This is the recording of the large Kermadec Islands earthquake
of June 6, 2011, recorded on our schools seismology SEP
seismometer here at Keele University in the UK. Being at an
epicentral distance of 155.3° from the epicentre, the first
arrival is the PP phase as the direct P-wave is obstructed by
the core. The event is somewhat unusual as one would normally
expect an earthquake of this magnitude along a subduction system
to be a low-angle thrust event along the plate boundary between
the Pacific and Australian Plates dipping gently to the west.
However, the CMT focal mechanism determined is one of a normal
fault. This event is right on the trench so it was possibly
caused as the subducting Pacific Plate is forced to flex in
order to subduct beneath the Australian Plate. The historical
data show a magnitude 6.6 event from 2008, also on the position
of the trench with a similar mechanism. The ‘quake seems to have
caused a ~1m tsunami wave locally but a more regional tsunami
alert was cancelled. Further detai