We identify and characterize four different low‐frequency earthquake (LFE) families (LFEs 1–4, ordered updip to downdip) that span the width of the transition zone in the Cascadia Subduction Zone beneath western Washington State. We find that LFE swarm duration, recurrence interval, and event size decrease systematically with increasing depth. LFE moments are observed to follow an exponential distribution rather than a power law distribution, allowing us to determine a characteristic event size for each family. Absolute locations of these LFE families place them very near the inferred Juan de Fuca‐North America plate interface. Relative relocation of individual LFEs within each family reveal elongation parallel to the plate convergence direction and narrow depth distributions with dips corresponding to plate interface models. We interpret the behavior of our LFE families as indicative of slow slip at those locations on the plate interface. The tidal response of LFE activity at LFEs 1, 2, and 3 during a large episodic tremor and slip (ETS) event follows a previously observed evolution, in which tidal response during the initial 1 to 2 days of slip at an LFE patch is much weaker than that during the next few days of slip. The brief bursts of activity at LFE4, although little affected by large episodic tremor and slip events, respond strongly to tidal stresses. Our observed along‐dip contrasts in recurrence behavior and tidal response provide evidence for stress transfer and increased frictional strength with position updip through the tremor zone.
The seismic moments observed for low‐frequency earthquakes (LFEs) vary over multiple orders of magnitude, even where the LFEs occur within families of similar events. Although this variability is typically interpreted to record a scale‐limited process at the LFE source, neither the slip per LFE nor the rupture area can be determined from seismological constraints. Here, we examine incrementally developed slickenfibers that have been proposed to record LFEs in exhumed subduction zones. These structures form through repeated, micron‐scale slip events across dilational irregularities in the fault plane, which are punctuated by cementation and sealing in the interstitial space. By statistically analyzing the geometry of inclusion trails delineating slip‐parallel mineral‐growth increments, we constrain the variability in slip per inferred LFE and test end‐member hypotheses regarding the controls on LFE moments. We find that that the slickenfibers exhibit characteristic slip increments, favoring a “slip‐limited” model that requires large variability in LFE rupture areas.
more » « less- Award ID(s):
- 2051565
- NSF-PAR ID:
- 10362995
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 49
- Issue:
- 2
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract -
more » « less
SUMMARY Earthquake ruptures are complex physical processes that may vary with the structure and tectonics of the region in which they occur. Characterizing the factors controlling this variability would provide fundamental constraints on the physics of earthquakes and faults. We investigate this by determining finite source properties from second moments of the stress glut for a global data set of large strike-slip earthquakes. Our approach uses a Bayesian inverse formulation with teleseismic body and surface waves, which yields a low-dimensional probabilistic description of rupture properties including the spatial deviation, directivity and temporal deviation of the source. This technique is useful for comparing events because it makes only minor geometric constraints, avoids bias due to rupture velocity parametrization and yields a full ensemble of possible solutions given the uncertainties of the data. We apply this framework to all great strike-slip earthquakes of the past three decades, and we use the resultant second moments to compare source quantities like directivity ratio, rectilinearity, average moment density and vertical deviation. We find that most strike-slip earthquakes have a large component of unilateral directivity, and many of these earthquakes show a mixture of unilateral and bilateral behaviour. We notice that oceanic intraplate earthquakes usually rupture a much larger width of the seismogenic zone than other strike-slip earthquakes, suggesting these earthquakes may often breach the expected thermal boundary for oceanic ruptures. We also use these second moments to resolve nodal plane ambiguity for the large oceanic intraplate earthquakes and find that the rupture orientation is usually unaligned with encompassing fossil fracture zones.
