Philippines (PHL)

Authors:

PHIVOLCS, K. Johnson, R. Styron

Summary

The model covering the Philippines was developed jointly by the Philippine Institute of Volcanology and Seismology (PHIVOLCS) and the Global Earthquake Model (GEM) in the OpenQuake (OQ) engine format.

Updates

In 2019, this model was updated to v. 2019.1.0. The significant changes to the model are:

  • The Sulu, Negros, and East Luzon Trenches/Troughs were added to the model, as well as the Cotabato slab.
  • Coupling coefficients were interface segments were reassigned based on more appropriate literature
  • Distributed crustal seismicity across the Philippine archipelago was subdivided into three source zones
  • The shallow fault model was revised, linking together some faults with continuous or semicontinuous surface traces and consistent kinematics, and using Youngs and Coppersmith MFDs for faults with Mmax>7.0
  • Completeness tables were re-evaluated manually, and MFDs recomputed

The model is documented in more detail in:

Peñarubia, H., Johnson, K. L., Styron, R. H., Bacolcol, T. C., Bonita, J. D., Narag, I. C., Perez, J. S., Sevilla, W. I. G., Solidum Jr., R. G., Pagani, M., and Allen, T. I., (accepted). Probabilistic Seismic Hazard Analsyis model for the Philippines. Earthquake Spectra

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

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

Tectonic overview

The Philippines sits in one of the world's most tectonically active locations, with the subduction of several oceanic plates beneath the crust of the islands, and a very rapidly-slipping and distributed fault system running throughout the islands. The eastern margin of the archipelago contains the East Luzon and Philippines Trenches, where the Philippine Sea Plate subducts obliquely underneath the islands. The western edge of the major islands is an active margin where the Sunda plate subducts at the Manila and Negros trenches; in between these, the buoyant Palawan block resists subduction and forces E-W contraction farther east. Subduction continues around the southern island of Mindanao with many short trenches such as the Cotobato and Halmahera, though relative plate velocities seem to be low and these features are poorly understood. The obliquity of convergence between the bounding Sunda and Philippine Sea plates results in a major strike-slip fault system bisecting the islands, the left-lateral Philippine Fault System. This system has net slip rates of almost 30 mm/yr, rivaling continental plate boundary transforms such as the Alpine and San Andreas faults. In places, this system has transtensional or transpressional bends or jogs, causing more distributed deformation. Additional, smaller faults are distributed throughout the archipelago, such as reverse faults in the Visayas and normal to strike-slip faults in southern Luzon; many of these are capable of generating high magnitude 7 earthquakes, which can (and have been) very damaging to proximal population centers.

Basic Datasets

Earthquake Catalogues

We use the magnitude-homogonized ISC-GEM extended catalogue of Weatherill et al. (2016) clipped to the Philippines region (bounds of ~112.5-134°E and 1°S-24°N). The catalogue includes 45,750 earthquakes Mw>2.82 from 1905-2014.

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

Fault Database

A database of 117 crustal faults was compiled based on decades of investigation by PHIVOLCS. These faults have been integrated into the GEM Active Faults Database.

Hazard 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>6.5 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 model subduction interfaces for the Manila, Philippines, and Cotobato trenches. We divide the trenches and slabs into segments using boundaries defined by the seafloor bathymetry, seismicity patterns, convergence rate changes, and any other information available within the literature as follows:

  • Manila: three segments based on a coupling model by Hsu et al. (2012) and a north-to-south decrease in convergence rates, with boundares at ~14° N, where the trench axis bends by ~45°, and ~16.5° N at the Scarborough Seamount and a sediment thickness contrast thought to affect subduction mechanics (Hsu et al., 2012)
  • Philippine: two segments separated at ~5° N, where the Halmahera slab (see next section: Intraslab) and Philippine slab begin to interact
  • Cotobato: unsegmented

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).

Trench segments are numbered with increasing distance along strike following the right-hand rule.

