-Oki earthquake from anisotropy of magnetic susceptibility

The Integrated Ocean Drilling Program (IODP) Expedition 343, Japan Trench Fast Drilling Project (JFAST) drilled and cored across the plate-boundary décollement at Site C0019 near the Japan Trench, where large slip occurred during the 2011 Tohoku-Oki earthquake (Mw 9.0). Anisotropy of magnetic susceptibility (AMS) data obtained from core samples show a striking change in magnetic fabric across the décollement. In the frontal prism above the décollement, the maximum AMS axes display a strong preferred orientation in the northeast–southwest direction, with the intermediate and minimum AMS axes distributed in the vertical plane. In the underthrust sediments below the décollement, the maximum and intermediate AMS axes are subhorizontal but variable in direction, and the minimum AMS axes display an approximately vertical preferred orientation. The AMS ellipsoids for all samples have an oblate component, but the AMS ellipsoids of the underthrust sediments are generally less oblate than those of the frontal prism. The magnetic fabric of the sediments in the prism are consistent with horizontal tectonic shortening nearly parallel to the plate convergence direction in the Japan Trench, while that in the underthrust sediments represents a vertical, uniaxial strain. The magnetic fabrics of the frontal prism and underthrust sediments indicate an abrupt change in strain across the décollement at shallow depths, and imply that large co-seismic slip occurred along a weak plate-boundary décollement that is mainly decoupled over the long-term.

Seismotectonic Model of the Kraljevo 3 November 2010 Mw 5.4 Earthquake Sequence

Online Material: Additional Kraljevo sequence data, details. An M w 5.4 earthquake occurred on 3 November 2010 (Fig. 1) near the city of Kraljevo, Serbia, the population of which is more than 100,000 people. By the end of March 2011, the earthquake had been followed by a sequence of more than 650 aftershocks of magnitude greater than 1.0. Despite the moderate magnitude of the event, two people were killed, many others were injured, and the total damage to the city has been evaluated as more than 100 million EUR (United Nations, Kraljevo Earthquakes Report, 2010). Liquefaction and rockslides occurred in several places (Serbian Seismological Survey (SSS) Report, 2011). The earthquake also caused changes in groundwater circulation: some existing wells have totally dried out and new geothermal springs have appeared. Figure 1. Geodynamics of the Central Mediterranean region: Major plates (Africa, Eurasia, and Anatolia) and their boundaries, important fault systems, and orogenic belts (after Dilek, 2010). Thick black arrows, plate convergence directions; white star, the location of the 3 November 2010 Kraljevo earthquake. Symbols, seismological station networks from which data were used in this study. Virtual Regional Seismic Network includes data from the following networks: Italy, Romania, Croatia, Former Yugoslav Republic of Macedonia, Republic of Serbia, and Albania. The Kraljevo 3 November 2010 earthquake was recorded by seismological stations worldwide. Although different seismological agencies show consistency in determination of magnitude ( M w 5.3 or 5.4), proposed locations for the mainshock vary greatly. In particular, proposed depths vary from 2 to 30 km. In this article, we jointly determine the location of the mainshock and aftershocks that occurred between 3 November and 31 December 2010, along with a new P ‐ and S ‐wave 1D velocity model for that area. Focal mechanism solutions are obtained for selected events using both local and regional data. The …

Analysis of normal fault populations in the Kumano Forearc Basin, Nankai Trough, Japan: 1. Multiple orientations and generations of faults from 3‐D coherency mapping

Analyses of normal faults in the Kumano forearc basin of the Nankai Trough reveal multiple normal fault populations in a region generally thought to be under compression. Most faults have offsets of less than 20 m and dips of 60–70° and show no growth structures, indicating that the faults were active for short periods of time. The oldest generation of faults is older than ~0.9 Ma and strikes ~50–60°. The next oldest faults strike ~160–170°, are older than 0.44 Ma, and are related to local uplift along the western edge of the region. The youngest faults cut the seafloor; shallow faults near the SE margin of the basin curve from ~100° in the middle of the survey area to ~145° at the SE corner of the area. The pattern of the two youngest fault populations is consistent with the regional stress pattern (maximum horizontal stress subparallel to the trench). Orientations of older fault populations are caused by uplift of the underlying accretionary prism, implying that the forearc basin region is not as stable as previously thought. Reconstruction of displacements on the youngest faults shows that the overall horizontal extension is less than 2%, concentrated near the seaward edge of the basin. The active normal faults distributed throughout the basin support the idea that the horizontal stress parallel to the plate convergence direction does not reach the critical stress to activate or form thrust faults and produce horizontal shortening within the shallow portion of the inner wedge.

