TOWARDS A PRECISE MODELLING OF THE EL SALVADOR FAULT ZONE USING GEODETIC TECHNIQUES

The El Salvador Fault Zone (ESFZ) comprises a set of a strike-slip faults, extending through the Central American VolcanicArc within El Salvador, where the Cocos plate subducts under the Caribbean plate. These structures act as a boundarybetween the forearc sliver and the western margin of the Chortís block, accommodating the relative movement betweenthem. The ESFZ has been responsible for several shallow, destructive earthquakes in El Salvador, thus posing a seriousthreat for millions of inhabitants. Understanding its seismic potential and the behaviour of its different segments results ofgreat importance for the assessment and mitigation of seismic risk in the region. Geodetic techniques, such as GNSS andInSAR, are useful tools for measuring surface deformation related to tectonic activity. We are in the process of updatingand densifying the existing GNSS velocity field in El Salvador, aiming to characterise the individual faults in the region bydetermining their slip rates and locking depth. Additionally, we will process InSAR data, trying to obtain a continuousmeasurement of the interseismic deformation. The combination of this information with other data (e.g. seismological andgeological) through kinematic models will allow us to better understand the factors controlling the seismogenic behaviourof the ESFZ faults, evaluate their seismic potential and improve the seismic hazard assessment.

Evidence for a Recent Formation of the Isthmus of Panama 0.6 Mya

The exact age of the final formation of the Isthmus of Panama is a critical reference point for oceanographic, climatic, biogeographic, and evolutionary hypotheses. Geotectonic evidence suggests that the isthmus was completed between 12 Mya and 3 Mya, and an age of 3–4 Mya has been used as a benchmark in hundreds of studies over the past 30 years. Phylogeographic data indicate the existence of marine connections across the isthmus much more recently, however. I reconsider the available geotectonic, biostratigraphic, oceanographic, and paleoclimatic data and show that multiple lines of indirect evidence suggest that four transisthmian seaways may have persisted until as recently as the onset of the Middle Pleistocene (~ 0.6 Mya). Subduction of the Cocos Plate beneath one transisthmian seaway (the only seaway featuring a deep sill) caused rapid tectonic shoaling and reorganisation of oceanic currents, which coincided with a major glacioeustatic sea level fall ~ 950–917 Mya that led to a temporary closing of the Bering Strait. This resulted in unusual and contrasting climate phenomena, including the “900-Kyr (cold) event” and the “greening” of South Greenland during the MIS 22 glacial maximum. The concurrence of the final formation of the Isthmus of Panama with the mid-Pleistocene Transition of glacial/interglacial periodicity suggests a tight relationship between these events.

Preliminary results on the characterisation of the Guaycume fault (El Salvador) from geodetic and seismological data

<p>The Guaycume fault is a right-lateral strike-slip structure located in Western El Salvador, within the El Salvador Fault Zone (ESFZ). The ESFZ consists of a strike-slip fault system extending through the Central American Volcanic Arc, on the western margin of the Chortís block, where the Cocos plate subducts under the Caribbean plate.</p><p>The Guaycume fault has been proposed as a possible source for the Mw 6.4 1917 El Salvador destructive earthquake, presenting high seismic potential in close proximity to San Salvador (Alonso-Henar et al., 2018). Its geomorphological expression has been clearly identified (Martinez-Diaz et al., 2016); however, few specific studies are currently published, and its behaviour and kinematics remain widely unknown. Notably, there is a lack of precise information about the amount of deformation that this fault currently absorbs of the westward movement (relative to the Chortís block) of the forearc sliver.</p><p>We process GNSS data in the area from 2007 to 2020 in order to retrieve the GNSS velocity field surrounding the Guaycume fault. We use these data to perform a thorough kinematic study, updating the previously existing slip rates (Staller et al., 2016). Combined with seismological data, this information allows us to understand the seismic cycle of the fault to a better extent, thus leading to a better comprehension of its seismic potential.</p>

The Mw7.1 September 19th Puebla-Morelos (Mexico) earthquake triggered by brucite and antigorite dewatering

<p>Subduction controls key geological processes at convergent margins including seismicity and resultant seismic hazard. The September 19th 2017 Mw7.1 Mexican earthquake nucleated ~250 km from the trench within the Cocos plate near its Moho, ~57 km below Earth’s surface. The prevailing hypothesis suggests that this earthquake resulted from bending stresses occurring at the flat-to-steep subduction transition. Here, we present an alternative, but not mutually exclusive, hypothesis: the dehydration reaction brucite + antigorite = olivine + H2O in the slab mantle controls intermediate-depth seismicity along the flat portion of the subducted Cocos plate. This reaction releases a substantial amount of H2O, resulting in a large positive volume change, and thus in an increase in pore fluid pressure at the appropriate depth–temperature conditions to cause the Puebla-Morelos and other intraslab earthquakes in Mexico. The amount of H2O released by this reaction depends on the degree of serpentinization of the oceanic mantle prior to subduction. Only oceanic mantle with > 60% serpentinization—as expected along abundant deep extensional faults at the mid-ocean-ridge or where the plate bends at the outer rise—will stabilize brucite, and thus, will experience this reaction at the same depths where the September 19th 2017 earthquake nucleated.</p>

