  The General Relativity Effective Field Theory (GREFT) introduces
higher-derivative interactions to parameterise the gravitational effects of
massive degrees of freedom which are too heavy to be probed directly. The
coefficients of these interactions have recently been constrained using
causality: both from the analytic structure of 4-point graviton scattering and
the time delay of gravitational waves on a black hole background. In this work,
causality is used to constrain the quasi-normal mode spectrum of GREFT black
holes. Demanding that quasi-normal mode perturbations decay faster in the GREFT
than in General Relativity -- a new kind of causality condition which stems
from the analytic structure of 2-point functions on a black hole background --
leads to further constraints on the GREFT coefficients. The causality
constraints and compact expressions for the GREFT quasi-normal mode frequencies
presented here will inform future parameterised gravitational waveforms, and
the observational prospects for gravitational wave observatories are briefly
discussed.
