Southern Pacific Islands (PAC)

Authors:

K. Johnson, M. Pagani

Summary

The Southern Pacific Islands model covers islands in the area of ~30-0°S and 150-200°E, including the Solomon Islands, Vanuatu, New Caledonia, Fiji, Samoa/American Samoa, and Tonga. The model was built for the OpenQuake (OQ) engine by the GEM Secretariat.

Information about the OQ model versions and input files can be found on the Results and Dissemination page.

Updates

In 2019, this model was updated to v2018.1.0. The main changes were:

  • minor updates to crustal faults with updated criteria to distinguish between categories of faults
  • some source zone and subduciton source MFDs were recalibrated using refined completness tables.

A manuscript corresponding to v2018.1.0 was submitted to Geophysical Journal International. In 2020, as part of the revision process, the model was updated to v2018.2.0. The main changes were:

  • epistemic uncertainty was added to the subduction source model, varying interface-intraslab cutoff depth, segmentation, Mmax, and magnitude scaling relationship
  • The subduction Zhao et al. (2006) GMPEs were replaced with Abrahamson et al. (2015)

For more information about v2018.2.0, please see:

K. L. Johnson, M. Pagani, R. H. Styron (accepted), PSHA of the southern Pacific Islands, Geophysical Journal International, https://doi.org/10.1093/gji/ggaa530

Interactive Viewer

The viewer below depicts the seismic sources and hazard results in terms of PGA for a return period of 475 years. Click on the menu in the upper right corner to select the layer.

viewer_legend

Regional tectonics

The southern Pacific Islands region is tectonically complex and seismically very active. Since 1900, ~350 earthquakes M>7.0 have occurred, of which 11 were M>8. The greatest hazard posed by these earthquakes is triggered tsunamis, however, past events have also caused shaking related damage and fatalities.

Most of the regional seismic hazard is attributable to interface and intraslab earthquakes along the >6000 km of subduction zones. Along the ~north-south trending Kermadec and Tonga trenches, the Pacific plate subducts beneath the Australian plate, converging at an increasing rate from ~80 mm/yr in the south to ~220 mm/yr in the north (Bird, 2003). At the point of peak convergence – the northern tip of the Tonga Trench – the plate boundary rotates counterclockwise to approximately parallel the plate motion. West of here, along a semi-continuous network of three trenches, the Australian plate subducts beneath the Pacific plate. Convergence rates range from ~35-120 mm/yr on the New Hebrides trench (Calmant et al., 2003); ~100 mm/yr on the South Solomon trench (Wallace, 2005); and ~50-130 mm/yr on the New Britain trench (Bird, 2003).

In addition to subduction hazards, seismicity occurs in the rapidly deforming Fiji Platform due to back arc spreading and clockwise rotation along left-lateral fracture zones (e.g. Rahiman, 2009). Some large earthquakes (M>7) also occur in the outer rise, and there is widespread distributed shallow seismicity.

Basic Datasets

Earthquake Catalogues

We use the magnitude-homogonized ISC-GEM extended catalogue of Weatherill et al. (2016) clipped to the Pacific Islands region (bounds of 45°S, 4°N, 145°E, and 160°W). The catalogue includes ~110,000 earthquakes Mw>2.8 from 1900-2014.

We also use Global Centroid Moment Tensor (GCMT) focal mechanisms from 1976-2015 (Dziewonski et al., 1981; Ekström et al., 2012).

Fault Database

We use the GEM Active Faults Database, which includes mostly oceanic structures (spreading ridges and transform faults), but also the Fiji Fracture zone.

Model

Seismic Source Characterisation

The source model includes varying source typologies for the different tectonic settings. These include:

  • Interface seisimicity with Mw>6 modeled as complex faults
  • Instraslab seismicity with Mw>6 and depth < 300 km modeled as nonparametric ruptures
  • Active shallow crustal faults producing earthquakes Mw>6.5 modeled as simple faults
  • Distributed active shallow seismicity modeled as a grid of point sources

The source surface projections are displayed on the interactive map (page bottom).

