Applied physics: Gravity measurements on chips
Gravimeters have applications ranging from oil exploration to the detection of underground tunnels, but size and lack of portability have limited their field use. A device the size of a postage stamp promises fresh opportunities. See Letter p.614
 Gravimeters can measure tiny variations in the local gravitational acceleration, with sensitivity high enough to measure Earth's tides: the elastic deformation of the Earth's crust caused by the changing relative phases of the Sun, Earth and Moon. Existing gravimeters are bulky and costly, but now Giles Hammond and colleagues have developed a compact and inexpensive microelectromechanical gravimeter with a novel 'anti-spring' design. They use it to measure gravitational acceleration with sufficient sensitivity and stability to detect Earth's tides. The small size and low cost of the device mean it could be mounted on a drone for surveying and exploration roles, used to monitor volcanoes or built into multi-pixel density-contrast imaging arrays.
One-step silicon substrate through-wafer etching process for high precision MEMS application
In order to get a low mechanical noise and high sensitivity, warpage during fabrication process which comes from the mismatching of thermal expansion between different materials is considered as a major issue for high precision measurement MEMS devices. For the purpose of warpage reduction, we proposed a one-step silicon substrate through-wafer etching process. The process is composed of a normal DRIE (Deep Reactive Ion Etching) process for an etching depth of 450μm and an optimized low frequency etching process for the residual silicon (50μm) removal. The deformation was suppressed within 2μm for a spring-mass structure with dimension of 20mm×26mm, which will be very attractive for nano-g MEMS accelerometer application.
Volcanic signatures in time gravity variations during the volcanic unrest on El Hierro (Canary Islands)
Gravity changes occurring during the initial stage of the 2011-2012 El Hierro submarine eruption are interpreted in terms of the preeruptive signatures during the episode of unrest. Continuous gravity measurements were made at two sites on the island using the relative spring gravimeter LaCoste and Romberg gPhone-054. On 15 September 2011, an observed gravity decrease of 45 mu Gal, associated with the southward migration of seismic epicenters, is consistent with a lateral magma migration that occurred beneath the volcanic edifice, an apparently clear precursor of the eruption that took place 25 days later on 10 October 2011. High-frequency gravity signals also appeared on 6-11 October 2011, pointing to an occurring interaction between a magmatic intrusion and the ocean floor. These important gravity changes, with amplitudes varying from 10 to -90 mu Gal, during the first 3 days following the onset of the eruption are consistent with the northward migration of the eruptive focus along an active eruptive fissure. An apparent correlation of gravity variations with body tide vertical strain was also noted, which could indicate that concurrent tidal triggering occurred during the initial stage of the eruption.