While I was back in the Ridgecrest, CA area further documenting our slickenline observations from the 5 July 2019 M 7.1 Ridgecrest Earthquake, there was a tsunamigenic earthquake in the Molucca Sea near Halmahera, Indonesia. Some of my earliest earthquake reports were from this region, but I have not had the opportunity to write anything up for earthquakes in this area for a few years.
The Halmahera Strait/Molucca Sea region is interesting as there is a pair of divergent subduction zones here. Basically, one dips to the east and one dips to the west, though it is a little more complicated.
McCaffrey et al. (1980) presented one of the first views of the subduction/thrust tectonics in this region. Since then, advances in marine geophysical methods have furthered our understanding and have generally re-enforced the early hypotheses rather well.
There was a minor tsunami recorded at a tide gage that was only 135 km (65 miles) from the epicenter. Here are those data plotted relative to time. The wave has a wave height of about 20 cm. The waves lasted several hours (it appears that maybe the waves resonated in the harbor).
In the interest of keeping this simple, I first present the interpretive poster, then I present some key figures that provide a seismotectonic context to this sequence.
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 1919-2019 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.
- Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.
- In the upper right corner are a series of panels from Zhang et al. (2017). The upper panel shows the major plate boundaries (subduction zones). I place a yellow star in the general location of this M 7.1 temblor. The middle panel shows a cross section of the Halmahera Strait where we can see the Mulucca Sea plate is diving to the east under the Philippine Sea plate and to the west under the Eurasia plate. This matches early cross sections (e.g. McCaffrey et al., 1980). The cross section is located along the red line on the poster (A-B). The lower panel shows seismicity for this area and we can see earthquakes trend deeper in bothe directions (east and west), though deeper to the west (red dots). Lines A-A’, B-B’, C-C’, D-D’, and E-E’ represent the locations of profiles included in the upper left figure.
- In the upper left corner is a series of cross sections A-A’, B-B’, C-C’, D-D’, and E-E’. These are profiles showing material properties of the Earth based on seismic tomographic analysis. Seismic tomography is based on the same principles as CT scans. So, we can imagine that these profiles are like X-Ray scans of the Earth’s interior. Blue represents materials with faster seismic velocity (older and colder oceanic crust) and red represents materials with slower seismic velocity (generally hotter crust).
- In the lower left corner is a plot of the tide gage data from Bitung, Sulawesi (location shown on interpretive poster).
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
- Here is a map that shows shaking intensity using the MMI scale. The colors and black contours are from the USGS shaking intensity model. I also include some of the “Did You Feel It?” report observations (e.g. labeled “DYFI 4.9”).
- In the upper left corner is a plot from the USGS that includes both modeled data (the orange and green lines) and DYFI data (the points and whisker plots). The legend informs us about the source of these different data.
Some Relevant Discussion and Figures
- Here is a tectonic map for this part of the world from Zahirovic et al., 2014. 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 the figures from Zhang et al. (2017). First I present the tectonic overview figure, with captions below.
- Here is the seismic tomography profile figure.
- If we look at the first Zhang et al. (2017) figure above, the seismicity shows that the slab dipping to the west has earthquakes that extend much deeper than the zone dipping to the east. These authors hypothesized about why these subduciton zones are assymetrical (possibly due to mantle flow around the downgoing ocean crustal slabs). Zhang et al. (2017) conducted numerical analysis of mantle flow.
- Here is a figure that shows some illustrations depicting possible tectonic configurations for this region.
- Zhang et al., 2017 present below different possible configurations of double subduction zones.
- Based on their analyses, Zhang et al. (2017) make some conclusions about the divergent subduction zones in this region.
(a) Sketch of the Molucca Sea subduction zone and its vicinity. (b) Cross section (A-B) demonstrating the structure of arc-arc collision zone (modified after Hall and Smyth ). (c) Distribution of earthquake events (2011.1.1–2015.12.31) and focal mechanism; the insert shows the vertical profile of epicenters sliced at position B-B0 . The black
dashed line in Figure 1a is the position of cross section A-B in Figure 1b, and the red solid lines in Figure 1c are the cross sections of the seismic tomographic velocity model in Figure 2.
Vertical slices of seismic velocity beneath the Molucca Sea and its surrounding regions, in which positive velocity anomalies outline the unique shape of the subducting Molucca Sea plate. The tomographic images are sliced from the global P wave velocity anomaly model UU-P07 [Amaru, 2007] along positions shown in Figure 1c.
