An official website of the United States government
Abstract Geologic and geodetic observations provide constraints on tectonic and earthquake cycle kinematics. Block models offer one approach to integrating the effects of plate rotations, elastic strain accumulation, applied basal displacements, internal block strain, and idealized pressure sources. Here, we describe the construction of block models where spatially variable slip rates are parameterized by distance‐weighted eigenmodes operating over meshes of triangular dislocation elements. This dimensionally reduced model is recast as a quadratic programming problem with upper and lower bounds on both geologic fault slip rates and spatially variable slip deficit rates. We propose iterating over successive quadratic programming estimates with evolving slip rate bounds to find a solution consistent with specified coupling at all points on geometrically complex fault surfaces.
more »
« less
Surface deformation associated with fractures near the 2019 Ridgecrest earthquake sequence
Contemporary earthquake hazard models hinge on an understanding of how strain is distributed in the crust and the ability to precisely detect millimeter-scale deformation over broad regions of active faulting. Satellite radar observations revealed hundreds of previously unmapped linear strain concentrations (or fractures) surrounding the 2019 Ridgecrest earthquake sequence. We documented and analyzed displacements and widths of 169 of these fractures. Although most fractures are displaced in the direction of the prevailing tectonic stress (prograde), a large number of them are displaced in the opposite (retrograde) direction. We developed a model to explain the existence and behavior of these displacements. A major implication is that much of the prograde tectonic strain is accommodated by frictional slip on many preexisting faults.
more »
« less
- Award ID(s):
- 1834807
- PAR ID:
- 10199806
- Publisher / Repository:
- American Association for the Advancement of Science (AAAS)
- Date Published:
- Journal Name:
- Science
- Volume:
- 370
- Issue:
- 6516
- ISSN:
- 0036-8075
- Page Range / eLocation ID:
- p. 605-608
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract In extending volcanic arcs such as the Aegean, tectonic processes exert a significant control on magmatism. Spanning scales from 1 to 10s of km, volcanic vents, edifices, and eruptive centers follow the orientation of, and are located near, fault zones. Whether this tectonic control on magmatism results from individual faults/fractures weakening the crust or because regional stresses control magma input into the crust is debated. Here we investigate the scales of tectonic and magmatic interactions, specifically focusing on the role of local‐scale (<10 km) faults/fractures in controlling magmatism. We infer local‐scale fault/fracture orientations from anisotropic active‐source P‐wave travel‐time tomography to investigate tectonic and magmatic interactions in the upper crust of Santorini Volcano, Greece, and the actively deforming region to the east. We use the anisotropy magnitude and seismic velocity reduction to model the relative distribution of both consistently oriented and randomly oriented faults/fractures. Our results show that oriented faulting/fracturing resulting from regional‐scale (>10 km) tectonic stresses is distributed broadly across the region at 2–3 km depth, approximately paralleling volcanic/magmatic features. On a local‐scale, magmatism is neither localized in areas of higher oriented fault/fracture density, nor is it accommodating enough extensional strain to inhibit oriented faulting/fracturing of host rock. The alignment of magmatic features shows strong tectonic control despite the lack of correlation with local oriented fault/fracture density. These results suggest that magmatic processes are strongly influenced by regional‐scale, not local‐scale, tectonic processes. We infer regional processes have a greater impact on magmatism than local features due to their greater effect at depth.more » « less
-
Abstract Understanding the stress state before and after an earthquake is essential to study how stress on faults evolves during the seismic cycle. This study integrates wellbore failure analysis, laboratory experiments, and edge dislocation model to study the stress state before and after the Chi‐Chi earthquake. The post‐earthquake in‐situ stress state observed along boreholes of the Taiwan Chelungpu‐fault Drilling Project (TCDP) is heterogeneous due to lithological variations. Along the borehole, we observe that drilling‐induced tensile fractures are only present in sandstones, whereas breakouts are mostly present in silt‐rich rocks. Laboratory experiments on TCDP cores also show that tensile and compressive strength are weaker in sandstones than in silt‐rich rocks. These observations imply that both maximum and minimum horizontal principal stresses are higher in silt‐rich intervals. Extended leak‐off tests in the TCDP borehole also show lower minimum horizontal stress in sand‐rich intervals, consistent with the above observations. We combine these observations to estimate a profile of stress magnitudes along the well which explains the variability of stress states found in previous studies. The stress heterogeneity we observed underlines the importance of acknowledging the spatial scale that the stress data represent. We then use an edge dislocation model constrained by GPS surface displacements obtained during Chi‐Chi earthquake to calculate the coseismic stress changes. Our inferred pre‐earthquake stress magnitudes, obtained by subtracting the coseismic stress change from the post‐earthquake stress, suggest subcritical stress state before the earthquake despite the large displacements observed during the Chi‐Chi earthquake in the region where TCDP encountered the fault.more » « less
-
Abstract Ice shelves are assumed to flow steadily from their grounding lines to the ice front. We report the detection of ice‐propagating extensional Lamb (plate) waves accompanied by pulses of permanent ice shelf displacement observed by co‐located Global Navigation Satellite System receivers and seismographs on the Ross Ice Shelf. The extensional waves and associated ice shelf displacement are produced by tidally triggered basal slip events of the Whillans Ice Stream, which flows into the ice shelf. The propagation velocity of 2,800 m/s is intermediate between shear and compressional ice velocities, with velocity and particle motions consistent with predictions for extensional Lamb waves. During the passage of the Lamb waves the entire ice shelf is displaced about 60 mm with a velocity more than an order of magnitude above its long‐term flow rate. Observed displacements indicate a peak dynamic strain of 10−7, comparable to that of earthquake surface waves that trigger ice quakes.more » « less
-
Cascading Hazards in a Migrating Forearc‐Arc System: Earthquake and Eruption Triggering in NicaraguaAbstract Strain partitioning in oblique convergent margins results in margin‐parallel shear in the overriding plate. Margin‐parallel shear is often accommodated by margin‐parallel strike‐slip faults proximal to active volcanic arcs. Along the Nicaraguan segment of the Central American Forearc (CAFA) in the Cocos‐Caribbean plate convergent margin, there are no well‐expressed right‐lateral faults that accommodate CA‐CAFA relative motion. Instead, historical earthquakes and mapped fault orientations indicate that the ∼12 mm/yr of dextral motion is accommodated on arc‐normal, left‐lateral faults (i.e., bookshelf faults). We investigate three upper‐plate earthquakes; the 10 April 2014 (Mw6.1), 15 September 2016 (Mw5.7), and 28 September 2016 (Mw5.5), using Global Position System co‐seismic displacements and relocated earthquake aftershocks. Our analyses of the three earthquakes indicate that the 10 April 2014 earthquake ruptured an unmapped margin‐parallel right‐lateral fault in Lago Xolotlán (Managua) and the September 2016 earthquakes ruptured arc‐normal, left‐lateral and oblique‐slip faults. These earthquakes represent a triggered sequence whereby the 10 April 2014 earthquake promoted failure of the faults that ruptured in September 2016 by imparting a static Coulomb stress change (ΔCFS) of 0.02–0.07 MPa. Likewise, the 15 September 2016, earthquake additionally promoted failure (ΔCFS of 0.08–0.1 MPa) on sub‐parallel faults that ruptured in two subsequent earthquakes. We also present an instance of magma‐tectonic interaction whereby the 10 April 2014 earthquake dilated (10s of μStrain) the shallow magmatic system of Momotombo Volcano, which led to magma injection, ascent, and eruption on 1 December 2015, after ∼100 years of quiescence.more » « less
