At this week's AGU meeting, at least one of the seismologists was aware of the tidal/earthquake connection and this is a paper that she told me to look into: “Abnormally Strong Daily-Cycle S1 Strain Tide: Observation and Physical Mechanism” — http://onlinelibrary.wiley.com/doi/10.1002/2017JB014383/full
> "Tidal signals are analyzed in the Plate Boundary Observatory borehole strainmeters in western North America. While the extracted diurnal strain tidal constituents (except daily-cycle S1) respond to the tidal forces in similar proportion, the S1 strain tide exhibits an abnormally strong amplitude, **reaching tens to hundreds times larger than theoretical prediction.**"
This is a paper that explains : "Influence of Tidal Forces on the Triggering of Seismic Events" -- https://link.springer.com/article/10.1007/s00024-017-1563-5
> "Results of calculations prove that stress increases as a function of depth reaching a value around some kPa at the depth of 900–1500 km, well below the zone of deep earthquakes. At the depth of the overwhelming part of seismic energy accumulation (around 50 km) the **stresses of lunisolar origin are only (0.0–1.0)·10^3 Pa**. Despite the fact that these values are much smaller than the earthquake stress drops (**1–30 MPa**) (Kanamori in Annu Rev Earth Planet Sci 22:207–237, 1994) this does not exclude the possibility of an impact of tidal forces on outbreak of seismic events. Since the tidal potential and its derivatives are coordinate dependent and the zonal, tesseral and sectorial tides have different distributions from the surface down to the CMB, the lunisolar stress cannot influence the break-out of every seismological event in the same degree. The influencing lunisolar effect of the solid earth tides on earthquake occurrences is connected first of all with stress components acting parallel to the surface of the Earth. The influence of load tides is limited to the loaded area and its immediate vicinity."
So it is possible that the theory is undershooting what the evidence is showing in terms of strain needed to trigger an earthquake.