North America Plate Research Articles

Slip history of the La Cruz fault: Development of a late Miocene transform in response to increased rift obliquity in the northern Gulf of California

The Gulf of California rift has accommodated oblique divergence of the Pacific and North America plates in northwestern México since Miocene time. Due to its infancy, its rifted margins preserve a rare onshore record of early continental break-up processes and an opportunity to investigate the role of rift obliquity in strain localization. We map rift-related structures and syn-tectonic basins on southern Isla Tiburón, a proximal onshore exposure of the rifted North America margin. We integrate analysis and geochronology of syn-tectonic sedimentary basins and mapping of crosscutting relationships to characterize the style and timing of fault activity. On southern Isla Tiburón, an early phase of extension initiated between ~19–17Ma and ~12.2Ma. Subsequently, these normal faults and related basins were cut by the La Cruz strike-slip fault and buried by deposits of the La Cruz basin, an elongate, fault-controlled trough coextensive with the La Cruz fault. Crosscutting relationships show that the NW-striking La Cruz fault accrued 5±2km of dextral slip ~8–4Ma. The La Cruz fault and parallel Tiburón transform were kinematically linked to detachment faulting that accommodated latest Miocene to Pliocene oblique opening of the offshore Upper Tiburón pull-apart basin. The onset of strike-slip faulting on Isla Tiburón was synchronous with the ~8–6Ma onset of transform faulting and basin formation along >1000km of the reconstructed Pacific-North America plate boundary. This transition coincides with the commencement of a clockwise azimuthal shift in Pacific-North America relative plate motion that increased the obliquity of the Gulf of California rift and formed the Gulf of California shear zone. The record from the proto-Gulf of California illustrates how highly oblique rift geometries, where transform faults are kinematically linked to pull-apart basins, enhance the ability of continental lithosphere to rupture and, ultimately, hasten the formation of new oceanic rift basins.

Toroidal, Counter-Toroidal, and Upwelling Flow in the Mantle Wedge of the Rivera and Cocos Plates: Implications for IOB Geochemistry in the Trans-Mexican Volcanic Belt

We carried out analog laboratory modeling at a scale 1:4,000,000 and computer rendering of the flow patterns in a simulated western Middle American subduction zone. The scaled model consists of a transparent tank filled with corn syrup and housing two conveyor belts made of polyethylene strips. One of the strips dips 60° and moves at a velocity of 30 mm/min simulating the Rivera plate. The other one dips 45°, moves at 90 mm/min simulating the subduction of the Cocos plate. Our scaled subduction zone also includes a gap between the simulated slabs analogous to a tear recently observed in shear wave tomography studies. An acrylic plate 3 mm thick floats on the syrup in grazing contact with the polyethylene strips and simulates the overriding North America plate. Our experiments reveal a deep toroidal flow of asthenospheric mantle through the Cocos–Rivera separation. The flow is driven by a pressure gradient associated with the down-dip differential-motion of the slabs. Similarly, low pressure generated by the fast-moving Cocos plate creates a shallow counter-toroidal flow in the uppermost 100 km of the mantle wedge. The flow draws mantle beneath the western Trans-Mexican Volcanic Belt to the Jalisco block, then plunges into the deep mantle by the descending poloidal cell of the Cocos slab. Moreover, our model suggests a hydraulic jump causes an ~250 km asthenosphere upwelling around the area where intra-arc extensional systems converge in western Mexico. The upwelling eventually merges with the shallow counter-toroidal flow describing a motion in 3D space similar to an Archimedes’ screw. Our results indicate the differential motion between subducting slabs drives mixing in the mantle wedge of the Rivera plate and allows the slab to steepen and retreat. Model results are in good agreement with seismic anisotropy studies and the geochemistry of lavas erupted in the Jalisco block. The model can explain the eruption of OIB lavas in the vicinity of the City of Guadalajara in western Mexico, and the south shoulder in the central part of the Tepic-Zacoalco fault system.

