ALBERT

Description: On April 1st, 2014, a Mw 8.2 (U.S. Geological Survey moment magnitude) earthquake occurred in the subduction zone offshore northern Chile. In the two weeks leading up to the earthquake, a sequence of foreshocks, starting with a Mw 6.7 earthquake on March 16th and including three more Mw 6.0+ events, occurred predominantly south of the April 1st mainshock epicenter and up-dip of the area of significant slip during the mainshock. Using earthquake locations and source parameters derived in a previous study (Hayes et al., 2014) and a Coulomb failure stress change analysis of these events, we assess in detail the hypothesis that the earthquakes occurred as a cascading sequence, each event successively triggering the next, ultimately triggering the rupture of the mainshock. Following the initial Mw 6.7 event, each of the three largest foreshocks (Mw 6.4, 6.2 and 6.3), as well as the hypocenter of the mainshock, occurred in a region of positive Coulomb stress change produced by the preceding events, indicating these events were brought closer to failure by the prior seismicity. In addition, we reexamine the possibility that aseismic slip occurred and what role it may have played in loading the plate boundary. Using horizontal GPS displacements from along the northern Chile coast prior to the mainshock, we find that the foreshock seismicity alone likely does not account for the observed signals. We perform a grid search for the location and magnitude of an aseismic slip patch that can account for the difference between observed signals and foreshock-related displacement, and find that a slow slip region with slip corresponding to a Mw ∼ 6.8 earthquake located coincident with or up-dip of the foreshock seismicity can best explain this discrepancy. Additionally, such a slow slip region positively loads the mainshock hypocentral area, enhancing the positive loading produced by the foreshock seismicity.

Description: The seismic gap theory1 identifies regions of elevated hazard based on a lack of recent seismicity in comparison with other portions of a fault. It has successfully explained past earthquakes (see, for example, ref. 2) and is useful for qualitatively describing where large earthquakes might occur. A large earthquake had been expected in the subduction zone adjacent to northern Chile3, 4, 5, 6, which had not ruptured in a megathrust earthquake since a M ~8.8 event in 1877. On 1 April 2014 a M 8.2 earthquake occurred within this seismic gap. Here we present an assessment of the seismotectonics of the March–April 2014 Iquique sequence, including analyses of earthquake relocations, moment tensors, finite fault models, moment deficit calculations and cumulative Coulomb stress transfer. This ensemble of information allows us to place the sequence within the context of regional seismicity and to identify areas of remaining and/or elevated hazard. Our results constrain the size and spatial extent of rupture, and indicate that this was not the earthquake that had been anticipated. Significant sections of the northern Chile subduction zone have not ruptured in almost 150 years, so it is likely that future megathrust earthquakes will occur to the south and potentially to the north of the 2014 Iquique sequence.

Description: Greater landward velocities were recorded after six megathrust earthquakes in subduction zone regions adjacent to the ruptured portion. Previous explanations invoked either increased slip deficit accumulation or plate bending during postseismic relaxation, with different implications for seismic hazard. We investigate whether bending can be expected to reproduce this observed enhanced landward motion (ELM). We use 3D quasi-dynamic finite element models with periodic earthquakes. We find that afterslip downdip of the brittle megathrust exclusively produces enhanced trenchward surface motion in the overriding plate. Viscous relaxation produces ELM when a depth limit is imposed on afterslip. This landward motion results primarily from in-plane elastic bending of the overriding plate due to trenchward viscous flow in the mantle wedge near the rupture. Modeled ELM is, however, incompatible with the observations, which are an order of magnitude greater and last longer after the earthquake. This conclusion does not significantly change when varying mantle viscosity, plate elasticity, maximum afterslip depth, earthquake size, megathrust locking outside of the rupture, or nature and location of relevant model boundaries. The observed ELM consequently appears to reflect faster slip deficit accumulation, implying a greater seismic hazard in lateral segments of the subduction zone.

Description: Prior to July 2020, the Alaska-Aleutian subduction zone near the Shumagin Islands had not experienced a large, Mw7.0+ earthquake in over 75 years. The previous major events in this region were the great (Mw8.0+) 1938 and 1946 earthquakes. These events occurred on either side of the “Shumagin Seismic Gap”, a region that has not hosted a great megathrust earthquake in recorded history. Attention to this region was renewed with the occurrence of three large earthquakes: the 22 July 2020 Mw7.8 Simeonof megathrust event, the 17 October 2020 Mw 7.6 Sand Point intraslab (strike-slip) event, and the 29 July 2021 Mw8.2 Chignik megathrust event. The two 2020 events and subsequent aftershock activity were shown to be consistent with a transition from high plate interface coupling east of the Shumagin Gap to a generally uncoupled megathrust in the Shumagin Gap. The 2021 Chignik earthquake was located in a section of the subduction zone that hosted previous large earthquakes and that was brought closer to failure by the seismicity of the previous year. Analyses of the Chignik earthquake sequence and the previous large earthquakes provide additional constraints on the transition from coupled to uncoupled along the Aleutian megathrust and implications for earthquake processes. In particular, the evolution of events in this sequence is compatible with occurring in the stress field produced by the coupling transition, which also allowed more complete elastic strain release during the large earthquakes.

Description: Heat flow data collected along the San Andreas plate boundary and in the subduction to translation transition at the Mendocino Triple Junction (MTJ) have played a defining role for the geodynamic processes associated with this fundamental change in plate boundary tectonics. In the 1980s, heat flow data led to the recognition that the frictional strength of the San Andreas fault is low (Lachenbruch and Sass, 1980), and there is a slab window in the wake of MTJ migration (Zandt and Furlong, 1982). More recently, heat flow data constrain crustal deformation along the San Andreas corridor (Mendocino Crustal Conveyor; Furlong and Govers, 1999; Guzofski and Furlong, 2002; and Furlong and Schwartz , 2004). These results explain the magnitude and spatial wavelength of the transition from low heat flow within the Cascadia subduction zone to elevated heat flow along the San Andreas plate boundary. Within this broad low-to-high heat flow pattern, there are locations of anomalous heat flow that remain enigmatic. In the region straddling the MTJ, observed heat flow values (75 mW/m2) are substantially higher than are seen both north and south of that region (~ 35 - 50 mW/m2). Combining these heat flow data with new crustal and upper-mantle seismic tomography and tectono-thermal modeling, we constrain the processes of MTJ-related crustal deformation and place the elevated seismicity in the vicinity of the MTJ into its seismo-tectonic context. The elevated heat flow is most likely a consequence of rapid exhumation following the passage of the southern edge of the subducting slab.