The interface and intraslab geometries are built using the GEM Subduction Toolkit. Surfaces are cut at 50 km depth to separate the shallower interface from the deeper slab top.

The occurrence rates were determined using the following methodologies, which vary by source typology. For rates derived from seismicity, we use subcatalogues classified to the respective tectonic settings, declustered using Uhrhammer (1985) windowing and filtered for completeness.

Interface

We segment each subduction zone accodring to past megathrust earthquakes, current seismicity patterns, trench convergence rates and kinematics, and assistance from thorough structural and tectonic regional studies. The sources are built such that ruptures do not propagate across the defined boundaries. The trench segments from west to east are:

  • New Britain: Unsegmented, and extending as in the GEM Faulted Earth Project (Christophersen et al., 2015).
  • South Solomon: Three segments based on the supersegments of Chen et al. (2011), which uses seafloor geomorphology, seismicity patterns, and uplift patterns from coral reefs.
  • New Hebrides: Four segments based on Power et al. (2011), Baillard et al. (2015), and the GEM Faulted Earth Project (Christophersen et al., 2015) Along one segment, the convergence transfers mostly to the backarc thrust belt.
  • Kermadec-Tonga: Three segments, following the model of Bonnardot et al. (2007). Convergence rate of the segments decreases from north to south, and interface seismicity drastically decreases within the central segment, which encompasses the bouyant Louisville Seamount Chain.

We derive a magnitude-frequency distribution (MFD) for each interface segment using a hybrid approach that combines statistics from observed seismicity with a characteristic component derived from tectonics. Source characteristics with references are summarized in Table 1. The listed characteristic magnitude (Mchar) is the median magnitude computed from the scaling relationship Thingbaijam and Mai (2017).

Subduction zone Segment a-Value b-Value Mmax,obs M*char*** Convergence rate (mm/yr) Coupling
New Britain 1 3.984 0.683 8.10 8.42 90[^1] 0.24[^4]
South Solomon 1 2.336 0.467 7.95 8.13 70[^1] 0.6[^2]
2 5.216 0.977 8.07 8.22 80[^2] 0.5[^2]
3 4.771 0.892 8.00 8.04 100[^2] 0.7[^4]
New Hebrides 1 0.131 0.218 7.75 7.58 40[^1] 0.11[^4]
2 4.954 0.840 8.14 8.49 120[^1] 0.4[^5]
3 4.169 0.780 7.60 8.12 35[^3] 0.9[^5]
4 3.280 0.600 8.02 8.25 120[^1] 0.25[^4]
Kermadec-Tonga 1 7.551 1.247 7.88 8.90 110[^1] 0.21[^4]
2 6.396 1.206 7.20 8.29 120[^1] 0.12[^4]
3 8.624 1.422 7.97 8.98 200[^1] 0.65[^6]

Intraslab

In our source model, segmentation boundaries from the interface extend into the slab, and are not meant to suggest barrier to rupture within the downgoing slab volume, but instead to allow spatial variability in observed seimsicity while still using non-parametric ruptures. We model the rupture geometries and rates following the standard GEM methodology for slab earthquakes, described here. Results are in Table 2.

Subduction zone Segment a-Value b-Value Mmax,obs
NewBritain 1 5.373 0.934 7.50
SouthSolomon 1 4.116 0.898 6.95
2 5.804 1.190 6.47
3a 4.284 0.882 6.33
3b 2.852 0.564 7.70
NewHebrides 1 1.875 0.484 7.00
2 3.796 0.662 7.85
3 4.515 0.792 7.70
4 5.046 0.850 7.90
Kermadec-Tonga 1 5.019 0.864 7.44
2 3.643 0.731 7.84
3 6.711 1.096 7.80

Figure X. Surface projection of the subduction interface and slab models with epicenters of earthquakes *M*>6 since 1900 classified as slab, shallow slab, or interface.

