We show that the spatial coherency of the ambient seismic field can be used for attenuation tomography in the western United States. We evaluate the real portion of the spatial coherency with an ...elastic geometric spreading term (a Bessel function) and a distance dependent decay (an attenuation coefficient). In order to invert the spatial coherency, a weight stack inversion technique is applied. We recover phase velocity and attenuation coefficient maps at periods of 8–32s, which correspond to the elastic and anelastic structure at crustal and upper mantle depths. The phase velocity maps obtained by this method are of similar resolution to more standard two‐station methods. The attenuation results provide an important complement to the information extracted from earthquake‐based tomography. Several geological features are easily identifiable in the attenuation coefficient maps, such as the highly attenuating sedimentary basins along the West Coast of the United States, and the highly attenuating Yellowstone region, and the boundaries of the Snake River Plains.
Key Points
Lateral variations in seismic attenuation can be obtained from ambient noise
Seismic attenuation structure is illuminated beneath the western United States
Empirical greens functions contain amplitude content for attenuation studies
Seismicity at the northern terminus of the Nazca subduction is diffused over a wide area containing the puzzling seismic feature known as the Bucaramanga nest. We relocate about 5000 earthquakes ...recorded by the Colombian national seismic network and produce the first 3‐D velocity model of the area to define the geometry of the lithosphere subducting below the Colombian Andes. We found lateral velocity heterogeneities and an abrupt offset of the Wadati‐Benioff zone at 5°N indicating that the Nazca plate is segmented by an E‐W slab tear, that separates a steeper Nazca segment to the south from a flat subduction to the north. The flat Nazca slab extends eastward for about 400 km, before dip increases to ∼50° beneath the Eastern Cordillera, where it yields the Bucaramanga nest. We explain this puzzling locus of intermediate‐depth seismicity located beneath the Eastern Cordillera of Colombia as due to a massive dehydration and eclogitization of a thickened oceanic crust. We relate the flat subducting geometry to the entrance at the trench at ca. 10 Ma of a thick ‐ buoyant oceanic crust, likely a volcanic ridge, producing a high coupling with the overriding plate. Sub‐horizontal plate subduction is consistent with the abrupt disappearance of volcanism in the Andes of South America at latitudes > 5°N.
Key Points:
This is the first high resolution image of seismicity and tomography of the Colombian Andes
Deep seismicity and velocity anomalies define tears and flattening of the Nazca slab
We explain the Bucaramanga seismicity nest with massive dehydration of the subducting Nazca plate
Subduction beneath the northernmost Andes in Colombia is complex. Based on seismicity distributions, multiple segments of slab appear to be subducting, and arc volcanism ceases north of 5° N. Here, ...we illuminate the subduction system through hypocentral relocations and Vp and Vs models resulting from the joint inversion of local body wave arrivals, surface wave dispersion measurements, and gravity data. The simultaneous use of multiple data types takes advantage of the differing sensitivities of each data type, resulting in velocity models that have improved resolution at both shallower and deeper depths than would result from traditional travel time tomography alone. The relocated earthquake dataset and velocity model clearly indicate a tear in the Nazca slab at 5° N, corresponding to a 250-km shift in slab seismicity and the termination of arc volcanism. North of this tear, the slab is flat, and it comprises slabs of two sources: the Nazca and Caribbean plates. The Bucaramanga nest, a small region of among the most intense intermediate-depth seismicity globally, is associated with the boundary between these two plates and possibly with a zone of melting or elevated water content, based on reduced Vp and increased Vp/Vs. We also use relocated seismicity to identify two new faults in the South American plate, one related to plate convergence and one highlighted by induced seismicity.
•Joint inversion provides a clearer image of subduction than body-wave-only tomography.•Normal-dip subduction of Nazca beneath volcanogenic southern Colombia.•Non-volcanogenic flat-slab subduction of Nazca and Caribbean plates further north.•Slab tear between normally dipping and flat sections of Nazca plate.
We use data from the Meso‐America Subduction Experiment to detect and locate low‐frequency earthquakes (LFEs) in the Mexican subduction zone. We use visually‐identified templates to perform a network ...waveform correlation search that produced ~17,000 robustly detected LFEs that form 15 distinct families. Stacking an LFE family's corresponding detections results in seismograms with high signal‐to‐noise ratios and clear P and S wave arrivals; we use these travel times to locate the sources. The resulting locations superpose a previously identified region of permanent non‐volcanic tremor (NVT) activity. Husker et al. (2012) called this region a Sweet Spot, suggesting that the local conditions are adequate to continuously generate NVT. The LFE hypocenters have been located at a depth of 40–45 km in an area that is surrounding the upper slab‐plate interface. We characterize their focal mechanisms by comparing their stacked seismograms to synthetic seismograms. This analysis reveals a common low‐dipping focal mechanism.
