A rare foreshock sequence of the 20 January 2007 Odaesan, Korea, earthquake to measure the existence of preseismic velocity changes

Dilatancy theory and laboratory studies predict that there may be changes in the velocity of seismic waves in the Earth's crust preceding large earthquakes due to cracks opening up in response to stress changes. These changes, however, have been extremely difficult to find in the field. Because we do not know where or when the next earthquake will strike, often there is not sufficient instrumentation to capture such a signal if it exists with active sources. But natural sources such as repeating earthquakes offer hope of measuring a preseismic signal. To date, however, they have not measured a preseismic signal, perhaps due to insufficient temporal sampling, especially if the signal is short lived. We examine a rare foreshock sequence of nine near‐repeating events with fine temporal sampling all occurring within 3 days of the Mw 4.6 main shock with the last event occurring an hour before. The nearest event occurred within 114 m of the main shock hypocenter. This presents a unique opportunity to measure preseismic velocity changes at the depth of the main shock, which is 9.4 km. Because the foreshocks are not exact repeats, slight position differences bias the velocity change measurements. The locations of these events are known precisely, however, which allows for us to correct for these biases by treating the events as a source array, improving the measurement precision by up to an order of magnitude. We observe no apparent preseismic velocity change signal, but we are able to place an upper bound on its existence ranging from 0.01% to 0.08%. Key Points Rare opportunity to measure preseismic changes as close as 114 m from mainshock Fine temporal sampling, events occur within 3 days, the last an hour before No apparent preseismic velocity change observed, upper bound as low as 0.01%