Earthquake Report: Java!

This morning (my time) there was a deep earthquake along the subduction zone beneath Java. The M 6.5 earthquake hypocentral depth is deeper than the subduction zone megathrust fault, so it is the downgoing Australia plate (AP).

My initial interpretation was that this earthquake is a strike-slip earthquake related to reactivated transform faults/fracture zones in the subducting AP. So, I did a little searching for prior historic earthquakes in the region to see what they might tell us about the tectonics of the subducting AP in this region. Given my knowledge of the fracture zones in the AP to the west, these fracture zones are ~north-south in orientation. Thus, my first interpretation was that this M 6.5 earthquake was a left-lateral strike-slip earthquake on a north-northwest striking (oriented) fault.

However, looking into these historic earthquakes, there are two good analogues. On 2001.05.25 there was an earthquake with magnitude of M 6.3 to the east of today’s M 6.5 earthquake, with a similar depth relation to the downgoing plate (it was also within the AP). This M 6.3 has a similarly oriented moment tensor.

Then I found a deeper earthquake (that plots closer to the depth of the downgoing AP, but does not have a thrust moment tensor, so is probably in the AP). This earthquake has a fault slip model from the USGS, where they inverted seismic data to interpret the M 7.5 earthquake to be a left-lateral strike-slip earthquake on an ~east-west fault. This did not fit my hypothesis about north-south fracture zones. So, I realized I needed to look at the magnetic anomaly data (and any other sources about the structures in the AP south of Java).

The fracture zones are differently oriented south of Java. In the Hall (2011) inset map in the interpretive poster, the fracture zones are oriented to the northwest, and the normal faults associated with spreading ridge tectonics are oriented to the northeast. So, perhaps the M 7.5 earthquake was on a reactivated spreading ridge fault and the 2017 M 6.5 and 2001 M 6.3 are on reactivated fracture zone faults. This leads me to my original interpretation, that the M 6.5 earthquake is a left-lateral strike-slip fault earthquake. Of course, this is still just an hypothesis. Since the earthquake is so deep, we will never be able to observe offset geomorphic features like we can for earthquakes on land.

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 include earthquake epicenters from 1917-2017 with magnitudes M > 6.0.
I plot the USGS fault plane solutions (moment tensors in blue) for the M 6.5 and some earlier 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. The depth is probably not very well constrained due to the geometry and lack of seismometer coverage in the oceanic setting.
  • Here are the USGS pages for the earthquakes with fault plane solutions plotted on the interpretive poster below.
  • 2017.12.15 M 6.5
  • 2007.07.08 M 7.5
  • 2001.05.25 M 6.3
  • I include some inset figures.

  • In the upper right corner I include a small scale (upper panel) and a large scale (bottom panel) view of the regional tectonics (Zahirovic et al., 2014). Plate boundary fault symbology (and other features, like fracture zones) is shown in the legend. I place a blue star on the map in the general location of this earthquake epicenter.
  • In the lower right corner is a figure from Krabbenhoeft et al. (2010) that shows the tectonic land forms associated with the subduction zone offshore of Java (forearc basins).
  • In the upper left corner are some figure insets from Jones et al. (2010). This is a report on the regional seismicity. The panel on the right is a map showing seismicity vs. depth (color of circle) and magnitude (diameter of circle). There are two cross sections (A-A’ and B-B’) that sample seismicity limited to the rectangular boxes shown on the map. The seismicity cross sections show the general location of the India-Australia slab as it subducts beneath the Sunda plate. On the left are legends for the map and the cross sections. I place a blue star for the general location of the epicenter of this M 6.5 earthquake on the map and on the cross section. I also place a blue line labeled A-A’ in the general location of the cross section on the map.

  • Here is the same map, but with the Magnetic Anomaly map as a base (Meyer et al., 2017). Note how the anomalies are oriented subparallel to the spreading ridge related structures.

  • Here is the figure from Hall (2011) showing the structures in some of the oceanic plates in this region.

  • Black lines show general trends of deep structures in NW Australia and predicted orientation of deep structures in Indonesia at the present-day if these faults were brought with accreted blocks from NW Australia according to the reconstructions of Figures 4 and 5.

  • Here is the plate tectonic map from Zahirovic et al (2014).

  • 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). ANI – Andaman and Nicobar Islands, BD– Billiton Depression, Ba – Bangka Island, BI – Belitung (Billiton) Island, BiS – Bismarck Sea, BP – Benham Plateau, CaR – Caroline Ridge, CS – Celebes Sea, DG– Dangerous Grounds, EauR – Eauripik Ridge, FIN – Finisterre Terrane, GoT – Gulf of Thailand, GR– Gagua Ridge, HAL– Halmahera, HBa – Huatung Basin, KB–Ketungau Basin, KP – Khorat Platform, KT – Kiilsgaard Trough, LS – Luconia Shoals, MacB – Macclesfield Bank, ManTr – Manus Trench, MaTr – Mariana Trench, MB– Melawi Basin, MDB– Minami Daito Basin, MG– Mangkalihat, MIN – Mindoro, MN– Mawgyi Nappe, MoS – Molucca Sea, MS– Makassar Straits, MTr – Mussau Trench, NGTr – New Guinea Trench, NI – Natuna Islands, ODR– Oki Daito Ridge, OJP –Ontong Java Plateau, OSF – Owen Stanley Fault, PAL – Palawan, PhF – Philippine Fault, PT – Paternoster Platform, PTr – Palau Trench, PVB – Parece Vela Basin, RB – Reed Bank, RMF– Ramu-Markham Fault, RRF – Red River fault, SEM– Semitau, ShB – Shikoku Basin, Sol. Sea – Solomon Sea, SPK – Sepik, SPT – Sabah–Palawan Trough, STr – Sorol Trough, Sul – Sulawesi, SuS – Sulu Sea, TPAA– Torricelli–Prince Alexander Arc, WB–West Burma, WCT–W Caroline Trough, YTr –Yap Trough.

  • Here is the base map without inset figures. The 1917-2017 USGS seismicity is included for reference.

  • 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.

  • In addition to the orientation of relative plate motion (that controls seismogenic zone and strain partitioning), the Indo Australia plate varies in crustal age (Lasitha et al., 2006). I include their figure caption below as a blockquote.

  • Tectonic sketch map of the Sumatra–Java trench-arc region in eastern Indian Ocean Benioff Zone configuration. Hatched line with numbers indicates depth to the top of the Benioff Zone (after Newcomb and McCann13). Magnetic anomaly identifications have been considered from Liu et al.14 and Krishna et al.15. Magnitude and direction of the plate motion is obtained from Sieh and Natawidjaja11. O indicates the location of the recent major earthquakes of 26 December 2004, i.e. the devastating tsunamigenic earthquake (Mw = 9.3) and the 28 March 2005 earthquake (Mw = 8.6).


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