Causes of diurnal ground movements at Omokoroa, New Zealand

Abstract

This study investigated processes acting on the Bramley Drive Landslide, located on the Omokoroa Peninsula. The landslide, first failed in 1979, and has become an important field site for assessing the processes affecting sensitive volcanic soils in the Bay of Plenty region that contribute to slope failure. Researchers from the University of Waikato and the University of Bremen, Germany observed daily displacements in a borehole inclinometer located on site that were attributed to amplification of Earth tide movements.

Potential factors contributing to exaggerated diurnal solid Earth tide strains were investigated, primarily: temperature, rainfall, Earth tides and microseisms. Measurements of displacements and forcing processes were obtained using a 3-axis 0.65 g precision accelerometer/seismometer, borehole inclinometer and weather station.

The accelerometer data indicate that microseisms, consistent with breaking waves on the open coast of Matakana Island, and a diurnal thermal component occur within the top 5cm of the ground profile. The measured ground accelerations are greatest in warmer summer months due to the larger changes in temperature throughout a day associated with higher insolation and reduced soil moisture.

The displacements within the borehole are a linear function of depth, but involve a series of sharp deviations, or steps, throughout the 42 m length measured. These steps coincide with boundaries between different stratigraphic units, usually where there is a change in soil type and soil strength. Therefore, the effects of the horizontal strain due to the solid Earth tide is exaggerated by differential and lagged displacement of the units.

This suggests that Earth tidal stress, may cause minor (usually temporary) shear surfaces at depths that may be the final trigger for an “at risk” slope that is already preloaded and ready to fail. Several examples where this may be a factor were identified, including all the events at Bramley Drive with a known time of failure. Failure occurred when the rate of vertical Earth tide displacement peaked close to the time of maximum horizontal displacement, which corresponds to a change in the direction of the stress caused by the combined Moon and Sun tide generating potential. Predicting the time of slope failure may be beneficial both locally and globally.