The spectral-element method (SEM) for simulating wave propagation is widely used with adjoint methods for full-waveform inversion. Typically, SEM is used to compute forward and adjoint wavefields, which is then applied to evaluate the Fréchet derivatives for updating the seismic structural model. The Hessian is rarely computed as the high computational and storage costs, although it can improve the accuracy of the model update and model convergence. Instead the approximate Hessian is determined, which is obtained with less computational effort. We present a method for simultaneously constructing Fréchet and Hessian kernels on the fly, which we call Multi-solver spectral-element and adjoint methods (Multi-SEM). Rather than storing all the wavefields, Multi-SEM is computed on the fly and requires only about a 2-fold computational cost when compared to the computation of Fréchet kernels. Numerical examples demonstrate the functionality of the method and the computer codes are provided with this contribution.

The crust and upper mantle structure of the Greater and Lesser Antilles Arc provides insights into key subduction zone processes in a unique region of slow convergence of old slow-spreading oceanic lithosphere. We use ambient noise tomography gathered from island broadband seismic stations and the temporary ocean bottom seismometer network installed as part of the VoiLA experiment to map crustal and upper mantle shear-wave velocity of the eastern Greater Antilles and the Lesser Antilles Arc. We find sediment thickness, based on the depth to the 2.0 km/s contour in the Grenada and Tobago basins up to 15 km in the south, with thinner sediments near the arc and to the north. We observe thicker crust, based on the depth to the 4.0 km/s velocity contour, beneath the arc platforms with the greatest crustal thickness of around 30 km, likely related to crustal addition from arc volcanism through time. There are distinct low velocity zones (4.2-4.4 km/s) in the mantle wedge (30-50 km depth), beneath the Mona Passage, Guadeloupe-Martinique, and the Grenadines. The Mona passage mantle anomaly may be related to ongoing extension there, while the Guadeloupe-Martinique and Grenadine anomalies are likely related to fluid flux, upwelling, and/or partial melt related to nearby slab features. The location of the Guadeloupe-Martinique anomaly is slightly to the south of the obliquely subducted fracture zones. This feature could be explained by either three-dimensional mantle flow, a gap in the slab, variable slab hydration, and/or melt dynamics including ponding and interactions with the upper plate.

The combined gravitational pulls from the moon and the sun result in periodical tidal stresses at rates potentially exceeding the tectonic ones. Yet, tidal triggering of earthquakes in critically stressed faults is still under debate and controversial results have been obtained, depending upon specific physical properties and geological settings. Although no universal triggering pattern between earthquakes and tides has been observed in oceanic environments, previous research implies relation between increased seismicity rates and low tides at particular sites at fast-spreading ridges in the Pacific. We present a dataset of 4719 microearthquakes (-1.4≤ML≤4.0) recorded by an Ocean Bottom Seismometer (OBS) network at the slow-spreading equatorial Mid-Atlantic Ridge from March 2016 to February 2017. We use a single-station template matching technique to focus on a small volume, spreading within a ~5km radius from the station. The origin time of the events and their epicentral location is sufficiently determined for a robust comparison with the ocean tides. Our analysis suggests a significant correlation between seismic potential and tidal forces, with the majority of events occurring during or towards low tides, i.e., during maximized extensional stress and maximized extensional stress rate. The tidal dependence of magnitude distribution is also investigated. Although the b-values are generally lower at low tides, the differences are not sufficiently large to achieve statistical significance. However, seismic bursts (enhanced activity rate clusters), occurring at rates above the reference seismicity, are exclusively initiated at extensional stress rates. Coulomb stress modelling implies that slip is promoted during low tides at low-angle normal faults. Local morphology, seismicity distribution and focal mechanisms suggest the existence of high angle faults at shallower depths. Coulomb modelling suggests slip on these faults should not be triggered at low tides unless another factor is considered. One possibility is the presence of a shallow magma chamber. Such a chamber has also been suggested by previous seismic imaging results. Overall, the result yields new insight into magmatic – tectonic cycles and seismicity triggering at mid-ocean ridges.

