What are the Plausible Earthquake
Hazards at your Home or Business?

Your home or work place response to earthquake shaking occurs over the time of few seconds. The
shaking differs from one building to another because of the variations in fault slippage, rock characteristics
through which the waves pass, and the soil properties at the building site. The seismic performance of a
building at a site also differs because of the variations in structural systems, building materials, number of
stories, size, configuration, quality of construction, and age.

Background

Earthquake surface waves develop inertial forces within a building. Newton's Second Law of Motion
states that the inertial force equals the mass multiplied by the acceleration. The mass is equivalent
to the weight of the building, which explains why a wood frame house tends to perform better in
earthquakes than other heavier construction materials such as masonry, concrete, or steel.

Period is the time in seconds or fractions of a second that is needed to complete one cycle of a
seismic wave. During an earthquake, the ground vibrates at its natural period in which rock or stiff
soils will experience short period vibration while very soft soils may have a period of up to 2
seconds.

Frequency is the inverse of the period and is measured in “Hertz”. It is the number of seismic wave
cycles that will occur in a second.

The ground acceleration, velocity, and displacement vary with the frequency of the earthquake wave
motion. High–frequency waves (higher than 10 hertz) tend to have high amplitudes of acceleration
but small amplitudes of velocity and displacement. The reverse is true for low–frequency waves.

A soft soil layer may result in amplification factor from 1.5 to 6 times the shaking at the base rock
according to depth of soil layers above the base rock and the properties of each soil layer. This
amplification is most pronounced at longer periods, and may not be so significant at short periods
as shown in Figure 1.

The fundamental period of low-to-medium rise residential buildings and office buildings equals
0.1–1.2 seconds for structures not exceeding 12 stories. As a rule of thumb stated in various
building codes, the fundamental period can be determined as the number of stories divided by 10.
This range is well within that of the natural period of ground vibration. Therefore, resonance is quite
possible causing the structure to encounter accelerations of perhaps 1g when the ground is only
vibrating with accelerations of 0.2g, where g is the acceleration due to gravity.

The Engineering Approach

The approach described below is simplified for home owners and business owners to understand. Only
licensed professional or structural engineers have the technical knowledge of seismic design and/or
structural analysis.

Structural engineers determine the spectral acceleration parameters utilizing the hazard maps
developed by the United States Geological Survey (USGS). The USGS maps are based on the
reference Site Class B, described as “rock”.

Structural engineers also use the seismic site class determined by a geotechnical engineer based
on the soil properties of the site of concern. The site shall be classified as Site Class A (hard rock),
B (rock), C (very dense soil and soft rock), D (stiff soil), E (soft clay soil), or F (soils require site
response analysis, such as liquefiable soils).

It is recommended by various building codes that: “Where the soil properties are not known in
sufficient detail to determine the site class, Site Class D shall be used.”

Structural engineers determine the site coefficients that represent the soil amplification at the site of
concern. These coefficients are tabulated in building codes and widely range from 0.80 for Site
Class A to 3.5 for Site Class E.

The design response spectrum for the maximum credible earthquake can be developed as shown
in Figure 1, for 4 zip codes in California and 4 zip codes in the Midwest. These zip codes actually
represent the location of their City Halls. The spectrum shall include 2 design spectral response
acceleration parameters at 0.2 second and at 1 second periods which are used in determining the
lateral forces on the structure.

Structural engineers determine the earthquake lateral forces acting on a structure which is a
percentage of its dead weight according to:

Earthquake-resisting structural system (steel or concrete moment-resisting frames,
concentrically or eccentrically braced frames, concrete or masonry shear walls, etc.)

The design spectral response acceleration parameters at 0.2 second and at 1 second periods.

The fundamental period of the structure based mainly on the height of the structure.

Occupancy importance factor (values of 1.0 for ordinary structures, 1.25 for schools and certain
health care facilities, or 1.5 for essential facilities such as police and fire stations, and
hospitals).

Structural engineers develop a computer model that represents the main structural system,
distribute the total earthquake lateral force on various levels of the structure, then analyze and
design it using available commercial structural analysis software.
Earthquake Awareness and Preparedness
An emerging feature in earthquake
engineering for structures within 20 miles of
an active fault, namely near-fault effects, has
been studied nationwide. For near-fault
critical structures such as bridges and
essential facilities, it shall be designed on
the combined effect of the two orthogonal
motions.

Figure 2 illustrates the spectral accelerations
developed from synthetic rock motions within
the New Madrid Seismic Zone (NMSZ) as
compared to the USGS Site Class B
spectrum. Synthetic motions shall be
generated because of the lack of strong
motion records of major earthquakes in New
Madrid. In Figure 2, two components are
shown, one is parallel and the other is
perpendicular to the southwestern segment
of the New Madrid faults.
Figure 1 – Spectral Accelerations at Different Locations in California and the Midwest
Figure 2 – Spectral Accelerations from Synthetic
Rock Motions in the New Madrid Seismic Zone
To Conclude:

As mentioned above, the engineering approach required the expertise of a structural engineer and may be
a geotechnical engineer, which would be costly. However, one can conclude from the above discussions
and from Figures 1 and 2 that the soil amplification would not be significant for residential buildings or
office buildings of less than 3 stories as their fundamental period is less than 0.3 second. The only
exception is for structures built on Site Class F where liquefaction or land sliding is susceptible.
Simple Approach for Estimating the
Hazards utilizing the USGS Maps

Go to the 2008 interactive national seismic  
hazard maps developed by the USGS.

Select the hazard map for the “Peak Horizontal
Acceleration (%g) with 2% Probability of
Exceedance in 50 Years” as shown on Figure 3.

Click on “Get Hazard by Lat / Lon” from the “hazard
values” right icon in the “Tools” menu. The
alternative “Get Hazard by Zip Code” of the left
icon would definitely be an easier option but the
function is not yet enabled. The latitude and
longitude of your home or business can be easily
obtained from internet search engines by entering
the name of the closest landmark building in your
neighborhood (e.g. school, government building,
etc).
The value of the peak horizontal acceleration for your specific latitude and longitude will pop up and
“black star” will also be shown on the overall map where you can zoom in to make sure that the
input data for your home or business location is correct (zooming capabilities can also show state
counties in addition to state and county borders).

Figure 4 illustrates the zoomed-in contour hazard maps for the peak accelerations at the location of
City Halls of Los Angeles (Lat: 34.0537 & Lon: -118.2430), San Francisco (Lat: 37.7793 & Lon:
-122.4165), and New Madrid (Lat: 36.5864 & Lon: -89.5278). Their specific values are 92.652%,
74.256%, and 186.753%, respectively, of the acceleration due to gravity, g.

Generally speaking, peak accelerations above 50%g represented in the USGS maps by contours of
orange, red, brown or other brown shades shall consider all retrofitting measures (foundation,
cripple walls, soft story, wall connections, and chimney) as soon as possible to withstand forces
generated by moderate-to-large earthquakes and keep up with current seismic design codes.

Peak accelerations ranging between 30%g – 50%g represented in the USGS maps by contours of
yellow or other yellow shades shall consider retrofitting measures one step at a time in the order
shown above.

You may select the map for the “Peak Horizontal Acceleration (%g) with 10% Probability of
Exceedance in 50 Years” if you know that you will relocate within the next few years.
Figure 3 - Peak Horizontal Acceleration
Contours with 2% Probability of Exceedance
in 50 Years
Figure 4 – Zoomed-In Contour Maps for the Peak Accelerations at various Locations
(source: USGS web site)
  City Hall at Los Angeles                    City Hall at San Francisco                    City Hall at New Madrid
Click now to get PREPARED!