Download preparing for a new view of u.s. earthquake risk

PREPARING FOR A NEW VIEW
OF U.S. EARTHQUAKE RISK
October 2015
The latest scientific studies are set to change vendor earthquake catastrophe models
over the next 18 months. But will these changes be significant to your catastrophe risk
management process?
Executive Summary
The 2014 National Seismic Hazard Maps (NSHM) and the 2015 Uniform California Earthquake Rupture Forecast (UCERF3)
describe the current scientific view of seismic hazard in the United States. These studies are the result of a massive amount of
scientific research and debate that has occurred over the past six years. This academic work has formed the foundation of the
catastrophe model updates that will be introduced by vendor modeling firms in 2016-17, and ultimately will have a significant
impact on the models’ view of risk to Property and Workers’ Compensation portfolios.
Willis Re reviewed the results of these studies with the goal of drawing preliminary conclusions on what they mean for the
insurance industry and what impact they could have on catastrophe model loss estimates.
Five key conclusions from our review include:
1.
2.
3.
4.
5.
The greatest magnitude changes in the modeled seismic risk can be expected in
California, with significant but lesser degree changes in the Pacific Northwest
Overall, key return period loss estimates for portfolios concentrated in Southern
California are expected to decrease and in some parts of Northern California to increase
Tail Value-at-Risk (TVAR) measures of loss estimates are expected to change for almost
all regions: in particular, they are expected to increase in the Pacific Northwest for
residential lines and to decrease in the Eastern U.S. for large commercial lines
Moderate to minor change in risk for portfolios concentrated in the Central and Eastern
U.S., including the New Madrid region
Potential increase in earthquake risk in Las Vegas and a decrease in Salt Lake City
The following graph shows the ranges of change in seismic hazard between 2008 and 2014. The magnitude changes in seismic
hazard furnished in the chart shows wide ranges for each region and it varies by location within a region. The actual change
specific to an insurance portfolio is highly dependent on where the exposure is located and the concentration of exposure within
each region.
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Ranges of change in seismic hazard between 2008 and 2014
Willis Re can help you
As a first step towards proactively managing the impact of potential model changes, we have developed a technique to more
specifically assess the changes to your portfolio loss estimates based on the changes to the USGS NSHM. We encourage you to
contact us to explore what these changes mean to your portfolios.
Willis Re will continue to closely follow information related to U.S. Earthquake model upgrades in 2016-17. Over the next 12-18
months, we will provide additional updates on how the models are changing to prepare you for how they may affect your
company’s underwriting guidelines, capital requirements and portfolio management strategies.
In these communications, we will continue to offer balanced advice based on our range of skills, from model development to the
practical implementation of portfolio management and underwriting objectives. This report is designed to be a point of reference
as more information becomes available from the model vendors.
Introduction
The United States Geological Survey (USGS) released the latest version of its National Seismic Hazard Maps (NSHM) in October
2014. These maps, last updated in 2008, incorporate the best available science on fault slip rates, paleoseismic data, earthquake
catalogs and ground motion models; they define the latest scientific view of earthquake hazard at varying probability levels across
the United States. While the 2008 maps served as the basis for many public and private policies regarding earthquakes, including
seismic-design regulations for buildings, bridges, highways, railroads and other structures, the new maps will drive the earthquake
catastrophe risk model updates from AIR, EQECAT and RMS that the insurance industry uses for portfolio risk quantification.
NSHM are based on time-independent (long-term view) estimates of location and size of future earthquakes. The seismic hazard
maps show the shake hazard for buildings of different heights built on a uniform firm-rock site, at the peak horizontal ground
acceleration estimated for two return period levels.
Peak Ground Acceleration (PGA) describes how abrupt the ground motion is during an earthquake. As its name states, it refers to
the movement of the ground itself, not the movement felt by buildings. PGA is the most relevant indicator of ground shaking
intensity for a structure located at ground level, such as in-ground or surface pipelines and railways.