Key Points
Detected and characterized 15 low‐frequency earthquake families in Mexico
Sources locate to a region known for constant non‐volcanic tremor activity
Focal mechanisms reflect shearing on the subducting plate interface
Intermediate‐depth earthquakes occur at depths where temperatures and pressures exceed those at which brittle failure is expected. There are two leading candidates for the physical mechanism behind ...these earthquakes: dehydration embrittlement and self‐localizing thermal shear runaway. A complete energy budget for a range of earthquake sizes can help constrain whether either of these mechanisms might play a role in intermediate‐depth earthquake rupture. The combination of high stress drop and low radiation efficiency that we observe for Mw 4–5 earthquakes in the Bucaramanga Nest implies a temperature increase of 600–1000°C for a centimeter‐scale layer during earthquake failure. This suggests that substantial shear heating, and possibly partial melting, occurs during intermediate‐depth earthquake failure. Our observations support thermal shear runaway as the mechanism for intermediate‐depth earthquakes, which would help explain differences in their behavior compared to shallow earthquakes.
Key Points
Evidence for thermal shear runaway mechanism for intermediate‐depth earthquakes
Stress drop scaling and low radiation efficiencies observed for Mw 3.5–5.2
Fracture energies larger than for shallow earthquakes with similar slip
Continuous monitoring of engineering structures provides a crucial alternative to assess its health condition as well as evaluate its safety throughout the whole service life. To link the field ...measurements to the characteristics of a building, one option is to characterize and update a model, against the measured data, so that it can best describe the behavior and performance of the structure. In this paper, we present a novel computational strategy for Bayesian probabilistic updating of building models with response functions extracted from ambient noise measurements using seismic interferometry. The intrinsic building impulse response functions (IRFs) can be extracted from ambient excitation by deconvolving the motion recorded at different floors with respect to the measured ambient ground motion. The IRF represents the representative building response to an input delta function at the ground floor. The measurements are firstly divided into multiple windows for deconvolution and the IRFs for each window are then averaged to represent the overall building IRFs. A hierarchical Bayesian framework with Laplace priors is proposed for updating the finite element model. A Markov chain Monte Carlo technique with adaptive random-walk steps is employed to sample the model parameters for uncertainty quantification. An illustrative example is studied to validate the effectiveness of the proposed algorithm for temporal monitoring and probabilistic model updating of buildings. The structure considered in this paper is a 21-storey concrete building instrumented with 36 accelerometers at the MIT campus. The methodology described here allows for continuous temporal health monitoring, robust model updating as well as post-earthquake damage detection of buildings.
•A novel computational strategy is proposed for continuous monitoring of buildings.•The travel waves are extracted from ambient noises using seismic interferometry.•The wave velocity, frequencies, mode shapes and damping ratios can be directly estimated from the extracted waves.•The building model is characterized using a hierarchical Bayesian inference framework.•The parameter uncertainties are quantified using a Markov chain Monte Carlo technique.
The Caribbean plate subducts beneath northwest South America at a shallow angle due to a large igneous province that added up to 12 km of buoyant crust. The overriding plate lacks volcanism and ...exhibits Laramide‐style uplifts over 500 km from the trench. Here, we illuminate the subduction structures through finite frequency teleseismic P‐wave tomography and connect those structures to the Laramide‐style deformation on the overriding plate. We use a new data set collected from the Caribbean‐Mérida Andes seismic experiment comprised of 65 temporary broadband stations integrated with permanent stations from the Colombian and Venezuelan national networks. We identify three segments of subducting Caribbean plate with one segment completely detached from the surface. The timing of the detachment aligns with other regional events, including the uplift of the Mérida Andes, about 10 Ma. Slab buoyancy post‐detachment likely resulted in recoupling with the overriding plate, reactivation of Jurassic‐aged rift structures and subsequent uplift of the Mérida Andes. Mantle counterflow over the broken segment induced by rollback of the attached slab likely contributed to the uplift of the Mérida Andes. We conclude that the northern limit of subduction lies south of the Oca‐Ancón fault, though the fault itself may be the surface expression of the boundary. The southern limit of subduction lies south of our study area.
