Surface temperature and precipitation affecting GPS signals before the 2009 L'Aquila earthquake (Central Italy)

An Mw 6.1 normal faulting earthquake struck Central Italy in 2009 April, which unfortunately nucleated right below the town of L'Aquila, causing more than 300 casualties and widespread damage. The main shock was preceded by a foreshock sequence lasting ~6 months. It has been claimed that an analysis of continuous Global Positioning System (GPS) data shows that during the foreshock sequence a 5.9 Mw slow slip event (SSE) occurred along a decollement located beneath the reactivated normal fault system. This hypothesized SSE that started in the middle of 2009 February and lasted for almost two weeks would have eventually loaded the largest foreshock and the main shock. We show that the strain signal that the SSE would have generated at two laser strainmeters operating at about 20 km NE from the SSE source was essentially undetected. We then propose an alternative interpretation for the displacement observed in the GPS data. A transient signal is present in temperature and precipitation time-series recorded close to the GPS station that has largest signal referred to the SSE, implying that these contaminated the GPS record. This work illustrates how environmental noise may be relevant when investigating small strain signals, showing the importance of having data from weather stations and water level sensors colocated with GPS stations.