Earthquake Report: New Hebrides

There was just a M 7.2 earthquake between New Caledonia and Fiji. There is a strike slip plate boundary fault in this region that connects the New Hebrides Trench with Fiji. Here is the USGS website for this earthquake.
Below is my interpretive map. I plot the earthquake epicenters from the last week as circles with diameters related to their magnitude and color representing time. I plot the moment tensor for this M 7.2 earthquake.
I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
I interpret this earthquake to be left-lateral (synistral) strike-slip. This is based upon its proximity to a transform plate boundary (HFZ – Hunter fracture zone). Without more information, I cannot rule out the alternative solution.

I include some inset figures in the poster.
• In the upper right corner, from left to right, are three figures from Richards et al. (2011). First is a tectonic map showing the plate boundaries with seismicity colored vs depth. Note how the New Hebrides earthquakes deepen to the east and the Tonga Trench earthquakes deepen to the west. Next is a map that shows the location of downgoing slabs (chunks of oceanic lithosphere), in red, that the authors have interpreted. Finally is a series of illustrations showing the Richards et al. (2011) interpretation of the evolution of the slabs related to the New Hebrides subduction zone.
• On the left, above the moment tensor legend, I place a subset of the USGS tectonic map poster for this region of the Pacific Ocean. This is from the USGS Open File Report 2010-1083-I (Benz et al., 2011). Hypocenters are plotted as cross sections to show the geometry of the subducting slabs.

Here are a series of recent earthquakes in this region of the western Pacific
• 2016.04.06 M 6.9 Vanuatu Update #1
• 2016.04.03 M 6.9 Vanuatu
• 2015.11.18 M 6.8 Solomon Islands
• 2015.05.24 M 6.8, 6.8, 6.9 Santa Cruz Islands
• 2015.05.05 M 7.5 New Britain

The New Hebrides subduction zone dips to the east and turns into a transform fault (Richards et al., 2011). I include the figure caption below as a blockquote.

Topography, bathymetry, and major tectonic element map of the study area. The Tonga and Vanuatu subduction systems are shown together with the locations of earthquake epicenters discussed herein. earthquakes between 0 and 70 km depth have been removed for clarity. Remaining earthquakes are color-coded according to depth. Earthquakes located at 500–650 km depth beneath the North Fiji Basin are also shown. Plate motions for Vanuatu are from the U.S. Geological Survey, and for Tonga from Beavan et al. (2002) (see text for details). Dashed line indicates location of cross section shown in Figure 3. NFB—North Fiji Basin; HFZ—Hunter Fracture Zone.

The New Hebrides subduction zone dips to the east and turns into a transform fault (Richards et al., 2011). I include the figure caption below as a blockquote.

Map showing distribution of slab segments beneath the Tonga-Vanuatu region. West-dipping Pacific slab is shown in gray; northeast-dipping Australian slab is shown in red. Three detached segments of Australian slab lie below the North Fiji Basin (NFB). HFZ—Hunter Fracture Zone. Contour interval is 100 km. Detached segments of Australian plate form sub-horizontal sheets located at ~600 km depth. White dashed line shows outline of the subducted slab fragments when reconstructed from 660 km depth to the surface. When all subducted components are brought to the surface, the geometry closely approximates that of the North Fiji Basin.

This figure shows Richards et al. (2011) Figure 4, that displays their interpretation of how the plates came to be configured here. They propose that the Australia plate detached and collided with the Pacific slab about 4 million years ago. I include the figure caption below as a blockquote.

Simplified plate tectonic reconstruction showing the progressive geometric evolution of the Vanuatu and Tonga subduction systems in plan view and in cross section. Initiation of the Vanuatu subduction system begins by 10 Ma. Initial detachment of the basal part of the Australian slab begins at ca. 5–4 Ma and then sinking and collision between the detached segment and the Pacific slab occur by 3–4 Ma. Initial opening of the Lau backarc also occurred at this time. Between 3 Ma and the present, both slabs have been sinking progressively to their current position. VT—Vitiaz trench;
dER—d’Entrecasteaux Ridge.

Here is a map from the USGS report linked above. Read more about this map on the USGS website. Earthquakes are plotted with color related to depth and circle diameter related to magnitude.

• This is the legend.




• Here are two cross sections showing the seismicity along swatch profiles F-F’ and G-G’.
  • F-F’

  
  

  • G-G’

  
  

An earthquake swarm happened on an analogous transform plate boundary to the north, along a strike-slip fault system in May, 2015. This is also a left lateral strike-slip system. Here is the Earthquake Report for this swarm of three earthquakes with magnitudes of 6.8, 6.8, and 6.9.

