ShakeMap Scientific Background

For additional background information, download the ShakeMap Manual


Introduction

A ShakeMap is a representation of ground shaking produced by an earthquake. The information it presents is different from the earthquake magnitude and epicenter that are released after an earthquake because ShakeMap focuses on the ground shaking produced by the earthquake, rather than the parameters describing the earthquake source. So, while an earthquake has one magnitude and one epicenter, it produces a range of ground shaking levels at sites throughout the region depending on distance from the earthquake, the rock and soil conditions at sites, and variations in the propagation of seismic waves from the earthquake due to complexities in the structure of the Earth's crust.

Part of the strategy for generating rapid-response ground motion maps is to determine the best format for reliable presentation of the maps given the diverse audience, which includes scientists, businesses, emergency response agencies, media, and the general public. In an effort to simplify and maximize the flow of information to the public, we have developed a means of generating not only peak ground acceleration and velocity maps, but also an instrumentally-derived, estimated Modified Mercalli Intensity map. This map makes it easier to relate the recorded ground motions to the expected felt and damage distribution. The Instrumental Intensity map is based on a combined regression of recorded peak acceleration and velocity amplitudes. (see Intensity Maps)

Even with the current seismic station distribution in Calfornia, data gaps are common, particularly for events outside the densely-instrumented metropolitan regions surrounding Los Angeles and San Francisco. In order to stabilize contouring and minimize the misrepresentation of the ground motion pattern due to data gaps, we augment the data with predicted values in areas without data. Given the epicenter and magnitude (and for larger earthquakes, fault geometry if avaliable), peak motion amplitudes in spare regions are estimated from the ground motion prediction equations.

The instrumental intensity map shows station symbols as open triangles in order to see the underlying intensity value. The legend bar at the bottom explains the colors (and see Intensity Maps below). For the intensity map as with other maps, station locations are the best indicator of where the map is most accurate: Near seismic stations the shaking is well constrained by data; far from such stations, the shaking is estimated using standard seismological inferences and interpolation.

Note: ShakeMaps are generated automatically following moderate and large earthquakes. These are preliminary ground shaking maps, normally posted within several minutes of the earthquake origin time. The acceleration and velocity values are raw and are at least initially, NOT checked by humans. Further, since ground motions and intensities typically can vary significantly over small distances, these maps are only APPROXIMATE. At small scales, they should be considered unreliable. Finally, the input data is raw and unchecked, and may contain errors. (See Disclaimer)


Maps - General Information

When viewing the peak ground motion maps using a Javascript-enabled browser, additional information about the earthquake epicenter and recording seismic stations can be viewed. A brief summary line is displayed when the mouse pointer is over the epicenter symbol or a station symbol. If the symbol is clicked, a small window with a table of information will be opened. By selecting the epicenter, the earthquake information includes the event date, time, location coordinates in degrees latitude and longitude, and hypocentral depth in kilometers; selecting a seismic station jumps to that stations entry in the table. This window can be moved to a preferred location, and clicking on the tab bar to see another map will close the current information window.

The station information includes the station code and name, the agency that manages the station, the station location coordinates in degrees latitude and longitude, and the peak acceleration, velocity, and spectral acceleration for each component of ground motion (when available). Spectral acceleration maps are only made for larger earthquakes (normally, magntidue greater than 5.5). When the peak ground motion maps are made, the value from the peak horizontal component of ground motion is used as the value for the station. This value is highlighted in bold in the station information.

Components from many stations are defined by three letter codes. The last letter indicates the orientation (Z = vertical, N = horizontal north, E = horizontal east). The first two letters indicate the instrument class:

Code Description
VL low gain channels on the analog network
VH high gain channels on the analog network
AS FBA's on the analog network
HL FBA's on the digital network
HN FBA's on the digital network
BH broadband data streams
HH broadband data streams

FBA's (force balance accelerometers) are designed to record extremely large ground motions and can accurately record waves from very large earthquakes. However, ground motions from small and moderate earthquakes are often too small to trigger these instruments or rise above instrument noise. On the other hand, Broadband seismic sensors can record extremely small ground motions and accurately record waves from earthquakes that range from very small up to moderately large. A number of stations have both FBA and broadband sensors. For ShakeMap, the network tends to emphasize FBA recordings for large ground motions and broadband recordings for small ground motions.

