We present a new global model of plate motions and strain rates in plate boundary zones constrained by horizontal geodetic velocities. This Global Strain Rate Model (GSRM v.2.1) is a vast improvement ...over its predecessor both in terms of amount of data input as in an increase in spatial model resolution by factor of ∼2.5 in areas with dense data coverage. We determined 6739 velocities from time series of (mostly) continuous GPS measurements; i.e., by far the largest global velocity solution to date. We transformed 15,772 velocities from 233 (mostly) published studies onto our core solution to obtain 22,511 velocities in the same reference frame. Care is taken to not use velocities from stations (or time periods) that are affected by transient phenomena; i.e., this data set consists of velocities best representing the interseismic plate velocity. About 14% of the Earth is allowed to deform in 145,086 deforming grid cells (0.25° longitude by 0.2° latitude in dimension). The remainder of the Earth's surface is modeled as rigid spherical caps representing 50 tectonic plates. For 36 plates we present new GPS‐derived angular velocities. For all the plates that can be compared with the most recent geologic plate motion model, we find that the difference in angular velocity is significant. The rigid‐body rotations are used as boundary conditions in the strain rate calculations. The strain rate field is modeled using the Haines and Holt method, which uses splines to obtain an self‐consistent interpolated velocity gradient tensor field, from which strain rates, vorticity rates, and expected velocities are derived. We also present expected faulting orientations in areas with significant vorticity, and update the no‐net rotation reference frame associated with our global velocity gradient field. Finally, we present a global map of recurrence times for Mw=7.5 characteristic earthquakes.
Key Points
A data set of ∼22,500 horizontal geodetic velocities is compiled
Geodetic plate motions for 36 plates are estimated
A new velocity gradient tensor field for plate boundary zones is modeled
We used an expanded PEER NGA-West2 database to develop a new ground motion prediction equation (GMPE) for the average horizontal components of PGA, PGV, and 5% damped linear pseudo-absolute ...acceleration response spectra at 21 periods ranging from 0.01 s to 10 s. In addition to those terms included in our now superseded 2008 GMPE, we include a more-detailed hanging wall model, scaling with hypocentral depth and fault dip, regionally independent geometric attenuation, regionally dependent anelastic attenuation and site conditions, and magnitude-dependent aleatory variability. The NGA-West2 database provides better constraints on magnitude scaling and attenuation of small-magnitude earthquakes, where our 2008 GMPE was known to be biased. We consider our new GMPE to be valid for estimating horizontal ground motion from shallow crustal continental earthquakes in an active tectonic domain for rupture distances ranging from 0 km to 300 km and magnitudes ranging from 3.3 to 7.5-8.5, depending on source mechanism.
NGA-West2 research project Bozorgnia, Yousef; Abrahamson, Norman A; Al Atik, Linda ...
Earthquake spectra,
08/2014, Volume:
30, Issue:
3
Journal Article
Peer reviewed
The NGA-West2 project is a large multidisciplinary, multi-year research program on the Next Generation Attenuation (NGA) models for shallow crustal earthquakes in active tectonic regions. The ...research project has been coordinated by the Pacific Earthquake Engineering Research Center (PEER), with extensive technical interactions among many individuals and organizations. NGA-West2 addresses several key issues in ground-motion seismic hazard, including updating the NGA database for a magnitude range of 3.0-7.9; updating NGA ground-motion prediction equations (GMPEs) for the "average" horizontal component; scaling response spectra for damping values other than 5%; quantifying the effects of directivity and directionality for horizontal ground motion; resolving discrepancies between the NGA and the National Earthquake Hazards Reduction Program (NEHRP) site amplification factors; analysis of epistemic uncertainty for NGA GMPEs; and developing GMPEs for vertical ground motion. This paper presents an overview of the NGA-West2 research program and its subprojects.
Plate tectonics requires the formation of plate boundaries. Particularly important is the enigmatic initiation of subduction: the sliding of one plate below the other, and the primary driver of plate ...tectonics. A continuous, in situ record of subduction initiation was recovered by the International Ocean Discovery Program Expedition 352, which drilled a segment of the fore-arc of the Izu-Bonin-Mariana subduction system, revealing a distinct magmatic progression with a rapid timescale (approximately 1 million years). Here, using numerical models, we demonstrate that these observations cannot be produced by previously proposed horizontal external forcing. Instead a geodynamic evolution that is dominated by internal, vertical forces produces both the temporal and spatial distribution of magmatic products, and progresses to self-sustained subduction. Such a primarily internally driven initiation event is necessarily whole-plate scale and the rock sequence generated (also found along the Tethyan margin) may be considered as a smoking gun for this type of event.