Here are the USGS web pages for the three largest earthquakes in this series:
• 2015.05.20 Mw 6.8
• 2015.05.22 Mw 6.9
• 2015.05.22 Mw 6.8

In April 2016, there was a swarm along the New Hebrides Trench to the north of today’s earthquake. The largest earthquakes were all in the upper M 5 to upper M 6 range. Here is my Earthquake Report for these earthquakes.

Here are the USGS websites for the largest earthquakes plotted below.
• 2016.04.03 M 6.9 08:23:53
• 2016.04.06 M 6.7 06:58:48
• 2016.04.06 M 5.9 07:57:38
• 2016.04.06 M 5.3 06:54:54
• 2016.04.07 M 6.7 03:32:53

Here is a cool video from IRIS that discusses the tectonics of this region.
• Here is a direct link to the embedded video below (10 MB mp4)

Here is an animation that shows the seismicity for this region from 1960 – 2016 for earthquakes with magnitudes greater than or equal to 7.0.
• I include some figures mentioned in the posters above, in addition to a plot from Cleveland et al. (2014). In the upper right corner, Cleveland et al. (2014) on the left plot a map showing earthquake epicenters for the time period listed below the plot on the right. On the right is a plot of earthquakes (diameter = magnitude) of earthquakes with latitude on the vertical axis and time on the horizontal axis. Cleveland et al (2014) discuss these short periods of seismicity that span a certain range of fault length along the New Hebrides Trench in this area. Above is a screen shot image and below is the video.




• Here is a link to the embedded video below (6 MB mp4)

Here are the two figures from Cleveland et al. (2014).
• Figure 1. I include the figure caption below as a blockquote.




(left) Seismicity of the northern Vanuatu subduction zone, displaying all USGS-NEIC earthquake hypocenters since 1973. The Australian plate subducts beneath the Pacific in nearly trench-orthogonal convergence along the Vanuatu subduction zone. The largest events are displayed with dotted outlines of the magnitude-scaled circle. Convergence rates are calculated using the MORVEL model for Australia Plate relative to Pacific Plate [DeMets et al., 2010]. (right) All GCMT moment tensor solutions and centroids for Mw ≥ 5 since 1976, scaled with moment. This region experiences abundant moderate and large earthquakes but lacks any events with Mw >8 since at least 1900.

• Figure 17. I include the figure caption below as a blockquote.




One hundred day aftershock distributions of all earthquakes listed in the ISC catalog for the 1966 sequence and in the USGS-NEIC catalog for the 1980, 1997, 2009, and 2013 sequences in northern Vanuatu. The 1966 main shocks are plotted at locations listed by Tajima et al. [1990]. Events of the 1997 and 2009 sequences were relocated using the double difference method [Waldhauser and Ellsworth, 2000] for P wave first arrivals based on EDR picks. The event symbol areas are scaled relative to the earthquake magnitudes based on a method developed by Utsu and Seki [1954]. Hypocenters of most aftershock events occurred at <50 km depth.

• Figure 17. I include the figure caption below as a blockquote.




(right) Space-time plot of shallow (≤ 70 km) seismicity M ≥ 5.0 in northern Vanuatu recorded in the NEIC catalog as a function of distance south of
10°N, 165.25°E. (left) The location of the seismicity on a map rotated to orient the trench vertically.

References:
• Benz, H.M., Herman, Matthew, Tarr, A.C., Furlong, K.P., Hayes, G.P., Villaseñor, Antonio, Dart, R.L., and Rhea, Susan, 2011. Seismicity of the Earth 1900–2010 eastern margin of the Australia plate: U.S. Geological Survey Open-File Report 2010–1083-I, scale 1:8,000,000.
• Cleveland, K.M., Ammon, C.J., and Lay, T., 2014. Large earthquake processes in the northern Vanuatu subduction zone in Journal of Geophysical Research: Solid Earth, v. 119, p. 8866-8883, doi:10.1002/2014JB011289.
• Richards, S., Holm., R., Barber, G., 2011. When slabs collide: A tectonic assessment of deep earthquakes in the Tonga-Vanuatu region, Geology, v. 39, pp. 787-790.
• Music Reference (in 19600-2016 seismicity video): Bumba Crossing Kevin MacLeod (incompetech.com) | Licensed under Creative Commons: By Attribution 3.0 License | http://creativecommons.org/licenses/by/3.0/