Postseismic deformation following subduction earthquakes includes the combined effects of afterslip surrounding the coseismic rupture areas and viscoelastic relaxation in the asthenosphere and provides unique and valuable information for understanding the rheological structure. Because the two postseismic mechanisms are usually spatiotemporally intertwined, we developed an integrated model combining their contributions, based on 5 years of observations following the 2016 Pedernales (Ecuador) earthquake. The results show that the early, near‐field postseismic deformation is dominated by afterslip, both updip and downdip of the coseismic rupture, and requires heterogeneous interface frictional properties. Viscoelastic relaxation contributes more to far‐field displacements at later time periods. The best‐fit integrated model favors a 45‐km thick lithosphere overlying a Burgers body viscoelastic asthenosphere with a Maxwell viscosity of 3 × 1019 Pa s (0.9–5 × 1019 Pa s at 95% confidence), assuming the Kelvin viscosity equal to 10% of that value. In addition to the postseismic afterslip, the coastal displacement of sites north and south of the rupture clearly require extra slip in the plate motion direction due to slow slip events that may be triggered by the coseismic stress changes (CSC) but are not purely driven by the CSC. Spatially variable afterslip following the Pedernales event, combined with the SSEs during the interseismic period, demonstrate that spatial frictional variability persists throughout the whole earthquake cycle. The interaction of adjacent fault patches with heterogeneous properties may contribute to the clustered large earthquakes in this area.
The heterogeneous seafloor topography of the Nazca Plate as it enters the Ecuador subduction zone provides an opportunity to document the influence of seafloor roughness on slip behavior and megathrust rupture. The 2016 Mw7.8 Pedernales Ecuador earthquake was followed by a rich and active postseismic sequence. An internationally coordinated rapid response effort installed a temporary seismic network to densify coastal stations of the permanent Ecuadorian national seismic network. A combination of 82 onshore short and intermediate period and broadband seismic stations and six ocean bottom seismometers recorded the postseismic Pedernales sequence for over a year after the mainshock. A robust earthquake catalog combined with calibrated relocations for a subset of magnitude ≥4 earthquakes shows pronounced spatial and temporal clustering. A range of slip behavior accommodates postseismic deformation including earthquakes, slow slip events, and earthquake swarms. Models of plate coupling and the consistency of earthquake clustering and slip behavior through multiple seismic cycles reveal a segmented subduction zone primarily controlled by subducted seafloor topography, accreted terranes, and inherited structure. The 2016 Pedernales mainshock triggered moderate to strong earthquakes (5 ≤ M ≤ 7) and earthquake swarms north of the mainshock rupture close to the epicenter of the 1906 Mw8.8 earthquake and in the segment of the subduction zone that ruptured in 1958 in a Mw7.7 earthquake.
more » « less- NSF-PAR ID:
- 10375950
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 125
- Issue:
- 2
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract -
So far in this century, six very large–magnitude earthquakes ( M W ≥ 7.8) have ruptured separate portions of the subduction zone plate boundary of western South America along Ecuador, Peru, and Chile. Each source region had last experienced a very large earthquake from 74 to 261 y earlier. This history led to their designation in advance as seismic gaps with potential to host future large earthquakes. Deployments of geodetic and seismic monitoring instruments in several of the seismic gaps enhanced resolution of the subsequent faulting processes, revealing preevent patterns of geodetic slip deficit accumulation and heterogeneous coseismic slip on the megathrust fault. Localized regions of large slip, or asperities, appear to have influenced variability in how each source region ruptured relative to prior events, as repeated ruptures have had similar, but not identical slip distributions. We consider updated perspectives of seismic gaps, asperities, and geodetic locking to assess current very large earthquake hazard along the South American subduction zone, noting regions of particular concern in northern Ecuador and Colombia (1958/1906 rupture zone), southeastern Peru (southeasternmost 1868 rupture zone), north Chile (1877 rupture zone), and north-central Chile (1922 rupture zone) that have large geodetic slip deficit measurements and long intervals (from 64 to 154 y) since prior large events have struck those regions. Expanded geophysical measurements onshore and offshore in these seismic gaps may provide critical information about the strain cycle and fault stress buildup late in the seismic cycle in advance of the future great earthquakes that will eventually strike each region.more » « less
