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Repeating earthquakes (REs) rupture the same fault patches at different times allowing temporal variations in the mechanical behavior of specific areas of the fault to be interrogated over the earthquake cycle. We study REs that reveal fault weakening after a large megathrust earthquake in Costa Rica, followed by fault recovery. We find shorter RE recurrence intervals and larger slip areas immediately following the mainshock that both gradually return to pre-earthquake values. RE seismic moments remain nearly constant throughout the earthquake cycle. This implies a balance between fault weakening (reducing slip) and transient embrittlement (increasing rupture area by converting regions from aseismic to seismic slip), induced by the increased loading rate following the mainshock. This interpretation is consistent with positive, negative, and constant moment versus RE recurrence interval trends reported in other studies following large earthquakes and with experimental work showing slip amplitudes and stress drop decrease with loading rate., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).)

In subduction zones, elevated pore fluid pressure, generally linked to metamorphic dehydration reactions, has a profound influence on the mechanical behavior of the plate interface and forearc crust through its control on effective stress. We use seismic noise-based monitoring to characterize seismic velocity variations following the 2012 Nicoya Peninsula, Costa Rica earthquake [M w (moment magnitude) 7.6] that we attribute to the presence of pressurized pore fluids. Our study reveals a strong velocity reduction (~0.6%) in a region where previous work identified high forearc pore fluid pressure. The depth of this velocity reduction is constrained to be below 5 km and therefore not the result of near-surface damage due to strong ground motions; rather, we posit that it is caused by fracturing of the fluid-pressurized weakened crust due to dynamic stresses. Although pressurized fluids have been implicated in causing coseismic velocity reductions beneath the Japanese volcanic arc, this is the first report of a similar phenomenon in a subduction zone setting. It demonstrates the potential to identify pressurized fluids in subduction zones using temporal variations of seismic velocity inferred from ambient seismic noise correlations.

Naccobus aberrans is a major pest of the potato crop in the Andean regions of Argentina, Bolivia, and Perú. It is endemic in northwest Argentina and is also found in lowlands. The resistance of eleven Andean potato landraces and three accessions of the wild tuber-bearing species Solanum acaule, S. infundibuliforme, and S. megistacrolobum were evaluated against a population of N. aberrans from Coctaca, Jujuy province, while Solanum tuberosum ssp. tuberosum 'Spunta', 'Kennebec', and 'Frital INTA' were evaluated against a population from the southeast of Buenos Aires province. The presence, the number of galls, and the number of individuals were recorded. In addition, a reproduction factor was calculated and races were determined. Results showed that the N. aberrans population from Coctaca corresponded to race 2 and the population from the lowlands belonged to the sugar beet group. Landrace Azul, one genotype of S. megistacrolobum, and two genotypes of S. acaule showed resistance towards the race from Coctaca while no infection was recorded in potato cultivars with the Naccobus race from the lowland area.