Plate boundary segmentation in the northeastern Caribbean from geodetic measurements and Neogene geological observations

The Caribbean–North America plate boundary in the northeastern Caribbean shows a remarkable example of along-strike transition from plate boundary–normal subduction in the Lesser Antilles, oblique subduction with no strain partitioning in Puerto Rico, and oblique subduction/collision with strain partitioning further west in Hispaniola. We show that this segmentation is well marked in the interseismic strain, as measured using space geodetic data, and in the Neogene deformation regime, as derived from geological observations. Hence, interseismic segmentation, which reproduces the geological segmentation persistent over a long time interval, is inherited from the geological history and long-term properties of the plate boundary. This result is relevant to the assessment of seismic hazard at convergent plate boundaries, where geodetic measurements often show interseismic segmentation between fully–and partially–coupled plate interface regions.

P-Wave Velocity Tomography from Local Earthquakes in Western Mexico

In western Mexico, the subduction of the Rivera and Cocos plates beneath the North America plate has deformed and fragmented the overriding plate, forming several structural rifts and crustal blocks. To obtain a reliable subsurface image of the continental crust and uppermost mantle in this complex area, we used P-wave arrivals of local earthquakes along with the Fast Marching Method tomography technique. We followed an inversion scheme consisting of (1) the use of a high-quality earthquake catalog and corrected phase picks, (2) the selection of earthquakes using a maximum location error threshold, (3) the estimation of an improved 1-D reference velocity model, and (4) the use of checkerboard testing to determine the optimum configuration of the velocity nodes and inversion parameters. Surprisingly, the tomography results show a very simple δVp distribution that can be described as being controlled by geologic structures formed during two stages of the separation of the Rivera and Cocos plates. The earlier period represents the initial stages of the separation of the Rivera and Cocos plates beneath western Mexico; the later period represents the more advanced stage of rifting where the Rivera and Cocos plates had separated sufficiently to allow melt to accumulate below the Colima Volcanic complex. During the earlier period (14 or 10–1.6 Ma), NE–SW-oriented structures/lineaments (such as the Southern Colima Rift) were formed as the two plates separated. During the second period (1.6 Ma to the present), the deformation is attributed to magma, generated within and above the tear zone between the Rivera and Cocos plates, rising beneath the region of the Colima Volcanic Complex. The rising magma fractured the overlying crust, forming a classic triple-rift junction geometry. This triple-rift system is confined to the mid- to lower crust perhaps indicating that this rifting process is still in an early stage. This fracturing, along with fluid circulation and associated heat advection within the fractures, can easily explain the observed distribution of δVp, as well as many of the results of previous seismological studies. Also surprisingly, we find no evidence at deep crustal depths to support either a trenchward migration of the volcanic arc or toroidal asthenospheric flow through the slab tears bounding the Jalisco Block to the NW and SE.

Chapter 3 Active tectonics in the central and eastern Azores islands along the Eurasia–Nubia boundary: a review

Abstract The geodynamic setting of the Azores archipelago, straddling the triple junction between the North America, Eurasia and Nubia plates, is reflected in frequent volcanic and tectonic activity. A review of neotectonics is presented for the islands forming the central and eastern groups of the Azores (Faial, Pico, São Jorge, Graciosa, Terceira, São Miguel and Santa Maria). The geometry and kinematics of active faults displacing stratigraphic and geomorphological markers of Pleistocene and Holocene age are presented. Slip-rates were determined using the available ages for the displaced markers. Maximum expected moment magnitudes were estimated using empirical correlations between magnitude and fault length, fault area and maximum observed surface displacement during surface-rupturing palaeoearthquakes. Neotectonic parameters show that the faults are in most cases very to moderately active, with slip-rates usually ranging from a few tenths of millimetres to a few millimetres per year, while maximum expected magnitudes vary from M w 6 to 7. These magnitudes are in agreement with the instrumental and historical seismic record in the region. Neotectonic data define a dextral transtensive stress regime acting on the region and contribute to characterizing the complexity of the geodynamic processes that dominate the western-most segment of the Eurasia–Nubia plate boundary.