Distributed seismicity

We model distributed seismicity with an approach that combines area sources with smoothed seismicity, incorporating methods from Frankel (1995), with the source zone approach commonly used to build GEM models. We build a source model for the crustal subcatalogue encompassed by each source zone polygon, with occurrence rates at a bin spacing of M=0.1. We compute the smoothed seismicity for a grid of 0.1° spacing. The MFDs for grid points near faults are truncated at MW6.5 to prevent double counting (see Crustal Faults description). Source zone characteristics are described in Table 3.

Source zone a-Value (zonal) b-Value Mmax,obs Description
1 3.821 0.726 7.55 fore/backarc seismicity in New Britain subduction zone
2 3.549 0.643 8.00 region of complex spreading centers and strike slip faulting
3 4.266 0.817 7.65 further backarc of South Solomon trench (Pacific Plate) where seismicity rates are lower and less dense
4 5.066 0.905 7.91 fore/backarc seismicity of New Britain and South Solomon subduction zones
5 4.791 0.894 7.50 outer rise extended of New Britain trench where it merges with the South Solomon trench
6 4.563 0.886 6.62 oceanic crustal region characterized by midocean ridges and transform faults
7 3.493 0.706 7.11 oceanic crustal region characterized by midocean ridges and transform faults
8 3.449 0.629 7.64 fore/backarc of hinge between New Hebrides and South Solomon subduction zones (Pacific Plate)
9 5.381 0.958 7.67 New Hebrides outer rise seismicity (Australian plate)
10 4.785 0.825 7.70 fore/backarc of New Hebrides (Pacific Plate)
11 4.688 0.836 7.08 North Fiji Basin; spreading ridges and transform faults
12 5.303 0.870 7.59 Fiji Platform, part of Fiji Fracture zone, Lua Ridge; zone of rotation between the two subduction zones with mostly spreading ridge and transform faulting
13 6.326 1.136 6.64 zone of strike slip seismicity that aligns with distinct lineaments
14 6.028 1.057 8.10 crustal seismicity where boundary is rotating from subduction to strike slip
15 6.083 1.068 7.60 Shallow seismicity in fore/backarc (Australian plate)
16 5.955 1.040 8.20 Kermadec-Tonga outer rise (Pacific Plate)
17 8.101 1.592 6.20 dispersed seismicity in oceanic crust
18 5.906 1.203 6.99 dispersed seismicity in oceanic crust

Crustal faults

From the GEM Active faults database, we keep transform faults that offset mid-ocean ridges, and other seafloor faults. Ridge-bounding normal faults are excluded due to their thin seismogenic coupling zone and assumed Mmax~5.8 (Bird, 2002). We link together continuous fault segments with the same slip type, and similar strike and sense of motion, choosing representative parameters. Using the standard fault modelling methodology of the GEM Secretariat, we create OQ simple fault sources with double-truncated Gutenberg-Richter MFDs (bin spacing M=0.1), keeping faults with Mmax>=6.5. In total, we keep 24 faults. b-values of the MFDs correspond to the encoupassing source zone (see above description of Distributed seismicity).

The figure below shows crustal sources.

Figure X. Input sources that model active crustal seismicity in the Pacific Islands. Yellow polygons bound the source zones, and encompass 0.1°-spaced grids of point sources. Red lines are active faults. Grey circles show crustal seismicity *M*>6 since 1900, scaled by magnitude.

Ground Motion Characterisation

The ground motion model includes three tectonic regimes, and uses weighted GMPEs for each to account for epistemic uncertainties. Due to the scarcity of land in the Pacific Islands region, station coverage is sparse, and regional ground motion models have not been developed. Residual analysis (described here) is challenging, as there are very few records available with source-site distances <300 km (our hazard model source distance cutoff, and the limit of data used to develop many of the GMPEs). We use the same GMPEs and weightings for each tectonic domain as the Ghasemi et al. (2016) model for Papua New Guinea. Ghasemi et al. (2016) selected GMPEs based on the recommendations of Bommer et al. (2010), and the residual analysis of Petersen et al. (2012) – which, while thorough, included recordings from earthquakes and stations throughout the whole Pacific Rim – to assign weights.

