Earthquake Report: Banda Sea

Earlier this week there was a moderate earthquake along a strike-slip fault that appears to adjoin the Banda/Timor/Java Arc with New Guinea. This strike-slip fault appears to cross oblique to the subduction zone that forms the Timor Trench to the south and the Seram Trench to the north. Various researchers portray the faulting in this region differently. Given earthquake moment tensor and focal mechanism from the USGS, this earthquake supports the interpretation that this fault system is left-lateral (synistral) strike-slip. A focal mechanism from an earthquake in 1938 (magnitude M 8.5) provides evidence that is a little more confusing. But, this region is a complicated region.
UPDATE: 2017.03.05 (23:00 local time): Interesting, I was reading my tweet after Lila Lisle noticed a mistake. I fixed that, but later realized that the possible fault in the downgoing plate is not the Sorog fault, but a fault that might intersect with the Sorong fault. I did not delete the tweet since this mistake is a topic of conversation and not really a key part of the story.
Here is the USGS earthquake website for this M 5.5 earthquake.

Here are the USGS websites for the major earthquakes in this region from the past century.

• 1938.02.01 M 8.5 Banda Sea
• 1963.11.04 M 8.1 Banda Sea
• 1987.06.17 M 7.1 Banda Sea
• 2005.03.02 M 7.1 Banda Sea
• 2012.12.10 M 8.1 Banda Sea
• 2017.03.01 M 5.5 Banda Sea

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 also include seismicity from 1917-2017 for earthquakes with magnitudes M ≥ 7.0. I show the fault plane solutions for some of these earthquakes.

• moment tensors for 2005, 2012, and 2017 (USGS)
• focal mechanisms for 1987 (USGS) and 1938 (Okal and Reymond, 2003)

• The fault plane solutions for the 1963, 1987, 2005, and 2012 earthquakes are all very similar to the 2017 M 5.5. However, these earthquakes are form two depth “populations.” The 1963 and 1987 earthquakes are at ~65 km depth, while the 2005, 2012, and 2017 are between 150-200 km. There are some earthquakes that are much shallower depth eastward along this possibly strike-slip fault. Near where this fault comes on land at the base of the Bird’s Head in New Guinea, there are some earthquakes from the past couple of decades that also have strike-slip fault–plane solutions. The earthquake depths along this Sorong fault (Hall, 2011) appear to show that the Sorong fault is active beneath the Sunda plate. The 1938 earthquake may be the result of some form of strain partitioned faulting(?). Alternatively, these earthquakes may be unrelated to the Sorong fault. There may be some internal structure in the Australia plate that is interacting with the subduction zones or other faults (some preexisting structure that is optimally oriented to reactivate with the .
• 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. The hypocentral depth plots this close to the location of the fault as mapped by Hayes et al. (2012).

I include some inset figures in the poster.

• In the upper left corner is a general tectonic map for this part of the world. I placed a green star in the location of this M 5.5 earthquake (Zahirovic et al., 2014).
• In the upper right corner is a low-angle oblique view of the plate boundaries in the northern part of this region (Hall, 2011). The upper part of the diagram shows the opposing vergent subduction zones along that strike north-south along the Molucca Strait (Halmahera, Philippines). The lower panel shows the downgoing Australia plate along the Timor Trench and Seram Trench. Note the location of the Bird’s Head, the northwestern part of New Guinea. I have also labeled this region in the main map for comparison. The strike-slip fault at the northern boundary of New Guinea is the Sorong fault and this is labeled in this Hall (2011) figure.
• Below the Hall (2011) figure is a figure from Baldwin et al. (2012) that shows the regional seismicity and faulting as they are related to different geologic types in New Guinea. I placed a green star in the location of this M 5.5 earthquake.
• In the lower right corner is a part of the USGS Poster that reviews the seismicity of this region for the past century or so (Benz et a., 2011). The map shows seismicity with depth, along with some cross section locations. I place a green star at the location of this M 5.5 earthquake. I present three of the cross sections from this poster, A-A’, B-B’, and C-C.’ Of particular interest is the section B-B’ because this is placed near the M 5.5 earthquake. I have placed a green star that represents the hypocentral location on cross section B-B.’ The hypocentral depth suggests this M 5.5 earthquake is in the downgoing Australia plate slab.
• In the lower left corner is a diagram showing the subducting Australia plate at the Java Trench (Yves Descatoire).
• To the right of the Java Trench figure presents a detailed view of the faulting along the eastern Java and western Timor trenches (Hangesh and Whitney, 2016). They present evidence for oblique motion along the Timor trough. And present evidence for a backthrust on the northern side of Timor and the Indonesia islands east of Java.

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

• Here is the map from Baldwin et al. (2012)




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 phiolite; SB, Sepik Basin block; SDB, Sunda block; SBS, South Bismarck plate; SIB, Solomon Islands block; WP, Wandamen p ninsula; WDK, Woodlark microplate; YQ, Yeleme quarries.

• Here is the tectonic map from Hangesh 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.

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.
• 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.
• Hayes, G.P., Wald, D.J., and Johnson, R.L., 2012. Slab1.0: A three-dimensional model of global subduction zone geometries in, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524
• Hayes, G.P., Smoczyk, G.M., Benz, H.M., Villaseñor, Antonio, and Furlong, K.P., 2015. Seismicity of the Earth 1900–2013, Seismotectonics of South America (Nazca Plate Region): U.S. Geological Survey Open-File Report 2015–1031–E, 1 sheet, scale 1:14,000,000, http://dx.doi.org/10.3133/ofr20151031E.
• 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.
• 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