Paleomagnetism of the Miocene Jantetelco Granodiorites and Tepexco Volcanic Group and inferences for crustal block rotations in central Mexico — Reevaluation

Miocene igneous rocks from a location in central Mexico provide paleomagnetic data that allow re-evaluation of previous data by Urrutia-Fucugauchi (1981) that were interpreted to suggest a counter clockwise rotation of a crustal block by about 50°. We sampled 31 sites from the Miocene trondhjemitic Chalcatzingo domes, the Tepexco volcanic group, and the Xalostoc diorite, covering the same area of approximately 50km2 about 90km SE of Mexico City. Magnetic analysis shows that these rocks contain magnetic phases of variable composition, with Curie temperatures characteristic for magnetite, but often accompanied by lower Curie temperature components. Magnetic hysteresis measurements point to the presence of PSD particles, and in combination these properties suggest that these rocks are also suitable for paleomagnetic study. Scanning electron microscope analysis supports the rock magnetic results, indicating the presence of magnetite and high-intermediate titanium titanomagnetite in many samples. Demagnetization experiments showed in most cases characteristic remanence directions of reasonable to good quality, and for 26 sites mean directions could be determined. Of these 14 (12) are of normal (reverse) polarity. Overall mean directions of normal and reverse sites are antipodal and pass a reversal test at the 95% probability level. The paleodirection from 26 sites is D=348.1°E, I=35.7°, α95=7.4°, and the paleopole is located at Lat=78.3°N, Long=184.2°E, A95=7.0°, which is indistinguishable from the 20Ma reference pole for the stable North America plate. These data do not support any tectonic deformation of the sampling area since the Miocene. Based on the number of sites studied, their rock magnetic characteristics, and the quality of the magnetic remanence that results in a positive reversal test, we consider our data to be reliable and we therefore suggest that this result supersedes that of Urrutia-Fucugauchi (1981). The reason for this previous and much different result remains unknown.

Multifractal detrended fluctuation analysis of earthquake magnitude series of Mexican South Pacific Region

The multifractality of the earthquake magnitude series of seismicity occurred on the Mexican South Pacific Coast was investigated. The area is composed by five seismic regions that are characterized by different tectonic subduction features, due to the interactions between the La Rivera and Cocos plates with the North America plate. Among the five seismic regions, Jalisco is tectonically characterized by the existence of active spreading center (the East Pacific Rise). The multifractal analysis shows that all the five seismic regions are characterized by very close multifractal properties; however, Jalisco is featured by a higher persistence of the magnitude series.

Stratigraphy and structural development of the southwest Isla Tiburón marine basin: Implications for latest Miocene tectonic opening and flooding of the northern Gulf of California