Occassionally, station channels will be flagged due to problems with the station or possibly anomalous peak values. In this case, the popup window of station information will indicate the flagging with the following codes:

Code Description
M Manually flagged
T Outlier
G Glitch (clipped or below noise)
I Incomplete time series
N Not in list of known stations

Map Types

Peak Acceleration Maps

Peak horizontal acceleration at each station is contoured in units of percent-g (where g = acceleration due to the force of gravity = 981 cm/s/s). The peak values of the vertical components are not used in the construction of the maps because they are, on average, lower than the horizontal amplitudes and ground motion prediciton equations used to fill in data gaps between stations are based on peak horizontal amplitudes. The contour interval varies greatly and is based on the maximum recorded value over the network for each event.

For moderate to large events, the pattern of peak ground acceleration is typically quite complicated, with extreme variability over distances of a few km. This is attributed to the small scale geological differences near the sites that can significantly change the high-frequency acceleration amplitude and waveform character. Although distance to the causative fault clearly dominates the pattern, there are often exceptions, due to local focussing and amplification. This makes interpolation of ground motions at one site to a nearby neighbor somewhat risky. Peak acceleration pattern usually reflects what is felt from low levels of shaking up to to moderate levels of damage.

Peak Velocity Maps

Peak velocity values are contoured for the maximum horizontal velocity (in cm/sec) at each station. As with the acceleration maps, the vertical component amplitudes are disregarded for consistency with the regression relationships used to estimate values in gaps in the station distribution. Typically, for moderate to large events, the pattern of peak ground velocity reflects the pattern of the earthquake faulting geometry, with largest amplitudes in the near-source region, and in the direction of rupture (directivity). Differences between rock and soil sites are apparent, but the overall pattern is normally simpler than the peak acceleration pattern. Severe damage, and damage to flexible structures is best related to ground velocity. For reference, the largest recorded ground velocity from the 1994 Northridge (Magnitude 6.7) earthquake made at the Rinaldi Receiving station, reached 183 cm/sec.

Spectral Response Maps

Following earthquakes larger than magnitude 5.5, spectral response maps are made. Response spectra portray the response of a damped, single-degree-of-freedom oscillator to the recorded ground motions. This data representation is useful for engineers determining how a structure will react to ground motions. The response is calculated for a range of periods. Within that range, the Uniform Building Code (UBC) refers to particular reference periods that help define the shape of the "design spectra" that reflects the building code.

ShakeMap spectral response maps are made for the response at three UBC reference periods: 0.3, 1.0, and 3.0 seconds. For each station, the value used is the peak horizontal value of 5% critically damped pseudo-acceleration.

Rapid Instrumental Intensity Maps

As an effort to simplify and maximize the flow of information to the public, we have developed a means of generating estimated Modified Mercalli Intensity maps based on instrumental ground motion recordings (Wald et al., 1999). These "Instrumental Intensities" are based on a combined regression of peak acceleration and velocity amplitudes vs. observed intensity for eight significant California earthquakes (1971 San Fernando, 1979 Imperial Valley, 1986 North Palm Springs, 1987 Whittier, 1989 Loma Preita, 1991 Sierra Madre, 1992 Landers, and 1994 Northridge).

From the comparison with observed intensity maps, we find that a regression based on peak velocity for intensity > VII and on peak acceleration for intensity < VII is most suitable. This is consistent with the notion that low intensities are determined by felt accounts (sensitive to acceleration). Moderate damage, at intensity VI-VII, typically occurs in rigid structures (masonry walls, chimneys, etc.) which also are sensitive to high-frequency (acceleration) ground motions. As damage levels increase, damage also occurs in flexible structures, for which damage is proportional to the ground velocity, not acceleration. By relating recorded ground motions to Modified Mercalli intensities, we can now estimate shaking intensities within a few minutes of the event based on the recorded peak motions made at seismic stations.

A descriptive table of Modified Mercalli Intensity is available from ABAG (Association of Bay Area Governments). A table of intensity descriptions with the corresponding peak ground acceleration (PGA) and peak ground velocity (PGV) values used in the ShakeMaps is given below. ShakeMap uses PGA to estimate intensities lower than V, it linearly combines PGA & PGV for intensities greater than V and less than VII, and it uses PGV for intensities greater than VII (See Wald et al., 1999b, for more details).

Modified Mercalli Intensity Scale

Global Earthquake ShakeMaps

For regions around the world were 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.

