C.-H. Chan, T. Ornthammarath
The seismic hazard model for Continental Southeast Asia (SEA) covers Myanmar, Thailand, Vietnam, Laos, Cambodia, Singapore, and West Malaysia. The model was developed by a group of scientists from Singapore, Thailand, Myanmar and Vietnam. Two seismic source models were independently developed by Chan et al. (2017) and Ornthammarath et al. (2020). The source model of Chan et al. (2017) covers a region that is larger than the extent of the SEA model used in the global mosaic, with sources extending east to Papua New Guinea and south to Indonesia, while the source model of Ornthammarath et al. (2020) covers the same extent of the SEA model used in the global mosaic. Both source models are included in the final SEA model by using a source model logic tree, where the models are assigned equal weights. The Ground Motion Characterisation for the two models is the same. The ground motion logic tree contains ground motion prediction equations for active shallow crust and, subduction interface and intraslab. The SEA model was originally created for the OpenQuake (OQ) engine. Previous studies in the region include Petersen et al., 2004.
Information about the OQ model versions and input files can be found on the Results and Dissemination page.
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.
Southeast Asia lies at the confluence of several plate boundary systems. The study area itself lies on the Sunda Plate, which is separated from greater Eurasia by a system of strike-slip faults, primarily the sinistral Xianshuehe- Red River Fault and a conjugate set of distributed right-lateral faults near the Chinese border with northern Vietnam, Cambodia, Myanmar and Laos. The plate is then bound on its northwestern margin by the right-lateral Sagaing Fault, which separates it from India. The Sagaing Fault is a north-striking transform boundary between the Sunda Plate and the Indo-Australian Plate or plates (depending on the definition) which becomes increasingly more convergent to the south into Indonesia as the boundary wraps around to the southeast; here it becomes primarily a subduction zone off of Sumatra and Java, though Sagaing-type deformation continues into Sumatra as the Great Sumatran Fault. The eastern margin of the Sunda plate is composed of a complicated set of plate boundaries that are primarily subduction zones between microplates and mobile belts in the Philippines south through Sulawesi and into Papua; these produce very frequent moderate to large magnitude earthquakes but are distant enough from the Indochina mainland to pose somewhat muted hazard. Relatively slowly-slipping active faults are distributed throughout the interior of the Sunda Plate, but these also produce smaller and less frequent earthquakes than the major plate boundaries described above.
The two models utilise a recently compiled set of shallow active faults.
See Chan et al. (in prep) and Ornthammarath et al. (in prep) for descriptions of the datasets used for developing the hazard model.
Seismic Source Characterisation
The seismic source characterisation (SSC) is the combination of two seismic source models (SSM) independently developed by Chan et al. (in prep) and Ornthammarath et al. (in prep), which are hereinafter referred to as SSM1 and SSM2, respectively. Both SSMs consists of various seismic source typologies to describe earthquake occurrence in different tectonic settings. Distributed seismicity is used to model both active shallow and deep intraslab seismicity, while fault sources are used to model seismicity occurring on shallow crustal faults and large subduction interface events.
The OQ implementation for SSM1 uses four source typologies (two in OQ). The shallow background and intraslab seismicity are modelled using Area Sources, while crustal faults and subduction interface faults are modelled using Simple Fault Sources. The OQ implementation for SSM2 uses four OQ source typologies. The background seismicity for active shallow crust is modelled using Point Sources with spatially variable properties. Deep intraslab seismicity is also modelled using Point Sources. Crustal faults are modelled using Simple Fault Sources, and Characteristic Fault Sources (with simple fault geometry). Subduction faults are modelled using Complex Fault Sources. The OQ sources are depicted in the interactive viewer.
Epistemic uncertainties are described using a logic tree. This permits the use of both SSMs, each represented by one branch. SSM1 and SSM2 are assigned equal weights of 0.5. Additionally, SSM2 also includes epistemic uncertainty of the maximum magnitude modelled on the Burma Sumatra Megathrust.
Ground Motion Characterisation
Table 1 shows the ground motion logic tree for SEA. The logic tree is the same for SSM1 and SSM2, and distinguishes between five main tectonic regions. Only three are within the extent of the SEA model used in the global mosaic, and they are: Active Shallow Crust, Subduction Interface, and Subduction IntraSlab.
For every tectonic region, epistemic uncertainty is considered by using multiple GMPEs, each with an associated logic tree weight.
|Active Shallow Crust||Weight|
|Philippine Active Shallow Crust||Weight|
Table 1 - GMPEs used in the SEA model.
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 30224 sites (spaced at approximately 10 km) with reference soil conditions with 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.
Chan C.-H-, Wang Y., Shi X., Ornthammarath T., Warnitchai P., Kosuwan S., Thant M., Nguyen P.H., Nguyen L. M., Solidum Jr. R., Irsyam M., Hidayati S., Sieh K. (2017). Toward uniform probabilistic seismic hazard assessments for Southeast Asia. 2017 AGU Fall Meeting Abstract #238207.
Ornthammarath T, Warnitchai P, Chan C-H, Wang Y, Shi X, Nguyen PH, Nguyen JM, Kosuwan Sand Thant M (2020). Probabilistic Seismic hazard assessments for Northern Southeast Asia (Indochina): Smooth seismicity approach. Earthquake Spectra 36(S1): 69–90.
Petersen, M.D., Dewey, J., Hartzell, S., Mueller, C., Harmsen, S., Frankel, A.D., and Rukstales, K., 2004, Probabilistic seismic hazard analysis for Sumatra, Indonesia and across the southern Malaysian Peninsula: Tectonophysics, v. 390, p. 141–158.