Earthquake Report: Banda Sea

Posted on November 10, 2023 by Jay Patton

The other evening (my time) I and many others noticed a series of earthquakes in the Banda Sea region.

As is typical, people want as much information about these earthquakes as possible as soon as possible.

There were two quakes within about a minute of each other (first M 6.7, then M 7.1), so it was possible that there was actually just one earthquake (or so we thought).

The reason for this is that sometimes the automated earthquake location algorithms get it wrong. That is when real people step in to review the data from the available seismic networks (data from seismometers that measure seismic waves from earthquakes).

Long story short, these two earthquakes were actually different earthquakes. After a while, the magnitudes stopped changing. And, a while later, another M 6.7 earthquake happened.

Here are the these three earthquakes:

2023-11-08 04:52:51 (UTC) M 6.7: https://earthquake.usgs.gov/earthquakes/eventpage/us7000l9h2/executive
2023-11-08 04:53:50 (UTC) M 7.1: https://earthquake.usgs.gov/earthquakes/eventpage/us7000l9h4/executive
2023-11-08 13:02:06 (UTC) M 6.7: https://earthquake.usgs.gov/earthquakes/eventpage/us7000l9ku/executive

In this part of the world, there are several different plate boundary faults that interact.

To the south of the Banda Sea, there is a subduction zone. This is a convergent plate boundary fault where an oceanic plate (Australia plate) “subducts” beneath the Sunda plate (part of the Eurasia plate).

This subduction zone extends laterally from between Australia and the Timor Islands, westward through Java and Sumatra, up to Myanmar/Burma. This plate boundary is part of the Alpide Belt, a convergent plate boundary that spans this region, all the way westward past Portugal and Morroco!

There are also some strike-slip faults in the upper plate here. These may be related to faults that extend further to the east, heading into Papua New Guinea and the “Bird’s Head.” One of these faults is the Sorong fault.

When I first saw this earthquake, I thought about the previous Earthquake Reports I had developed for events in this region. Every one of them were for intermediate depth earthquakes within the subducted Australia plate.

The mechanism for this M 7.1 was strike-slip and matched many of the mechanisms from these earlier earthquakes. However, the depth was set at 10 km. This is one of the default earthquake depths, so I thought that it might be updated later to be much deeper. Though, when earthquakes are so much deeper, their initial depths are often not set at the default 10 km depth.

So, I posted to social media that this may be one of these intermediate depth intraslab (within the slab of the plate) earthquakes. I suggested that the depth may be updated, as I mention here ^^^.

Well, it is always good to be receptive to one being incorrect. I was incorrect. That is OK! In the early hours (even days) after an earthquake, everyone is trying to figure out what happened. I don’t want to hold back my imagination from conceiving of hypotheses. But, I do want to be open minded that I need to change my interpretations given more information as that rolls in.

One of these pieces of information was data from a nearby tide gage, a gage located on the Island of Damar (less than 100 km from the epicenter). This tide gage showed a tsunami being recorded.

Strike-slip earthquakes are generally thought to not generate tsunami. Though, the 1906 San Francisco, the 1999 Izmit, and the 2018 Dongalla-Palu earthquakes did. Generally they are simply smaller in size (Dongalla-Palu being an exception, given the configuration of Palu Bay).

If the moderate sized strike-slip earthquake were intermediate depth, it is extremely unlikely to generate a tsunami as there would be very little deformation of the seafloor. Because this M 7.1 generated a tsunami, it supported the shallow depths and a crustal source for this earthquake.

So, this series of earthquakes was indeed shallower, within the upper plate (not the Australia plate). AND there is this strike-slip fault that is already mapped, probably the source for these earthquakes.

A second thing about the tide gage plots is that we can see that the second M 6.7 earthquake also generated a tsunami! Much smaller, but a signal that can be found on several tide gage records.

Indonesia operates an extensive tide gage network. Here one can locate all the gages in their network.

Below I present an interpretive poster and some supporting material.

Below is my interpretive poster for this earthquake

I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1923-2023 with magnitudes M ≥ 7.0 in one version.
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

In the upper right corner is a map showing plate boundary fault lines and the major tectonic plates.
To the left of the plate tectonic map is an inset, larger scale map, showing the aftershocks from a couple days. I outline what may be the main fault rupture area, plus a secondary rupture for triggered earthquakes. Based on Wells and Coppersmith (1994), the aftershocks span a region larger in size than expected for a 7.1. So, there may be several faults involved here. Perhaps the second M 6.7 was also a triggered earthquake (on a different fault), not an aftershock(?).
In the center right is a low angle oblique view of the tectonic plate configuration (Pownall et al., 2014). The previous earthquakes in the poster that have mechanisms plotted all appear to be in the subducted Australia plate, shown here diving downwards.
In the lower right corner is a view of the tectonic plate configuration (Koulali et al., 2014). I place a blue star in the location of the M 7.1 earthquake. Note that there is an earthquake from 1763 with a magnitude 7 located along the strike-slip fault mapped on this map. This earthquake generated a tsunami.
In the upper left corner are maps that show the seismic hazard and seismic risk for Indonesia. I spend more time explaining this below.
In the center top-left is a map that shows earthquake intensity using the Modified Mercalli Intensity (MMI) Scale.
In the bottom center I plot the tide gage data from Damar Island.

Here is the map with a month’s seismicity plotted.

Tsunami Hazard

Here are two maps that show the results of probabilistic tsunami modeling for the nation of Indonesia (Horspool et al., 2014). These results are similar to results from seismic hazards analysis and maps. The color represents the chance that a given area will experience a certain size tsunami (or larger).
The first map shows the annual chance of a tsunami with a height of at least 0.5 m (1.5 feet). The second map shows the chance that there will be a tsunami at least 3 meters (10 feet) high at the coast.

Annual probability of experiencing a tsunami with a height at the coast of (a) 0.5m (a tsunami warning) and (b) 3m (a major tsunami warning).

Here are two tide gage records that include observations from the M 7.1 and M 6.7 earthquakes. There were a couple other gages in the region that include the tsunami but the tsunami was very small and difficult to accurately measure.

Some Relevant Discussion and Figures

Here is a tectonic map for this part of the world from Zahirovic et al., 2014. They show a fracture zone where the M 7.3 earthquake happened. I left out all the acronym definitions (you’re welcome), but they are listed in the paper.

Regional tectonic setting with plate boundaries (MORs/transforms = black, subduction zones = teethed red) from Bird (2003) and ophiolite belts representing sutures modified from Hutchison (1975) and Baldwin et al. (2012). West Sulawesi basalts are from Polvé et al. (1997), fracture zones are from Matthews et al. (2011) and basin outlines are from Hearn et al. (2003).

