Historical tsunami earthquakes in the Southwest Pacific: An extension to Δ > 80° of the energy-to-moment parameter Θ

Emile A. Okal*, Nooshin Saloor

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

8 Scopus citations


We extend to distances beyond 80° the computation of the energy-to-moment slowness parameter Θ introduced by Newman and Okal, by defining a regional empirical correction based on recordings at distant stations for events otherwise routinely studied. In turn, this procedure allows the study of earthquakes in a similar source-station geometry, but for which the only available data are located beyond the original distance threshold, notably in the case of historical earthquakes predating the development of dense networks of short-period seismometers. This methodology is applied to the twin 1947 earthquakes off the Hikurangi coast of NewZealand forwhichwe confirmslowness parameters characteristic of tsunami earthquakes. In addition, we identify as such the large aftershock of 1934 July 21 in the Santa Cruz Islands, which took place in the immediate vicinity of the more recent 2013 shock,which also qualifies as a tsunami earthquake. In that subduction zone, the systematic compilation of Θ for both recent and pre-digital events shows a diversity in slowness correlating with local tectonic regimes controlled by the subduction of fossil structures. Our methodology is also well adapted to the case of analogue records of large earthquakes for which short-period seismograms at conventional distances are often off-scale.

Original languageEnglish (US)
Pages (from-to)852-873
Number of pages22
JournalGeophysical Journal International
Issue number2
StatePublished - Aug 1 2017


  • Earthquake source observations
  • NewZealand
  • Pacific Ocean
  • Tsunamiwarning

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology


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