SEISMIC EVALUATIONS

The Southern California area is located within a tectonically active portion of the earth's crust and has had a long history of both small and devastating earthquakes. While it is not be possible to completely insulate your site and structures from the potential of damage due to future earthquakes, there are certain precautions which can be taken to reduce the potential damage.

Escrow Seismic Evaluations

As part of a typical escrow geology evaluation, the seismic conditions effecting the site and structures are reviewed by the geologist. There are several tools which are used in making this evaluation. The geologist's primary tool in seismic evaluation is research of available published documents and maps. Information concerning potential seismic hazards can also be obtained from various websites, such as the Southern California Earthquake Center. The first determination is whether or not the site is located within an Alquist-Priolo Special Studies Zone. The Alquist-Priolo Act was passed in 1971 and authorized the State of California to determine active faults which are capable of producing an earthquake. Zones have been established around active faults which require special seismic studies to be performed prior to any construction. These zones also represent a higher than average risk of structural damage during an earthquake generated along the active fault within the zone. The California Real Estate disclosure laws now require the notification to the buyer if the property is located within a special studies zone.

In 1991, the State of California empowered the Department of Conservation, Division of Mines and Geology, to produce a set of maps covering portions of the State of California delineating areas which are considered to be susceptible to seismically-induced liquefaction or seismically-induced slope instability. These maps are reviewed in preparation of an escrow geology report and their applicability to the site is evaluated and discussed.

Once all of the published and visual seismic information pertaining to the site has been reviewed, an evaluation can be made regarding future seismic risks and methods which may be available to reduce future seismic damage.

Steps that the homeowner can take to reduce seismic damage include the following:

Seismic Bolting - The most important aspect of reducing potential seismic damage to a structure is to ensure that the residence is properly secured to the foundation system.  The purpose of seismic bolting is to prevent the structure's framing system from shifting off the foundation system during strong seismic shaking.  Residences constructed prior to 1935 often are not seismically bolted.  After the devastating 1933 Long Beach Earthquake, seismic bolting of residential structures was required by the Department of Building and Safety.  Over the years, changes have been made to seismic bolting requirements.  Many of the homes constructed in the 1940's, 1950's and 1960's have been bolted, however the bolting does not meet current standards.  Seismic bolting typically consists of anchor bolts which are embedded into the concrete foundation and extend through the overlying wood sill plate.  The residence framing system is secured to the foundation by tightening a threaded nut on the anchor bolt.  The size and spacing of anchor bolts has has been increased over the years when large scale earthquakes have shown that earlier designs were not enough.  While the geologist may be able to verify the presence of seismic bolting, evaluating its effectiveness and/or conformance with current regulations will also require consultation with a structural engineer or foundation contractor. Voluntary upgrading of the seismic bolting can be performed.  The process is commonly referred to as seismic retorfitting. 
 

Grade Beam Connections - Many residences in the Los Angeles area are constructed over steep descending hillsides.  Utilization of a caisson or friction pile and grade beam system is commonly employed to support hillside residences.  Homes built in the 1950's. 1960's and 1970's often employed sloping grade beams.  The wood framing on the residence would be bolted to the sloping grade beam.  This condition has been shown to be a potential seismic hazard.  Today, grade beams are required to be stepped down the slope, resulting in a level surface which supports the wood framing of the residence above.  Structural reinforcement may be necessary for structures which employ sloping grade beams.  Seismic connections between the grade beams and residence framing system have also changed over the years.  Voluntary upgrading to current seismic standards is possible. Grade beam connection evaluations are performed by licensed structural engineers.
 

Shear Resistance - Design of some hillside residences involves the use of cripple walls.  Cripple walls are wood-framed structural elements which are constructed between the bottom of the wood flooring and the top of the concrete foundation.  Unbraced cripple walls represent a structural weakness which respected to shear forces during and earthquake.  Severe damage can result if failure of the cripple wall system occurs.  To correct this condition, cripple walls can now be reinforced, typically by installing plywood sheeting.  Cripple wall evaluations and reinforcement must be performed under the direction of a structural engineer or foundation contractor and must be completed in accordance with building code regulations.
 

