Ghost tilt signal during transient ground surface deformation events: Insights from the September 3, 2010 Mw7.1 Darfield earthquake, New Zealand

Abstract

[1] This work assesses whether tiltmeters can reliably measure ground tilt in the 2–20 s band, similar to that of very–long period volcanic earthquakes. Seismic waves recorded with co–located high–rate GPS, borehole seismometer and bubble tiltmeter during the 2010 Mw7.1 Darfield earthquake in New Zealand are used to determine the response of the tiltmeter. The joint analysis of this dataset shows that, at least for periods within the aforementioned band, the observed tilt signal was in fact not related to ground tilt but, instead, to translational ground acceleration caused by passing S and surface waves. If no independent control on translational motion is provided for recorded very–long period volcanic earthquakes within the 2–20 s band, the signal recorded on bubble tiltmeters may therefore be misinterpreted. The use of co–located high–rate GPS data provides such control and is essential to discriminate translational motion from ground tilt on the recorded seismic and tiltmeter signals.