Paleogeographic and tectonic controls on the evolution of Cenozoic basins in the Altiplano and Western Cordillera of southern Peru

Integrated studies of stratigraphy, sedimentology, paleogeography and tectonic controls on Cenozoic basins provide the basis for a series of time-slice reconstructions of basin evolution in the Andes of southern Peru. The Altiplano and adjacent margin of the Western Cordillera are characterized by several Paleocene–Miocene synorogenic continental basins with thicknesses locally exceeding 10km. The evolution of these basins has been controlled by NW-trending tectonic features that mark the Altiplano–Western Cordillera and Altiplano–Eastern Cordillera boundaries and the Condoroma structural high. Sedimentary deposits of Paleocene age preserved in the Altiplano are the result of nonmarine sedimentation in a distal foreland basin. During the early Eocene, predominantly dextral strike-slip movements in the Altiplano between the Cusco–Lagunillas and Urcos–Ayaviri fault systems created the transpressional Kayra basin. The Soncco and Anta basins (middle Eocene–early Oligocene) are related to NE shortening (43–30Ma) and represent proximal, wedge-top and foredeep basin environments preserved on the Altiplano. At ~29–28Ma, a change to predominantly E–W shortening produced sinistral strike-slip motion along NW-striking faults, resulting in intermontane, transpressional basins. In the Altiplano, the Tinajani and Punacancha (29–5Ma), and Paruro (12–6Ma) basins were controlled by the Cusco–Lagunillas and the Urcos–Ayaviri fault systems. The Maure, Tincopalca–Huacochullo and Condoroma basins (22–5Ma) of the Western Cordillera developed between the Condoroma high and the Cusco–Lagunillas fault system. Oligocene–Miocene sedimentation commonly evolved from proximal (alluvial) facies along the borders to distal (lacustrine) facies. These basins were linked to sinistral strike-slip faults that evolved into reverse-sinistral structures. Plate kinematics may play a role in Andean basin evolution, with deformation influenced by major preexisting faults that dictated paleogeographic trends, but orientations of regional compression that appear to coincide with the plate convergence direction. However, the processes of slab flattening and steepening exerted a primary control on regional crustal shortening and filling of synorogenic basins.

Change in stress field after the 2011 great Tohoku-Oki earthquake

Temporal change in the stress field after the 2011 great Tohoku-Oki earthquake was observed by using stress tensor inversions of earthquakes near the source region. In the upper plate (hanging wall) before the Tohoku-Oki earthquake, the maximum compressive stress (σ1) axis was oriented in the direction of plate convergence, specifically, in the direction of an estimated large near-trench slip area. The stress field in the upper plate was completely transformed after the earthquake, with the minimum compressive stress (σ3) axis being oriented in the direction of the large near-trench slip area, even near the trench axis. In the near-coast area within the Pacific plate (i.e., in the footwall), the σ1 axis was oriented in the direction of plate convergence prior to the earthquake, as within the upper plate; however, the orientations of the principal stress axes in this area showed no change after the earthquake. After the earthquake, the σ3 axis was oriented in the plate-convergence direction in areas within the Pacific plate below the central to near-trench portions of the mainshock rupture area. These observations provide evidence for large near-trench slip at very shallow depths during the mainshock rupture. Moreover, the trend of the σ1 axis in the upper plate before the Tohoku-Oki earthquake (i.e., in the direction of an estimated large near-trench slip area) strongly suggests that strain accumulation is possible even in the near-trench area at very shallow depths. The observed temporal change in the stress field of the upper plate associated with the Tohoku-Oki earthquake indicates differential stress magnitudes as small as 5–15MPa.