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The shaking experienced by a building depends on its height, which relates to its resonant frequency. Spectral Acceleration (SA) is
used to distinguish the hazard experienced by buildings of different height. SA is expressed at different periods, such as 0.2 sec or
1.0 sec. As a rule of thumb, we can approximate the building height by multiplying the time period by 10, so that 0.2 sec period ≈ 2
stories and 1.0 sec period ≈ 10 stories.
PGA and SA are commonly measured in units of gravity (g = 9.8 ms-2). Current vendor earthquake catastrophe models calculate
hazard in terms of spectral acceleration at different time periods, and relate building loss ratios to spectral acceleration
corresponding to specific building types.
USGS uses the following standard reference points:
• Spectral Acceleration
–
SA = 0.2 sec

Resonant frequency for a roughly 2-story building

Provides insight about effect on low-rise (1-3 story) exposures
–
SA = 1.0 sec

Resonant frequency for a roughly 10-story building

Provides insight about effect on high-rise (>8 story) exposures
• Return Time
–
10% probability of exceedance in 50 years

475 year return period

Provides insight about changes that might occur to modeled loss in the 250-500 year range
–
2% probability of exceedance in 50 years

2,475 year return period

Provides indications about the further tail of the curve (e.g., TVAR)
USGS hazard for
0.2 sec SA (2-story buildings)
USGS hazard for
1.0 sec SA (10-story buildings)
2,475 yr Hazard Return Period
(2% exceeding probability in 50 years)
475 yr Hazard Return Period
(10% exceeding probability in 50 years)
The following maps show national patterns of earthquake risk per the USGS update. Locations with negligible potential damage to
buildings due to ground shaking are not highlighted in these maps. The areas of greatest earthquake risk remain along the West
Coast (particularly California) and in the New Madrid region (at the intersection of Missouri, Illinois, Kentucky, Tennessee and
Arkansas). Other areas with notable earthquake hazard include Salt Lake City, Charleston and portions of New England.
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Although at this point we cannot precisely predict how modeled losses will change, we can anticipate potential impacts based on
how vendors might implement the USGS technical updates. Given the nature of the relationship between building damage and
ground shaking intensity, a 10% change in hazard can mean a 15-35% change in expected damage – so modeled loss may change by
a much larger amount than the USGS hazard change. Additionally, the seismic hazard changes presented in this report may be
offset or amplified by changes to other modeling components, such as building damage functions.
We now examine the effects for three geographic regions: California, Western U.S. (WUS) and Central and Eastern U.S. (CEUS).
California Region
Differences between the 2008 and 2014 NSHM maps show complex patterns across California with hazard varying locally. The
change in shake hazard is primarily due to updates to earthquake source parameters, ground motion models (GMM, a.k.a.
“attenuation equations”) and revised smoothed seismicity of background sources.
At this point, we cannot precisely predict how modeled losses will change. However, based on the technical updates implemented
by the USGS, our analysis indicates decreases for exposures in Southern California and increases in some parts of Northern
California. Statewide, we can expect decreases in risk estimates at lower return periods (100 to 250yr) and increases at high return
periods (1000 to 5000yr). Local hotspots of large risk increases may result from the changes to gridded background seismicity and
the new smoothing technique. Changes to the earthquake source parameters overshadow the smaller effects from changes to ground
motion models in some cases. Risk in the northwest California is expected to increase due to the revised assumptions for higher
earthquake rates and the addition of possible Mw ≥8.0 earthquakes on the southern portion of the Cascadia Subduction Zone (CSZ).
The change in the amount of shaking experienced by low-rise (1-3 story) and high-rise (>8 story) buildings at the 475 year and
2,475 year return periods are shown below. Cool colors indicate decreases in hazard, while warm colors indicate increases in
hazard. Only those areas where hazard is significant enough to result in building damage are shown.
