Friday Seminar (Oct. 5th) by Prof. B.L.N. Kennett (Research School of Earth Sciences, The Australian National University)

Starting from the hypocentre, the point of initiation of seismic energy, the subsequent trajectory of the points of emission of high-frequency energy is tracked in 3-D - these points of maximum energy emission are termed ‘evocentres’. These evocentres are found as a function of time by energy stacking for putative points on a 3-D grid around the hypocentre that is expanded as time progresses, selecting the location of maximum energy release as a function of time. The spatial resolution in the neighbourhood of a target point can be simply estimated by spatial mapping using the properties of isochrons from the available stations. The mapping of a seismogram segment to space is by inverse slowness, and thus more distant stations have a broader spatial contribution. As in hypocentral estimation, the inclusion of a wide azimuthal distribution of stations significantly enhances 3-D capability. The interpretation of the evocentres is simplest in the case of unilateral rupture.

The approach is illustrated for a number of major eearthquakes, including the 2007 Mw 8.1 Solomons islands event that ruptured across a plate boundary, the 2013 Mw 8.3 event 610 km beneath the Sea of Okhotsk, the 2014 Mw 8.1 event on the Macquarie Ridge and the major events offshore Sumatra including the Mw 8.6 and 8.2 events in 2012. In each case we are able to
provide estimates of the evolution of high-frequency energy that tally well with alternative
schemes (where available), but also to provide information on the 3-D characteristics that is not available from backprojection from distant networks. The evocentre estimates can be of considerable value where events are out of range of any dense networks e.g. in the Southern Ocean.

The major characteristics of event rupture can be captured using just a few azimuthally distributed stations, which opens the opportunity for the approach to be used in a rapid mode immediately after a major event to provide guidance for, for example tsunami warning for megathrust events.