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During the last two decades, quasi‐periodic long‐term slow slip events (SSEs) of magnitude up to Mw7.5 have been observed about every 4 years in the Guerrero Seismic Gap, Mexico. We present numerical simulations of the long‐term SSE cycles along the 3D slab geometry of central Mexico. Our model accounts for the hydrated oceanic crust in the framework of rate‐and‐state friction and captures the major source characteristics of the long‐term SSEs occurring between 2001 and 2014, as inferred from geodetic observations. Synthetic surface deformation calculated from simulated fault slip is in good agreement with the cumulative GPS displacements. Our results suggest that the flat‐slab segment of the Cocos plate aids the large magnitudes and long recurrence interval of the long‐term SSEs. We conclude that 3D slab geometry is an important factor in improving our understanding of the physics of slow slip events. Plain Language Summary: Slow slip events (so‐called "silent earthquakes") have been detected worldwide in circum‐Pacific subduction zones, such as Cascadia and southwest Japan. Long‐term slow slip events occur about every 4 years in the Guerrero Seismic Gap (Mexico) where tectonic plate movement is largely accommodated by aseismic slip and no large earthquakes have been observed since 1911. We build a numerical model incorporating a realistic 3D geometry of the subducting slab and lab‐derived friction laws to investigate the physics of these slow slip events. The simulated events have slip patterns, magnitudes, and recurrence intervals comparable with the observed ones. Our study demonstrates that plate geometry is an important factor to account for when studying the initiation, propagation and arrest of slow slip. Key Points: We model sequences of long‐term slow slip events in the Guerrero Seismic Gap using a geometrically flexible three‐dimensional (3D) boundary integral methodOur model reproduces the source characteristics and surface deformation of the four long‐term slow slip events (SSEs) inferred from geodetic observationsThe flat segment of the Cocos plate likely aids the large magnitudes and long recurrence interval of the slow slip events in Guerrero [ABSTRACT FROM AUTHOR]

We analyzed new recordings of SPdKS seismic waveforms from a global set of broadband seismograms and horizontal tiltmeters from the Hi‐net array in Japan from 26 earthquakes in the Central American region. The anomalous waveforms are consistent with the presence of at least three ultralow‐velocity zones (ULVZs), on the core‐mantle boundary beneath northern Mexico and the southeastern United States. These ULVZs ring an area of high seismic wave speeds observed in tomographic models that has long been associated with past subduction. Waveform modeling using the PSVaxi method suggests that the ULVZs have S and P wave velocity decreases of 40% and 10%, respectively. These velocity decreases are likely best explained by a partially molten origin where the melt is generated through melting of mid‐ocean ridge basalt atop the subducted slab. Plain Language Summary: We use a set of seismic observations recorded globally to investigate the lower mantle beneath Central America. The deepest mantle in this region has been associated with the final resting place of subducted slab material from subduction that initiated approximately 200 million years ago. This ancient subducted material is associated with high seismic wave speeds in the lowermost mantle just above the core‐mantle boundary. We find that patches of highly reduced seismic wave speeds, referred to as ultralow‐velocity zones (ULVZs), appear to be associated with the border of the high wave speed region, along the border of the subducted slab material. These ULVZ patches are consistent with being regions of partial melt. A possible scenario for their creation is that mid‐ocean ridge basalt (MORB), comprising the crust of the subducted slab material, has a low melting point at conditions in the deep earth and may be melting as the slabs reach the bottom of the mantle. Previous experimental work has suggested that MORB will likely partially melt in the deep mantle, yet little evidence for the existence of MORB partial melt has previously been found. Key Points: Subducted slab material resting on the core‐mantle boundary is ringed by ultralow‐velocity zonesA partially molten origin to these ultralow‐velocity zones is likelyMelting of mid‐ocean ridge basalt may give rise to ultralow‐velocity zones [ABSTRACT FROM AUTHOR]