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Key implications for catastrophe risk managers
•
•
•
Risk relativities between various regions, sub regions and internal rating territories may change significantly (e.g.,
Southern California to Northern California)
Business rules that are based on the distance to a fault, such as exposure aggregate thresholds, underwriting guidelines or
insurance rates, will be moderately affected by these changes
Tail view of risk to change significantly
Key scientific advancements driving the hazard changes
The USGS incorporated many new methods in its 2014 National Seismic Hazard Maps. The key technical updates are:
• Revised seismic source model based on the third version of the Uniform California Earthquake Rupture Forecast
(UCERF3)
• Revised fault parameters and earthquake recurrence rates for various faults
• Revised multi-segment rupture scenarios and earthquake recurrence rates; a revised smoothed seismicity model for
background sources
• Accounts for earthquakes that rupture multiple faults yielding larger magnitudes than those in the previous model
• Updated ground motion prediction equation and weights for active shallow crustal earthquakes, subduction zone-related
interface and intra-slab earthquakes
Time-dependent view
The national seismic hazard maps described in the prior section are based on a time-independent forecast, in which the probability
of each earthquake rupture is independent of the timing of all others. It is generally accepted, however, that a time-dependent
model provides a more accurate representation of the risk in California where most faults have been well studied. Time-dependent
models are based on the concept of stress renewal – where the probability of a fault rupture drops immediately after a large
earthquake releases tectonic stress on the fault and rises again as the stress is regenerated by continuous tectonic loading. In a
time-dependent earthquake forecast, the probabilities of a future event are conditioned on known previous earthquakes.
The Working Group on California Earthquake Probabilities (WGCEP) is the multi-disciplinary team of scientists and engineers
that develop time dependent earthquake forecasts for California. The time-dependent earthquake probabilities report for the third
Uniform California Earthquake Rupture Forecast (UCERF3) was released in April 2015. UCERF3 is an updated version of 2008
UCERF2.
The UCERF study describes the probability of an earthquake of various magnitudes occurring on various well known faults across
California. However, the UCERF study does not detail the likelihood of amount of shaking caused by these quakes (“seismic
hazard”). This is an important distinction between NSHM and UCERF, because even in areas with a low probability of a local fault
rupture, strong shaking and damage from distant powerful earthquakes can occur.
Key implications for catastrophe risk managers
•
•
•
•
•
•
There is near certainty (essentially 100% probability) for an Mw≥ 6.7 earthquake occurring in California in the next 30
years. There is a 48% and 7% probability for an Mw≥ 7.5 and Mw≥ 8.0 earthquake occurring in California in the next 30
years, respectively
The chance of an Mw≥7.5 earthquake occurring in Southern California (36% chance in 30 years) is higher than the chance
for a similar earthquake in Northern California (28% chance in 30 years)
Time-dependent probabilities for an Mw≥ 7.5 and Mw≥ 8.0 earthquake occurring in Southern California are 22% and 25%
higher than the time-independent probabilities, respectively
Time-dependent probabilities for an Mw≥ 7.5 and Mw≥ 8.0 earthquake occurring in Northern California are 1% and 14%
higher than the time-independent probabilities, respectively
The chance of an M≥6.7 earthquake in the Los Angeles area is 60% in the next 30 years, which is 14% higher than the
time-independent probability
The chance of an M≥6.7 earthquake in the San Francisco area is 72% in the next 30 years, which is 12% higher than the
time-independent probability
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Technical highlights of the 2015 UCERF time-dependent earthquake probabilities
The UCERF framework comprises a sequence of four model types: a fault model that gives the physical geometry of the larger
known faults, a deformation model that gives slip rates and aseismicity factors to each fault section, an earthquake rate model that
gives the long-term rate of all earthquakes of magnitude five or greater (Mw ≥ 5) throughout the region and a probability model
that gives a probability of occurrence for each earthquake during a specified future time interval. The latest versions of these
models are used in developing the time-independent earthquake rate model used in the 2014 national seismic hazard maps and the
UCERF time-dependent model for California. UCERF3 employs a new procedure for computing elastic-rebound based earthquake
probabilities and supports magnitude-dependent aperiodicity and epistemic uncertainties with a logic tree producing 5,760
different forecasts.