Here are a series of maps from Koulali et al. (2014).
This first one shows the major historic earthquakes at the time of publication (also, this is the map in the interpretive poster).

Seismotectonic setting of the Sunda-Banda arc-continent collision, East Indonesia. Major faults (thick black lines) [Hamilton, 1979]. Topography and bathymetry are from Shuttle Radar Topography Mission (http://topex.ucsd.edu/www_html/srtm30_plus.html). Focal mechanisms are from the Global Centroid Moment Tensor. Blue mechanisms correspond to earthquakes with Mw>7 (brown transparent ellipses are the corresponding rupture areas for Flores 1992 and Alor 2004 earthquakes), while the green focal mechanism shows the highest magnitude recorded in Sumbawa. Red dots indicate the locations of major historical earthquakes [Musson, 2012].

This map shows the plate velocities as measured by GPS/GNSS instruments.
Note the blue arrows show the convergent nature of the plate boundary to the south of this earthquake.

GPS velocities determined in this study with respect to Sunda Block. Uncertainty ellipses represent 95% confidence level. The inset figure corresponds to the area of the dashed rectangle in the map. Light blue arrows show the velocities for East and West Makassar Blocks.

This map shows the results of their plate tectonic modeling.
Koulali et al. (2014) basically modeled the different tectonic blocks and made estimates of how much each plate boundary fault would need to move based on these block motions.
Here we can see that there is lateral motion required for the strike-slip fault that may be the causative fault for this M 7.1 earthquake sequence.

Relative slip vectors across block boundaries, derived from our best fit model. Arrows show motion of the hanging wall (moving block) relative to the footwall (fixed block) with 95% confidence ellipses. The tails of arrows is located within the “moving” block. Black thick lines show well-defined boundaries we use as active faults in our model and dashed lines show less well-defined boundaries (green : free-slipping boundaries and black: fixed locked faults). Principal axes of the horizontal strain tensor estimated for the SUMB, EMAK, and EJAV are shown in pink. The thick pink arrow shows the relative motion of Australia with respect to Sunda (AUST/SUND). Abbreviations are Sumba Block (SUMB), West Makassar Block (WMAK), East Makassar Block (EMAK), East Java Block (EJAV), and Timor Block (TIMO). The background seismicity is from the International Seismological Centre catalog with magnitudes ≥5.5 and depths <40 km.

This is a great visualization showing the Australia plate and how it formed the largest forearc basin on Earth (Pownall et al., 2014).
The maps on the left show a time history of the tectonics. The low angle oblique view on the right shows the dipping crust (north is not always up, as in this figure).
In the lower right, they show how there is strike-slip faulting along the Seram trough also (I left out the figure caption for E).

Reconstructions of eastern Indonesia, adapted from Hall (2012), depict collision of Australia with Southeast Asia and slab rollback into Banda Embayment. Yellow star indicates Seram. Oceanic crust is shown in purple (older than 120 Ma) and blue (younger than 120 Ma); submarine arcs and oceanic plateaus are shown in cyan; volcanic island arcs, ophiolites, and material accreted along plate margins are shown in green. A: Reconstruction at 15 Ma. B: Reconstruction at 7 Ma. C: Reconstruction at 2 Ma. D: Visualization of present-day slab morphology of proto–Banda Sea based on earthquake hypocenter distribution and tomographic models

Here is a map and some cross sections showing seismic tomography (like C-T scans into the Earth using seismic waves instead of X-Rays). The map shows the location of the cross sections (Spakman et al., 2010).

The Banda arc and surrounding region. 200 m and 4,000 m bathymetric contours are indicated. The numbered black lines are Benioff zone contours in kilometres. The red triangles are Holocene volcanoes (http://www.volcano.si.edu/world/). Ar=Aru, Ar Tr=Aru trough, Ba=Banggai Islands, Bu=Buru, SBS=South Banda Sea, Se=Seram, Sm=Sumba, Su=Sula Islands, Ta=Tanimbar, Ta Tr=Tanimbar trough, Ti=Timor, W=Weber Deep.

Tomographic images of the Banda slab. Vertical sections through the tomography model along the lines shown in Fig. 1. Colours: P-wave anomalies with reference to velocity model ak135 (ref. 30). Dots: earthquake hypocentres within 12 km of the section. The dashed lines are phase changes at ~410 km and ~660 km. The sections are plotted without vertical exaggeration; the horizontal axis is in degrees. The labelled positive anomalies are the Sunda (Su) and Banda (Ba) slabs: BuDdetached slab under Buru, FlDslab under Flores, SDslab under Seram, TDslab under Timor. a, The Sunda slab enters the lower mantle whereas the Banda embayment slab is entirely in the upper mantle with the change under Sulawesi. b–e, Banda slab morphology in sections parallel to Australia plate motion shows a transition from a steep slab with a flat section (fs) (b) to a spoon shape shallowing eastward (c–e).

Here is the map from Benz et al. (2011).

Here is the tectonic map from Hengesh and Whitney (2016)

Illustration of major tectonic elements in triple junction geometry: tectonic features labeled per Figure 1; seismicity from ISC-GEM catalog [Storchak et al., 2013]; faults in Savu basin from Rigg and Hall [2011] and Harris et al. [2009]. Purple line is edge of Australian continental basement and fore arc [Rigg and Hall, 2011]. Abbreviations: AR = Ashmore Reef; SR = Scott Reef; RS = Rowley Shoals; TCZ = Timor Collision Zone; ST = Savu thrust; SB = Savu Basin; TT = Timor thrust; WT =Wetar thrust; WASZ = Western Australia Shear Zone. Open arrows indicate relative direction of motion; solid arrows direction of vergence.

Here is the Audley (2011) cross section showing how the backthrust relates to the subduction zone beneath Timor. I include their figure caption in blockquote below.

Cartoon cross section of Timor today, (cf. Richardson & Blundell 1996, their BIRPS figs 3b, 4b & 7; and their fig. 6 gravity model 2 after Woodside et al. 1989; and Snyder et al. 1996 their fig. 6a). Dimensions of the filled 40 km deep present-day Timor Tectonic Collision Zone are based on BIRPS seismic, earthquake seismicity and gravity data all re-interpreted here from Richardson & Blundell (1996) and from Snyder et al. (1996). NB. The Bobonaro Melange, its broken formation and other facies are not indicated, but they are included with the Gondwana mega-sequence. Note defunct Banda Trench, now the Timor TCZ, filled with Australian continental crust and Asian nappes that occupy all space between Wetar Suture and the 2–3 km deep deformation front north of the axis of the Timor Trough. Note the much younger decollement D5 used exactly the same part of the Jurassic lithology of the Gondwana mega-sequence in the older D1 decollement that produced what appears to be much stronger deformation.