Plain Language Summary
At the convergent boundary between the southern Caribbean and northwest South America the Caribbean plate dips, or subducts, beneath South America at angles less than 30° for up to ∼300 km inland before steepening. Such low‐angle subduction systems are unique because they produce mountains hundreds of kilometers inland. Using new data collected by the Caribbean‐Mérida Andes seismic array, which continuously recorded earthquakes for two years, we imaged the Caribbean plate to depths of 660 km and identified at least three segments, including a segment detached from the surface. The timing of the detachment ∼5–15 million years ago aligns with the uplift of the Mérida Andes of Venezuela ∼500 km inland. Both the attached and detached segments interacted with the underside of South America, compressing parts of it, and causing uplift of the Mérida Andes. The northern limit of subduction lies south of the Oca‐Ancón fault of northern Colombia and Venezuela. The southern expanse of subduction includes the Bucaramanga earthquake cluster, but the boundary lies south of our study area. By understanding how the actively subducting plate, the Caribbean, deforms the upper plate, South America, we can better assess seismic risk and understand similar systems that are no longer active.
Key Points
At least three Caribbean segments subduct under South America, one detached under the Merida Andes. Detachment due to lithospheric weakness
Post‐break recoupling and rollback of slab contribute to uplift of Mérida Andes
Northern subduction boundary identified
We investigate the global rupture parameters for deep and intermediate‐depth earthquakes. From measurements of rupture duration and radiated seismic energy we estimate stress drop, apparent stress, ...and radiation efficiency and obtain a detailed earthquake energy budget. From scaling of the source parameters we highlight differences between crustal and deep seismicity, with the latter showing larger fracture energies. The observed increase of radiation efficiency with depth suggests that rupture mechanism for deep and intermediate‐depth events differs. In agreement with previous studies we observe along‐strike variability of rupture properties for deep and intermediate‐depth earthquakes, correlating with slab morphology, plate age, or presence of volcanic structures.
Key Points
Systematical analysis of rupture properties for deep and intermediate‐depth earthquakes
Scaling of stress drop and apparent stress is different from the ones estimated for crustal earthquakes
Strong frictional resistance characterizes deep and intermediate‐depth earthquakes
Thank You to Our 2023 Peer Reviewers Rajaram, Harihar; Aiyyer, Anantha; Camargo, Suzana ...
Geophysical research letters,
16 May 2024, Letnik:
51, Številka:
9
Journal Article
Recenzirano
Odprti dostop
On behalf of the journal, AGU, and the scientific community, the editors of Geophysical Research Letters would like to sincerely thank those who reviewed manuscripts for us in 2023. The hours reading ...and commenting on manuscripts not only improve the manuscripts, but also increase the scientific rigor of future research in the field. With the advent of AGU's data policy, many reviewers have also helped immensely to evaluate the accessibility and availability of data, and many have provided insightful comments that helped to improve the data presentation and quality. We greatly appreciate the assistance of the reviewers in advancing open science, which is a key objective of AGU's data policy. We particularly appreciate the timely reviews in light of the demands imposed by the rapid review process at Geophysical Research Letters. We received 4,512 submissions in 2023 and 5,112 reviewers contributed to their evaluation by providing 8,587 reviews in total. We deeply appreciate their contributions.
Plain Language Summary
Individuals in italics provided three or more reviews for GRL in 2023.
Key Points
The editors thank the 2023 peer‐reviewers
Coda wave interferometry is applied to data from Community Seismic Network MEMS accelerometers permanently installed on nearly every floor of a 52-story steel moment-and-brace frame building in ...downtown Los Angeles. Wavefield data from the 2019 M7.1 Ridgecrest, California earthquake sequence are used to obtain impulse response functions, and time-varying damping ratios and shear-wave velocities are computed from them. The coda waves are used because of their increased sensitivity to changes in the building’s properties, and the approach is generalized to show that a building’s nonlinear response can be monitored through time-varying measurements of representative pseudo-linear systems in the time domain. The building was not damaged, but temporary nonlinear behavior observed during the strong motions provides a unique opportunity to test this method’s ability to map time-varying properties. Reference damping parameters and velocities are obtained from a month-long period during which no significant seismic activity had occurred. Damping ratios measured over narrow frequency bands increase by up to a factor of 4 over short time durations spanning the main shock, as well as M > 4.5 aftershocks and a foreshock. The largest damping ratio increases occur for the highest frequencies, and the increase is attributed to friction associated with structural and non-structural surface discontinuities which experience relative motion and impact during shaking, resulting in energy loss. Shear-wave velocities in the building’s east–west and north–south directions are found by applying a waveform stretching method to the direct and coda waves. The broadband velocities are reduced by as much as 10% during building shaking, and their restoration to pre-earthquake levels is found to be a function of shaking amplitudes. Until recently, these techniques had been limited by temporal and spatial sparsity of measurements, but in this study, variations of the impulse response functions are resolved over time scales of tens of seconds and on a floor-by-floor spatial scale.
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