The probability of an Mw≥ 6.7 earthquake occurring on various main faults in the next 30 years is shown in the table below,
relative to the time-independent probability and the results from the last working group studies for comparison purposes. Timedependent probabilities for an Mw ≥ 6.7 earthquake to occur on the southern San Andreas Fault (near Los Angeles) and HaywardRodgers Creek (near Oakland) are 45% and 52% higher than the time-independent probabilities, respectively. The time-dependent
probability for an Mw ≥ 6.7 earthquake occurring on the northern San Andreas Fault (near San Francisco) is about 4% lower than
the time-independent probability.
The southern San Andreas Fault has the highest probability (53%) in California of generating at least one Mw≥6.7 earthquake in
the next 30 years. In northern California, the Hayward-Rodgers Creek Fault and the northern San Andreas Fault has almost the
same probability (33%) of generating at least one Mw≥6.7 earthquake in the next 30 years. An earthquake of this size can be a
significant loss event for the insurance industry, as it saw with the 1989 Loma Prieta earthquake (Mw=6.9, northern San Andreas)
and the 1994 Northridge earthquake (Mw=6.7, southern San Andreas).
Probability of an Mw≥ 6.7 earthquake occurring
on various main faults in the next 30 years
Time-independent
(Poisson) probability
Fault Name
Time-dependent mean
probability (min - max)
% Elevated above timeindependent model
2008 UCERF
2015 UCERF
2008 UCERF
2015 UCERF
2008 UCERF
2015 UCERF
San Andreas – South
(near Los Angles)
49.3%
36.2%
60.3%
(22.8% - 94.1%)
52.7%
(17.1% - 92.8%)
22.3%
44.5%
San Jacinto
(near San Bernardino)
31.4%
10.9%
32.1%
(14.0% - 55.3%)
9.3%
(0.3% - 35.1%)
2.2%
(16.7%)
Elsinore
(near Lake Elsinore)
13.0%
5.4%
11.8%
(5.3% - 25.2%)
5.5%
(1.1% - 16.6%)
(9.6%)
0.1%
Garlock (near Mojave)
5.9%
6.5%
6.2%
(3.1% - 12.6%)
8.3%
(0.2% - 36.6%)
6.0%
15.8%
Hayward-Rodgers Creek
(near Oakland)
23.9%
21.3%
31.9%
(12.2% - 68%)
32.3%
(13.7% - 54.0%)
33.5%
51.7%
San Andreas North
(new San Francisco)
24.2%
33.3%
21.2%
(6.4% - 40.3%)
33.0%
(0.7% - 73%)
(12.5%)
(4.0%)
Calaveras
(near San Jose)
7.8%
17.5%
7.7%
(1.5% - 22.2%)
25.5%
(10.3% - 54.4%)
(1.9%)
45.6%
Earthquake probabilities for many parts of California are similar to those in previous studies. However, the new probabilities for
the San Jacinto Fault in southern California are about one third of the previous predictions, and probabilities for the Calaveras
fault in northern California increased three-fold. Earthquake probabilities statewide are almost the same between time-dependent
and time-independent (Poisson) models (time-independent rates are 2% to 3% higher for Mw7.5). At a state level, these differences
are not significant relative to the overall modeling uncertainties. The difference between the time-dependent and the timeindependent views of risk can be significant for portfolios with exposure concentrations in specific regions and/or near specific
faults.
Page 6 of 10
Western United States
The change in shake hazard in the Pacific Northwest (PNW) and Inter-Mountain West (IMW) is largely due to the updated ground
motion models for crustal and Cascadia Subduction Zone (CSZ) earthquakes, revised Cascadia Subduction Zone source model,
updated crustal fault models, revised source parameters and seismicity rates.
Specifically, the hazard is increasing along Northwest Washington and Southwest Oregon due to the changes to ground motion
models and the CSZ source model. Higher earthquake rate assumption and addition of possible Mw ≥8.0 earthquakes to the
southern portion of the CSZ source is increasing the shake risk in Southwest Oregon and Northwest California. The revised
subduction zone ground motion models attenuate faster with distance from the source. Therefore, shake hazard is decreasing for
locations that are away from the PNW coast.