Structural evolution of an inner accretionary wedge and forearc basin initiation, Nankai margin, Japan

Core recovered during Integrated Ocean Drilling Program (IODP) Expedition 319 from below the Kumano forearc basin of Japan's Nankai margin provides some of the only in situ samples from an inner accretionary wedge, and sheds light on the tectonic history of a seismically hazardous region. The 84m of core comprises Miocene-age well-bedded muds, silts, and volcaniclastic sediments. Beds increase in dip with depth, and are cut by (i) soft-sediment deformation bands (“vein structures”), (ii) ∼1-cm thick shear zones within ∼10-cm thick regions of high shear strain, and (iii) <1-mm thick slickensided faults which are the youngest structures in the core and highly localized. Microstructural analyses of the shear zones suggest that they formed via multiple increments of shear localization and a mixed granular and cataclastic flow. Kinematic analysis of slip indicators in shear zones further reveals that they formed via north–south shortening. In contrast, the faults cut the shear zones with mixed slip kinematics, and accommodated northwest–southeast shortening, roughly parallel to the modern shortening direction. The entire section was also rotated ∼15° counterclockwise about a roughly vertical axis. Therefore the principle strain axes and stratigraphic section rotated during or postdating development of the major sub-basin (∼5.6–3.8Ma) unconformity, a time that generally coincides with a change in the Philippine Sea plate convergence direction. Forearc basin development therefore postdates a protracted geologic evolution of shear-zone development, tectonic rotations, and inner-wedge development, the last of which coincides with a rheological evolution toward localized frictional faulting.

In-situ stress and fault reactivation associated with LNG injection in the Tiechanshan gas field, fold-thrust belt of Western Taiwan

The Tiechanshan gas field located in the fold-thrust belt of western Taiwan was depleted and converted for underground storage of Liquefied Natural Gas (LNG) decades ago. Recently, CO2 sequestration has been planned at shallower depths of this structure. We characterize the in-situ stresses from over 40 wells and assess the leakage potential through fault reactivation in response to LNG-injection increased pore-pressure. The formation pore pressure (Pf), vertical stress (Sv), and minimum horizontal stress (Shmin) are measured from repeated formation tests, density logs, and hydrofrac data including leak-off tests and fluid injection. Formation pore pressures are hydrostatic above depths of 2km, and increase with local gradients of 14.02 and 21.26MPa/km above and below 3.2km, respectively. Extremely high pore pressures (λp=0.8) are observed at depths below 3.8km. Lower than normal pressures (average 9.47MPa/km) are observed in the gas-bearing reservoir of the Talu A-sand. The gradient of Shmin is ∼17.46MPa/km or equivalent to 0.74 of Sv (∼23.60MPa/km). A detailed structure contour map of the top of the A-sand, combined with the measured Shmin and Sv, show that the stress state in the Tiechanshan field is predominantly strike-slip stress regime (SHmax>SV>Shmin). An upper-bound value of the maximum horizontal stress (SHmax) constrained by frictional limits and the coefficient of friction (μ=0.6) is about 27.36MPa/km. Caliper logs from two wells show that the mean azimuth of preferred orientation of borehole breakouts are in ∼028°N. Consequently, the maximum horizontal stress axis tends 118°N, which is sub-parallel to the far-field plate-convergence direction. Geomechanical analyses of the reactivation of pre-existing faults at the depths of the LNG reservoir sand indicate that all faults are stable at the present stress state. Sensitivity analyses indicate that ∼5.9MPa excess pore pressure would be required to cause the optimal oriented f1 fault to reactivate. This corresponds to LNG column heights of ∼760m (density=∼790kg/m3), whereas the height of structural closure of the A-sand does not exceed 400m. Therefore, it is unlikely that LNG injection will reactivate f1 fault.

Evidence for active strike-slip faulting along the Eurasia-Africa convergence zone: Implications for seismic hazard in the southwest Iberian margin

New seismic imaging and seismotectonic data from the southwest Iberian margin, the site of the present-day boundary between the European and African plates, reveal that active strike slip is occurring along two prominent lineaments that have recently been mapped using multibeam bathymetry. Multichannel seismic and subbottom profiler images acquired across the lineaments show seafloor displacements and active faulting to depths of at least 10 km and of a minimum length of 150 km. Seismic moment tensors show predominantly WNW–ESE right-lateral strike-slip motion, i.e., oblique to the direction of plate convergence. Estimates of earthquake source depths close to the fault planes indicate upper mantle (i.e., depths of 40–60 km) seismogenesis, implying the presence of old, thick, and brittle lithosphere. The estimated fault seismic parameters indicate that the faults are capable of generating great magnitude (Mw ≥ 8.0) earthquakes. Such large events raise the concomitant possibility of slope failures that have the potential to trigger tsunamis. Consequently, our findings identify an unreported earthquake and tsunami hazard for the Iberian and north African coastal areas.