Here is a figure showing the regional geodetic motions (Bock et al., 2003). I include their figure caption below as a blockquote.

Topographic and tectonic map of the Indonesian archipelago and surrounding region. Labeled, shaded arrows show motion (NUVEL-1A model) of the first-named tectonic plate relative to the second. Solid arrows are velocity vectors derived from GPS surveys from 1991 through 2001, in ITRF2000. For clarity, only a few of the vectors for Sumatra are included. The detailed velocity field for Sumatra is shown in Figure 5. Velocity vector ellipses indicate 2-D 95% confidence levels based on the formal (white noise only) uncertainty estimates. NGT, New Guinea Trench; NST, North Sulawesi Trench; SF, Sumatran Fault; TAF, Tarera-Aiduna Fault. Bathymetry [Smith and Sandwell, 1997] in this and all subsequent figures contoured at 2 km intervals.

Whitney and Hengesh (2015) used GPS modeling to suggest a model of plate blocks. Below are their model results.

Plate boundary segments in the Banda Arc region from Nugroho et al (2009). Numbers inside rectangles show possible micro-plate blocks near the Sumba Triple Junction (colored) based on GPS velocities (black arrows) with in a stable Eurasian reference frame.

Here is the conceptual model from Whitney and Hengesh (2015) that shows how left-lateral strike-slip faulting can come into the region.

Schematic map views of kinematic relations between major crustal elements in the Sumba Triple Junction region. CTZ= collisional tectonic zone. Red arrow size designates schematic plate motion relations based on geological data relative to a fixed Sunda shelf reference frame (pin).

Seismic Hazard and Seismic Risk

These are the two maps shown in the map above, the GEM Seismic Hazard and the GEM Seismic Risk maps from Pagani et al. (2018) and Silva et al. (2018).

The GEM Seismic Hazard Map:

The Global Earthquake Model (GEM) Global Seismic Hazard Map (version 2018.1) depicts the geographic distribution of the Peak Ground Acceleration (PGA) with a 10% probability of being exceeded in 50 years, computed for reference rock conditions (shear wave velocity, VS30, of 760-800 m/s). The map was created by collating maps computed using national and regional probabilistic seismic hazard models developed by various institutions and projects, and by GEM Foundation scientists. The OpenQuake engine, an open-source seismic hazard and risk calculation software developed principally by the GEM Foundation, was used to calculate the hazard values. A smoothing methodology was applied to homogenise hazard values along the model borders. The map is based on a database of hazard models described using the OpenQuake engine data format (NRML). Due to possible model limitations, regions portrayed with low hazard may still experience potentially damaging earthquakes.
Here is a view of the GEM seismic hazard map for Indonesia.

The GEM Seismic Risk Map:

The Global Seismic Risk Map (v2018.1) presents the geographic distribution of average annual loss (USD) normalised by the average construction costs of the respective country (USD/m2) due to ground shaking in the residential, commercial and industrial building stock, considering contents, structural and non-structural components. The normalised metric allows a direct comparison of the risk between countries with widely different construction costs. It does not consider the effects of tsunamis, liquefaction, landslides, and fires following earthquakes. The loss estimates are from direct physical damage to buildings due to shaking, and thus damage to infrastructure or indirect losses due to business interruption are not included. The average annual losses are presented on a hexagonal grid, with a spacing of 0.30 x 0.34 decimal degrees (approximately 1,000 km² at the equator). The average annual losses were computed using the event-based calculator of the OpenQuake engine, an open-source software for seismic hazard and risk analysis developed by the GEM Foundation. The seismic hazard, exposure and vulnerability models employed in these calculations were provided by national institutions, or developed within the scope of regional programs or bilateral collaborations.

Here is a view of the GEM seismic risk map for Indonesia.

Indonesia | Sumatra

General Overview

M 9.2 Andaman-Sumatra subduction zone 2014 Earthquake Anniversary
M 9.2 Andaman-Sumatra subduction zone SASZ Fault Deformation
M 9.2 Andaman-Sumatra subduction zone 2016 Earthquake Anniversary

Earthquake Reports

2023.11.08 M 7.1 Banda Sea
2023.08.28 M 7.1 Lombok & Bali, Indonesia
2023.04.24 M 7.1 Sumatra
2022.11.18 M 6.9 Sumatra
2022.02.25 M 6.2 Sumatra
2020.05.06 M 6.8 Banda Sea
2019.08.02 M 6.9 Indonesia
2019.06.23 M 7.3 Banda Sea
2019.04.12 M 6.8 Sulawesi, Indonesia
2018.09.28 M 7.5 Sulawesi
2018.10.16 M 7.5 Sulawesi UPDATE #1
2018.08.19 M 6.9 Lombok, Indonesia
2018.08.05 M 6.9 Lombok, Indonesia
2018.07.28 M 6.4 Lombok, Indonesia
2017.12.15 M 6.5 Java
2017.08.31 M 6.3 Mentawai, Sumatra
2017.08.13 M 6.4 Bengkulu, Sumatra, Indonesia
2017.05.29 M 6.8 Sulawesi, Indonesia
2017.03.14 M 6.0 Sumatra
2017.03.01 M 5.5 Banda Sea
2016.10.19 M 6.6 Java
2016.03.02 M 7.8 Sumatra/Indian Ocean
2015.07.22 M 5.8 Andaman Sea
2015.11.08 M 6.4 Nicobar Isles
2012.04.11 M 8.6 Sumatra outer rise
2004.12.26 M 9.2 Andaman-Sumatra subduction zone

Social Media

Here is my main thread, which includes many intermediate tweets (one can see the process of us figuring this sequence out in real time):

#EarthquakeReport for M6.9 #Gempa #Earthquake offshore in the #BandaSea

Probably intraslab earthquake within Australia plate

In almost same location as 2020 earthquake: https://t.co/DBTKHT6oYn https://t.co/yE3fOxqZoO pic.twitter.com/nNAIGl7EaL

— Jason "Jay" R. Patton (@patton_cascadia) November 8, 2023

#EarthquakeReport #TsunamiReport for M 7.1 #Gempa #Earthquake sequence in the #BandaSea

