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Recent Earthquakes
Earthquake Real-Time Feed Courtesy of the
U.S. Geological Survey

Hazards & Risk Assessment

Legg Geophysical has accumulated more than 30 years experience in assessing Effects and Consequences for several types of Natural and Human-Caused Hazards. Expertise gained in earthquake and tsunami research over the past three decades provide the requisite background to understand and assist clients in evaluation of potential losses from these catastrophic events.

We have completed several thousand Seismic Risk Assessments required by lending and government agencies which estimate the direct (and sometimes indirect) losses from earthquakes that may affect commercial and government properties. Hazard assessments for major facilities and infrastructure, both existing and planned for construction, have been performed to investigate all forms of hazards relevant to earthquake (shaking, ground rupture, liquefaction, landslide, inundation) and for other geological hazards such as lava flow inundation on the Island of Hawaii. Human-Caused hazards and risk assessments performed by Legg Geophysical have focused on Blast effects, and in particular, atmospheric propagation of blast waves and sonic boom that may damage property or injure humans exposed to the threat. We have also conducted HAZOP studies for investigation of hazards related to release of toxic materials (often through earthquake events) or rupture of underground pipelines. Vibration Hazards due to human activities such as construction activities, freeway traffic, seismic exploration, blasting, or drilling have been investigated for sensitive facilities (hospitals), commercial properties, and residential neighborhoods.

Earthquake Risk Assessment

Probable Maximum Loss

Risk = Hazard * Vulnerability * Exposure

Risk: Probable Maximum Loss (PML) provides a measure of expected losses from future earthquakes at a site and typically is based on direct structural damage. Contents loss and business interruption costs may also be estimated as needed.

Hazard is nominally based on shaking levels published in the National Seismic Hazard Maps, but site-specific analyses are provided for more detailed studies. Secondary hazards including liquefaction, landslide, and fault rupture ground failures are included to represent a more complete hazard estimate. The nominal probability considered is the 10% chance of exceedance for a 50-year building exposure - this is equivalent to a 475-year average recurrence event. For critical facilities, more conservative risk may be considered. Earthquake response spectra and time-histories may be provided for large structures and critical facilities.

Vulnerability is estimated based on fragility curves or other data based on earthquake experience (empirical data). In more detailed analyses, collaboration with structural engineers enables development of more site-specific vulnerability models.

Exposure data are provided by the client or by a seismic walkdown at the site with our collaborating structural engineer. Available building design plans are important to provide more accurate estimation of the structural configuration and earthquake vulnerability.

Typical Level 1 Seismic Risk Assessments (PML) have been provided for clients with commercial properties including office buildings, multifamily residential properties, commercial retail and manufacturing buildings. These evaluations meet the requirements for HUD, Fannie Mae and Freddie Mac, and follow the FEMA and ASCE standards for Seismic Evaluation of Buildings.

Earthquake Design Criteria for New Construction

Response Spectra
Probabilistic Seismic Hazard Assessment

Explosive Blast

Distant Focusing of Overpressure - Atmospheric conditions including inversion layers and wind shear can focus the acoustic waves from explosive blast at distant locations from the source resulting in structural damage and casualties. High overpressures from sonic boom can also be focused due to flight path and atmospheric refraction effects. Typical damage includes broken windows that may injure occupants if exposed to impact from flying shards. Dr. Legg produced software to calculate the focusing effects of blast waves from high-order detonations at rocket launch sites.

Business collaborators: Acta, Inc.

Other Natural Hazards

Legg Geophysical has a broad background in earth sciences including geology, geophysics, oceanography and meteorology that provide requisite expertise to investigate a wide variety of natural hazards. Historical research of extreme events provides important background data to evaluate the potential for future disasters. Meteorological hazards include tornado, hurricane wind and storm surge, and riverine flooding may be evaluated.

Geological hazards include landslide, expansive soil, liquefaction, ground settlement, shallow gas.

For information regarding Legg Geophysical's experience with tsunami hazards, click here.

Man-Made Hazards

Vibrations - Blasting, traffic, construction activities, drilling and seismic exploration induce ground vibrations that may be damaging to nearby structures or disturbing to people in the area. Legg Geophysical has conducted vibration studies to evaluate these hazards.