Cartoon of end-member models illustrating effects of the order of subduction initiation, the mobility and thickness of the overriding plates on slab morphology, and migration of the overriding plates during DDS. Idealized DDS features (a) symmetrical subduction of slabs and (b–d) asymmetrical plate shape resulted from influence of order of subduction initiation, mobility, and thickness of overriding plates, respectively. (e) Tentative interpretation of formation of the asymmetrical DDS observed in the Molucca Sea region. Size of arrows indicate relative scale of the subduction-induced mantle flow.
Cartoons illustrating several forms of double subduction. (a) Divergent double subduction (descripted in this study and by Soesoo et al. , Di Leo et al. , Li et al., , etc.). (b) Double subduction with opposite dipping directions (descripted by Maruyama et al. [ 2007]). (c and d) Convergent double subduction (descripted by Jagoutz et al.  and Billen ). In all of these case, sustainable subduction of the oceanic plate(s) requires smooth escape (Figures 17a and 17c) of the slab-trapped mantle or replenishment of external materials of mantle into the void space left by slab rollback (Figures 17b and 17d). The toroidal mantle flow (orange arrows) plays a dominant role in
redistribution of material during all these types of double subduction.
- The self-sustaining, asymmetrical subduction of the Molucca Sea plate may drive the convergence of the overriding plates and collision of magmatic arcs, even in an extensional setting of SE Asia.
- The earlier and faster subduction on the western Sangihe side with respect to the eastern Halmahera side predominantly led to formation of the present-day asymmetrical shape of the subducting Molucca Sea plate.
- The relative immobility of the western overriding Eurasian plate may have promoted the westward migration of the Halmahera arc in the Molucca Sea subduction zone.
- Bending of arcs was probably a consequence of the toroidal mantle flow induced by rollback of the subducting Molucca Sea plate on its both sides.
- DDS is unsustainable without effective escape of the slab-trapped mantle via toroidal flow. It is therefore likely that DDS is confined to narrow and short oceanic plates and is related to closure of archipelagic oceans and accretion of arcs in accretionary orogenic belts.
- Here are maps showing the regional tectonics (Smoczyk et al., 2013).
Along its western margin, the Philippine Sea plate is associated with a zone of oblique convergence with the Sunda plate. This highly active convergent plate boundary extends along both sides the Philippine Islands, from Luzon in the north to Sulawesi in the south. The tectonic setting of the Philippines is unusual in several respects: it is characterized by opposite-facing subduction systems on its east and west sides; the archipelago is cut by a major transform fault, the Philippine Fault; and the arc complex itself is marked by volcanism, faulting, and high seismic activity. Subduction of the Philippine Sea plate occurs at the eastern margin of the archipelago along the Philippine Trench and its northern extension, the East Luzon Trough. The East Luzon Trough is thought to be an unusual example of a subduction zone in the process of formation, as the Philippine Trench system gradually extends northward (Hamburger and others, 1983).
- This shows Global Positioning System (GPS) velocities at various locations. These plate motions are represented as vectors in mm/yr. (see legend) Note that the plate motion vectors on either side of the Halmahera Strait are opposing each other, evidence of the contraction/convergence/compression across these plate boundary faults. Below I include the text from the original figure caption in 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, ew 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
- This is one of my favorite figures of all time (Hall, 2011). Today’s earthquake sequence happened in the center of the middle panel, between the cyan (Molluca plate) and yellow plates. Read below for more details.
3D cartoon of plate boundaries in the Molucca Sea region modified from Hall et al. (1995). Although seismicity identifies a number of plates there are no continuous boundaries, and the Cotobato, North Sulawesi and Philippine Trenches are all intraplate features. The apparent distinction between different crust types, such as Australian continental crust and oceanic crust of the Philippine and Molucca Sea, is partly a boundary inactive since the Early Miocene (east Sulawesi) and partly a younger but now probably inactive boundary of the Sorong Fault. The upper crust of this entire region is deforming in a much more continuous way than suggested by this cartoon.
- Here is a map and cross section presented by Waltham et al. (2008). They use a variety of data sources as a basis for their interpretations (seismic reflection data, gravity data). Note how the Molucca Sea plate subducts both to the west and to the east. Below I include the text from the original figure caption in blockquote.