M6.7 foreshock to 7.1 both left-lateral strike-slip earthquakes in the Sunda block crust
triggered a M 6.7
both the 7.1 and triggered 6.7 caused tsunami

read reporthttps://t.co/V9Y1zX4sRr pic.twitter.com/MEot8TVrlU

— Jason "Jay" R. Patton (@patton_cascadia) November 10, 2023

Mw=7.5, BANDA SEA (Depth: 16 km), 2023/11/08 04:52:52 UTC – Full details here: https://t.co/UPXunwhyVI pic.twitter.com/fPFfoh3zUu

— Earthquakes (@geoscope_ipgp) November 8, 2023

This seismic signal actually shows two earthquakes in approx. the same location in the Banda Sea, ~1 min apart – the first a M7.0, the second a M7.1. Our recorders were still picking up a signal >30 mins later. https://t.co/TLOo9ts3yM pic.twitter.com/ztEZGUEECx

— Dr. Dee Ninis (@DeeNinis) November 8, 2023

data monitoring paras muka air laut di stasiun pengamamatan Damar Maluku 12:53 WIB 08/11/2023 cc: @widjokongko pic.twitter.com/oXIklbZtsG

— INFOMITIGASI (@infomitigasi) November 8, 2023

Rabu 08 Nov 2023 pukul 11.52.53 WIB Laut Banda diguncang gempa tektonik. Hasil analisis BMKG menunjukkan gempa ini memiliki parameter update M7,1. Episenter gempa terletak di laut pada jarak 255 Km arah Barat Laut Tanimbar, Maluku pada kedalaman 45 km. pic.twitter.com/AgO1mx94Ka

— DARYONO BMKG (@DaryonoBMKG) November 8, 2023

Today, 7.2 Eq. Banda Sea (Eastern #INDONESIA 🇮🇩), located in the "Inner Banda Arc" (close to Extinct Damar Spreading Basin), crustal strike-slip faulting suggest reactivation of "Damar Extinct Spreading Back-arc Basin (NW-SE structure?).

🔹️Damar Basin :https://t.co/7mSB8NnO4R pic.twitter.com/ZZF6AVeC6n

— Abel Seism🌏Sánchez (@EQuake_Analysis) November 8, 2023

Watch the earthquake waves from these events in Indonesia sweep across North America. (GMV from @EarthScope_sci) https://t.co/ALTlPHqNyt https://t.co/21Y7is9EhQ pic.twitter.com/ywo6DLGUcq

— Wendy Bohon, PhD 🌏 (@DrWendyRocks) November 8, 2023

Recent Earthquake Teachable Moment for the M7.1 earthquake in the Banda Sea.

Teachable Moments presentations capture the opportunity to bring knowledge, insight, and critical thinking to the classroom following a newsworthy earthquake.

➡️ https://t.co/zRd4mRCHc4 pic.twitter.com/fpLiQLuz3E

— EarthScope Consortium (@EarthScope_sci) November 9, 2023

Very interesting series of M6.7, 7.1, and 6.7 earthquakes in the remote Banda Sea yesterday – among the largest shallow strike-slip events recorded in the region!

The tectonics of the region are complex, with many open questions.

Read more on our blog – link in my bio. pic.twitter.com/6B7PLGR7K1

— Dr. Judith Hubbard (@JudithGeology) November 9, 2023

References:

Basic & General References

Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
Jessee, M.A.N., Hamburger, M. W., Allstadt, K., Wald, D. J., Robeson, S. M., Tanyas, H., et al. (2018). A global empirical model for near-real-time assessment of seismically induced landslides. Journal of Geophysical Research: Earth Surface, 123, 1835–1859. https://doi.org/10.1029/2017JF004494
Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
Pagani,M. , J. Garcia-Pelaez, R. Gee, K. Johnson, V. Poggi, R. Styron, G. Weatherill, M. Simionato, D. Viganò, L. Danciu, D. Monelli (2018). Global Earthquake Model (GEM) Seismic Hazard Map (version 2018.1 – December 2018), DOI: 10.13117/GEM-GLOBAL-SEISMIC-HAZARD-MAP-2018.1
Silva, V ., D Amo-Oduro, A Calderon, J Dabbeek, V Despotaki, L Martins, A Rao, M Simionato, D Viganò, C Yepes, A Acevedo, N Horspool, H Crowley, K Jaiswal, M Journeay, M Pittore, 2018. Global Earthquake Model (GEM) Seismic Risk Map (version 2018.1). https://doi.org/10.13117/GEM-GLOBAL-SEISMIC-RISK-MAP-2018.1
Storchak, D. A., D. Di Giacomo, I. Bondár, E. R. Engdahl, J. Harris, W. H. K. Lee, A. Villaseñor, and P. Bormann (2013), Public release of the ISC-GEM global instrumental earthquake catalogue (1900–2009), Seismol. Res. Lett., 84(5), 810–815, doi:10.1785/0220130034.
Zhu, J., Baise, L. G., Thompson, E. M., 2017, An Updated Geospatial Liquefaction Model for Global Application, Bulletin of the Seismological Society of America, 107, p 1365-1385, https://doi.org/0.1785/0120160198