-
more » « less
Abstract Fault‐damage zones comprise multiscale fracture networks that may slip dynamically and interact with the main fault during earthquake rupture. Using 3D dynamic rupture simulations and scale‐dependent fracture energy, we examine dynamic interactions of more than 800 intersecting multiscale fractures surrounding a listric fault, emulating a major listric fault and its damage zone. We investigate 10 distinct orientations of maximum horizontal stress, probing the conditions necessary for sustained slip within the fracture network or activating the main fault. Additionally, we assess the feasibility of nucleating dynamic rupture earthquake cascades from a distant fracture and investigate the sensitivity of fracture network cascading rupture to the effective normal stress level. We model either pure cascades or main fault rupture with limited off‐fault slip. We find that cascading ruptures within the fracture network are dynamically feasible under certain conditions, including: (a) the fracture energy scales with fracture and fault size, (b) favorable relative pre‐stress of fractures within the ambient stress field, and (c) close proximity of fractures. We find that cascading rupture within the fracture network discourages rupture on the main fault. Our simulations suggest that fractures with favorable relative pre‐stress, embedded within a fault damage zone, may lead to cascading earthquake rupture that shadows main fault slip. We find that such off‐fault events may reach moment magnitudes up to
M w ≈ 5.5, comparable to magnitudes that can be otherwise hosted by the main fault. Our findings offer insights into physical processes governing cascading earthquake dynamic rupture within multiscale fracture networks. -
more » « less
Abstract Seismological fracture or breakdown energy represents energy expended in a volume surrounding the advancing rupture front and the slipping fault surface. Estimates are commonly obtained by inverting ground motions and using the results to model slip on the fault surface. However, this practice cannot identify contributions from different energy‐consumption processes, so our understanding of the importance of these processes comes largely from field‐ and laboratory‐based studies. Here, we use garnet fragment size data to estimate surface‐area energy density with distance from the fault core in the damage zone of a deeply exhumed strike‐slip fault/shear zone. Estimated energy densities per fragmentation event range from 2.87 × 103to 2.72 × 105 J/m3in the outer and inner portions of the dynamic damage zone, respectively, with the dynamic zone being inferred from the fractal dimensions of fragment size distributions and other indicators. Integrating over the ∼105 m width of the dynamic damage zone gives fracture surface‐area energy per unit fault area ranging from a lower bound of 6.63 × 105 J/m2to an upper bound of 1.63 × 107 J/m2per event. This range overlaps with most geological, theoretical, and kinematic slip‐model estimates of energy expenditure in the source volume for earthquakes characterized by seismic moments >1017 N·m. We employ physics‐based fragmentation models to estimate equivalent tensile strain rates associated with garnet fragmentation, which range from 5.42 × 102to 1.04 × 104 s−1per earthquake in the outer and inner portions of the dynamic damage zone, respectively. Our results suggest that surface‐energy generation is a nonnegligible component of the earthquake energy budget.
-
more » « less
Abstract High‐resolution biogenic and geologic proxies in which one increment or layer is formed per year are crucial to describing natural ranges of environmental variability in Earth's physical and biological systems. However, dating controls are necessary to ensure temporal precision and accuracy; simple counts cannot ensure that all layers are placed correctly in time. Originally developed for tree‐ring data, crossdating is the only such procedure that ensures all increments have been assigned the correct calendar year of formation. Here, we use growth‐increment data from two tree species, two marine bivalve species, and a marine fish species to illustrate sensitivity of environmental signals to modest dating error rates. When falsely added or missed increments are induced at one and five percent rates, errors propagate back through time and eliminate high‐frequency variability, climate signals, and evidence of extreme events while incorrectly dating and distorting major disturbances or other low‐frequency processes. Our consecutive Monte Carlo experiments show that inaccuracies begin to accumulate in as little as two decades and can remove all but decadal‐scale processes after as little as two centuries. Real‐world scenarios may have even greater consequence in the absence of crossdating. Given this sensitivity to signal loss, the fundamental tenets of crossdating must be applied to fully resolve environmental signals, a point we underscore as the frontiers of growth‐increment analysis continue to expand into tropical, freshwater, and marine environments.