Changes to smoothed gridded seismicity and the earthquake catalog are creating hotspots of increased risk in the IMW. Risk
around Salt Lake City is changing, predominantly due to a decrease in event rates for the Great Salt Lake City fault. Hazard is
decreasing near Reno, Nevada and increasing around Las Vegas, Nevada. Earthquake catalog changes for the IMW will impact
hazard estimates at high return periods more than at low return periods. Changes to ground motion models and source parameters
are causing complex pattern of changes for many locations.
The maps below show the change in the amount of shaking experienced by low-rise (1-3 story) and high-rise (>8 story) buildings at
the 475 year and 2,475 year return periods. Only those areas where hazard is significant enough to result in damage are shown.
Key implications for catastrophe risk managers
•
•
•
•
TVAR measures for low-rise residential and small commercial portfolios is expected to increase in PNW
Moderate reductions (-10% or more) to key return period loss estimates can be expected for large commercial portfolios
concentrated in the Pacific Northwest
Earthquake risk around Tacoma, WA is expected to increase for all return periods
Risk to portfolios concentrated in Las Vegas, NV is expected to increase and in Salt Lake City, UT to decrease at key return
periods
Page 7 of 10
Key scientific advancements driving the hazard changes
The USGS incorporated many new methods in its 2014 National Seismic Hazard Maps. The key technical updates are:
1. Revised fault parameters such as magnitude-frequency distributions, maximum magnitude, slip rate and dip uncertainties
2. Updated earthquake catalog and treatment of magnitude uncertainty in rate calculations
3. New models for Cascadia earthquake-rupture geometries and rates based on onshore and offshore studies
4. Updated model for deep (intra-slab) earthquakes along the coasts of Oregon and Washington, including a new depth
distribution for intra-slab earthquakes
5. A new Tacoma fault source and revised South Whidbey Island fault source in Washington
6. An updated Wasatch fault zone under Salt Lake City and reduced event rates for the Great Salt Lake fault
7. New and revised ground motion models for active crustal earthquakes and Cascadia Subduction Zone (interface and intraslab) earthquakes
Central and Eastern United States
The change in shake hazard in the Central and Eastern United States (CEUS) is primarily due to the updated ground motion
models, earthquake catalog updates and revised sources parameters for fault and grid based earthquake sources. In particular, new
ground motion models decay more quickly with distance than the previous versions.
Updates to the New Madrid Seismic Zone (NMSZ) decrease shake hazard at higher return periods and increase hazard at low
return periods. Updates specific to the northern segment of NMSZ are decreasing hazard for locations closer to the northern
segment.
Increases to shake hazard near Oklahoma City and Charleston are primarily due to the changes to the source parameters of Meers
Fault and Charleston seismic region respectively. Changes to gridded seismicity and the earthquake catalog are creating hot and
cold spots of hazard changes in the CEUS as well. Along the east coast, large commercial exposures may experience relatively large
decreases compared to small residential due to the revised ground motion models.
Page 8 of 10
Key implications for catastrophe risk managers
•
•
•
•
The tail risk view of the New Madrid region is expected to change
Potential increase to loss estimates under 500 year return periods for exposures close to the New Madrid fault
Risk relativities between various territories / sub-regions within the CEUS may change
Business rules such as exposure aggregate thresholds, underwriting guidelines and insurance rates that are based on the
distance to a fault may need to be revised due to the potential changes to shake hazard
Key scientific advancements driving the hazard changes
1.
2.
3.
4.
5.
6.
7.
A new earthquake catalog based on moment magnitude (Mw) replacing the body wave magnitude (mb)-based catalog
used in earlier versions of NSHM
Revised catalog completeness and magnitude uncertainty estimates
Revised maximum magnitude (Mmax) distribution and seismicity smoothing algorithms for background earthquake
sources
A new four-zone model based on the Central and Eastern United States seismic source characterization for nuclear
facilities project (CEUS–SSCn, 2012) delineating the craton, Paleozoic margin, Mesozoic margin and Gulf Coast
Revised New Madrid source model for fault geometry, large earthquake recurrence rates, and alternative magnitudes
range from Mw6.6 to 8.0 (average weighted magnitude Mw=7.5 is same between new and old versions)
Revised Charleston (South Carolina), Wabash (Indiana and Illinois), Charlevoix (Eastern Canada), Commerce lineament
(Arkansas – Indiana), East Rift Margin (western Tennessee) and Marianna (east and central Arkansas) seismic sources
New and revised ground motion models, and revised model weights, supported by preliminary Electric Power Research
Institute (EPRI) ground motion study
How might vendor model changes and USGS differ?