Specific References

Audley-Charles, M.G., 1986. Rates of Neogene and Quaternary tectonic movements in the Southern Banda Arc based on micropalaeontology in: Journal of fhe Geological Society, London, Vol. 143, 1986, pp. 161-175.
Audley-Charles, M.G., 2011. Tectonic post-collision processes in Timor, Hall, R., Cottam, M. A. &Wilson, M. E. J. (eds) The SE Asian Gateway: History and Tectonics of the Australia–Asia Collision. Geological Society, London, Special Publications, 355, 241–266.
Baldwin, S.L., Fitzgerald, P.G., and Webb, L.E., 2012. Tectonics of the New Guinea Region in Annu. Rev. Earth Planet. Sci., v. 41, p. 485-520.
Benz, H.M., Herman, Matthew, Tarr, A.C., Hayes, G.P., Furlong, K.P., Villaseñor, Antonio, Dart, R.L., and Rhea, Susan, 2011. Seismicity of the Earth 1900–2010 New Guinea and vicinity: U.S. Geological Survey Open-File Report 2010–1083-H, scale 1:8,000,000.
Given, J. W., and H. Kanamori (1980). The depth extent of the 1977 Sumbawa, Indonesia, earthquake, in EOS Trans. AGU., v. 61, p. 1044.
Gusnman, A.R., Tanioka, Y., Matsumoto, H., and Iwasakai, S.-I., 2009. Analysis of the Tsunami Generated by the Great 1977 Sumba Earthquake that Occurred in Indonesia in BSSA, v. 99, no. 4, p. 2169-2179, https://doi.org/10.1785/0120080324
Hall, R., 2011. Australia-SE Asia collision: plate tectonics and crustal flow in Geological Society, London, Special Publications 2011; v. 355; p. 75-109 doi: 10.1144/SP355.5
Hangesh, J. and Whitney, B., 2014. Quaternary Reactivation of Australia’s Western Passive Margin: Inception of a New Plate Boundary? in: 5th International INQUA Meeting on Paleoseismology, Active Tectonics and Archeoseismology (PATA), 21-27 September 2014, Busan, Korea, 4 pp.
Koulali, A., S. Susilo, S. McClusky, I. Meilano, P. Cummins, P. Tregoning, G. Lister, J. Efendi, and M. A. Syafi’i, 2016. Crustal strain partitioning and the associated earthquake hazard in the eastern Sunda-Banda Arc in Geophys. Res. Lett., 43, 1943–1949, doi:10.1002/2016GL067941
Okal, E. A., & Reymond, D., 2003. The mechanism of great Banda Sea earthquake of 1 February 1938: applying the method of preliminary determination of focal mechanism to a historical event in EPSL, v. 216, p. 1-15.
Osada, M. and Abe, K., 1981. Mechanism and tectonic implications of the great Banda Sea earthquake of November 4, 1963 in Physics of the Earth and Plentary Interiors, v. 25, p. 129-139
Pownall, J.M., Hall, R., Armstrong,, R.A., and Forster, M.A., 2014. Earth’s youngest known ultrahigh-temperature granulites discovered on Seram, eastern Indonesia in Geology, v. 42, no. 4, p. 379-282, https://doi.org/10.1130/G35230.1
Spakman, W. and Hall, R., 2010. Surface deformation and slab–mantle interaction during Banda arc subduction rollback in Nature Geosceince, v. 3, p. 562-566, https://doi.org/10.1038/NGEO917
Whitney, B.B. and Hengesh, J.V., 2015. A new model for active intraplate tectonics in western Australia in Proceedings of the Tenth Pacific Conference on Earthquake Engineering Building an Earthquake-Resilient Pacific 6-8 November 2015, Sydney, Australia, paper number 82
Zahirovic, S., Seton, M., and Müller, R.D., 2014. The Cretaceous and Cenozoic tectonic evolution of Southeast Asia in Solid Earth, v. 5, p. 227-273, doi:10.5194/se-5-227-2014

Return to the Earthquake Reports page.

Sorted by Magnitude
Sorted by Year
Sorted by Day of the Year
Sorted By Region

Earthquake Report: M 6.9 Papua New Guinea

Posted on October 8, 2023 by Jay Patton

Early this morning my time, there were several earthquakes in and offshore of Papua New Guinea.

The two main earthquakes are magnitude M 6.7 anf 6.9.

2023.10.07 M 6.7: https://earthquake.usgs.gov/earthquakes/eventpage/us6000ldqd/executive
2023.10.07 M 6.9: https://earthquake.usgs.gov/earthquakes/eventpage/us6000ldqf/executive

There are several different interpretations for the main tectonic features in this part of the world. So there appears to be some uncertainty about the geometry of the plates here.

The reason for this uncertainty is partly because of the potentially complicated ways that the plates are oriented relative to each other.

These earthquakes happened on a plate boundary fault system (they were intraplate reverse earthquakes, compressional earthquakes along a subduction zone megathrust fault). At least, that is how I am interpreting them.

There has been some seismicity along this plate boundary system over the past few years, but the fault geometry has been somewhat still unresolved (?).

In this region is the Huon Peninsula, a famous location where a Pleistocene sea-level curve was reconstructed from uplifted and subsided prehistoric marine terraces and shorelines.

Along the southern boundary of Huon Peninsula, is a compressional fault system (the Ramu-Markham fault system) that may be the surface trace of a subduction zone fault that dips to the north (??) (or collision?).

Holm and Richards (2013) has a figure that includes cross sections showing this subducted slab. On the poster, I include the cross section B-B’ and I placed a yellow star for the two earthquakes.

Note the depths for these earthquakes. The M 6.9 is 76 km and the M 6.7 is 53.5 km deep. The M 6.9 is to the north of the M 6.7.

These two earthquakes show that they are on a north dipping fault (and this matches the Holm and Richards (2013) figure).

We can take this lesson one step further. If we look at the M 6.3 earthquake on 13 March 2023, it is to the north of today’s earthquake and is deeper still.

This M 6.3 earthquake led me to take a second look at a M 7.6 earthquake further to the south from 14 September 2022. This earthquake reminds us of how confusing the slabs are and how difficult it is to interpret where these earthquakes are (especially the M 7.6). Here is the Earthquake Report for this M 7.6 earthquake.

Because these earthquakes are not close to the surface, the intensities were not too destructive. The USGS Pager Alerts for these earthquakes suggest a very low likelihood for damage or casualties. (click the link to the usgs earthquake pages and navigate to the One Pager tab for these earthquake pages to see more).

Below is my interpretive poster for this earthquake

I plot the seismicity from the past 1 week, with diameter representing magnitude (see legend). I include earthquake epicenters from 1923-2023 with magnitudes M ≥ 6.5 in one version.
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

In the upper right corner is a map showing the plate tectonic boundaries (from the GEM) and seismicity from the past century (from the USGS).
To the left of the plate tectonic boundary map is a great figure showing the generalized plate tectonic boundaries in this region of the equatorial Pacific Ocean (Holm et al., 2016). I place a yellow star in the general location of the M 6.9 earthquake (also plotted in other inset figures).
In the lower right corner are two maps that show the M 6.7 and M 6.9 earthquake intensity using the modified Mercalli intensity scale. Earthquake intensity is a measure of how strongly the Earth shakes during an earthquake, so gets smaller the further away one is from the earthquake epicenter. The map colors represent a model of what the intensity may be. The USGS has a system called “Did You Feel It?” (DYFI) where people enter their observations from the earthquake and the USGS calculates what the intensity was for that person. The dots with yellow labels show what people actually felt in those different locations.
In the upper left corner are two maps showing the possibility of earthquake induced liquefaction for these two earthquakes.
In the lower left center is a figure from Baldwin et al. (2012). This figure shows a series of cross sections along this convergent plate boundary from the Solomon Islands in the east to Papua New Guinea in the west. Cross section ‘C’ is the most representative for the earthquakes today. I present the map and this figure again below, with their original captions. Above the map is cross sections C-C’ and D-D’ that shows the slabs and the crustal structures in the region. I placed the yellow star marking today’s M 6.9. The earthquake actually is closer to cross section D-D’ but that cross section does not include a view of the slabs.
In the bottom center is the Holm and Richards (2013) cross section B-B.’ Note the yellow stars that show the 6.9 is deeper and to the north of the 6.7.

