Time-Domain Observations of a Slow Precursor to the 1994 Romanche Transform Earthquake

Low-frequency spectral anomalies have indicated that some large earthquakes are preceded by extended episodes of smooth moment release, but the reality of these slow precursors has been debated because they have not been directly observed in the time domain. High-gain seismograms from the 14 March 1994 Romanche Transform event (moment magnitude M w 7.0) show a precursory ramp with a moment of 7 × 10 18 newton meters beginning about 100 seconds before the arrival of the high-frequency P waves. This precursor was the initial phase of a slow component of slip that released nearly half of the total moment of the earthquake. Such behavior may be typical for large earthquakes on the oceanic ridge-transform system.