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1999 Applied Technology Council

Earthquake AftershocksEntering Damaged Buildings

Ronald P. Gallagher, R. P. Gallagher Associates, Inc.Paul A. Reasenberg, U. S. Geological Survey

Chris D. Poland, Degenkolb Engineers

Funded by the U.S. Geological Survey as part of theATC-35 Research Utilization Project

Summary

Earthquake aftershocks can cause signifi-cant damage to buildings. Occasionally,they can result in building collapse. Thisrisk is highest for previously damaged

buildings (Figure 1).Entry into damaged buildings as soon

as possible is often necessary for a varietyof emergency reasons, including searchand rescue, building stabilization andrepair, and salvage and retrieval of posses-sions. Because people entering damagedbuildings are at risk should an aftershockoccur, the decision to permit entry mustconsider both the level of initial damageand the probability of aftershocks.

This TechBrief offers guidelines forentering damaged buildings under emer-gency conditions as a function of timeafter the initial damaging event. These

guidelines are based on aftershockresearch carried out by the U.S. Geologi-cal Survey and the postearthquake build-ing safety evaluation procedures of ATC-20 (ATC, 1989, 1995).

After a damaging earthquake, localbuilding departments inspect and postbuildings as INSPECTED, RESTRICTEDUSE, or UNSAFE using the ATC-20 pro-cedures. Table 1 summarizes these post-ings and provides recommended guide-

Figure 1: Buildings such as this office building in Kobe, Japan, are generally unstableand may collapse in an aftershock.

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lines for emergency entry of damagedbuildings. For buildings posted UNSAFE(red placard), entry depends on whetherthe building is considered stableorunstable. (Guidelines for classifying abuilding as unstableare provided inTable 2.)

For buildings considered stable, waittimes depend on the main shock magni-

tude and the duration of occupancy, asboth of these factors affect the probabil-ity of a large aftershock occurring duringthe period of occupancy. Table 3 pro-vides recommended days to wait beforeentering buildings posted UNSAFE, butstable.

Table 1: Guidelines for Emergency Entry of Damaged Buildings

Posting Placard Color Condition Entry Allowed a

None (not yetinspected)

Serious structural damage Only for search and rescue, and atown risk.

INSPECTED Green Minor structural damage Yes.

RESTRICTED USE Yellow Some structural damage, gener-ally of limited severity

Yes, but according to restrictions.Entry into the restricted area onlywith permission of the local build-ing department.

UNSAFE Red Structure has serious structuraldamage, but is stable

Yes, according to Table 3 guidelines.

UNSAFE Red Structure has serious structuraldamage and is unstable

No. Table 3 does not apply. Entryonly with written permission of thelocal building department.

UNSAFE Red Posting due to otherthan struc-

tural damage

No. Table 3 does not apply. Entry

only with written permission of thelocal building department.

a. During the first 24 hours, entry into seriously damaged buildings should be avoided in case the damaging shock is

a foreshock and a subsequent event is the main shock.

Table 2: Guidelines for Classifying Damaged Buildings as Unstable

UNSAFE Buildings that Have at Least One of the Following Characteristics Should be Classified as Unstable

1. May collapse or partially collapse under its own weight.

2. Likely to collapse in a strong aftershock, from additional damage.3. Ongoing (progressive) lean.

4. Ongoing creep or structural deterioration.

5. So heavily damaged that its stability cannot readily be determined.

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Safety Evaluation of Earthquake-Damaged Buildings

After a large earthquake, a state of sus-pended animation often exists. If themainshock has caused widespread dam-age, some buildings may have collapsedand others may be poised to collapse.Many buildings may be damaged andsome may have falling hazards (that is, ahazardous situation exists from an itempoised to fall) caused by broken chim-neys or damage to other nonstructuralcomponents. Seriously damaged build-ings, such as the one shown in Figure 1,

are posted UNSAFE, and entry is prohib-ited.

