3.6.1. General Disclaimer¶
Some USGS information accessed through this page may be preliminary in nature and presented prior to final review and approval by the Director of the USGS. This information is provided with the understanding that it is not guaranteed to be correct or complete, and conclusions drawn from such information are the sole responsibility of the user. In addition, ShakeMaps are automatic, computer-generated maps and have not necessarily been checked by human oversight, so they may contain errors. Further, the input data is raw and unchecked, and may contain errors.
- Contours can be misleading, since data gaps may exist. Caution should be used in deciding which features in the contour patterns are required by the data. Ground motions and intensities can vary greatly over small distances, so these maps are only approximate.
- Locations within the same intensity area will not necessarily experience the same level of damage, since damage depends heavily on the type of structure, the nature of the construction, and the details of the ground motion at that site. For these reasons, more or less damage than described in the intensity scale may occur. The ground motion levels and descriptions associated with each intensity value are based on recent damaging earthquakes. There may be revisions in these parameters as more data become available or due to further improvements in methodologies.
- Large earthquakes can generate very long-period ground motions that can cause damage at great distances from the epicenter; although the intensity estimated from the ground motions may be small, significant effects to large structures (e.g., bridges, tall buildings, storage tanks) may be notable.
- The utilization of DYFI data on ShakeMap, in addition to using recorded peak ground motions, is standard-operating-procedure for some ShakeMap operations, including the Global ShakeMap (GSM) system operated out of the USGS NEIC. The algorithmic strategy for including these data in ShakeMap is documented in the Technical Guide. As described by Wald et al. (2011), the ultimate discretion to use, filter, or overrule specific internet-based or historic intensities (or specific suspect strong motion data, for that matter) remains with the ShakeMap operators. A number of filtering and quality control strategies are in place (e.g., Wald et al., 2011), but erroneous or suspect data can not always be culled immediately. While we make efforts to provide consistent quality control of the data, the DYFI system depends upon open, citizen-science based input from the public. A number of studies have shown these data to be generally reliable, but the data reliability may vary from event to event. Moreover, macroseismic intensities are fundamentally non-unique: Differing polygonal aggregations for computing Community Decimal Intensities (CDI, using geocoded boxes or ZIP codes, for example, Wald et al., 2011) may yield varying intensity values at specific locations. [Historic or modern MMI or EMS-98 intensity assignments are also non-unique; the assignment can vary from expert to expert, the selection of areas may vary, and occasionally different structure types may indicated alternative intensities]. Changes to the size of the areas used to aggregate CDIs often trade off a greater number of responses per polygon (hence greater confidence in the derived intensity) against a more precise spatial location. DYFI data are routinely used on the GSM systems and other regional ShakeMap systems of the ANSS. DYFI data are not currently (as of 2016) used in the Northern or Southern California ShakeMap systems, in part due to the adequacy of strong-motion station coverage there.
3.6.2. ShakeMap Update Policy¶
ShakeMaps are preliminary in nature and will be updated as data arrives from a variety of distributed sources. Our practice is to improve the maps as soon as possible, but there are factors beyond our control that can result in delayed updates.
- For regions around the world, where there are insufficient near–real-time strong-motion seismic stations to generate an adequate strong-ground-motion data-controlled ShakeMap, we can still provide a very useful estimate of the shaking distribution using the ShakeMap software. Site amplification is approximated from a relationship developed between topographic gradient and shear-wave velocity. Additional constraints for these predictive maps come primarily from additional earthquake source information, particularly fault rupture dimensions, observed macroseismic intensities (including via the USGS “Did You Feel It?” system), and observed strong ground motions, when and where available.
- There is no formal “final” version of any ShakeMap. Version Control is up to users. ShakeMap version numbers and timestamps are provided on the maps, webpages, grid files, and metadata.
- Our strategy is to update ShakeMaps as warranted from a scientific perspective. We reserve the option to update ShakeMaps as needed to add data or to improve scientific merit and/or presentation of the maps in any way beneficial. This most typical update is after new data arrive, finite-fault models get established or revised, magnitude gets revised, or as improved site amplification maps, ground motion prediction equations, or even interpolation or other procedures become available.
Updates to Online Maps
- Real-time ShakeMap Updates. Changes can be tracked with the ShakeMap version numbers and timestamps found in the metadata, the info.xml and grid.xml files, and on the maps themselves (time generated). The info.xml file contains timestamps, number of stations used, GMPE information, and many other attributes that could have changed from version to version. Often a text statement is provided that notes significant changes for a particular version.
- ShakeMap Atlas Updates. The ShakeMap Atlas uses version numbers for each Atlas event; the overall Atlas collections is also Versioned. Currently ShakeMap Atlas Version 2.0 is online in the ComCat database, and the older Atlas (Version 1.0) can be found online on the legacy ShakeMap Archives pages.
- Scenario Revisions. ShakeMap Scenario collections uses version numbers for each event; the overall scenario collections is named according to their source. Currently ShakeMap Atlas Version 2.0 is online in the USGS Comprehensive Catalogue (ComCat) Earthquake database. Some older scenarios are online on the legacy ShakeMap Archives pages. Scenario ShakeMaps will be revised when the underlying probabilistic seismic map fault segmentation or other particulars (like GMPE selection) change. Older versions will be archived in ComCat.