Earthquake Report: Papua New Guinea: Update #1

The aftershocks are still coming in! We can use these aftershocks to define where the fault may have slipped during this M 7.5 earthquake. As I mentioned yesterday in the original report, it turns out the fault dimension matches pretty well with empirical relations between fault length and magnitude from Wells and Coppersmith (1994).

The mapped faults in the region, as well as interpreted seismic lines, show an imbricate fold and thrust belt that dominates the geomorphology here (as well as some volcanoes, which are probably related to the slab gap produced by crust delamination; see Cloos et al., 2005 for more on this). I found a fault data set and include this in the aftershock update interpretive poster (from the Coordinating Committee for Geoscience Programmes in East and Southeast Asia, CCOP).

I initially thought that this M 7.5 earthquake was on a fault in the Papuan Fold and Thrust Belt (PFTB). Mark Allen pointed out on twitter that the ~35km hypocentral depth is probably too deep to be on one of these “thin skinned” faults (see Social Media below). Abers and McCaffrey (1988) used focal mechanism data to hypothesize that there are deeper crustal faults that are also capable of generating the earthquakes in this region. So, I now align myself with this hypothesis (that the M 7.5 slipped on a crustal fault, beneath the thin skin deformation associated with the PFTB. (thanks Mark! I had downloaded the Abers paper but had not digested it fully.)

Below is my interpretive poster for this earthquake

I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend).
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 7.5 earthquake, in addition to some relevant historic earthquakes.

  • 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 also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.
  • I include some inset figures.

  • In the upper right corner is a general overview of the plate boundaries and mapped faults in the region. I place a blue star in the general location of the M 7.5 epicenter. The fault lines on this figure also come from CCOP.
  • In the lower right corner is a plot showing vertical land motion for GPS sites along a north-south profile. Basically, this shows that the sites north of the FTB are currently uplifting at about 5 mm.yr and the sites north of the Bewani fault zone are uplifting an additional 10 mm/yr. This means that the crustal shortening associated with the collision of Australia with the Pacific/Caroline plates is partly being accumulated as elastic strain in the crust and is localized on these fault systems. While this profile is several tens of kilometers to the west of the M 7.5, this process is likely also happening where the M 7.5 occurred.
  • On the left are three figures from Abers and McCaffrey (1988).
    • The upper panel shows the extent of a portion of their analysis that is cogent for the M 7.5 sequence. The extent of this box is also outlined in a dashed yellow rectangle on the main map. The blue star represents the general location of the M 7.5 earthquake. There are no backthrusts mapped on this figure (the hypothesis for the M 7.5 source fault promoted in my original report and on social media).
    • This is a north-south cross section showing the focal mechanisms for 3 of the earthquakes in the map. This shows a south vergent fault as a possible source for the M ~5.x earthquakes studied by Abers and McCaffrey (1988). I am starting to favor an interpretation that the M 7.5 fault is south vergent.
    • The lowest panel shows the interpretation from Abers that these deeper crustal faults are responsible for the seismicity they studied (and I thank mark again that I may posit that these faults are responsible for the current seismicity).

  • Here is the original interpretive poster from my initial report here.

  • The same map without historic seismicity.

Some Relevant Discussion and Figures

  • Here is the tectonic map from Loulali et al. (2015).

  • Tectonic setting of the Papua New Guinea region. Topography and bathymetry are from SRTM( Faults are mostly from the East and Southeast Asia (CCOP) 1:2000000 geological map (downloaded from AFTB, Aure Fold-and-Thrust Belt; OSZF, Owen Stainly fault zone; GF, Gogol fault; BTFZ, Bewani-Torricelli fault zone; RMFZ, Ramu-Markham fault zone; BSSL, Bismarck Sea Seismic Lineation.

  • Here is a map from Abers and McCaffrey (1988) that shows all the earthquakes included in their study (and the focal mechanisms). Inset “a” is the region shown on the aftershock poster above.

  • Map of focal mechanisms determined here, locations of cross sections in Figure 11, and shallow seismicity. Focal mechanisms are shown as lower hemisphere projections with the compressional quadrants shaded, and the P and T axes shown as solid and open circles, respectively. The sizes of the focal spheres are scaled to log (MO), according to the scale in the upper right, and are labeled by the event numbers in Table 1. Seismicity is from the ISC catalog, 1964-1984, and includes all events listed as being shallower than 70 km recorded by 25 or more stations, with M b • 5.0, and with standard deviations in latitude, longitude, or depth each not exceeding 20 km. Inset in lower left shows all large (M • 7.0), shallow (! 70 km) earthquakes in the period 1900-1985, from the catalog compiled by Everingham [1974] for events before 1971 and from Ganse and Nelson [1981, with supplement] for more recent events. Faults are labelled on Figure 1.

  • Here are all the 3 cross sections from Abers and McCaffrey (1988). The upper section is a and the lower section is c (from the above map).

  • Cross sections of seismicity and topography: a, b, and c refer to the profile locations on Figure 2. Vertical exaggeration is 10x for topography and lx for seismicity, as indicated by the vertical scale bars on right. Horizontal scale, indicated on profile a, is the same for all profiles. Focal spheres are plotted as back hemisphere projections, and compressional quadrants are filled.

  • This is the money shot, showing their interpretation (Abers and McCaffrey, 1988).

  • Cartoon showing how thin-skinned faulting mapped in PNG might be related to faulting in the basement, inferred from the earthquakes and other evidence discussed in the text. See Figure 11a for comparison to actual topography and earthquake mechanisms.

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