Here is the map with 1 week’s seismicity plotted.

Some Relevant Discussion and Figures

Here is the Holm et al. (2016) figure.

Topography, bathymetry and regional tectonic setting of New Guinea and Solomon Islands. Arrows indicate rate and direction of plate motion of the Australian and Pacific plates (MORVEL, DeMets et al., 2010); Mamberamo thrust belt, Indonesia (MTB); North Fiji Basin (NFB)

These maps from Holm et al. (2016) show the tectonic plate boundaries and plates/microplates.
The lower panel includes symbology for the magmatic centers associated with the different arcs analyzed in their study.

Tectonic setting of Papua New Guinea and Solomon Islands. a) Regional plate boundaries and tectonic elements. Light grey shading illustrates bathymetry b2000mbelow sea level indicative of continental or arc crust, and oceanic plateaus; 1000mdepth contour is also shown. Adelbert Terrane (AT); Bismarck Sea fault (BSF); Bundi fault zone (BFZ); Feni Deep (FD); Finisterre Terrane (FT); Gazelle Peninsula (GP); Kia-Kaipito-Korigole fault zone (KKKF); Lagaip fault zone (LFZ); Mamberamo thrust belt (MTB); Manus Island (MI); New Britain (NB); New Ireland (NI); North Sepik arc (NSA); Ramu-Markham fault (RMF); Weitin Fault (WF);West Bismarck fault (WBF); Willaumez-Manus Rise (WMR). b) Magmatic arcs and volcanic centers related to this study.

The this map and cross section pair shows the Holm et al. (2016) interpretation of the oceanic crust in this region in the current position.

a) Present day tectonic features of the Papua New Guinea and Solomon Islands region as shown in plate reconstructions. Sea floor magnetic anomalies are shown for the Caroline plate (Gaina and Müller, 2007), Solomon Sea plate (Gaina and Müller, 2007) and Coral Sea (Weissel and Watts, 1979). Outline of the reconstructed Solomon Sea slab (SSP) and Vanuatu slab (VS)models are as indicated. b) Cross-sections related to the present day tectonic setting. Section locations are as indicated. Bismarck Sea fault (BSF); Feni Deep (FD); Louisiade Plateau (LP); Manus Basin (MB); New Britain trench (NBT); North Bismarck microplate (NBP); North Solomon trench (NST); Ontong Java Plateau (OJP); Ramu-Markham fault (RMF); San Cristobal trench (SCT); Solomon Sea plate (SSP); South Bismarck microplate (SBP); Trobriand trough (TT); projected Vanuatu slab (VS); West Bismarck fault (WBF); West Torres Plateau (WTP); Woodlark Basin (WB).

Koulali et al (2015) use GPS data to resolve the kinematics of the central-eastern Papua New Guinea region. The first figure below is a map that shows the GPS velocities in this region There are two cross section profiles labeled on the map (the M 7.0 earthquake happened to the east of A-A’). Note the complicated and detailed fault mapping (the balck lines). The convergence is generally perpendicular to the PFTB in the east and more oblique to the PFTB on the western portion of this map.

The GPS velocity field and 95 per cent confidence interval ellipses with respect to the Australian Plate. Red and blue vectors are the new calculated field and black vectors are from Wallace et al. (2004). The dashed rectangle shows the area of Fig. 3. The blue dashed lines correspond to the location of profiles shown in Fig. 4. Note that the velocity scales for the red and blue vectors are different (see the lower right corner for scales). The black velocities are plotted at the same scale as the red vectors.

Here are the two profiles. The red and blue lines plot vertical land motion (VLM) rates in mm/yr and show strain accumulates across the region. Today’s earthquake happened in the region labeled ‘Highland FTB.’ The plot shows that ~5 mm/yr of strain accumulates in this fault system.

Profiles A–A& and B–B& from Fig. 2 showing model fit to GPS observations. Red symbols and lines are the GPS observed and modelled velocities, respectively, for the profile-normal component. Blue symbols and lines correspond to the profile-parallel component. The green and pink lines corresponds to the model using the Ramu-Markham fault geometry from Wallace et al. (2004), south of Lae. Grey profiles show the projected topography. The seismicity is from the ISC catalogue for events > Mw 3.5 (1960–2011).

This is a Cloos et al. (2005) map.
Something that came up this week during a tsunami workshop/meeting was about the activity for each plate boundary that has a potential to generate trans-Pacific tsunami impacting the U.S. and U.S. territories.
Over long periods of time, the plate boundaries around the world change shape. At some times, the relative plate motion between plates is localized one fault system. At other times, the active plate boundary fault is along a different fault system.
The map below includes information about the activity of the plate boundary faults. The active convergent zones are the New Britain subduction zone, the Ramu-Markham fault zone (RMFZ), the Seram subduction zone, part of the Papuan Fold and Thrust Belt, and parts of the New Guinea subduction zone. The strike-slip zones are the Bewani-Torricelli fault zone, the Mamberamo deformation zone, the Yapen fault zone, the Sorong fault zone, and the Tarera-Aiduna fault zone.
This map shows evidence for several different paleo-plate boundaries. Imagine how each subduction zone once had a pair of plates and those plates are still there. Even while inactive, earthquakes can occur on these faults.

Tectonic map of New Guinea, adapted from Hamilton (1979), Cooper and Taylor (1987), Dow et al. (1988), and Sapiie et al. (1999). AFTB—Aure fold and thrust belt, FTB—fold-and-thrust belt, IOB—Irian Ophiolite Belt, TFB—thrust-and-fold belt, POB—Papuan Ophiolite Belt, BTFZ—Bewani-Torricelli fault zone, MDZ—Mamberamo deformation zone, YFZ—Yapen fault zone, SFZ—Sorong fault zone, WO—Weyland overthrust. Continental basement exposures are concentrated along the southern fl ank of the Central Range: BD—Baupo Dome, MA—Mapenduma anticline, DM—Digul monocline, IDI—Idenberg Inlier, MUA—Mueller anticline, KA—Kubor anticline, LFTB—Legguru fold-and-thrust belt, RMFZ—Ramu-Markham fault zone, TAFZ—Tarera-Aiduna fault zone. The Tasman line separates continental crust that is Paleozoic and younger to the east from Precambrian to the west.

This is the Cloos et al. (2005) cross section, showing a different interpretation of the delaminated slab.