Accurate information on the timing of earliest marine incursion into the Gulf of California (northwestern Mexico) is critical for paleogeographic models and for understanding the spatial and temporal evolution of strain accommodation across the obliquely divergent Pacific–North America plate boundary. Marine strata exposed on southwest Isla Tiburon (SWIT) have been cited as evidence for a middle Miocene marine incursion into the Gulf of California at least 7 m.y. prior to plate boundary localization ca. 6 Ma. A middle Miocene interpretation for SWIT marine deposits has played a large role in subsequent interpretations of regional tectonics and rift evolution, the ages of marine basins containing similar fossil assemblages along ∼1300 km of the plate boundary, and the timing of marine incursion into the Gulf of California. We report new detailed geologic mapping and geochronologic data from the SWIT basin, an elongate sedimentary basin associated with deformation along the dextral-oblique La Cruz fault. We integrate these results with previously published biostratigraphic and geochronologic data to bracket the age of marine deposits in the SWIT basin and show that they have a total maximum thickness of ∼300 m. The 6.44 ± 0.05 Ma (Ar/Ar) tuff of Hast Pitzcal is an ash-flow tuff stratigraphically below the oldest marine strata, and the 6.01 ± 0.20 Ma (U/Pb) tuff of Oyster Amphitheater, also an ash-flow tuff, is interbedded with marine conglomerate near the base of the marine section. A dike-fed rhyodacite lava flow that caps all marine strata yields ages of 3.51 ± 0.05 Ma (Ar/Ar) and 4.13 ± 0.09 Ma (U/Pb) from the base of the flow, consistent with previously reported ages of 4.16 ± 1.81 Ma (K-Ar) from the flow top and (K-Ar) 3.7 ± 0.9 Ma from the feeder dike. Our new results confirm a latest Miocene to early Pliocene age for the SWIT marine basin, consistent with previously documented latest Miocene to early Pliocene (ca. 6.2–4.3 Ma) planktonic and benthic foraminifera from this section. Results from biostratigraphy and geochronology thus constrain earliest marine deposition on SWIT to ca. 6.2 ± 0.2 Ma, coincident with a regional-scale latest Miocene marine incursion into the northern proto–Gulf of California. This regional marine incursion flooded the northernmost, >500-km-long portion of the Gulf of California shear zone, a narrow belt of localized strike-slip faulting, clockwise block rotation, and subsiding pull-apart basins. Oblique Pacific–North America relative plate motion gradually localized in the >1000-km-long Gulf of California shear zone ca. 9–6 Ma, subsequently permitting the punctuated south to north flooding of the incipient Gulf of California seaway.

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.

Crustal structure of northern Fossa Magna and off Hokuriku area: Does the plate boundary between North America and Eurasia plate exist in central Japan?

Crustal structure of northern Fossa Magna and off Hokuriku area: Does the plate boundary between North America and Eurasia plate exist in central Japan?

Geology of the coastal Chiapas (Mexico) Miocene plutons and the Tonalá shear zone: Syntectonic emplacement and rapid exhumation during sinistral transpression

Late Miocene plutons in coastal Chiapas, Mexico, represent the roots of an extinct magmatic arc. Miocene granitoids of calc-alkaline composition and arc chemistry intruded into and were deformed within the Tonala mylonite belt in the middle to upper crust. The mylonite belt is a crustal-scale shear zone extending along the western margin of the Chiapas Massif for ∼150 km. Deformation is characterized by a dominantly subhorizontal lineation and subvertical foliation along a strikingly linear zone that trends ∼310°. Mylonitic fabrics contain ambiguous but dominantly sinistral shear indicators. Intrusions are interpreted as syntectonic on the basis of similar U-Pb zircon crystallization age estimates (ca. 10 Ma) and the cooling age estimates obtained on neoformed micas in the mylonite. The plutons are elongated, their long axis is parallel to shear zone, and some plutons show markedly asymmetric outcrop patterns, with sheared tails that trail behind the intrusions and that are consistent with sinistral displacement. Parts of plutons were mylonitized by continuous deformation in the Tonala shear zone, locally developing intricate pseudotachylyte and cataclasite veins slightly oblique to the mylonite foliation. Outside of the shear zone, plutons preserve magmatic fabrics. These observations are consistent with features common to syntectonic granites interpreted to have been emplaced along strike-slip shear zones in a transpressional setting. We interpret the Tonala mylonites as representing a relict transform boundary that was slightly oblique to the Polochic-Motagua fault system, which accommodated over 100 km of sinistral displacement between the Chortis block (on the Caribbean plate) and Chiapas (on the North America plate) in late Miocene time.

A Crustal Velocity Model for the Southern Mexicali Valley, Baja California, Mexico

In northern Baja California, the two largest regions with different geological characteristics are the granitic Peninsular Ranges of Baja California (PRBC) and the sedimentary environment of the Mexicali Valley (Lomnitz et al., 1970). The boundary of these two regions is the Main Gulf Escarpment (Fig. 1). The northern Baja California peninsula has active normal and strike‐slip faults originating from the transtensional limit between the Pacific and North America plates (Stock et al., 1991).

