The scatter of time-delays in shear-wave splitting above small earthquakes

Measurements of time- delays in seismic shear- wave splitting above small earthquakes typically display a scatter of often as much as +/- 80 per cent about the mean. Changes in the average time-delay appear to be related to changes of stress, but applications of this potentially powerful tool have been handicapped by the previously inexplicable scatter in time- delays above earthquakes. In contrast, measurements of shear- wave time- delays in controlled- source exploration seismics are typically well controlled and display little scatter. Previous estimates of possible causes of scatter cannot produce sufficient variation specifically above earthquakes. Here we show that 90degrees -flips in shear- wave polarizations due to fluctuating high pore- fluid pressures on seismically-active fault planes are the most likely cause of the scatter.