Age dependence of oceanic intraplate seismicity and implications for lithospheric evolution

We have determined depths for 16 oceanic intraplate earthquakes using body wave modeling. A data set composed of these and 11 other well‐constrained depths shows that the maximum depth of seismicity deepens with increasing lithospheric age and appears to be bounded by a 700°–800°C isotherm. The maximum depth to which intraplate earthquakes occur is approximately equal to the flexural elastic thickness but much less than the seismic thickness from surface wave dispersion. The maximum faulting depth is consistent with the predictions of rapid weakening at high temperatures for dry olivine rheologies determined by experimental rock mechanics. Oceanic intraplate earthquakes can occur at greater depths and temperatures than has previously been reported for continental crustal events, presumably because oceanic lithosphere is stronger at high temperatures. Both the number of earthquakes and the cumulative seismic moment decrease with lithospheric age. The cause of this trend is unclear: possibilities include nonrandom location of preexisting weak zones with respect to the age provinces, fault strengthening with age, or a decrease in the stress level. A decrease in intraplate stress with age is opposite that expected from “ridge push” stresses, which increase with age, but may result either from thermoelastic stresses which are greatest in young lithosphere or near‐ridge stress concentrations due to local heterogeneities in the spreading process.