The 2007 USGS Alaska Seismic Hazard Model

The 2007 USGS Alaska seismic hazard model is described by Wesson et. al. (2007) (see also the USGS Website). Here we present a short description of the OpenQuake-engine implementation of the model.

The seismic source model consists of different seismic source typologies to describe earthquake occurrence in different tectonic settings. The model defines gridded seismicity to model both active shallow and deep intraslab seismicity. Fault sources are instead defined for shallow crustal faults and large subduction interface events. Gridded seismicity models are implemented as collections of NRML pointSource objects. Crustal faults and subduction faults are instead modeled as NRML simpleFaultSource and NRML complexFaultSource objects, respectively.

The whole source model is divided into the following sub-models:

  • Active shallow crust gridded seismicity
  • Subduction intraslab gridded seismcity
  • Active shallow crust faults
  • Subduction interface faults

The map below depicts the annual occurrence rate per source (between minimum and maximum magnitudes) for the different source models included in the hazard model. Click the show map layers icon to view different source models and base layer maps.

operating instructions

Various functions are available as part of the map.

mouse/touch operation
  • moving by grabbing the map with a mouse-click you can move the map around
  • overview map using the + button in the bottom right of the map you can expand an overview map
  • zooming in and out using the + and - buttons in the top left of the map you can obtain more or less detail in the map
  • switching themes or maps clicking on the ≡ icon on the right-hand side of the map you can view and select available maps and themes
  • retrieving information the map may contain elements that contain more information, by clicking these a popup will show this information
  • fullscreen display using the ✈ button the map can be maximized to fullscreen display, use the ✕ button to return to page display.
keyboard operation

Keyboard operation becomes available after activating the map using the tab key (the map will show a focus indicator ring).

  • moving using the arrow keys you can move the map
  • overview map using the + button in the bottom right of the map you can expand an overview map
  • zooming in and out using the + and - buttons in the top left of the map or by using the + and - keys you can obtain more or less detail in the map
  • switching themes or maps clicking on the ≡ icon on the right-hand side of the map you can view and select available maps and themes
  • retrieving information the map may contain elements that contain more information, using the i key you can activate a cursor that may be moved using the arrow keys, pressing the enter will execute an information retrieval. press the i or the escape key to return to navigation mode
  • fullscreen display using the ✈ button the map can be maximized to fullscreen display, use the ✕ button to return to page display.

It's possible that some of the functions or buttons describe above have been disabled by the page author or the administrator

 

Total occurrence rate
(number of events / year)
  • < 1e-6
  • 1e-6 - 1e-5
  • 1e-5 - 1e-4
  • 1e-4 - 1e-3
  • 1e-3 - 1e-2
  • 1e-2 - 1e-1
  • 1e-1 - 1
  • 1 - 10
  • >= 10








The ground motion model distinguishes between three main tectonic regions:

  • Active Shallow Crust
  • Subduction Interface
  • Subduction Intraslab

For each tectonic region, the model considers multiple ground motion prediction equations organized in a logic tree structure.

Subduction Interface Weight
Youngs et. al. 1997 0.5
Sadigh et. al. 1997 0.5
Subduction Intraslab Weight
Youngs et. al. 1997 0.5
Atkinson and Boore 2003 0.5

Reference site condition

The NEHRP B/C site condition is assumed to be the reference site conditions for the 2007 Alaska National Seismic Hazard Model. This is equivalent to a Vs30 (shear wave velocity in the uppermost 30 meters) = 760 m/s. GMPEs that do not depend on Vs30 are used with coefficients corrected for the B/C site condition.

Hazard maps

The figures below represent hazard maps for peak ground acceleration, for 10% and 2% probabilities of exceedance in 50 years , using the OpenQuake-engine.

Comparison against USGS results

The figures below shows hazard maps, for 10% probability of exceedance in 50 years, provided by the USGS and computed with the OpenQuake-engine. A difference map is also shown which allows to more quantitatively appreciate the differences between the two maps. The largest differences are associated with subduction fault sources. Differences may be due to different approaches in modeling earthquake ruptures in complex geometries and magnitude scaling relationships. Further analysis is required to better constrain what are the sources of the discrepancies.

  • Wesson, Robert L., Boyd, Oliver S., Mueller, Charles S., Bufe, Charles G., Frankel, Arthur D., Petersen, Mark D., 2007, Revision of time-Independent probabilistic seismic hazard maps for Alaska: U.S. Geological Survey Open-File Report 2007-1043. Report

This table summarises the main characteristics of the original implementation of this model

1 Datasets availability
1.1 Earthquake catalogue Not available. The catalogue used to develop the model is a combination of several catalogues including the Centennial, PDE, ISC and Alaska Earthquake Information Center
1.2 Geological database Not available. Information comes from different sources (see Wesson et al., 2007 page 12). A good description of major fault is included in the report.
1.3 Strong-motion database Not available
1.4 Site characterization database Not available
Notes
2 Methodology for model development
2.1 Scientific participation (SSHAC levels) and review process Level 2
2.2 Documentation describing model preparation Wesson et. al. (2007) provides a general description of the methodology adopted for the creation of the hazard model.
2.3 Codes used for model preparation Not available
Notes
3 PSHA input model
3.1 Seismic Source Model
3.1.1 Area sources Not included
3.1.2 Grid sources Gridded seismicity is used to model distributed seismicity in the shallow active crust as well as to model inslab seismicity
3.1.3 Crustal faults Included
3.1.4 Subduction faults Subduction interface sources modelled as a faults
3.1.5 Non-parametric ruptures Not included
3.1.6 Magnitude-area scaling relationships Not explicitly defined in Wesson et. al. (2007)
3.2 Ground Motion Model
3.2.0 Tectonic regionalisation Sources are classified in accordance with the same tectonic regions used by the USGS to compute hazard for the conterminous United States.
3.2.1 Models for active shallow seismicity Included
3.2.2 Models for subduction interface Included
3.2.3 Models for subduction intraslab Included
3.2.4 Models for stable continental regions Not included
3.2.5 Models for deep non-subduction sources Not included
3.2.6 Models for volcanic areas Not included
3.3 Site Response Model
3.3.1 Based on GMPEs Yes, hazard is computed for a reference soil condition corresponding to NEHRP B/C boundary (Vs30=760 m/s)
3.3.2 Based on site-response analysis No
3.4 Epistemic uncertainties
3.4.1 Seismic Source Model Not included
3.4.2 Ground Motion Model Included using a logic tree (see the ground motion model section)
3.4.3 Site Response Model Not included
Notes
4 Hazard Input Description
4.1 Hazard input document Not available
4.2 Original input files Available (ASCII input files for the USGS fortran codes)
Notes
5 Calculation
5.1 Software Suite of fortran codes developed by the USGS
5.2 Results
5.2.1 Hazard curves Not directly available
5.2.2 Hazard maps Not directly available
5.2.3 Uniform hazard spectra Not directly available
5.2.4 Disaggregation Not directly available
5.2.5 Stochastic event sets Not available
5.2.6 Ground motion fields Not available
Notes Almost all the results of a Classical PSHA can be computed with the USGS fortran codes

The OpenQuake-engine input model (NRML format) can be downloaded at the link provided below - Please read the license and disclaimer attached to the model.

N.B. This is a model adapted by GEM Hazard Team to the OpenQuake-engine from the original model developed by the USGS. This explains minor differences you might encounter between the results presented in the OpenQuake platform and those disseminated by the original Organisation.

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  • alaska_2007_intro.txt
  • Last modified: 2016/10/07 09:29
  • by Armando Scarpati