Geophysical Equipment Facility Report Loan 802 Monitoring seismic slip on the Mam Tor landslide

Loan 802 had a two-fold aim: to ascertain whether seismic events were associated with the movement of the Mam Tor landslide and could help with our understanding of sliding mechanisms; to field test the then new Guralp DCM recorder. From the 19th December 2006 to 9th January 2007 a CMG-3T sensor was deployed in a vault on the Mam Tor slip, whilst a second control sensor was placed a few hundred metres away, off the slip, in farm buildings. Surprisingly, correlation of waveforms with significant slip events on the co-located Manchester University’s strainmeter showed no demonstrable evidence of seismic emissions. We deduce that Mam Tor is moving as an aseismic ‘slurry’ at rates out-with the period band of the CMG-3T (>>120s). An undergraduate at Leeds University determined receiver functions for the two sensors as part of his independent project. Despite the very different superficial conditions on and off the slip, he showed that the receiver functions for sensors only few hundred metres apart were in fact very similar. The velocity-depth structure of the crust determined by modelling the receiver functions was similar to the LISPB model. Specific developments with Guralp on the DCM resulting from this project include: 1) producing reliable communication with recorder, 2) enabling locking of 3T and 3TD sensors without serial communication. 3) an understanding of the optimal DCM data recording parameters, 4) using autonomous systems without battery charge capability. This preliminary loan led to a NERC small grant proposal (Sept 2007) entitled ‘Monitoring seismic slip on the Mam Tor landslide PIs (Prof. E. Rutter, Manchester University; Prof. G. Stuart, Leeds University; Dr Brisbourne, Seis-UK). Unfortunately this grant was unsuccessful. Background Annually since 1995, and more recently twice yearly, Manchester University have used electronic distance surveying methods on 40 survey stations on the Mam Tor landslide, Derbyshire, England to study its creep motion. This work substantially augmented earlier studies by Skempton et al. (1989) and Waltham & Nixon (2000). Since its formation ca 3000 years ago, the slipped mass, some 600 m downslope and 350 m across slope (Fig. 1a), has been moving continuously. Manchester University’s surveys, plus analysis of maps of the area extending back some 200 years, showed that slippage has been uniformly occurring at rates up to 500 mm per year in different parts of the slide. Rutter et al. (2003) used methodology for analysis of glacier sliding to determine the strain rate distribution within the landslide. Although there was an evident crude correlation of slip with annual rainfall, it became clear that high resolution monitoring of slip, rainfall and groundwater pressure would be needed to take full advantage of the opportunity presented by this unique natural laboratory. Long-term monitoring of the Mam Tor slip provides what is perhaps the best opportunity in the UK to assess the potential sensitivity of slope failure to extreme rainfall events that might be expected to arise due to global warming. A full high-resolution monitoring program (NERC sponsored) was operational on Mam Tor since summer 2006. This consisted of continuous records (at 3-hour intervals) of automatic measurements of creep (measured from 3 underground wire creep meters, to 0.2 mm resolution), rainfall, temperature and barometric pressure, and groundwater pressure and tilt at each of 2 depths in 2 boreholes. It was at the EGU, Vienna, Easter 2006 that Professors Stuart and Rutter discussed the possibility of deploying a broadband seismometers on and off the Mam Tor slip to establish whether short-duration creep 'events', which appear to be characteristic of early stages of movement periods (Fig 1b), could be detected seismically, in the same way that has recently been discovered to be characteristic of surging glaciers (e.g. Stuart et al. 2005). The project was thus initiated as a feasibility study for the use of broadband seismic stations for monitoring the Mam Tor land slip. SEIS-UK provided significant support for the fieldwork as this project presented an opportunity to thoroughly field-test the new 2 generation DCM datalogger system. Figure 1b shows data from the creep meters for the period May 2006 to July 2007. Creep is firmly localized in the period November through March, although the exceptional rainfall of June and July 2007 has produced an additional surge. Although creep correlates with groundwater pressure fluctuations, a normal summer at Mam Tor is on average no drier than the winter period. However it is during the autumn and winter that rain water can permeate the ground. This is due to the NERC Geophysical Equipment Facility View more reports on our website at http://gef.nerc. ac.uk/reports.php