We compute and analyze P wave spectra from earthquakes in southern California between 1989 and 2001 using a method that isolates source‐, receiver‐, and path‐dependent terms. We correct observed ...source spectra for attenuation using both fixed and spatially varying empirical Green's function methods. Estimated Brune‐type stress drops for over 60,000 ML = 1.5 to 3.1 earthquakes range from 0.2 to 20 MPa with no dependence on moment or local b value. Median computed stress drop increases with depth in the upper crust, from about 0.6 MPa at the surface to about 2.2 MPa at 8 km, where it levels off and remains nearly constant in the midcrust down to about 20 km. However, the results at shallow depths could also be explained as reduced rupture velocities near the surface rather than a change in stress drop. Spatially coherent variations in median stress drop are observed, with generally low values for the Imperial Valley and Northridge aftershocks and higher values for the eastern Transverse ranges and the north end of the San Jacinto fault. We find no correlation between observed stress drop and distance from the San Andreas and other major faults. Significant along‐strike variations in stress drop exist for aftershocks of the 1992 Landers earthquake, which may correlate with differences in main shock slip.
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
Geophysical interpretation of potential field data plays an important role in the integration of geological data. Estimation of density and magnetic susceptibility variations within the upper crust ...helps evaluating the continuity of geological structures in the field. In the present study we use gravity and magnetic data in NW Amazonian Craton in Colombia. Total horizontal gradient of the reduction to magnetic pole were used to delineate magnetic lineaments and domains showing four zones, each with its own features. Multiscale edge detection (worming) of the data help delineate upper crustal structures that we interpret as tectonic boundaries that correlate with the four zones identified. 3D density and magnetic susceptibility inversion showed high density and/or high magnetic susceptibility sources correlated with these crustal structures. Zone (1) is located south of the Guaviare River, with predominant NW-SE and NE-SW magnetic lineaments; zone (2), located from south of the Guaviare River to the north, present nearly E-W magnetic lineaments and a deep E-W edge interpreted as a possible shear zone parallel to Guaviare, Orinoco and Ventuari rivers; zone (3) from south of the Vichada River to the north, with NE-SW and NW-SE lineaments; N-S zone (4) cuts the zones (2) and (3), characterized by high density/magnetic susceptibility source bounded by N-S deep edges. A more complete tectonic evolution interpretation requires further work, but we speculate that the zone (4) could indicate an aborted rift/collision suture and that the zone (2) is indicative of a younger deformation event. Shear direction at (2) is not clear: geological maps show NEE-SWW right-lateral faulting, but geophysical anomalies suggest left-lateral displacement, highlighted by left dislocation of the Orinoco River. We also speculate that a N-S edge located at the SE of the area can be related with the Atabapo Belt and the limit of Ventuari-Tapajós and Rionegro geochronological provinces.
•Multiscale edge detection showed possible tectonic limits within NW Amazon Craton.•E-W edge is interpreted as a younger deformation event.•N-S high density/magnetic susceptibility source represents old suture zone.•Interpretation of major structures help to improve the tectonic model of the area.
SUMMARY
Here, we evaluate the improvement in noise correlation functions (NCFs) gained by dividing ambient seismic records into shorter, overlapping time windows before correlation and stacking ...(Welch's method). We compare waveform convergence of short duration NCF stacks (e.g. 2, 5, 15 and 50 d stacks) towards the long‐term (365 d) NCF stack. We observe short duration NCF improvement when applying Welch's method for non–pre‐processed and running normalized time‐series and short duration NCF degradation when applied to a ‘one‐bit’ normalized time‐series. Surprisingly, non–pre‐processed time‐series provides the quickest convergence to a robust (year‐long) NCF. Because of the simplicity of Welch's method, the improved NCF convergence and a minimal increase in computation, we recommend applying Welch's method for future ambient seismic field analyses. Using this approach will likely improve future NCF analyses, particularly for studies with limited duration recordings, high levels of intermittent local or site noise and studies attempting to evaluate temporal variations in subsurface structure.
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.
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
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
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
The waves generated by faulting represent the primary threat posed by most large earthquakes. The effect of complex geological structures, such as sedimentary basins, on earthquake ground motion is a ...source of particular concern. We show that it is possible to extract reliable phase and amplitude response that includes the effects of complex structure for the elastodynamic Green's function from the ambient seismic field using deconvolution. We demonstrate the accuracy of the approach by predicting complex ground motion for a moderate (Mw = 4.6) earthquake in southern California as recorded in the Los Angeles Basin. This suggests a novel approach to seismic hazard analysis in which ground motion from hypothetical future earthquakes is simulated directly, without the need for modeling the detailed heterogeneity of the Earth's crust as an intermediate step.
PDF