Structural Inspection - Having a qualified structural engineer inspect the site structures is a good way of identifying potential weaknesses in the structure which could result in damage in future earthquakes.  Seismic codes have changed significantly over the years and older structures may be at greater risk.  Architectural design in some older structures often results in seismic weakness.  Identifying these potential weaknesses is important to reduce the risk of future damage.  The structural engineer may also be able to provide advice on ways of improving existing conditions.
 

Unreinforced Masonry Structures - Many of the hillside properties I have inspected have unreinforced masonry structures which include brick retaining walls and walkways, stone and mortar walkways and retaining walls and unreinforced concrete walkways and retaining walls.  Unreinforced structures lack steel bars which are used to strengthen the construction material.  Unreinforced structures are more susceptible to damage and failure during strong seismic shaking.  Failure of these structures can also represent a safety concern if the unreinforced structure is part of the residence construction or if failure of structure can impact inhabited areas of the site.  To reduce potential damage, unreinforced masonry structures on the site should be replaced with properly designed and reinforced structures.   
 

Construction Seismic Evaluations

The seismic evaluation prepared for construction purposes is similar to the procedures described above for an escrow geology report with regards to research of available published information. In addition to office work, certain incorporated cities also require subsurface exploration to positively determine if any active faults are located on the subject property and also to evaluate the potential for liquefaction.

• Active Faults

A fault trench is sometimes necessary to determine if an active fault is suspected to be present on the subject property based upon review of published maps. A fault trench is excavated with the use of a backhoe or larger equipment and is sufficiently deep to penetrate any recent sediments. The trench is excavated perpendicular to the suspected trace of regional active faults. The fault trench is carefully logged by the geologist for any signs of recent seismic activity.  If active faulting is determined to be present on the site, a safe construction setback from the fault trace must be recommended by the geologist.

• Liquefaction

Liquefaction is a process which occurs within fine grained sandy sediments in an area which has a relatively high groundwater level (within the upper 50 feet). With a strong enough seismic event, pressure within the water table is elevated to a point where grain-to-grain contact of the soil is temporarily lost, resulting in an inability of the soil to support any structure. During liquefaction, structures can literally sink into the ground several inches to several feet, depending on the thickness of liquefiable materials and the strength and duration of the earthquake. Liquefaction can result in severe or complete structural damage to buildings and is therefore considered a great risk. Identifying the potential for liquefaction is required for construction of new residences, second story additions, or significant alterations of existing structures. The type of liquefaction study necessary to satisfy the local Department of Building and Safety will depend upon the scope of proposed work and whether or not the property is located within an official designated liquefaction zone.  If the property is located within an official Seismic Hazard Zone, a detailed liquefaction analysis is required by the State of California for residential structures which exceed two stories.  A two story residence with a basement will require a detailed study.  A detailed liquefaction study is coordinated with the services of an independent soils engineer and consists of drilling a boring on the site to a depth of at least 50 feet. Samples of the earth materials are collected from the boring and standard penetration blow counts are recorded to be utilized in the computer liquefaction analyses. Even if no groundwater is encountered, the historic high groundwater level must be researched to ensure that liquefaction is not possible. If liquefaction is determined by the geologist and independent soils engineer to be a potential hazard, design recommendations are provided to the structural engineer to mitigate the problem zone or layer.

In addition to the field exploration, a more detailed seismic evaluation is typically required for the construction of schools, government buildings and large commercial buildings. Evaluation of maximum probable and credible earthquakes and anticipated site accelerations are typically provided.

• Seismically-Induced Differential Settlement

In areas which are underlain by unconsolidated sediments, such as alluvium or fill, strong seismic shaking can cause differential settlement to occur.  When non-homogeneous sediments are present  in the near surface region, seismic shaking can cause varying degrees of compression of the sediments, resulting in areas of differential settlement.  Differential seismic settlement can be very damaging to site structures.  A quantitative analysis of a site's susceptibility to differential settlement requires excavation of a deep boring, sampling of the earth materials and engineering analysis coordinated with an independent soils engineer.