Lithospheric-scale cross section at 2 Ma. Plate motion is now focused along the Yapen fault zone in the center of the recently extinct arc. This probably occurred because this zone of weakness had a trend that could accommodate the imposed movements as the corner of the Caroline microplate ruptured, forming the Bismarck plate, and the corner of the Australian plate ruptured, forming the Solomon microplate. The collisional delamination-generated magmatic event ends in the highlands as the lower crustal magma chamber solidifies. Upwelled asthenosphere cools and transforms into lithospheric mantle. This drives a slow regional subsidence of the highlands that will continue for tens of millions of years or until other plate-tectonic movements are initiated. Deep erosion is still concentrated on the fl anks of the mountain belt. RMB—Ruffaer Metamorphic Belt, AUS—Australian plate, PAC—Pacific plate.

Here is the tectonic map figure from Sappie and Cloos (2004). Their work was focused on western PNG, so their interpretations are more detailed there (and perhaps less relevant for us for these eastern PNG earthquakes).

Seismotectonic interpretation of New Guinea. Tectonic features: PTFB—Papuan thrust-and-fold belt; RMFZ—Ramu-Markham fault zone; BTFZ—Bewani-Torricelli fault zone; MTFB—Mamberamo thrust-and-fold belt; SFZ—Sorong fault zone; YFZ—Yapen fault zone; RFZ—Ransiki fault zone; TAFZ—Tarera-Aiduna fault zone; WT—Waipona Trough. After Sapiie et al. (1999).

This is the two panel figure from Holm and Richards (2013) that shows how the New Britain trench megathrust splays into three thrust faults as this fault system heads onto PNG. They plot active thrust faults as black triangles (with the triangles on the hanging wall side of the fault) and inactive thrust faults as open triangles. So, either the NG trench subduction zone extends further east than is presented in earlier work or the Bundi Fault Zone is the fault associated with this deep seismicity.

Topography, bathymetry and major tectonic elements of the study area. (a) Major tectonic boundaries of Papua New Guinea and the western Solomon Islands; CP, Caroline plate; MB, Manus Basin; NBP, North Bismarck plate; NBT, New Britain trench; NGT, New Guinea trench; NST, North Solomon trench; PFTB, Papuan Fold and Thrust Belt; PT, Pocklington trough; RMF, Ramu-Markham Fault; SBP, South Bismarck plate; SCT, San Cristobal trench; SS, Solomon Sea plate; TT, Trobriand trough; WB,Woodlark Basin; WMT,West Melanesian trench. Study area is indicated by rectangle labelled Figure 1b; the other inset rectangle highlights location for subsequent figures. Present day GPS motions of plates are indicated relative to the Australian plate (from Tregoning et al. 1998, 1999; Tregoning 2002; Wallace et al. 2004). (b) Detailed topography, bathymetry and structural elements significant to the South Bismarck region (terms not in common use are referenced); AFB, Aure Fold Belt (Davies 2012); AT, Adelbert Terrane (e.g. Wallace et al. 2004); BFZ, Bundi Fault Zone (Abbott 1995); BSSL, Bismarck Sea Seismic Lineation; CG, Cape Gloucester; FT, Finisterre Terrane; GF, Gogol Fault (Abbott 1995); GP, Gazelle Peninsula; HP, Huon Peninsula; MB, Manus Basin; NB, New Britain; NI, New Ireland; OSF, Owen Stanley Fault; RMF, Ramu-Markham Fault; SS, Solomon Sea; WMR, Willaumez-Manus Rise (Johnson et al. 1979); WT, Wonga Thrust (Abbott et al. 1994); minor strike-slip faults are shown adjacent to Huon Peninsula (Abers & McCaffrey 1994) and in east New Britain, the Gazelle Peninsula (e.g. Madsen & Lindley 1994). Circles indicate centres of Quaternary volcanism of the Bismarck arc. Filled triangles indicate active thrusting or subduction, empty triangles indicate extinct or negligible thrusting or subduction.

Here is the slab interpretation for the New Britain region from Holm and Richards, 2013. I include the figure caption below as a blockquote.

3-D model of the Solomon slab comprising the subducted Solomon Sea plate, and associated crust of the Woodlark Basin and Australian plate subducted at the New Britain and San Cristobal trenches. Depth is in kilometres; the top surface of the slab is contoured at 20 km intervals from the Earth’s surface (black) to termination of slabrelated seismicity at approximately 550 km depth (light brown). Red line indicates the locations of the Ramu-Markham Fault (RMF)–New Britain trench (NBT)–San Cristobal trench (SCT); other major structures are removed for clarity; NB, New Britain; NI, New Ireland; SI, Solomon Islands; SS, Solomon Sea; TLTF, Tabar–Lihir–Tanga–Feni arc. See text for details.

This figure includes a map that shows the cross section I included in the interpretive poster.
Cross section B-B’ is just to the east of this M 6.9-6.9 sequence.

Interpretation of present day tectonic plate configuration and magmatic arc distribution in northeastern Papua New Guinea. Gravity anomaly map is provided as a base map. Bold black outlines illustrate the extent of the underthrust continental crust, formerly the leading edge of the Papua New Guinea mainland; and the associated correlation of the Bundi Fault Zone and Owen Stanley Fault in the under-thrust crust. CMT solutions are shown for the under-thrust margin between 40 and 90 km depth. Below the under-thrust margin, the distribution of subducted oceanic crust of the Solomon slab is shown for comparison, and is contoured at 40 and 100 km until termination to the slab. The ‘Bismarck arc’ is divided into the West Bismarck arc, New Britain arc, and mixing zone between the two; these are derived from continental crust, oceanic slab, and a combination of the two respectively. Cross-sections through the plate arrangement are provided to illustrate the 3-D framework of the new plate arrangement and context of corresponding fluid sources of the equivalent magmatic arc. See text for discussion.

Here are the forward models for the slab in the New Britain region from Holm and Richards, 2013. I include the figure caption below as a blockquote.

Forward tectonic reconstruction of progressive arc collision and accretion of New Britain to the Papua New Guinea margin. (a) Schematic forward reconstruction of New Britain relative to Papua New Guinea assuming continued northward motion of the Australian plate and clockwise rotation of the South Bismarck plate. (b) Cross-sections illustrate a conceptual interpretation of collision between New Britain and Papua New Guinea.

This map shows plate velocities and euler poles for different blocks. I include the figure caption below as a blockquote.