Subduction zone Segment a-Value b-Value Mmax,obs Mchar Convergence rate (mm/yr) Coupling
Manila 1 4.229 0.859 7.63 7.89 50[1] 0.20[1]
2 4.897 0.977 8.15 7.24 60[1]
3 7.264 1.287 7.07 8.49 90[1] 0.20[1]
Philippine 1 5.692 0.989 7.52 8.63 50[2] 0.12[3]
2 4.147 0.850 7.41 8.04 30[2] 0.12[3]
Cotobato 1 1.938 0.453 8.30 8.08 35[2] 0.41[3]

[1]: Hsu et al. (2012) [2]: Rangin, 2016 [3]: Heuret et al. (2011)

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.

An intraslab source model is included for each subuction segment in the interface source model, except the Cotobato subduction zone where a meaningful MFD for slab seismicity is not resolvable. Additionally, we include the Halmahera slab - which is fully subducted and and dips in both directions - modelled in two segments separated by the hinge axis. We divide the along-strike extent of Philippine interface Segment 2 into two subsegments, since at slab depths, the crustal body is discontinuous.

Subduction zone Segment a-Value b-Value Mmax,obs
Manila 1 6.608 1.268 6.69
2 2.473 0.597 6.85
3 2.701 0.618 6.9
Philippine 1 7.820 1.433 7.0
2a 1.503 0.428 6.25
2b 5.940 1.157 7.01
Halmahera 1 7.949 1.311 7.72
2 8.398 1.463 7.60

Crustal faults

The crustal faults are modelled from each fault's geometry, kinematics, and slip rate (parameters included in the GEM Active Faults Database) using the standard practice of the GEM Secretariat, described here. For most faults, we assigneed a fixed lower seismogenic depth of 25 km, minimum magnitude of MW6.5, and aseismic coefficient of 0.3. The exception is sections of the Philippines fault system on Leyte Island, which are shown to be mostly creeping; these faults were given an aseismic coefficient of 0.9.

The final MFDs are double-truncated Gutenberg-Richter distributions with a bin size of 0.1. The b-value is set to that of the encompassing source zone described in the next section (Distributed seismicity).

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 5.554 1.113 7.14 Manila trench outer rise
2 5.620 0.949 7.75 shallow thrust faulting above the Halamahera slab
3 4.734 0.917 7.76 Philippine trench outer rise
4 3.917 0.827 7.15 diffuse seismicity
5 3.576 0.688 7.69 fore- and backarc thrusting in the overriding plate of the Philippine trench
6 5.937 0.977 7.80 region of high-rate active crustal deformation

models

Ground Motion Characterisation

Active Shallow Crust Weight
BooreAtkinson2008 0.2
CampbellBozorgnia2008 0.2
ChiouYoungs2008 0.2
BooreEtAl2014 0.133
CampbellBozorgnia2014 0.133
ChiouYoungs2014 0.134
Subduction Interface Weight
YoungsEtAl1997SInter 0.15
AtkinsonBoore1995GSCBest 0.15
ZhaoEtAl2006SInter 0.3
AbrahamsonEtAl2015SInter 0.4
Subduction IntraSlab Weight
YoungsEtAl1997SSlab 0.333
AtkinsonBoore2003SSlabNSHMP2008 0.333
AtkinsonBoore2003SSlabCascadiaNSHMP2008 0.334

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 11401 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

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

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).

Hsu, Y. J., Yu, S. B., Song, T. R. A., & Bacolcol, T. (2012). Plate coupling along the Manila subduction zone between Taiwan and northern Luzon. Journal of Asian Earth Sciences, 51, 98-108.

Rangin, C. (2016). Rigid and non-rigid micro-plates: Philippines and Myanmar-Andaman case studies. Comptes Rendus Geoscience, 348(1), 33-41.

Thingbaijam, K. K. S., Martin Mai, P., & Goda, K. (2017). New Empirical Earthquake Source‐Scaling Laws. Bulletin of the Seismological Society of America, 107(5), 2225-2246.

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.