The lithosphere-asthenosphere system is fundamental to our understanding of mantle convection and plate tectonics. Seismic and electromagnetic methods are our primary means of determining its structure and physical properties. These independent constraints with different sensitivities to Earth’s properties hold promise for understanding the system. Here we use the shear velocity model from Rayleigh waves and the MT based resistivity model from near the equatorial Mid-Atlantic Ridge. Cross-plots of the models suggest a linear or near-linear trend that is also in agreement with petrophysical predictions. We therefore map the MT model to a new shear-wave starting model using the petrophysical relationship, which is then used to re-invert for shear-wave velocity. The resulting shear-wave velocity model fits the phase velocity data, and the correlation coefficient between the shear velocity and resistivity models is increased. Much of the model can be predicted by expectations for a thermal half-space cooling model, although some regions require a combination of higher temperatures, volatiles, or partial melt. We use the petrophysical predictions to estimate the melt fraction, melt volatile content, and temperature structure of the asthenospheric anomalies. We find up to 4% melt, with the lowest resistivities and shear velocities explained by up to 20% water or 20% CO2 in the melt or ~1% nearly pure sulfide melt, depending on the set of assumptions used. Melt is required in punctuated anomalies over broad depth ranges, and also in channels at the base of the lithosphere. Melt in the asthenosphere is dynamic, yet persistent on geologic time scales.

The gravitational pulls from the moon and the sun result in tidal forces which influence both Earth’s solid and water mass. These stresses are periodically added to the tectonic ones and may become sufficient for initiating rupture in fault systems critically close to failure. Previous research indicates correlations between increased seismicity rates and low tides for mid-ocean, fast-spreading ridges in Pacific ocean. Here, we present a microseismicity dataset (4719 events) from an Ocean Bottom Seismometer (OBS) network at the equatorial Mid-Atlantic Ridge, suggesting a significant correlation between seismic potential and tidal forces. We show that low as well as decreasing ocean water level results in elevated seismicity rates and lower b-values, translated into considerably increased probabilities of stronger event occurrence at or towards low tides. In addition, seismic bursts (enhanced activity rate clusters), occurring at rates fairly above the reference seismicity, are exclusively present during either high extensional stresses or high extensional stress rates. Our results exhibit remarkable statistical significance, supporting the previous findings for tidal triggering at low tides within normal-faulting regimes and extending the range of observations to slow-spreading ridges. Observed triggering of slip on low angle faults at low tides is predicted by Coulomb stress modelling. The triggering of slip on high angle faults observed here, is not easily explained without another factor. It may be related to fatigue and/or the presence of a shallow magma body beneath the ridge, as suggested by previous seismic imaging in the region.

Oceanic transform faults are intriguing in that they do not produce earthquakes as large as might be expected given their dimensions. We use 1-year of local seismicity recorded on an array of ocean bottom seismometers (OBS) and geophysical data to study the seismotectonic properties of the Chain transform, located in the equatorial Mid-Atlantic. We extend our analysis back in time by considering stronger earthquakes (MW ≥ 5.0) from global catalogs. We divide Chain into three areas (eastern, central, and western) based on multi-dimensional OBS seismicity cluster analysis. Seismic activity recorded by the OBS is the highest at the eastern area of Chain where there is a lozenge shaped topographic high, a negative rMBA gravity anomaly, and only a few historical MW ≥ 5.5 events. OBS seismicity rates are lower in the western and central areas. However, these areas accommodate the majority of seismic moment release, as inferred from both OBS and historical data. We find no evidence of remote dynamic triggering and only weak evidence of tidal and static stress triggering. Higher b-values are significantly correlated with lower rMBA and also with shallower bathymetry, potentially related to thickened crust. Our results suggest high lateral heterogeneity along Chain: Patches with moderate to low OBS seismicity rates that occasionally host MW ≥ 6.0 earthquakes are interrupted by segments with abundant OBS activity but few historical events with 5.5 ≤ MW < 6.0. This segmentation is possibly due to variable fluid circulation and alteration, which may also be variable in time.