Tectonic maps of the New Guinea region. (a) Seismicity, volcanoes, and plate motion vectors. Plate motion vectors relative to the Australian plate are surface velocity models based on GPS data, fault slip rates, and earthquake focal mechanisms (UNAVCO, http://jules.unavco.org/Voyager/Earth). Earthquake data are sourced from the International Seismological Center EHB Bulletin (http://www.isc.ac.uk); data represent events from January 1994 through January 2009 with constrained focal depths. Background image is generated from http://www.geomapapp.org. Abbreviations: AB, Arafura Basin; AT, Aure Trough; AyT, Ayu Trough; BA, Banda arc; BSSL, Bismarck Sea seismic lineation; BH, Bird’s Head; BT, Banda Trench; BTFZ, Bewani-Torricelli fault zone; DD, Dayman Dome; DEI, D’Entrecasteaux Islands; FP, Fly Platform; GOP, Gulf of Papua; HP, Huon peninsula; LA, Louisiade Archipelago; LFZ, Lowlands fault zone; MaT, Manus Trench; ML, Mt. Lamington; MT, Mt. Trafalgar; MuT, Mussau Trough; MV, Mt. Victory; MTB, Mamberamo thrust belt; MVF, Managalase Plateau volcanic field; NBT, New Britain Trench; NBA, New Britain arc; NF, Nubara fault; NGT, New Guinea Trench; OJP, Ontong Java Plateau; OSF, Owen Stanley fault zone; PFTB, Papuan fold-and-thrust belt; PP, Papuan peninsula; PRi, Pocklington Rise; PT, Pocklington Trough; RMF, Ramu-Markham fault; SST, South Solomons Trench; SA, Solomon arc; SFZ, Sorong fault zone; ST, Seram Trench; TFZ, Tarera-Aiduna fault zone; TJ, AUS-WDKPAC triple junction; TL, Tasman line; TT, Trobriand Trough;WD, Weber Deep;WB, Woodlark Basin;WFTB, Western (Irian) fold-and-thrust belt; WR,Woodlark Rift; WRi, Woodlark Rise; WTB, Weyland thrust; YFZ, Yapen fault zone.White box indicates the location shown in Figure 3. (b) Map of plates, microplates, and tectonic blocks and elements of the New Guinea region. Tectonic elements modified after Hill & Hall (2003). Abbreviations: ADB, Adelbert block; AOB, April ultramafics; AUS, Australian plate; BHB, Bird’s Head block; CM, Cyclops Mountains; CWB, Cendrawasih block; CAR, Caroline microplate; EMD, Ertsberg Mining District; FA, Finisterre arc; IOB, Irian ophiolite belt; KBB, Kubor & Bena blocks (including Bena Bena terrane); LFTB, Lengguru fold-and-thrust belt; MA, Mapenduma anticline; MB, Mamberamo Basin block; MO, Marum ophiolite belt; MHS, Manus hotspot; NBS, North Bismarck plate; NGH, New Guinea highlands block; NNG, Northern New Guinea block; OKT, Ok Tedi mining district; PAC, Pacific plate; PIC, Porgera intrusive complex; PSP, Philippine Sea plate; PUB, Papuan Ultramafic Belt ophiolite; SB, Sepik Basin block; SDB, Sunda block; SBS, South Bismarck plate; SIB, Solomon Islands block; WP, Wandamen peninsula; WDK, Woodlark microplate; YQ, Yeleme quarries.

This figure incorporates cross sections and map views of various parts of the regional tectonics (Baldwin et al., 2012). These deep earthquakes are nearest the cross section D (though are much deeper than these shallow cross sections). I include the figure caption below as a blockquote.

Oblique block diagram of New Guinea from the northeast with schematic cross sections showing the present-day plate tectonic setting. Digital elevation model was generated from http://www.geomapapp.org. Oceanic crust in tectonic cross sections is shown by thick black-and-white hatched lines, with arrows indicating active subduction; thick gray-and-white hatched lines indicate uncertain former subduction. Continental crust, transitional continental crust, and arc-related crust are shown without pattern. Representative geologic cross sections across parts of slices C and D are marked with transparent red ovals and within slices B and E are shown by dotted lines. (i ) Cross section of the Papuan peninsula and D’Entrecasteaux Islands modified from Little et al. (2011), showing the obducted ophiolite belt due to collision of the Australian (AUS) plate with an arc in the Paleogene, with later Pliocene extension and exhumation to form the D’Entrecasteaux Islands. (ii ) Cross section of the Papuan peninsula after Davies & Jaques (1984) shows the Papuan ophiolite thrust over metamorphic rocks of AUS margin affinity. (iii ) Across the Papuan mainland, the cross section after Crowhurst et al. (1996) shows the obducted Marum ophiolite and complex folding and thrusting due to collision of the Melanesian arc (the Adelbert, Finisterre, and Huon blocks) in the Late Miocene to recent. (iv) Across the Bird’s Head, the cross section after Bailly et al. (2009) illustrates deformation in the Lengguru fold-and-thrust belt as a result of Late Miocene–Early Pliocene northeast-southwest shortening, followed by Late Pliocene–Quaternary extension. Abbreviations as in Figure 2, in addition to NI, New Ireland; SI, Solomon Islands; SS, Solomon Sea; (U)HP, (ultra)high-pressure.

Here is the relevant cross section from Baldwin et al. (2012).

Across the Papuan mainland, the cross section after Crowhurst et al. (1996) shows the obducted Marum ophiolite and complex folding and thrusting due to collision of the Melanesian arc (the Adelbert, Finisterre, and Huon blocks) in the Late Miocene to recent.

Here is map that shows the tectonics in and to the east of Papua New Guinea from Ott and Mann (2015).
These authors use seismic reflection data and onshore geologic and GPS studies to look at the formation of the Aure-Moresby and Papuan fold and thrust belts.

Active tectonic setting of eastern Papua New Guinea showing the boundaries of the Woodlark microplate that includes previously proposed oceanic Solomon Sea plate, the Trobriand platform, and the Woodlark plate [Wallace et al., 2014]. The New Britain trench along the northern margin of the Woodlark plate is a rapidly subducting, 600 km long slab that generates a strong pull on the unsubducted Woodlark microplate [Weissel et al., 1982; Wallace et al., 2004, 2014]. Small circles around the Trobriand platform/Australia pole predict the described pattern of transpressional deformation along the Aure-Moresby fold-thrust belt and the formation of the adjacent, late Miocene to Recent Aure-Moresby foreland basin. Approximate location of the downdip limits of the subducted Solomon Sea slabs are shown by dashed lines and modified from Pegler et al. [1995], Woodhead et al. [2010], and Hayes et al. [2012]. Earthquake data are provided courtesy of the U.S. Geological Survey. Note that the tapering triangular shape of the extension in the Woodlark basin closely matches the size and shape of the thrusting observed in the Aure-Moresby fold-thrust belt and foreland basin.