Rescue workers, residents, and busi-ness personnel often have legitimatesometimes urgentneeds to enter dam-aged buildings to find and rescuetrapped occupants, to perform essentialfunctions, or to retrieve personal prop-erty. These factors, when placed in thecontext of chaos and poor communica-tion, create a potentially hazardous envi-ronment within an aftershock zone.

A major question becomes: Howsoon should a damaged building beentered? The answer is different for eachdamaged site. It depends on, amongother things, the degree of damage, theprobability of damaging aftershocks, andthe urgency of the need to enter.

Building officials are responsible fordetermining when damaged buildingsare unsafe to enter. Most jurisdictions in

California follow ATC-20 procedures(ATC, 1989, 1995). Using the ATC-20procedures, inspectors can identify whenthe damage is apparently not significant,when a buildings use should berestricted, or when the building is unsafeto enter and to post the building accord-ingly. Such postings are initially based ona rapid survey and may be changed aftera more detailed inspection.

The ATC-20 posting system wasdeveloped to inform owners, occupants,

and the public about the condition of adamaged building in terms of its suitabil-ity for occupancy and general use follow-ing an earthquake. The posting criteriatake into account the possibility thataftershocks will aggravate the existingdamage. The three posting classificationsare defined in the paragraphs below.

INSPECTED (Green Placard): Thebuilding has been inspected by the local

jurisdiction. It may or may not have been

damaged. If damaged, the observeddamage does not pose a significant safetyhazard. There is no limit on the use oroccupancy of the building.

RESTRICTED USE (Yellow Placard):The building has been inspected andfound to have damage or some othercondition (e.g., falling hazard) that pre-cludes unrestricted occupancy. The

Table 3: Recommended Days to Wait Before Emergency Entry of Buildings Posted UNSAFE, but Stablea,b

Mainshock Magnitude (M) Enter for 2 hours Enter for 8 hours Enter for 24 hoursc

Mequal to 6.5 or greater 1 day 3 days 8 days

Mequal to 6.0 or greater,

but less than 6.5

1 day 2 days 4 days

M less than 6.0 1 day 1 day 2 days

a. Refer to Table 1 for other posting conditions.

b. Recommended days to wait refers to the date of the mainshock, not the date of posting.

c. For continuous emergency access only. Full-time occupancy is permitted only when approved by the local build-ing department.

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building can be entered and used, butsome restrictions have been placed on itsuse. The house in Figure 2 has received aRESTRICED USE posting because of the

leaning chimney. The house can be occu-pied, but the fireplace may not be used,the room with the broken chimney maynot be entered, and an area outside thehouse and under the chimney isrestricted.

UNSAFE (Red Placard): The buildinghas been inspected and found to be seri-ously damaged or have a serious hazard(e.g., toxic spill). Generally, buildingsposted UNSAFE have serious structuraldamage. Many, but not all, are at risk ofpartial or complete collapse. SomeUNSAFE postings are made when nor-mal occupancy is inadvisable, such aswhen an old house has fallen off its foun-dation. Generally, entry into a buildingposted UNSAFE is not permitted with-

out the approval of the local jurisdiction.

The ATC-20 posting procedures providea valuable tool for building officials tocommunicate safety information imme-diately to the public. In the days that fol-low a damaging mainshock, there is anadditional need, though, to determinewhen the chance of aftershocks hasdiminished to the point that restrictionson entry and occupancy can be relaxed.

About Aftershocks

Foreshock, Mainshock, and Aftershock

Earthquakes typically occur in clusters.Seismologists have coined three terms todistinguish the events in a cluster: fore-shock, mainshock, and aftershock. In anycluster of earthquakes, the one with thelargest magnitude is called the main-shock. Earthquakes that occur before themainshock are called foreshocks, whilethose that occur after are called after-shocks. In this discussion, it is assumedthat the foreshocks have been inconse-quential, the damage has occurred in themainshock, and the safety concerns cen-ter on the aftershocks that are to come.

Aftershock Sequences

Generally speaking, the stress on theearthquake fault drops drastically dur-ing the mainshock and the small redistri-butions of stress and frictional strengthcause that fault to produce most of theaftershocks. The patterns that aftershocksequences follow can be described andused to estimate the probability of signif-icant aftershocks occurring. The specificlocation, time, and size of individualaftershocks, however, cannot be pre-dicted.

