A document by Carolina Hurtado-Pulido. Click on the document to view its contents.

The Coast of Louisiana is affected by accelerating sea level rise compounded by land subsidence, leading to land loss. Vertical crustal motions in the region are caused by natural and anthropogenic processes that vary temporally and spatially across the Gulf of Mexico. We investigate the role of growth faulting contributions to subsidence in a case study of Baton Rouge, where two E-W striking, down-to-the-south normal faults, the Denham Springs and Baton Rouge faults, cut compacted Pleistocene strata, and where sediment compaction should be minimal. We used InSAR time series and LiDAR differencing data spanning 1999-2020 to quantify modern vertical and horizontal displacements. After calibration with GNSS data, both methods reveal similar spatial patterns in ground motion, with the faults delimiting areas with different absolute rates. On average the area north of the Baton Rouge fault is subsiding faster than the south, opposite to the long-term sense of fault slip. LiDAR mean vertical rates range between -5 to -11 mm/y and -2.4 to -7 mm/y. InSAR time-series mean rates in the LOS direction range between -10.9 to -13.6 mm/y and -8 to -10.6 mm/y, respectively, for the north and south areas. Subsidence in the northern area likely is controlled by groundwater level changes caused by pumping as indicated by groundwater extraction models. The southern area average is likely influenced by the injection of fluids. Our results suggest volumetric changes caused by fluid extraction and injection in regions separated by growth faults that are creeping to accommodate the spatial variations in subsidence.

Constraints on chemical heterogeneities in the upper mantle may be derived from studying the seismically observable impedance contrasts that they produce. Away from subduction zones, several causal mechanisms are possible to explain the intermittently observed X-discontinuity (X) at 230-350km depth: the coesite-stishovite phase transition, the enstatite to clinoenstatite phase transition and/or carbonated silicate melting, all requiring a local enrichment of basalt. Africa hosts a broad range of terranes, from Precambrian cores to Cenozoic hotspots with or without lowermost mantle origins. With the absence of subduction below the margins of the African plate for >0.5Ga, Africa presents an ideal study locale to explore the origins of the X. Traditional receiver function (RF) approaches used to map seismic discontinuities, like common conversion-point stacking, ignore slowness information crucial for discriminating converted upper mantle phases from surface multiples. By manually assessing depth and slowness stacks for 1° radius overlapping bins, normalized vote mapping of RF stacks is used to robustly assess the spatial distribution of converted upper mantle phases. The X is mapped beneath Africa at 233-340km depth, revealing patches of heterogeneity proximal to mantle upwellings in Afar, Canaries, Cape Verde, East Africa, Hoggar, and Réunion with further observations beneath Cameroon, Madagascar, and Morocco. There is a lack of an X beneath southern Africa, and strikingly, the magmatic eastern rift branch of the southern East African Rift. With no relationships existing between depth and amplitudes of observed X and estimated mantle temperatures, multiple causal mechanisms are required across a range of continental geodynamic settings.

The subdued topography of the Turkana Depression separates the elevated Ethiopian and Kenyan Plateaus in East Africa. Mechanisms to explain its topography are debated because constraints on upper mantle structure and dynamics are lacking. Attempts to understand the role of the mantle below Turkana in the evolution of rifting between the Main Ethiopian and Southern East African rifts and the onset of Ethiopian Flood Basalt volcanism are also hindered by limited data availability. Here, recently deployed seismic networks in Turkana and neighboring Uganda enable us to develop a new absolute P-wavespeed tomographic model (AFRP21) to image mantle structure below the Turkana depression. Additionally, we use P-to-s receiver functions to map the mantle transition zone (MTZ) discontinuity structure. In the shallow mantle, broadly distributed slow wavespeeds reside below the Main Ethiopian rift. To the south, slow wavespeeds occur in a focused zone below the East African rift, but beneath the northern Turkana depression these are cross-cut by a narrow E-W band of fast wavespeeds. At upper MTZ depths slow wavespeeds are broadly continuous below the East African rift but begin to separate into two distinct anomalies at the base of the MTZ. While receiver functions reveal a broadly thinned MTZ below Cenozoic rift-related magmatism in East Africa, the thinnest transition zone exists below the Turkana Depression. Slow wavespeeds and a thinned MTZ below the Turkana Depression indicate hot upwelling material, thus its low-lying nature is not due to the lack of underlying dynamic support. Instead, the depressed topography may be better explained by Mesozoic-Cenozoic E-W rifting associated with the imaged shallow fast wavespeed band. Furthermore, the main eruptive phase of Ethiopian Flood basalt volcanism may be associated with the African plate’s position over the anomalously thinned MTZ in Turkana at ~30Ma.