The 2014 USGS NSHM and UCERF3 are comprehensive scientific studies and as such, have wide scientific and catastrophe
modeler adoption. They will drive updates to AIR, EQECAT and RMS U.S. Earthquake catastrophe risk models. However, the
modeling companies cannot directly take the end product of the NSHM and UCERF and simply implement it into their models.
These NSHM and UCERF studies must first be translated into an event-based catastrophe model that the insurance industry uses
for probabilistic loss estimation. Although all modeling firms may start their model development activities from a similar place,
their implementation of these studies will result in different answers to the same question.
The USGS maps form the basis of building code provisions. As such, the maps assume uniform soil conditions. When applying the
code, the design engineer evaluates the site-specific soil conditions to assess the potential amplifying effect of the local soils on
ground shaking intensity. Similarly, vendor models include local soil conditions in estimating the hazard at a given site.
The changes to vendor models will also likely be broader in scope including factors such as site-specific soil amplification, basin
effects, fire following, loss amplification, time dependency, engineering refinements, etc. Specifically, modeling companies can
incorporate lessons learned from the recent 2010 Tohoku, Japan, 2011 Christchurch, New Zealand and 2012 Maule, Chile
earthquakes. Vendors will also selectively differ from the USGS and from each other in their scientific assumptions. As the vendors
recalibrate their models, changes to damage functions may offset or amplify changes to the seismic hazard.
Lastly, vendors may choose to incorporate some new studies not released in time to be included in the USGS maps. For all these
reasons, changes to the USGS seismic hazard maps cannot exactly predict the changes in vendor model loss results.
Conclusions
Model changes will be significant for many portfolios, and the patterns of change will be complex and multifaceted. These changes
are expected to impact underwriting guidelines, capital requirements and portfolio management strategies. In addition, these
changes will affect the Workers’ Compensation and Fire Following Earthquake (FFEQ) model loss estimates as well as shake loss
estimates.
Changes to portfolio loss estimates will be highly influenced by the updates to fault parameters, earthquake catalogs, ground
motion models, soil amplification updates and potential updates to damage functions.
Page 9 of 10
References
2007 Working Group on California Earthquake Probabilities, 2008, The Uniform California Earthquake Rupture Forecast, Version
2 (UCERF 2): U.S. Geological Survey Open-File Report 2007-1437 and California Geological Survey Special Report 203
Field, E. H., G. P. Biasi, P. Bird, T. E. Dawson, K. R. Felzer, D. D. Jackson, K. M. Johnson, T. H. Jordan, C. Madden, A. J. Michael,
K. R. Milner, M. T. Page, T. Parsons, P. M. Powers, B. E. Shaw, W. R. Thatcher, R. J. Weldon, and Y. Zeng (2013). Uniform
California Earthquake Rupture Forecast, version 3 (UCERF3): The time-independent model, U.S. Geological Survey Open-File
Report 2013–1165 and California Geological Survey. Special Report 228
Field, E. H., G. P. Biasi, P. Bird, T. E. Dawson, K. R. Felzer, D. D. Jackson, K. M. Johnson, T. H. Jordan, C. Madden, A. J. Michael,
K. R. Milner, M. T. Page, T. Parsons, P. M. Powers, B. E. Shaw, W. R. Thatcher, R. J. Weldon, and Y. Zeng (2015). Long-Term
Time-Dependent Probabilities for the Third Uniform California Earthquake Rupture Forecast (UCERF3), Bulletin of the
Seismological Society of America, Vol. 105, No. 2A, pp. –, April 2015, doi: 10.1785/0120140093
Petersen, Mark D., Frankel, Arthur D., Harmsen, Stephen C., Mueller, Charles S., Haller, Kathleen M.,Wheeler, Russell L., Wesson,
Robert L., Zeng, Yuehua, Boyd, Oliver S., Perkins, David M., Luco, Nicolas,Field, Edward H., Wills, Chris J., and Rukstales,
Kenneth S., 2008, Documentation for the 2008 Update of the United States National Seismic Hazard Maps: U.S. Geological
Survey Open-File Report 2008–1128, 61 p.