Geometría de la zona sismogénica interplacas en el Sureste de Costa Rica a la luz de la secuencia de Golfito del 2018

Between August and November 2018, a seismic sequence took place in the vicinity of Golfito, a city in the Dulce Gulf in Southeastern Costa Rica. The main shock had a moment magnitude (Mw) of 6.1 and was widely felt in Costa Rica and Western Panama, with maximum Modified Mercalli intensities of VI. In this region, the oceanic Cocos Ridge, riding on top of the Cocos Plate, subducts beneath the Panama Microplate. Using the seismic records from the National Seismological Network of Costa Rica, in this work the seismicity is relocated using the double-difference technique, and an analysis of its temporal and geographic distribution together with the focal mechanism and intensities of the strongest events are presented. The results show that the sequence occurred at the interplate seismogenic zone, within the rupture area of the 1983 Golfito earthquake (7.4 Mw), between 12 and 27 km depth, in a cluster dipping 35º northeast underneath the Dulce Gulf. Based mainly on these results and on previous seismic sequences, it is here proposed that the seismogenic zone in Southeastern Costa Rica has an extension of ~160 x 45 km. Further, during the Golfito sequence, the rupture of an inverse fault (5.9 Mw) took place within the Cocos Plate beneath the Dulce Gulf, as well as of dextral strike-slip faults (4.6-5.6 Mw) in the Panama Microplate, 50 km away of the Dulce Gulf. The analysis of the interseismic interplate seismicity contributes to a better understating of the dynamics of the seismogenic zone. This is of particular relevance in Southeastern Costa Rica, where at least six damaging earthquakes of Mw > 7 have occurred since 1803, implying the impending risk of the next big earthquake in this region.

Conductive Channels in the Deep Oceanic Lithosphere Could Consist of Garnet Pyroxenites at the Fossilized Lithosphere–Asthenosphere Boundary

Magnetotelluric (MT) surveys have identified anisotropic conductive anomalies in the mantle of the Cocos and Nazca oceanic plates, respectively, offshore Nicaragua and in the eastern neighborhood of the East Pacific Rise (EPR). Both the origin and nature of these anomalies are controversial as well as their role in plate tectonics. The high electrical conductivity has been hypothesized to originate from partial melting and melt pooling at the lithosphere–asthenosphere boundary (LAB). The anisotropic nature of the anomaly likely highlights high-conductivity channels in the spreading direction, which could be further interpreted as the persistence of a stable liquid silicate throughout the whole oceanic cycle, on which the lithospheric plates would slide by shearing. However, considering minor hydration, some mantle minerals can be as conductive as silicate melts. Here I show that the observed electrical anomaly offshore Nicaragua does not correlate with the LAB but instead with the top of the garnet stability field and that garnet networks suffice to explain the reported conductivity values. I further propose that this anomaly actually corresponds to the fossilized trace of the early-stage LAB that formed near the EPR about 23 million years ago. Melt-bearing channels and/or pyroxenite underplating at the bottom of the young Cocos plate would transform into garnet-rich pyroxenites with decreasing temperature, forming solid-state high-conductivity channels between 40 and 65 km depth (1.25–1.9 GPa, 1000–1100 °C), consistently with experimental petrology.

Integrated Cretaceous–Cenozoic plate tectonics and structural geology in southern Mexico

The structural evolution of southern Mexico is described in the context of its plate tectonic evolution and illustrated by two restored crustal scale cross-sections through Cuicateco and the Veracruz Basin and a third across Chiapas. We interpret the Late Jurassic–Early Cretaceous opening of an oblique hyper-stretched intra-arc basin between the Cuicateco Belt and Oaxaca Block of southern Mexico where Lower Cretaceous deep-water sediments accumulated. These rocks, together with the hyper-stretched basement beneath them and the Oaxaca Block originally west of them, were thrust onto the Cretaceous platform of the Cuicateco region during a Late Cretaceous–Eocene orogenic event. The mylonitic complex of the Sierra de Juárez represents this hyper-stretched basement, perhaps itself an extensional allochthon. The Chiapas fold-and-thrust belt is mainly Neogene in age. Shallowing of the subduction angle of the Cocos Plate in the wake of the Chortis Block, suggested by seismicity and migrating arc volcanism, is thought to play an important role in the development of the Chiapas fold-and-thrust belt itself, helping to explain the structural dilemma of a vertical transcurrent plate boundary fault (the Tonalá Fault) at the back of an essentially dip-slip fold-and-thrust belt.

