A noticeable decrease in seismic noise was registered worldwide during the lockdown measures implemented in 2020 to prevent the spread of COVID-19. In Central America, strong lockdown measures started during March of 2020. In this study, we used seismic stations from Costa Rica, Guatemala, El Salvador, and Nicaragua to study the effects of these measures on seismic records by characterizing temporal variations in the high-frequency band (4–14 Hz) via spectral and amplitude analyses. In addition, we studied the link between the reduction in seismic noise and the number of earthquake detections and felt reports in Costa Rica and Guatemala. We found that seismic stations near the capitals of Costa Rica, Guatemala, and El Salvador presented a decrease in their typical seismic noise levels, from 200 to 140, from 100 to 80, and from 120 to 80 nm, respectively. Our results showed that the largest reduction of ∼ 50 % in seismic noise was observed at seismic stations near main airports, busy roads, and densely populated cities. In Nicaragua, the seismic noise levels remained constant (∼ 40 nm), as no lockdown measures were applied. We suggest that the decrease in seismic noise levels may have increased earthquake detections and the number of felt reports of low-magnitude earthquakes. However, the variations observed in several seismic parameters before and after the lockdown are not significant enough to easily link our observations or separate them from other contributing factors. Our results imply that the study of seismic noise levels can be useful to verify compliance with lockdown measures and to explore their effects on earthquake detection and felt reports.
Abstract. In multiply deformed terrains multiphase boudinage is common, but identification and analysis of these is difficult. Here we present an analysis of multiphase boudinage and fold structures in deformed amphibolite layers in marble from the migmatitic centre of the Naxos metamorphic core complex. Overprinting between multiple boudinage generations is shown in exceptional 3-D outcrop. We identify five generations of boudinage, reflecting the transition from high-strain high-temperature ductile deformation to medium- to low-strain brittle boudins formed during cooling and exhumation. All boudin generations indicate E–W horizontal shortening and variable direction of bedding parallel extension, evolving from subvertical extension in the earliest boudins to subhorizontal N–S extension during exhumation. Two phases of E–W shortening can be inferred, the first associated with lower crustal synmigmatic convergent flow and the second associated with exhumation and N–S extension, possibly related to movement of the North Anatolian Fault.
Abstract. A multidisciplinary study has been carried out in Hontomín (Spain) to determine the basement structural setting, its geometry and the geometry of the sedimentary succession of an area aimed to be the first Spanish pilot plant for CO2 storage. An integration of coincident 3-D seismic results, borehole data and unpublished microgravity data were used to reproduce the deep structure and topography of the basement and to quantify the thickness of the Triassic Keuper evaporites. The subsurface structure is characterized by a half-graben setting filled with Keuper evaporites (up to 2000 m thick), forming an extensional forced fold. All data sets clearly identify two main fault systems, compartmentalizing the main structural domain into three differentiated blocks. These faults have been interpreted to be reactivated normal faults that have led to the formation of the Hontomín dome.
The Antarctic bedrock is evolving as the solid Earth responds to the past and ongoing evolution of the ice sheet. A recently improved ice loading history suggests that the Antarctic Ice Sheet (AIS) has generally been losing its mass since the Last Glacial Maximum. In a sustained warming climate, the AIS is predicted to retreat at a greater pace, primarily via melting beneath the ice shelves. We employ the glacial isostatic adjustment (GIA) capability of the Ice Sheet System Model (ISSM) to combine these past and future ice loadings and provide the new solid Earth computations for the AIS. We find that past loading is relatively less important than future loading for the evolution of the future bed topography. Our computations predict that the West Antarctic Ice Sheet (WAIS) may uplift by a few meters and a few tens of meters at years AD 2100 and 2500, respectively, and that the East Antarctic Ice Sheet is likely to remain unchanged or subside minimally except around the Amery Ice Shelf. The Amundsen Sea Sector in particular is predicted to rise at the greatest rate; one hundred years of ice evolution in this region, for example, predicts that the coastline of Pine Island Bay will approach roughly 45 mm yr−1 in viscoelastic vertical motion. Of particular importance, we systematically demonstrate that the effect of a pervasive and large GIA uplift in the WAIS is generally associated with the flattening of reverse bed slope, reduction of local sea depth, and thus the extension of grounding line (GL) towards the continental shelf. Using the 3-D higher-order ice flow capability of ISSM, such a migration of GL is shown to inhibit the ice flow. This negative feedback between the ice sheet and the solid Earth may promote stability in marine portions of the ice sheet in the future.