Initially, a point source approximation (hypocenter and magnitude) is used to constrain region-specific empirical ground motion estimation. Site amplification is approximated from a relationship developed between topographic gradient and shear-wave velocity. Additional constraints for these predictive maps come primarily from three important sources, the availability of which varies depending on the region in which the earthquake occurred, as well as a function of time after the earthquake occurrence. These constraints include: (1) additional earthquake source information, particularly fault rupture dimensions, (2) observed macroseismic intensities (including via the USGS “Did You Feel It?” system, and (3) observed strong ground motions, where and when available.

When intensity data are used directly, the peak ground motion parameters are inferred from the macroseismic observations using the equations of Wald et al, (1999a). This is the opposite approach normally used in ShakeMaps where numerous seismic recording are available, and from them the intensities are then inferred.

Input data for global ShakeMaps are depicted with circles for intensity reports and triangles for strong motion data. Intensities are further separated by color: Blue circles indicate data collected via the USGS "Did You Feel It?" Community Internet Intensity Map system and Yellow circles indicate traditional Modifed Mercalli Intensity assignments. For the intensity maps, circles are open, allowing the underlying intensity color to show thru. On the other maps observation location symbols (circles, triangles) are color coded according to the data type mentioned above.


Shakemap Atlas

An atlas of maps of peak ground motions and intensity “ShakeMaps” has been developed for approximately 1,000 recent and historical global earthquakes (Allen and others, 2008). These maps are produced using established ShakeMap methodology (Wald and others, 1999c, 2005) and constraints from macroseismic intensity data, instrumental ground motions, regional topographically-based site amplifications (Wald and Allen, 2007), and published earthquake rupture models. The Atlas of ShakeMaps provides a consistent and quantitative description of the distribution of shaking intensity for recent global earthquakes (January 1973 – September 2007 for Version 1.0). We anticipate that the Atlas will be regularly updated with more data constraints for historical events and the addition of future significant events as time progresses.

The Atlas was developed specifically for calibrating global earthquake loss estimation methodologies to be used in the USGS Prompt Assessment of Global Earthquakes for Response (PAGER) Project. PAGER will employ these loss models to rapidly estimate the impact of global earthquakes as part of the USGS National Earthquake Information Center’s earthquake response protocol. Though developed primarily for PAGER, we anticipate many other uses for the historical ShakeMap Atlas, including disaster response planning, capacity building and outreach programs, in addition to calibration of other global loss methodology approaches.

The primary sources for instrumental data are:

Macroseismic intensity observations are gathered from several online sources, including:

Community Internet Intensity Maps data obtained from the USGS Did You Feel It? (DYFI?) system (Wald and others, 1999a; Atkinson and Wald, 2007) are a valuable ground-shaking constraint and are used for U.S. events since 1999 and for global events since 2003. Additional intensity data were gathered or digitized from numerous earthquake reconnaissance reports and peer reviewed publications, or through the generous contribution from colleagues around the world.

An important, consolidated source for finite fault models is provided by Martin Mai of the Swiss Seismological Service, Zurich. Other fault rupture dimensions were digitized from observations of surface displacement, finite-fault source inversions and slip distribution determined from teleseismic observations, or more recently, from InSAR observations. Less well-constrained faults have been estimated from earthquake aftershock distributions.

The ShakeMap Atlas aims to provide the best estimate of the shaking distribution for historical earthquakes. One important feature of the Atlas is that data constraints are not limited to those data produced by the National Earthquake Information Center, and we will openly accept data contributions that help to improve the representation of the shaking distribution for any of the events. References to all data sources in the Atlas of ShakeMaps (as of August 2008) are provided in Allen and others (2008). Please contact Trevor Allen or David Wald with any comments or data contributions.

Release Notes

Below we indicate the default values and predictive equations used in constructing the Atlas of ShakeMaps. Variations in ground-motion prediction equations (GMPEs) and other configurations may change on an event-by-event basis. Please see the event info.xml on individual ShakeMap download pages or Allen and others (2008) for more information.

Version 1
Date Range: January 1973 – September 2007
Active crust GMPE: Boore and others (1997)
Subduction zone GMPE (intraslab and interface): Youngs and others (1997)
Stable continent GMPE: Atkinson and Boore (2006)
Seismic site-conditions: Wald and Allen (2007) <http://earthquake.usgs.gov/vs30/>
Peak ground motion to macroseismic intensity conversions: Wald and others (1999b)

References