Natural Time Analysis of Seismicity within the Mexican Flat Slab before the M7.1 Earthquake on 19 September 2017

One of the most important subduction zones in the world is located in the Mexican Pacific Coast, where the Cocos plate inserts beneath the North American plate. One part of it is located in the Mexican Pacific Coast, where the Cocos plate inserts beneath the North American plate with different dip angles, showing important seismicity. Under the central Mexican area, such a dip angle becomes practically horizontal and such an area is known as flat slab. An earthquake of magnitude M7.1 occurred on 19 September 2017, the epicenter of which was located in this flat slab. It caused important human and material losses of urban communities including a large area of Mexico City. The seismicity recorded in the flat slab region is analyzed here in natural time from 1995 until the occurrence of this M7.1 earthquake in 2017 by studying the entropy change under time reversal and the variability β of the order parameter of seismicity as well as characterize the risk of an impending earthquake by applying the nowcasting method. The entropy change ΔS under time reversal minimizes on 21 June 2017 that is almost one week after the observation of such a minimum in the Chiapas region where a magnitude M8.2 earthquake took place on 7 September 2017 being Mexico’s largest quake in more than a century. A minimum of β was also observed during the period February–March 2017. Moreover, we show that, after the minimum of ΔS, the order parameter of seismicity starts diminishing, thus approaching gradually the critical value 0.070 around the end of August and the beginning of September 2017, which signals that a strong earthquake is anticipated shortly in the flat slab.

Geochemistry of noble gases and CO2 in mantle xenoliths and arc lavas from central Mexico

<p>The Ventura Espiritu Santo Volcanic Field (VESVF) and the Sierra Chichinautzin (SCN) are two monogenetic volcanic fields originated in different tectonic environments in the central portion of Mexico (continental rift and subduction). The VESVF is located 35 km NE of the city of San Luis Potosí in the south of the Basin and Range extensional province. This volcanic field was formed by the eruption of alkaline magmas of mafic composition transporting mantle xenoliths described as spinel lherzolites and pyroxenites (Luhr et al., 1989; Aranda -Gómez and Luhr, 1996). The SCN is a Quaternary volcanic field located in the Trans-Mexican Volcanic Belt (TMVB) between two Quaternary arc-volcanoes (Popocatepetl and Nevado de Toluca[AR1] ). Some authors believe that its origin has been related to the subduction of the Cocos plate beneath the North American plate (Marquez et al., 1999; Meriggi et al., 2008); however, the basalts present in the SCN are geochemically similar to OIBs.</p><p>New isotopic data of noble gases and CO<sub>2</sub> in fluid inclusions from the VESVF and SCN are presented in this work, since these two areas offer a great opportunity to study the local lithospheric mantle features and related processes (e.g., metasomatism, partial melting) occurring beneath Mexico. Twelve fresh xenoliths from the VESVF and two aliquots of olivine phenocrysts of andesites from SCN were selected. Based on the petrographic analysis, it was determined that the set of xenoliths exhibit same paragenesis: Ol> Opx>> Cpx> Spinel; all samples are plagioclase-free and are classified as spinel-lherzolites and harzburgites. Both the boundaries and the fractures of the crystals develop veins composed of yellowish glass and tiny crystals of carbonates. Lavas from SCVF were previously described as olivine andesites mainly aphanitic and porphyritic with few (<10%) phenocrysts of olivine and orthopyroxene (Marquez et al., 1999; Straub et al., 2011).</p><p>The mantle xenoliths and the olivine phenocrysts have comparable Rc/Ra values (where Rc/Ra is the <sup>3</sup>He/<sup>4</sup>He corrected for air contamination and normalized to air He). We find Rc/Ra compositions of 6.9-7.7 and 7.2-7.3, respectively, which are within the MORB-like upper-mantle range (Graham, 2002). The highest CO<sub>2</sub> concentrations are observed in olivine phenocrysts from SCN (9.2·10<sup>-7</sup> mol/g and 1.3·10<sup>-6</sup> mol/g), while the xenoliths cover a wide range of concentrations with values as high as 3.9·10<sup>-7</sup> mol/g in Cpx. The isotopic composition of CO<sub>2</sub> (d<sup>13</sup>C vs PDB) in the olivine phenocrysts is around -6.2‰ with CO<sub>2</sub>/<sup>3</sup>He ratios of 3.3·10<sup>9</sup>, which are comparable to MORB-like range (-8‰<d<sup>13</sup>C<-4‰); the mantle xenoliths in contrast, although displaying similar CO<sub>2</sub>/<sup>3</sup>He ratios (2.8·10<sup>9</sup>), exhibit more positive d<sup>13</sup>C signature between -1.0 and -2.7%. We propose that these differences testify for isotopic heterogeneity in the mantle beneath the two areas, with and reflect mantle metasomatism underneath VESVF driven by interaction with carbonate rich-melts (likely consequence of carbonate recycling during the subduction process), as also evidenced by the petrographic analysis.</p>