A combination of magmatic and tectonic processes occur on the western branch of the East African Rift System (EARS) driven by active volcanoes adjacent to active rift faults. Mt. Nyiragongo and Mt. Nyamuragira have the most recent eruptive histories of the 8 volcanoes in the Virunga Volcanic Zone (VVZ) located in a region between Rwanda, Uganda and Democratic Republic of Congo (DRC). On May 22nd 2021, Mt.Nyiragongo erupted the first major eruption following its 2002 eruption. This eruption didn’t have the common precursory seismic activity expected before an eruption as was observed in the 2002 eruption seismic record. Rather, there were numerous post-eruption earthquake events with the largest of those events being a magnitude ML 5.1. Around the region of the earthquake swarm, there was observable ground deformation in the city of Goma and Rubavu where surface fissures destroyed houses and split roads apart. This deformation appears to be related to a N-S striking dike intrusion from the volcano trending south towards and under Lake Kivu, according to observed seismicity. In collaboration with the Government of Rwanda, following the May Mt. Nyiragongo eruption, we established a network of 6 seismometers (2 Meridian Compact PH and 4 Trillium Compact PH) operating at 100 sps and two complimentary raspberry Shake and Booms (SBS) around Lake Kivu. This study will focus on characterizing deformation associated with the eruption and the subsequent seismic swarm. Here we present model results based on deformation during the May 2021 eruption as recorded through ALOS InSAR scenes to understand slip concentration during the dike intrusion. Using GTDef, a set of algorithms developed in MATLAB that can incorporate a wide range of geodetic data types to model deformation observed on the Earth’s surface, we model the slip distribution in this region based on the current hypothesis that the observed seismicity was a result of a dike intrusion defined by the southward propagation of the seismic swarm from Mt. Nyiragongo. Given an approximate source, we determine a preferred GNSS/GPS network design based on resolution-cost of additional stations at given locations and discuss first order characterization of the observed deformation.

Previous studies of the East African upper mantle have invoked one or more mantle upwellings with varying thermochemical nature to underly the distribution of surface volcanism. For example, Boyce and Cottaar (2021) suggest that a hot, chemically distinct upwelling beneath the southern East African Rift (EAR) is sourced from the African Large Low Velocity Province (LLVP), while magmatism in Ethiopia may lie above an additional purely thermal upwelling. Constraints on chemical heterogeneities in the upper mantle may be derived from studying the seismically observable impedance contrasts that they produce. Away from subduction zones, two causal mechanisms are possible to explain the X-discontinuity (X; 230-350km): the coesite-stishovite phase transition and/or carbonate silicate melting, both of which require entrainment of basalt from the lower mantle. Intriguingly, carbonate silicate melt was invoked by Rooney et al., (2012) to explain the discrepancy in upper mantle temperature anomalies predicted by seismic wavespeed and petrological estimates beneath East Africa. Further, active carbonatite magmatism occurs along the edge of the Tanzanian craton (Muirhead et al., 2020). Several recent regional to continental receiver function (RF) studies have identified potential observations of the X in East Africa. These studies are not focused on the presence of these upper mantle phases or lack the spatial sampling needed to robustly identify the X and its causal mechanism. Targeted high-resolution observations of the X are required to confirm the presence of exotic converted phases in the East African upper mantle and their relationship to mantle upwellings. We capitalise on the new TRAILS dataset from the Turkana depression (Bastow, 2019; Ebinger, 2018) and an adjacent network in neighbouring Uganda (Nyblade, 2017), to supplement our existing RF database and characterise the X across active continental rift setting in unprecedented detail. The prevalence of the X is mapped beneath East Africa, and subsequently compared to other areas of the African continent.

Seismometers were installed in the Haynesville basin and Sabine uplifts in northwestern Louisiana and eastern Texas with the purpose of tracking earthquakes in correlation with hydraulic fracturing and wastewater injections in the area. Project ISLA (Investigating Seismicity of Louisiana) consists of 10 seismometers installed in June 2019 and 4 in late 2020 combined to discover baseline seismicity in the area. This study which analyzes earthquakes from April 2021 between 2.5 and 3.2 in magnitude uses data extracted from the ISLA seismic array in Northwest Louisiana from 2019 to present. Additionally, in an attempt to decrease azimuthal gaps data is also extracted from permanent TEXNET and U.S. array stations in Arkansas and Oklahoma. Absolute locations are determined using hypoinverse and the velocity model from East Texas. Focal mechanisms are determined by picking the polarity of P arrivals and plotting them on a stereonet, using take off angles and azimuths from hypoinverse results. Using first motion data, nodal planes are determined, and focal mechanism solutions are revealed. Analyses will result in the ability to understand focal mechanisms and earthquake locations in the Haynesville basin and Sabine uplifts in the context of Mesozoic rift structures, hydraulic fracturing, and wastewater injections.