(A) Location and major tectonic features of the Molucca Sea region. Small, black-filled triangles are modern volcanoes. Bathymetric contours are at 200, 2000, 4000, and 6000 m. Large barbed lines are subduction zones, and small barbed lines are thrusts. (B) Cross section across the Halmahera and Sangihe Arcs on section line B. Thrusts on each side of the Molucca Sea are directed outward toward the adjacent arcs, although the subducting Molucca Sea plate dips east beneath Halmahera and west below the Sangihe Arc. (C) Inset is the restored cross section of the Miocene–Pliocene Weda Bay Basin of SW Halmahera on section line C, fl attened to the Pliocene unconformity, showing estimated thickness of the section
- Early work done in the region was presented by McCaffrey et al. (1980). Here is a map showing seismic refraction lines that they used to constrain the structures in this region. Below I include the text from the original figure caption in blockquote.
- Here is a cross section that shows the gravity model they used to interpret this region.
Map of the Molucca Sea, eastern Indonesia, showing I~tions of seismic refraction lines (solid straight lines) and gravity traverses (dashed-dotted lines). Thrust faults are shown with teeth on hanging wall. Triangles represent active volcanoes defining the Sangihe and Halmahera magmatic arcs. Isobath interval is 1 km from Mammericks et al. .
Gravity model for the central Molucca Sea. (II) Crustal model with layers designated by their density contrasts and refraction control points by open circles and vertical bars. (b) Mantle structure used in modeling the gravity profiles in the central Molucca Sea. Figure 124 fits into the small box at the apex of the inverted-V-ehaped lithosphere. Slab dimensions are controlled by earthquake foci (dots) from Hlltherton 11M Dickinaon [1969J, and mantle densities are taken from Grow 11M Rowin [1975J. The column at the left shows assumed densities for the range of depths between the tick marks. The small v pattern represents oceanic crust, and island arc crust is designated by a short parallel line pattern. East is to the right of the figure.
- Here is another tectonic map showing the Sorong fault and some splay faults (dashed lines running along Halmahera), one of which may be involved in today’s earthquake.
Location map and active faults of the Molucca Sea region. Fault colours: blue, convergence; red, transvergence; yellow, divergence; grey, uncertain motion. Fault abbreviations: CF, Catabato Fault; GF, Gorontalo Fault; NST, North Sulawesi Trench; PKF, Palu-Koro Fault; SF, Sorong Fault.
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 USGS Seismic Hazard Map:
- Here is another version of the seismic hazard for this region (Smoczyk et al., 2013). The GEM map suggests that the islands along the Halmahera Strait may have accelerations between 0.8-1.6 m2. This translates to 0.08 to 0.16 g. The GEM seismic hazard map shows a potential shaking of 0.20-0.35 g, slightly higher.
- 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 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).
- 2019.11.14 M 7.1 Halmahera
- 2019.07.14 M 7.3 Halmahera
- 2018.12.29 M 7.0 Philippines
- 2017.12.08 M 6.5 Caroline Ridge
- 2017.12.09 M 6.5 Caroline Ridge Update #1
- 2017.08.11 M 6.2 Philippines
- 2017.04.28 M 6.9 Philippines
- 2017.04.08 M 5.9 Philippines
- 2017.01.10 M 7.3 Celebes Sea
- 2016.07.29 M 7.7 Mariana
- 2015.03.17 M 6.2 Molucca Sea
- 2014.11.26 M 6.8 Molucca Sea
- 2014.11.21 M 6.5 Molucca Sea
- 2014.11.15 M 7.1 Molucca Sea
Philippines | Western Pacific
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- 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
- Smoczyk, G.M., Hayes, G.P., Hamburger, M.W., Benz, H.M., Villaseñor, Antonio, and Furlong, K.P., 2013. Seismicity of the Earth 1900–2012 Philippine Sea plate and vicinity: U.S. Geological Survey Open-File Report 2010–1083-M, 1 sheet, scale 1:10,000,000.
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- Zhang, Q., F. Guo, L. Zhao, and Y. Wu, 2017. Geodynamics of divergent double subduction: 3-D numerical modeling of a Cenozoic example in the Molucca Sea region, Indonesia, J. Geophys. Res. Solid Earth, 122, 3977–3998, doi:10.1002/2017JB013991.
- Zulkifli, M., Rudyanto, A., and Sakti, A.P., 2016. The View of Seismic Hazard in The Halmahera Region in proceedings from International Symposium on Earth Hazard and Disaster Mitigation (ISEDM) 2016 AIP Conf. Proc. 1857, 050004-1–050004-7; doi:10.1063/1.4987082
Basic & General References
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