An earthquake is said to have a short-term slow precursor when there is significant moment released in an interval immediately before its high-frequency origin time t0. A method is formulated for detecting slow precursors from observations of the earth's free oscillations. It assumes that the far-field radiation at low frequencies (≲10 mHz) is due to a compact source of constant orientation having a moment-release function which is zero prior to some start time t* and nondecreasing for t≥t*. Any moment-release function for which t0=t* (no precursor) must then satisfy the inequality &tgr;c<Δt1√2(1+√1+8&agr;) where Δt1=t1-t0 is the centroid time shift, &tgr;c is the characteristic duration, and &agr; is a dimensionless measure of the temporal skewness. A detector based on this inequality is applied to two events. Data from the shallow-focus Loma Prieta earthquake of 18 Oct 89 are consistent with no moment release prior to the high-frequency origin time, whereas the intermediate-focus Peru-Ecuador earthquake of 12 Apr 83 appears to have a significant slow precursor. The systematic analysis of large earthquakes using this detector may elucidate earthquake nucleation processes and help to assess, on a global scale, the feasibility of short-term earthquake prediction from near-field measurements. ¿American Geophysical Union 1991