What Will a Large Earthquake be Like?

Tom HeatonCaltech

Magnitude Paradox … Seismologist• Radiated energy increases by 32 times for each

unit of magnitude.• The number of earthquakes decreases by 10

times for each unit of magnitude.• In California, most of the energy is in earthquakes

with magnitudes larger than 7.4.• Large earthquakes do most of the work of plate

tectonics. Although they are infrequent, they are inevitable.

• After the M 6.7 1994 Northridge earthquake seismologists said, “this was only a moderate earthquake … wait till you see a great one.”

Magnitude Paradox … Engineer• Perhaps there can be larger motions, but these

are extreme examples of extraordinary events that shouldn’t be used for building design.

• Eyewitness reports of the 1906 earthquake indicate that the shaking was comparable to that in 1994, but it lasted longer and occurred over a larger area.

• Computer models shows that most of the risk comes from more frequent moderate size events.

• This building is designed for a M 8, the largest possible earthquake!

Current model says most of the threat to downtown LA is not from the San Andreas fault

1906 M 7.8 San Francisco earthquake rupture with large ground displacement. Notice that the farm buildings were largely intact.

Pt Reyes Station 1906

Current Building Code

• Current building codes are mostly prescriptive rules based on the building type and seismic zone.

• Codes have been developed by fixing deficiencies from past earthquakes.

• If you’ve got a good building code, who needs a seismologist?

How do buildings resist earthquake forces?

Front View

Top View

Image: Courtesy EERITall Building AnatomyTall Building Anatomy

stiff

Wooden Houses7-11 storesCaltech

High-rise buildings

flexible

Flexible or Strong?

• Stiff buildings tend to have high stresses, and must therefore be strong.

• Making a building strong increases the stiffness, which increases the stresses, which increases the required strength of the building (a vicious circle).

• Making a building flexible tends to decrease the stress, but it also decreases the strength of a building (another vicious circle).

• Tall buildings are always designed to be flexible.

Lateral stiffness relies on flexing beams

Large deformation should result in bent steel beams

From the lab of Chia-Ming Uang, UC San Diego

Tall buildings cannot withstand large Tilts

• Integrity of the columns is critical.• Gravity loads are normally axial compressional

loads on the columns.• Tilted columns result in bending forces on the

columns caused by the weight of the building.• Drift (i.e. column tilt) should not exceed 0.03

for tall MRF buildings.

• John Hall’s design of a 20-story steel MRF building that meets California 1994 code (zone IV, site class C)

20-story steel-frame building subjected to a 2-meter

near-source displacement pulse (from Hall) • triangles on the frame indicate the failures of welded column-beam

connections (loss of stiffness).

• Simulated deformation of 20-story steel frame for the M6.9 1989 Loma Prieta Earthquake

• JBC is Japanese building code

• UBC is 1994 California code

1906 ground motion simulation from Brad Aagaard (USGS)

• Simulated 20-story steel frame for a M 7.8 1906-like earthquake

• Yellow and red are damaged beyond repair

• Pink is simulated collapse

Large displacements can overwhelm base isolation systems

• 2-meter displacement pulse as input for a simulation of the deformation of a 3-story base-isolated building (Hall, Heaton, Wald, and Halling

• The Sylmar record from the 1994 Northridge earthquake also causes the building to collide with the stops

• Isolator displacements (m)

• Most isolated buildings cannot exceed 0.4 m

Frame buildings can have also be built with concrete columns and beams (as opposed to steel)

1971 San Fernando earthquake showed that many concrete frames were brittle

Potential for collapse at drifts of about 0.01 (lower than for steel buildings)

There are thousands of these buildings in California and occupants have not been notified

Olive View Hospital M 6.7 1971 San Fernando Earthquake

Northridge 118 FWY

Example of failure of a brittle concrete column (pre-1975 code)

Example of “ductile” behavior of concrete columns. Although the parking structure performed poorly, the exterior columns did not fail.

One of the great disappointments is that there has been little progress in the retrofitting of “nonductile” concrete frame buildings. Most people wholive or work in them are not aware of the serious risk involved.

Conclusions• Current probabilistic hazard analysis may seriously

underestimate the importance of large earthquakes.• Flexible buildings that rely on high ductility will be

damaged beyond repair in large earthquakes and many may collapse.

• Fix the pre-1994 steel welds!• Notify occupants of pre 1975 brittle concrete frame

buildings!• Base isolation systems may be overdriven by large

near-source ground motions.• Strong shear-wall construction is best suited to resist

large-magnitude earthquakes (your wooden house will perform well).

• If it doesn’t burn.