Petersen, M.D., Moschetti, M.P., Powers, P.M., Mueller, C.S., Haller, K.M., Frankel, A.D., Zeng, Yuehua, Rezaeian, Sanaz,
Harmsen, S.C., Boyd, O.S., Field, Ned, Chen, Rui, Rukstales, K.S., Luco, Nico, Wheeler, R.L., Williams, R.A., and Olsen, A.H.,
2014, Documentation for the 2014 update of the United States national seismic hazard maps: U.S. Geological Survey Open-File
Report 2014–1091, 243 p., http://dx.doi.org/10.333/ofr20141091.
Glossary
Attenuation: decrease in size, or amplitude, of seismic waves as they move away from the earthquake source.
Earthquake: a term used to describe both sudden slip on a fault, and the resulting ground shaking and radiated seismic energy
caused by the slip, or by volcanic or magmatic activity, or other sudden stress changes in the earth.
Epicenter: the point on the earth’s surface vertically above the hypocenter (or focus), point in the earth crust where a seismic
rupture begins.
Fault: a fracture along which the blocks of crust on either side have moved relative to one another parallel to the fracture.
Ground motion: the movement of the earth's surface from earthquakes or explosions. Ground motion is produced by waves that
are generated by sudden slip on a fault or sudden pressure at the explosive source and travel through the earth and along its
surface.
Intensity: a number (written as a Roman numeral) describing the severity of an earthquake in terms of its effects on the earth’s
surface and the built environment. Several scales exist, but Modified Mercalli (MMI) scale is most commonly used in the United
States. There are many intensities for an earthquake, depending on where you are – unlike the magnitude, which is one number
for each earthquake.
Interplate: pertains to processes between the earth’s crustal plates.
Intraplate: pertains to processes within the plates
Magnitude: depicts the relative size of an earthquake. Magnitude is based on measurement of the maximum motion recorded by a
seismograph. Several scales have been defined, but the most commonly used are (a) local magnitude (ML), commonly referred
to as “Richter magnitude,” (b) surface-wave magnitude (Ms), (c) body-wave magnitude (Mb), and (d) moment magnitude
(Mw). The moment magnitude (Mw) scale, based on the concept of seismic moment, is uniformly applicable to all sizes of
earthquakes but is more difficult to compute than the other types.
Peak Ground Acceleration (PGA): the largest acceleration experienced by the ground at a particular point away from the epicenter.
Spectral Acceleration (SA): approximately what is experienced by a building, as modeled by a particle on a massless vertical rod
having the same natural period of vibration as the building.
Subduction zone: the place where two plates come together, one riding over the other.
Tsunami: a sea wave of local or distant origin that results from large-scale seafloor displacements associated with large
earthquakes, major submarine slides, or exploding volcanic islands.
Contact us
Prasad Gunturi
Senior Vice President
Willis Re Inc.
7760 France Avenue South
Suite 450
Minneapolis, MN 55435
Phone: +1 952 841 6638
Email: [email protected]
The contents herein are provided for informational purposes only and do not constitute and should not be construed as professional advice. Any and all examples used
herein are for illustrative purposes only, are purely hypothetical in nature, and offered merely to describe concepts or ideas. They are not offered as solutions to produce
specific results and are not to be relied upon. The reader is cautioned to consult independent professional advisors of his/her choice and formulate independent
conclusions and opinions regarding the subject matter discussed herein. Willis is not responsible for the accuracy or completeness of the contents herein and expressly
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