Tides Only Tip Critical Faults

01The evidence

• Tidal triggering of earthquakes on the San Andreas Fault has been interpreted as evidence for poroelastic behaviour, in which the tide modulates pore pressure and therefore effective stress rather than adding directly to the rupture force.USGS

02The tension

Some faults and some sequences show clear tidal phase preference; others show none. The effect has been treated as a property of the fault rather than of its state, so it has not been usable as a monitoring signal. Proponents and sceptics have argued for decades without a discriminating prediction.

03The theory

The stress a tide applies is of order a kilopascal. A fault that is far from failure will not care. A fault that is within a small fraction of its failure stress cannot tell the difference between a tidal perturbation and any other nudge, so it will fail preferentially at a particular phase.

That makes tidal sensitivity a state variable. It should be absent in quiet periods, appear as pore pressure rises, strengthen as the fault approaches failure, and vanish after the event resets the stress. The transition should be sharp, because the sensitivity depends exponentially on the distance to failure.

Pore pressure is the control because it sets effective stress. A fault with high pore pressure sits closer to failure at the same tectonic load. So the prediction is not that tides trigger earthquakes in general, but that tidal sensitivity appears when a measurable proxy for pore pressure crosses a threshold.

The practical value is a cheap precursor. Phase preference can be computed continuously from a catalogue with no new instruments. If the threshold behaviour is real, induced seismicity and volcanic unrest both become monitorable with data that already exist.

04What this is not

05Predictions

• P1Tidal phase locking should increase in the weeks before moderate events in induced sequences, and drop after them.
• P2The effect should be absent when independent proxies place pore pressure far from critical.
• P3The transition should occur within a band of roughly ten per cent of failure stress, not gradually across the whole range.

06What would refute it

Tidal sensitivity uncorrelated with pore pressure proxies, or present uniformly regardless of distance to failure, would reduce the effect to a site property.

07What it would change

It would give induced seismicity and volcanic monitoring a free precursor, computed from catalogues rather than from new instruments.