Miocene basin development and volcanism along a strike-slip to flat-slab subduction transition: Stratigraphy, geochemistry, and geochronology of the central Wrangell volcanic belt, Yakutat-North America collision zone

New geochronologic, geochemical, sedimentologic, and compositional data from the central Wrangell volcanic belt (WVB) document basin development and volcanism linked to subduction of overthickened oceanic crust to the northern Pacific plate margin. The Frederika Formation and overlying Wrangell Lavas comprise >3 km of sedimentary and volcanic strata exposed in the Wrangell Mountains of south-central Alaska (United States). Measured stratigraphic sections and lithofacies analyses document lithofacies associations that reflect deposition in alluvial-fluvial-lacustrine environments routinely influenced by volcanic eruptions. Expansion of intrabasinal volcanic centers prompted progradation of vent-proximal volcanic aprons across basinal environments. Coal deposits, lacustrine strata, and vertical juxtaposition of basinal to proximal lithofacies indicate active basin subsidence that is attributable to heat flow associated with intrabasinal volcanic centers and extension along intrabasinal normal faults. The orientation of intrabasinal normal faults is consistent with transtensional deformation along the Totschunda-Fairweather fault system. Paleocurrents, compositional provenance, and detrital geochronologic ages link sediment accumulation to erosion of active intrabasinal volcanoes and to a lesser extent Mesozoic igneous sources. Geochemical compositions of interbedded lavas are dominantly calc-alkaline, range from basaltic andesite to rhyolite in composition, and share geochemical characteristics with Pliocene–Quaternary phases of the western WVB linked to subduction-related magmatism. The U/Pb ages of tuffs and 40 Ar/ 39 Ar ages of lavas indicate that basin development and volcanism commenced by 12.5–11.0 Ma and persisted until at least ca. 5.3 Ma. Eastern sections yield older ages (12.5–9.3 Ma) than western sections (9.6–8.3 Ma). Samples from two western sections yield even younger ages of 5.3 Ma. Integration of new and published stratigraphic, geochronologic, and geochemical data from the entire WVB permits a comprehensive interpretation of basin development and volcanism within a regional tectonic context. We propose a model in which diachronous volcanism and transtensional basin development reflect progressive insertion of a thickened oceanic crustal slab of the Yakutat microplate into the arcuate continental margin of southern Alaska coeval with reported changes in plate motions. Oblique northwestward subduction of a thickened oceanic crustal slab during Oligocene to Middle Miocene time produced transtensional basins and volcanism along the eastern edge of the slab along the Duke River fault in Canada and subduction-related volcanism along the northern edge of the slab near the Yukon-Alaska border. Volcanism and basin development migrated progressively northwestward into eastern Alaska during Middle Miocene through Holocene time, concomitant with a northwestward shift in plate convergence direction and subduction collision of progressively thicker crust against the syntaxial plate margin.

Seismicity within the actively deforming Eastern Sierras Pampeanas, Argentina

SUMMARY The Eastern Sierras Pampeanas of central Argentina have been interpreted by previous studies to represent the easternmost expression of continental deformation resulting from the flat-slab subduction of the Nazca Plate beneath the South American Plate. Yet, the seismic activity and deformation of this area, as well as its relationship to subduction more than 700 km to the west, are poorly understood. Using seismic data recorded by the Eastern Sierras Pampeanas broadband seismic array between 2008 August and 2009 August, we identified and located 445 local crustal earthquakes, most with magnitudes of 2.5 or less. We employed a double-difference location method to clarify the hypocentral distribution of local seismicity and to define zones of both clustered and diffuse seismicity. The relocated events define active structures in the region, including a horizontal zone of seismicity between 15 and 25 km depth, a prominent west-dipping structure beneath the central ranges and a vertical structure along the eastern margin of the study area. Overall, seismicity in this region during the study period is constrained to the upper ∼25 km of the crust and is mostly concentrated along minor structures and not along the major range-bounding faults. Focal mechanism solutions from the nine clearest events show mainly normal deformation with an oblique component and E–W oriented P-axes that likely result from shortening along structures with oblique orientations to the direction of plate convergence. This is the first seismic study to document that the Eastern Sierras Pampeanas are actively deforming in response to compression from the convergent margin.