Active Shallow Crust Weight
Boore and Atkinson 2008 0.3
Chiou and Youngs 2008 0.3
Zhao et al. 2006 0.4
Subduction Interface Weight
Atkinson and Boore 2003 0.3
Youngs et. al. 1997 0.3
Zhao et. al. 2006 0.4
Subduction Intraslab Weight
Atkinson and Boore 2003 0.3
Youngs et. al. 1997 0.3
Zhao et. al. 2006 0.4

Results

Hazard curves were computed with the OQ engine for peak ground acceleration (PGA) and spectral acceleration (SA) at 0.2s, 0.5s, 1.0s, and 2s. The computation was performed on a grid of 11825 sites (spaced at approximately 10 km) with reference soil conditions corresponding to a shear wave velocity in the upper 30 meters (Vs30) of 760-800 m/s.

The hazard map for PGA corresponding to a 10% probability of exceedance in 50 years (475 year return period), can be seen using the interactive viewer. For a more comprehensive set of hazard and risk results, please see the GEM Visualization Tools.

References

Baillard, Christian, et al. "Seismicity and shallow slab geometry in the central Vanuatu subduction zone." Journal of Geophysical Research: Solid Earth 120.8 (2015): 5606-5623.

Bird, Peter. "An updated digital model of plate boundaries." Geochemistry, Geophysics, Geosystems 4.3 (2003).

Bommer, Julian J., et al. "On the selection of ground-motion prediction equations for seismic hazard analysis." Seismological Research Letters 81.5 (2010): 783-793.

Bonnardot, M‐A., et al. "Seismicity and state of stress within the overriding plate of the Tonga‐Kermadec subduction zone." Tectonics 26.5 (2007).

Calmant, Stéphane, et al. "New insights on the tectonics along the New Hebrides subduction zone based on GPS results." Journal of Geophysical Research: Solid Earth 108.B6 (2003).

Chen, Ming-Chu, et al. "Arc segmentation and seismicity in the Solomon Islands arc, SW Pacific." Tectonophysics 507.1-4 (2011): 47-69.

Christophersen, Annemarie, Kelvin Berryman, and Nicola Jane Litchfield. The GEM Faulted Earth Project. GNS Science, Te Pū Ao, 2015.

Dziewonski, A. M., T.-A. Chou and J. H. Woodhouse, Determination of earthquake source parameters from waveform data for studies of global and regional seismicity, J. Geophys. Res., 86, 2825-2852, 1981. doi:10.1029/JB086iB04p02825

Ekström, G., M. Nettles, and A. M. Dziewonski, The global CMT project 2004-2010: Centroid-moment tensors for 13,017 earthquakes, Phys. Earth Planet. Inter., 200-201, 1-9, 2012. doi:10.1016/j.pepi.2012.04.002

Ghasemi, Hadi, et al. "Probabilistic seismic hazard map of Papua New Guinea." Natural Hazards 81.2 (2016): 1003-1025.

Heuret, A., Lallemand, S., Funiciello, F., Piromallo, C., & Faccenna, C. (2011). Physical characteristics of subduction interface type seismogenic zones revisited. Geochemistry, Geophysics, Geosystems, 12(1).

Lagabrielle, Yves, et al. "Coseismic and long‐term vertical displacement due to back arc shortening, central Vanuatu: Offshore and onshore data following the Mw 7.5, 26 November 1999 Ambrym earthquake." Journal of Geophysical Research: Solid Earth 108.B11 (2003).

Petersen, Mark D., et al. Seismic Hazard of American Samoa and Neighboring South Pacific Islands--methods, Data, Parameters, and Results. US Department of the Interior, US Geological Survey, 2012.

Scholz, C. H., & Campos, J. (2012). The seismic coupling of subduction zones revisited. Journal of Geophysical Research: Solid Earth, 117(B5).

Uhrhammer, Robert A. "SEISMICITY RECORD, M> 2.5, FOR THE CENTRAL." Clement F. Shearer (1985): 199.

Wallace, L. M., Fagereng, Å., & Ellis, S. (2012). Upper plate tectonic stress state may influence interseismic coupling on subduction megathrusts. Geology, 40(10), 895-898.