Assembly of a large earthquake from a complex fault system: Surface rupture kinematics of the 4 April 2010 El Mayor–Cucapah (Mexico) Mw 7.2 earthquake

The 4 April 2010 moment magnitude (M_w) 7.2 El Mayor–Cucapah earthquake revealed the existence of a previously unidentified fault system in Mexico that extends ∼120 km from the northern tip of the Gulf of California to the U.S.–Mexico border. The system strikes northwest and is composed of at least seven major faults linked by numerous smaller faults, making this one of the most complex surface ruptures ever documented along the Pacific–North America plate boundary. Rupture propagated bilaterally through three distinct kinematic and geomorphic domains. Southeast of the epicenter, a broad region of distributed fracturing, liquefaction, and discontinuous fault rupture was controlled by a buried, southwest-dipping, dextral-normal fault system that extends ∼53 km across the southern Colorado River delta. Northwest of the epicenter, the sense of vertical slip reverses as rupture propagated through multiple strands of an imbricate stack of east-dipping dextral-normal faults that extend ∼55 km through the Sierra Cucapah. However, some coseismic slip (10–30 cm) was partitioned onto the west-dipping Laguna Salada fault, which extends parallel to the main rupture and defines the western margin of the Sierra Cucapah. In the northernmost domain, rupture terminates on a series of several north-northeast–striking cross-faults with minor offset (<8 cm) that cut uplifted and folded sediments of the northern Colorado River delta in the Yuha Desert. In the Sierra Cucapah, primary rupture occurred on four major faults separated by one fault branch and two accommodation zones. The accommodation zones are distributed in a left-stepping en echelon geometry, such that rupture passed systematically to structurally lower faults. The structurally lowest fault that ruptured in this event is inclined as shallowly as ∼20°. Net surface offsets in the Sierra Cucapah average ∼200 cm, with some reaching 300–400 cm, and rupture kinematics vary greatly along strike. Nonetheless, instantaneous extension directions are consistently oriented ∼085° and the dominant slip direction is ∼310°, which is slightly (∼10°) more westerly than the expected azimuth of relative plate motion, but considerably more oblique to other nearby historical ruptures such as the 1992 Landers earthquake. Complex multifault ruptures are common in the central portion of the Pacific North American plate margin, which is affected by restraining bend tectonics, gravitational potential energy gradients, and the inherently three-dimensional strain of the transtensional and transpressional shear regimes that operate in this region.

Seismic evidence for rotating mantle flow around subducting slab edge associated with oceanic microplate capture

AbstractTectonic plate reorganization at a subduction zone edge is a fundamental process that controls oceanic plate fragmentation and capture. However, the various factors responsible for these processes remain elusive. We characterize seismic anisotropy of the upper mantle in the Explorer region at the northern limit of the Cascadia subduction zone from teleseismic shear wave splitting measurements. Our results show that the mantle flow field beneath the Explorer slab is rotating anticlockwise from the convergence‐parallel motion between the Juan de Fuca and the North America plates, re‐aligning itself with the transcurrent motion between the Pacific and North America plates. We propose that oceanic microplate fragmentation is driven by slab stretching, thus reorganizing the mantle flow around the slab edge and further contributing to slab weakening and increase in buoyancy, eventually leading to cessation of subduction and microplate capture.

What drives microplate motion and deformation in the northeastern Caribbean plate boundary region?

AbstractThe north Caribbean plate boundary zone is a broad deformation zone with several fault systems and tectonic blocks that move with different velocities. The indentation by the Bahamas Platform (the “Bahamas Collision”) is generally invoked as a cause of this fragmentation. We propose that a second driver of deformation is the western edge of the south dipping Puerto Rico slab moving sideways with the North America plate. The westward motion of the slab edge results in a push on the Caribbean plate farther west. We refer to this second mechanism for deformation as “Slab Edge Push.” The motion of the North America plate relative to the Caribbean plate causes both drivers to migrate from east to west. The Bahamas Collision and Slab Edge Push have been operating simultaneously since the Miocene. The question is the relative importance of the two mechanisms. We use mechanical finite element models that represent the two mechanisms from the late Oligocene (30 Ma) to the present. For the present, both models successfully reproduce observed deformation, implying that both models are viable. Back in time the Slab Edge Push mechanism better reproduces observations. Neither mechanism successfully reproduces the observed Miocene counterclockwise rotation of Puerto Rico. We use this rotation to tune a final model that includes fractional contributions of both mechanisms. We find that the Slab Edge Push was the dominant driver of deformation in the north Caribbean plate boundary zone since 30 Ma.

