Estimating radiated seismic energy for New Zealand and the South-West pacific

Resilience during and after large earthquakes depends heavily on the rapid characterisation of the events and their impacts. To achieve the best response possible, it is useful to estimate the dynamic rupture characteristics. The duration of rupture can help us assess tsunamigenic potential, by detecting the special class of “tsunami earthquakes” [Newman & Okal 1998], characteristically long-rupturing events that generate larger tsunamis than expected for their magnitude [Kanamori 1972]. Energy partitioning between high and low frequencies can guide analysis of potential damage to the built environment.

To better understand the links between the dynamic energy parameters and seismic impacts of earthquakes in the southwest Pacific, I have compiled a catalogue of historic and recent events, to use as a baseline for future near-real-time estimates and bring some more insight into seismic activities in this part of the Pacific.

We apply proven algorithms [Newman & Okal 1998, Newman & Convers 2013, Boatwright et al 2002], that rely on different type of waves and distance ranges, as well as the more recent updates to such estimations [Ebeling & Okal 2012, Saloor & Okal 2018], to select the approaches and input assumptions most fitting for this region. However, the remote location with sparse instrumental data brings limitations in gaining reliable estimations of radiated energy.

The technique from Newman & Okal [1998], relying on teleseismic P-waves, performs reliable estimates of Mw5.5+ events but needs some adjustment to reduce uncertainties for intermediate magnitude earthquakes in the region, due to large azimuthal gaps. Following a later approach from the author [Convers & Newman 2011], I design a network of consistent and reliable stations within the distance range and correct them for permanent deviations. Lastly, I reproduce the scheme from Ebeling and Okal [2012] in evaluating an empirical correction of the scaled radiated energy for stations closer than 35° epicentral distance, but on a regional scale instead. This approach makes use of stations within regional distances of the epicentre for faster results.

Our application of the S-wave approach of Boatwright et al. [2002], relies heavily on the densely sampled New Zealand broad-band seismic network. I use the existing velocity and attenuation models for New Zealand [Eberhart-Phillips et al. 2020] to test both 1D and 3D attenuation corrections, in terms of reliability of the results and computing time. We also refine our estimations of higher frequencies to better evaluate their contribution to the energy estimates in various subduction zone settings.

In this talk, I will present the current state of the radiated energy catalogue for the South-West Pacific and the automated energy analysis under the New Zealand RCET (Rapid Characterisation of Earthquakes and Tsunamis) program. The teleseismic estimates are in good agreement with previous evaluations for large earthquakes. The richer dataset largely correlates with what we know about regional tectonics and highlights some variations that could indicate large transitions in subduction-zone stress fields.