NNR‐MORVEL56, which is a set of angular velocities of 56 plates relative to the unique reference frame in which there is no net rotation of the lithosphere, is determined. The relative angular ...velocities of 25 plates constitute the MORVEL set of geologically current relative plate angular velocities; the relative angular velocities of the other 31 plates are adapted from Bird (2003). NNR‐MORVEL, a set of angular velocities of the 25 MORVEL plates relative to the no‐net rotation reference frame, is also determined. Incorporating the 31 plates from Bird (2003), which constitute 2.8% of Earth's surface, changes the angular velocities of the MORVEL plates in the no‐net‐rotation frame only insignificantly, but provides a more complete description of globally distributed deformation and strain rate. NNR‐MORVEL56 differs significantly from, and improves upon, NNR‐NUVEL1A, our prior set of angular velocities of the plates relative to the no‐net‐rotation reference frame, partly due to differences in angular velocity at two essential links of the MORVEL plate circuit, Antarctica‐Pacific and Nubia‐Antarctica, and partly due to differences in the angular velocities of the Philippine Sea, Nazca, and Cocos plates relative to the Pacific plate. For example, the NNR‐MORVEL56 Pacific angular velocity differs from the NNR‐NUVEL1A angular velocity by a vector of length 0.039 ± 0.011° a−1 (95% confidence limits), resulting in a root‐mean‐square difference in velocity of 2.8 mm a−1. All 56 plates in NNR‐MORVEL56 move significantly relative to the no‐net‐rotation reference frame with rotation rates ranging from 0.107° a−1 to 51.569° a−1.
Key Points
31 plates are added to MORVEL to describe geologically current plate motion
The no‐net‐rotation frame for these plates, NNR‐MORVEL56, is determined
NNR‐MORVEL56 differs significantly from NNR‐NUVEL1A and other realizations
We present four companion digital models of the age, age uncertainty, spreading rates, and spreading asymmetries of the world's ocean basins as geographic and Mercator grids with 2 arc min ...resolution. The grids include data from all the major ocean basins as well as detailed reconstructions of back‐arc basins. The age, spreading rate, and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading. Ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments. The age uncertainties for grid cells coinciding with marine magnetic anomaly identifications, observed or rotated to their conjugate ridge flanks, are based on the difference between gridded age and observed age. The uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities. Asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges, resulting in ridge jumps toward hot spots. We also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age‐depth relationships. The new set of grids helps to investigate prominent negative depth anomalies, which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow. A combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems.
This paper presents an analytical method to estimate surface and subsurface soil movements induced by deep excavations in the vertical and horizontal directions. Elastic and elastoplastic closed-form ...solutions for ground losses in the half-space are adopted, using the superposition method, to consider convergence and ovalization processes that occur within the soil mass due to displacements of the retaining wall; inputs include the shape and magnitude of the wall displacement profile to be selected based on design charts, monitoring data, or allowable level of resulting ground deformations. A parametric analysis illustrates (i) the effects on greenfield soil movements of the retaining wall’s deformation modes (cantilever, parabolic, composite, or kick-in), (ii) the influence of its displacement level, and (iii) the influence of aspects such as the soil’s ovalization and volumetric behaviour. Predictions of the proposed method are compared with empirical criteria, field data, and centrifuge results from the literature compiled for braced excavations in clays and sands. This comparison shows that the analytical approach provides reasonable predictions of excavation-induced soil deformations both at surface and subsurface levels, except for high normalized wall deflections when the analytical ground displacement field is wider and its maximum movements underestimate measured values. Finally, from the back-analyses of real data, a design guideline is suggested to select the relative ovalization depending on ground conditions and wall deflection mode.
•Predicting ground vertical and horizontal displacements due to deep excavations.•Solutions considering soil’s ovalization, volumetric behaviour and wall displacement.•Effects of retaining wall deformation mode on surface and subsurface soil movements.•Comparison with experimental and field data.
Here, we present the source mechanism and rupture process for the destructive 24 January 2020 Mw 6.7 Doğanyol–Sivrice earthquake at the East Anatolian Fault Zone (EAFZ, Turkey), obtained from ...seismological waveform analysis and space geodetic observations. Multi-data analyses and modelling in the present study provide fundamental data and strong constraints for retrieving complex source mechanism of an earthquake and its spatiotemporal slip characteristics along the ruptured segment of fault. The acquired slip model of this earthquake reveals heterogeneous slip distribution along strike N244°E of the fault plane dipping NW (68°) with duration of the source time function (STF) and low stress drop value (Δσ) of ~25 s and ~6 bars, respectively. Back-projection analysis validates fault length (L) stretching along strike for a distance of ~75 km and supports predominant south-westerly bilateral rupture propagation with a variable rupture velocity (Vr) of ~2.3–3.4 km/s along with two main patches, presumably a sequence of two asperities being ruptured following the surface trace of the EAFZ. The distribution of aftershocks based on the analysis of two months long data consistently confirms spreading of seismicity along the ruptured fault. The evaluation of Interferometric Synthetic Aperture Radar (InSAR) data reveals that left-lateral co-seismic slip and significant deformation extends for ~20 km on either side of the fault with evident post-seismic displacement. Yet, no significant vertical offsets were observed as GNSS stations detected only horizontal motions. Coda-wave analysis as an independent tool also confirms moment magnitude of Mw 6.7. Our results highlight a case of a damaging earthquake and enhance our understanding of earthquake mechanics, continental deformation and augmented earthquake risk on the EAFZ.
Display omitted
•Source model and slip history for the largest magnitude earthquake on the EAFZ are obtained.•Bilateral rupture contributed to directivity and the pattern of strong shaking.•Slow rupture speed on discrete patches of the EAFZ down to ~20 km depth is obtained.•No significant vertical offsets sensed as GNSS stations exposed horizontal motions.•Aftershocks extends for ~75 km along strike of N244°E, and co-seismic displacements ~20 km on either side of the fault.
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