Localized shear in the deep lithosphere beneath the San Andreas fault system

Research Article| April 01, 2014 Localized shear in the deep lithosphere beneath the San Andreas fault system Heather A. Ford; Heather A. Ford * 1Department of Geological Sciences, Brown University, 324 Brook Street, Box 1846, Providence, Rhode Island 02912, USA *Current address: Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven, Connecticut 06520-8109, USA; E-mail: heather.ford@yale.edu. Search for other works by this author on: GSW Google Scholar Karen M. Fischer; Karen M. Fischer 1Department of Geological Sciences, Brown University, 324 Brook Street, Box 1846, Providence, Rhode Island 02912, USA Search for other works by this author on: GSW Google Scholar Vedran Lekic Vedran Lekic 2Department of Geology, University of Maryland, College Park, Maryland 20742, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Heather A. Ford * 1Department of Geological Sciences, Brown University, 324 Brook Street, Box 1846, Providence, Rhode Island 02912, USA Karen M. Fischer 1Department of Geological Sciences, Brown University, 324 Brook Street, Box 1846, Providence, Rhode Island 02912, USA Vedran Lekic 2Department of Geology, University of Maryland, College Park, Maryland 20742, USA *Current address: Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven, Connecticut 06520-8109, USA; E-mail: heather.ford@yale.edu. Publisher: Geological Society of America Received: 19 Sep 2013 Revision Received: 31 Dec 2013 Accepted: 10 Jan 2014 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2014 Geological Society of America Geology (2014) 42 (4): 295–298. https://doi.org/10.1130/G35128.1 Article history Received: 19 Sep 2013 Revision Received: 31 Dec 2013 Accepted: 10 Jan 2014 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Heather A. Ford, Karen M. Fischer, Vedran Lekic; Localized shear in the deep lithosphere beneath the San Andreas fault system. Geology 2014;; 42 (4): 295–298. doi: https://doi.org/10.1130/G35128.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Seismic images of the base of the lithosphere across the San Andreas fault system (California, USA) yield new constraints on the distribution of deformation in the deep lithosphere beneath this strike-slip plate boundary. We show that conversions of shear to compressional waves (Sp) across the base of the lithosphere are systematically weaker on the western side of the plate boundary, indicating that the drop in seismic shear-wave velocity from lithosphere to asthenosphere is either smaller or occurs over a larger depth range. In central and northern California, the lithosphere-asthenosphere boundary changes character across a distance of <50 km, and does so directly beneath the San Andreas fault along its simple central segment, and beneath the Calaveras–Green Valley–Bartlett Springs faults to the north. Given the absolute velocities of the North America and Pacific plates, and low viscosities inferred for the asthenosphere, these results indicate the juxtaposition of mantle lithospheres with different properties across these faults. The spatial correlation between the central San Andreas fault and the laterally abrupt change in the velocity structure of the deepest mantle lithosphere points to the accommodation of relative plate motion on a narrow shear zone (<50 km in width), and a rheology that enables strain localization throughout the thickness of the lithosphere. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Strain partitioning along the western margin of North America