Oceanic Transform Faults (TF) comprise first order discontinuities bounded between mid-ocean ridge spreading centres. TF mainly accommodate strike slip motion, separating lithospheric plates of different age and thermal structure. Oceanic TF are intriguing in that they do not produce earthquakes as large as might be expected given their long length, with seismic slip corresponding only to a small fraction of the total tectonic slip. The relative geologic simplicity of oceanic TF means that they are an important analogue for more hazardous continental TF, with high potential for improving insights into the earthquake cycle. We investigate the earthquake properties along Chain, a ~300 km long TF in the equatorial MAR by combining both microseismic and teleseismic data. We use the ~1-year microseismicity data (total of 812 events) gathered during the PI-LAB (Passive Imaging of the Lithosphere-Asthenosphere Boundary) experiment and EURO-LAB (Experiment to Unearth the Rheological Lithosphere-Asthenosphere Boundary). We perform cluster analysis in multi-dimensional phase space, consisting of various seismic (epicentral coordinates, magnitude) and geophysical (gravity anomalies, bathymetry, tidal height) parameters. We investigate potential triggering mechanisms, including tidal, static and dynamic stresses. We extend our analysis back in time by considering stronger earthquakes (MW>~5.0) from Global Centroid Moment Tensor (GCMT) since 1976. We find three unique, 50-100 km long clusters or segments from our analysis going from east to west, separated by seismic gaps. Microseismic activity is highest at the eastern segment of Chain where there is the largest positive flower structure, negative rMBA gravity anomaly but very few M>5.5 events. The western segment has reduced seismicity rates relative to the eastern, and is associated with a positive rMBA and a few small flower structures. The central segment is bounded between two seismic gaps and demonstrates relatively high activity rates in the middle. Our result suggests that trans-pression of highly altered mantle/crust and/or high pore pressure due to hydrothermal fluid circulation in the eastern flower structure enhances seismic activity. Overall, we find the existence of consecutive locking and creeping segments, with some of the patches exhibiting hybrid behaviour, potentially causing their sporadic activation/reactivation.

Seismicity along transform faults provides important constraints for our understanding of the factors that control earthquake ruptures. Oceanic transform faults are particularly useful due to their relatively simple structure in comparison to continental counterparts. The seismicity of several fast-moving transform faults has been investigated by local networks, but as of today there have not been many studies of slower spreading centres. Here we present the first local seismicity catalogue based on event data recorded by a temporary broadband network of 39 ocean bottom seismometers located around the slow-moving Chain Transform Fault (CTF) along the Mid-Atlantic Ridge (MAR) from March 2016 to March 2017. Locations are constrained by simultaneously inverting for a 1-D velocity model informed by the event P- and S-arrival times. Depths and focal mechanisms of the larger events are refined using deviatoric moment tensor inversion. We find a total of 972 events in the area. Most of the seismicity is located at the CTF (700) and Romanche transform fault (94) and the MAR (155); a smaller number (23) can be observed on the continuing fracture zones or in intraplate locations. The ridge events are characterised by normal faulting and most of the transform events are characterised by strike slip faulting, but with several reverse mechanisms that are likely related to transpressional stresses in the region. CTF events range in magnitude from 1.1 to 5.6 with a magnitude of completeness around 2.3. Along the CTF we calculate a b-value of 0.81 ± 0.09. The event depths are mostly shallower than 15 km below sea level (523), but a small number of high-quality earthquakes (16) are located deeper, with some (8) located deeper than the brittle-ductile transition as predicted by the 600˚C-isotherm from a simple thermal model. The deeper events could be explained by the control of seawater infiltration on the brittle failure limit.