caused a tephra barrier to be formed across the outlet of Crater Lake. By 2005 seepage from the refilled lake into the barrier raised the
possibility of an eventual collapse of the barrier, releasing a catastrophic lahar down the mountain.
As part of an extensive monitoring programme of the tephra barrier, direct current (dc) resistivity surveys
were carried out on a number of lines along and across it in order to test whether the extent of the seepage
could be measured (and monitored) by geophysical means. Two dimensional inversion of measured apparent
resistivity data showed that between the initial measurements, made in January 2005, and February 2006,
there was a gradual decrease in resistivity above the old outlet from ~50–60 Ωmto ~30 Ωm. This gave the first
indication that lake water was seeping into the barrier. Between October and December 2006 there was a
rapid rise in lake level to only 2 mbelow the top of the barrier, and a further resistivity survey in January 2007
showed that there had been a further decrease in resistivity throughout the entire barrier with values
dropping to b10 Ωm. The extent of this low resistivity indicated that the barrier was now saturated. At this
stage lake water was penetrating the barrier and starting to cause erosion on its downstream side.
Catastrophic collapse occurred on 18 March 2007, accompanied by a lahar in the Whangaehu river valley.
Subsequent forward 3D numerical modelling of the resistivity structure of the barrier has confirmed that the
observed changes in measured resistivity were directly related to the progress of seepage of lake water into
the barrier.