GLOSSARY OF TERMS
Geology
SUBDUCTION ZONE
A boundary where two tectonic plates meet and one sinks below the other. The Cascadia Subduction Zone is expected to be the source of the next megathrust quake.
Thrust Fault
A break in the Earth’s crust that occurs when one portion of crust slips over another due to the formation of new rock. The Seattle Fault Zone is a series of east-west thrust faults.
Liquefaction
A phenomenon experienced by loosely packed soil during an earthquake, where the ground will lose its stability and move like water. Zones of potential liquefaction are important to identify when assessing seismic risks.
Deep Quake
An earthquake which originates more than 40 miles beneath the earth’s surface. Deep quakes may be less severe than shallow quakes or megathrust quakes, because energy has dissipated by the time it reaches the surface. However, they can still cause significant damage, as in the 2001 Nisqually Earthquake.
Shallow Quake
An earthquake which originates within 40 miles of the earth’s surface. Shallow quakes can be more damaging than deep quakes.
Megathrust Quake
A powerful earthquake that occurs at a subduction zone, where one tectonic plate is forced under another.
RISKS
UNREINFORCED MASONRY BUILDING (URM)
A building with load-bearing masonry (brick, stone, concrete block, tile, adobe) walls that are not braced by reinforcing material. Such structures may be brittle and are at risk of failure during earthquakes.
UNREINFORCED CONCRETE BUILDING
A building in which the primary structure is concrete with no embedded steel. Unreinforced concrete is susceptible to cracking under stress. Concrete buildings can be much larger than masonry buildings, and as such pose an especially high risk when unreinforced.
SOFT STORY
A building level that is less than 70% as stiff as the levels above it. Common in multi-story buildings with a garage or other open space at ground level. Soft story buildings are prone to collapse in moderate to severe earthquakes.
DUCTILITY/BRITTLENESS
Ductility describes a material’s ability to experience deformation without rupturing. A ductile material may undergo significant stress while maintaining some amount of strength. A material with low ductility could be described as brittle.
SHEAR STRESS
Cracks or buckling in a wall may indicate structural failure due to shear stress. Shear stress occurs along the plane of a wall when forces push or pull it in opposing directions. It is, on a building scale, similar to the geologic phenomenon of tectonic plates shifting and rupturing along the surface of the earth.
ROOF-TO-WALL CONNECTION
Inadequate connections or anchorage between the roof and wall of a building, common in older construction, may result in these components detaching under seismic forces, leading to building collapse.
SOlutions
PARAPET BRACING
Parapets and building appendages (like signs or even gargoyles) may be braced or tied back to the roof structure, to eliminate fall hazard in a seismic event.
CARBON-FIBER REINFORCED POLYMER (CFRP)
A strong and lightweight plastic made of a resin reinforced with carbon fibers. CFRP sheets are a low-profile way to add strength to existing structures.
NITINOL
A superelastic alloy made of nickel and titanium. It can return to its previously held shape and retain most of its strength following stretching or deformation. Currently used in medical applications, it can also be a structural material to maintain integrity in historic buildings without large braced frames.
Shear Wall
A structural component for resisting lateral forces. A typical low-cost shear wall includes plywood panels, though steel panels may also be used.
BASE ISOLATION
A seismic protection system that uses isolators (flexible pads in the building’s substructure) to allow a building to move independently of the shaking ground. It may be understood as a sort of large-scale shock absorber, and is one of the more successful, if more costly, techniques of seismic retrofitting.
SHOTCRETE
Sprayed concrete applied on a wire mesh, which may be used on the interior face of existing walls to strengthen them. Interior application of shotcrete is especially useful where changes are not permitted to the exterior of a historic building.
STRONGBACKING
Also known as strongback bracing. Secondary structural support with the purpose of reducing vibrations and deflection.