This paper describes an elastic block model for the interseismic horizontal crustal velocity field occurring in that part of the United States located west of longitude 100° W and between latitudes 31°N and 49°N. We developed the model by simultaneously inverting 6873 GPS-derived velocity vectors and 166 geological fault slip rates for the angular velocities (i.e. the Euler poles relative to the North America plate) of 46 elastic blocks, horizontal strain rate tensors for 38 of these blocks, and the spatially variable elastic coupling coefficients on faults that bound adjacent blocks.While the model covers all of the western United States located between Canada and Mexico, this paper focuses on the region residing south of Cape Mendocino where plate boundary deformation is accommodated predominantly by slip on the San Andreas fault system. Block strain rates (which account for deformation associated with distributed faults within blocks) are systematically higher in blocks located in the western part of the model and adjacent to the plate boundary. Strain rate magnitudes range from over 10−7/yr for some blocks adjacent to the San Andreas fault system to values of about 10−9/yr for blocks located in eastern Nevada and western Utah. Blocks adjacent to the San Andreas fault system are characterized by strain rate tensors that correspond to uniaxial contraction perpendicular to the local strike of the San Andreas. The highest rates of fault normal contraction are associated with the northern end of the fault (north of San Francisco) and in the southern end (south of Los Angeles). The central San Andreas (including the creeping segment of the fault) is characterized by strain rate tensors more consistent with dextral shear. Thus the northern and southern ends of the fault are consistent with a transpressional strain partitioning model with strike slip occurring on the San Andreas fault system and distributed shortening occurring within the blocks adjacent to this fault system. There is no evidence of strain partitioning in the central San Andreas.

An Overview of the 28 October 2012 Mw 7.7 Earthquake in Haida Gwaii, Canada: A Tsunamigenic Thrust Event Along a Predominantly Strike-Slip Margin

The boundary between the Pacific and North America plates along Canada’s west coast is one of the most seismically active regions of Canada, and is where Canada’s two largest instrumentally recorded earthquakes have occurred. Although this is a predominantly strike-slip transform fault boundary, there is a component of oblique convergence between the Pacific and North America plates off Haida Gwaii. The 2012 Mw 7.7 Haida Gwaii earthquake was a thrust event that generated a tsunami with significant run up of over 7 m in several inlets on the west coast of Moresby Island (several over 6 m, with a maximum of 13 m). Damage from this earthquake and tsunami was minor due to the lack of population and vulnerable structures on this coast.

Earthquake Ground Motion and 3D Georgia Basin Amplification in Southwest British Columbia: Shallow Blind-Thrust Scenario Earthquakes

Abstract Finite‐difference modeling of 3D long‐period (>2 s) ground motions for large ( M w 6.8) scenario earthquakes is conducted to investigate the effects of the Georgia basin structure on ground shaking in Greater Vancouver, British Columbia, Canada. Scenario earthquakes include shallow blind‐thrust North America (NA) plate earthquakes, simulated in locations congruent with linear clusters of shallow seismicity, that is, potential active faults. A slip distribution model of the M w 6.7 Northridge, California, blind‐thrust earthquake, with the hypocenter modified to 5 km depth, is used to characterize the source rupture process. Two sets of simulations are performed for a given scenario earthquake using models with and without Georgia basin sediments. The ratio of predicted peak ground velocity (PGV) for the two simulations is applied here as a quantitative measure of amplification due to 3D basin structure. A total of eight shallow blind‐thrust NA plate scenario earthquakes are simulated within 100 km of Greater Vancouver. Overall, predicted ground motions are higher in the down‐dip direction of each epicenter due to the source radiation pattern; hence, scenario earthquakes located south of Vancouver produce the highest motions in the city. The average maximum PGV at stiff soil sites across Greater Vancouver considering all eight scenario earthquakes is 17.8 cm/s (modified Mercalli intensity VII); the average increase in peak motion due to the presence of Georgia basin sediments is a factor of 4.1. The effective duration of moderate‐level (≥3.4 cm/s) shaking within Greater Vancouver is an average of 22 s longer when Georgia basin sediments are included in the 3D structure model. Online Material: Snapshots and videos of wave propagation, and peak ground velocity maps.

High-resolution estimates of Nubia–North America plate motion: 20 Ma to present

High-resolution estimates of Nubia–North America plate motion: 20 Ma to present