What precursor geological activity occurred prior to the Parícutin Volcano eruption in 1943?
The region around the Parícutin volcano is within the Trans-Mexican Volcanic Belt and is affected by the subduction of the Cocos and Rivera Plates (Gardine et al. 2011), as seen in the image below (from Langridge et al. 2013):



As the petrology is very recent, it has provided scientists an unprecedented opportunity to study a complete volcanic sequence. Petrological observations reported by Erlund et al. (2010) determined that there were 3 main distinct phases of volcanic activity, the initial phase, which lasted only about 4 months, is what is relevant to this question.

Erlund et al. (2010) extensively studied the basal tephra, their results suggest the following occurred immediately prior to the 1943 eruption:


magma ascended from depth through dyke and sill systems quick enough to prevent substantial fractionation from olivine settling.
magma was stalled at shallow depths under sufficient pressure to allow plagioclase phenocryst growth.


This magma movement is likely to have caused the ~45 days of seismic activity that was noted before the fissure opened up in the cornfield, initiating the Parícutin volcano eruption.

There is a more modern day analogue to the associated seismicity, based on models developed by Gardine et al. (2011) from an earthquake swarm that occurred near to the Parícutin volcano in 2006, where the low magnitude earthquake swarms occurred in clusters that demonstrated reasonably rapid ascent and lateral movement - suggesting not only a possible mechanism for magma ascent, but also demonstrates the region is still very active.

References

Erlund et al. 2010, Compositional evolution of magma from Parícutin Volcano, Mexico: The tephra record, Journal of Volcanology and Geothermal Research

Gardine et al. 2011, Dike emplacement near Parícutin volcano, Mexico in 2006, Bulletin of Volcanology

Langridge et al. 2013, Preliminary paleoseismic results from the Pastores fault and its role in the seismic hazard of the Acambay graben, Trans-Mexican Volcanic Belt, Mexico, Revista mexicana de ciencias geológicas