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SUMMARY The Ecuadorian forearc is a complex region of accreted terranes with a history of large megathrust earthquakes. Most recently, a Mw 7.8 megathrust earthquake ruptured the plate boundary offshore of Pedernales, Ecuador on 16 April 2016. Following this event, an international collaboration arranged by the Instituto Geofisico at the Escuela Politécnica Nacional mobilized a rapid deployment of 65 seismic instruments along the Ecuadorian forearc. We combine this new seismic data set with 14 permanent stations from the Ecuadorian national network to better understand how variations in crustal structure relate to regional seismic hazards along the margin. Here, we present receiver function adaptive common conversion point stacks and a shear velocity model derived from the joint inversion of receiver functions and surface wave dispersion data obtained through ambient noise cross-correlations for the upper 50 km of the forearc. Beneath the forearc crust, we observe an eastward dipping slow velocity anomaly we interpret as subducting oceanic crust, which shallows near the projected centre of the subducting Carnegie Ridge. We also observe a strong shallow positive conversion in the Ecuadorian forearc near the Borbon Basin indicating a major discontinuity at a depth of ∼7 km. This conversion is not ubiquitous and may be the top of the accreted terranes. We also observe significant north–south changes in shear wave velocity. The velocity changes indicate variations in the accreted terranes and may indicate an increased amount of hydration beneath the Manabí Basin. This change in structure also correlates geographically with the southern rupture limit of multiple high magnitude megathrust earthquakes. The earthquake record along the Ecuadorian trench shows that no event with a Mw >7.4 has ruptured south of ∼0.5°S in southern Ecuador or northern Peru. Our observations, along with previous studies, suggest that variations in the forearc crustal structure and subducting oceanic crust may influance the occurrence and spatial distribution of high magnitude seismicity in the region.more » « less
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SUMMARY The Ecuadorian convergent margin has experienced many large mega-thrust earthquakes in the past century, beginning with a 1906 event that propagated along as much as 500 km of the plate interface. Many subsections of the 1906 rupture area have subsequently produced Mw ≥ 7.7 events, culminating in the 16 April 2016, Mw 7.8 Pedernales earthquake. Interestingly, no large historic events Mw ≥ 7.7 appear to have propagated southward of ∼1°S, which coincides with the subduction of the Carnegie Ridge. We combine data from temporary seismic stations deployed following the Pedernales earthquake with data recorded by the permanent stations of the Ecuadorian national seismic network to discern the velocity structure of the Ecuadorian forearc and Cordillera using ambient noise tomography. Ambient noise tomography extracts Vsv information from the ambient noise wavefield and provides detailed constraints on velocity structures in the crust and upper mantle. In the upper 10 km of the Ecuadorian forearc, we see evidence of the deepest portions of the sedimentary basins in the region, the Progreso and Manabí basins. At depths below 30 km, we observe a sharp delineation between accreted fast forearc terranes and the thick crust of the Ecuadorian Andes. At depths ∼20 km, we see a strong fast velocity anomaly that coincides with the subducting Carnegie Ridge as well as the southern boundary of large mega-thrust earthquakes. Our observations raise the possibility that upper-plate structure, in addition to the subducting Carnegie Ridge, plays a role in the large event segmentation seen along the Ecuadorian margin.more » « less
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Abstract Slow slip events (SSEs) have been observed in spatial and temporal proximity to megathrust earthquakes in various subduction zones, including the 2014
M w7.3 Guerrero, Mexico earthquake which was preceded by aM w7.6 SSE. However, the underlying physics connecting SSEs to earthquakes remains elusive. Here, we link 3D slow‐slip cycle models with dynamic rupture simulations across the geometrically complex flat‐slab Cocos plate boundary. Our physics‐based models reproduce key regional geodetic and teleseismic fault slip observations on timescales from decades to seconds. We find that accelerating SSE fronts transiently increase shear stress at the down‐dip end of the seismogenic zone, modulated by the complex geometry beneath the Guerrero segment. The shear stresses cast by the migrating fronts of the 2014M w7.6 SSE are significantly larger than those during the three previous episodic SSEs that occurred along the same portion of the megathrust. We show that the SSE transient stresses are large enough to nucleate earthquake dynamic rupture and affect rupture dynamics. However, additional frictional asperities in the seismogenic part of the megathrust are required to explain the observed complexities in the coseismic energy release and static surface displacements of the Guerrero earthquake. We conclude that it is crucial to jointly analyze the long‐ and short‐term interactions and complexities of SSEs and megathrust earthquakes across several (a)seismic cycles accounting for megathrust geometry. Our study has important implications for identifying earthquake precursors and understanding the link between transient and sudden megathrust faulting processes.