The sequence of aftershocks thatoccurred after the 1994 Northridge, Cali-fornia, earthquake followed a typical pat-tern. All significant aftershocks occurred

Figure 2: A house with a broken chim-ney, while posted yellow, RESTRICTEDUSE, may be entered except for therestricted area.

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vicinity of the site and compare thisdamage with other damaged areas ofsimilar distance from the mainshockfault rupture. If damage was heavy in the

mainshock, the site is more likely toexperience additional damage in anaftershock. Such was the case in theMarina district of San Francisco after the1989 Loma Prieta, California, earth-quake. Conversely, in areas that sustainedlittle or no damage in the mainshock, theaftershock ground shaking hazard can beexpected to be lower. In general, it is rea-sonable to assume that if a building hasbeen inspected and postedRESTRICTED USE or UNSAFE, it may

experience significant additional dam-age if a large aftershock occurs nearby.

A mainshock large enough to causedamage will probably be followed by sev-eral felt aftershocks within the first hour.The rate of aftershocks dies off quicklywith time, as is illustrated visually inFigure 4, and the rate is inversely propor-tional to the time since the mainshock.Aftershocks at each magnitude leveldecrease with time at the same rate. Onaverage, the second day will have approx-

imately 1/2 the number of aftershocks ofthe first day, and the tenth day will haveapproximately 1/10the number of thefirst day. These patterns describe only theaverage behavior of aftershocks; theactual times, numbers, and locations ofthe aftershocks are random. One largeaftershock sometimes occurs as much assix months after the main event. Forexample, a magnitude 5.4 aftershock

occurred six months after the magnitude7.1 Loma Prieta mainshock.

Larger earthquakes have more andlarger aftershocks than smaller earth-

quakes. Smaller aftershocks are morenumerous than large ones. The differ-ence in magnitude between the main-shock and largest aftershock can be 3 ormore, but averages 1.2. In the 1987 Whit-tier Narrows earthquake sequence illus-trated in Figure 4, the largest aftershock(M5.3) was only 0.6 smaller than themainshock.

What Magnitude Aftershock CausesMore Damage?

The answer depends, among otherthings, on the site conditions, buildingtype, and distance from the aftershock.While any felt aftershock may causeadditional damage or create new fallinghazards, those of magnitude 5 and largerare generally considered likely to causesome significant new damage or toworsen existing damage. Seriously dam-aged buildings are, of course, particularlyvulnerable.

While the mainshock may have pro-duced widespread damage, the effects ofan aftershock will usually be confined toa smaller area. Within that area, though,the effects can occasionally be severe.Other damaged areas more distant froma specific aftershock or with better soilconditions will be less affected by it. Thelocation of the aftershocks, however, isnot predictable.

Figure 4: The aftershock sequence after theM5.9 1987 Whittier Narrows, California,earthquake. Each line represents an aftershock.

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Probability of Aftershocks

Figure 5 displays the probability of after-shocks of magnitude 5 or larger occur-ring in a 7-day period, as a function ofthe magnitude and time since the main-shock. While these curves were devel-oped for California earthquakes, they

can be useful elsewhere, in the absence of

more specific information. Point A inFigure 5 indicates that 10 days after amagnitude 7 earthquake, the probabilityof an aftershock with a magnitude of 5.0or greater in the next seven days, some-where in the aftershock zone, is slightlyless than 50%. Smaller magnitude after-shocks are more likely, and larger magni-

tude aftershocks less likely.

Guidelines for Entering Damaged Buildings

One of the most difficult problems facingbuilding officials and structural engi-neers is when to permit entry into dam-aged buildings, particularly those postedUNSAFE. Entry into an undamagedbuilding merely requires a determinationthat it has not been seriously damaged

and has been posted INSPECTED. How-ever, entry into a seriously damagedbuilding (i.e., one posted UNSAFE) car-ries a high risk, particularly during theperiod of frequent aftershocks. There is apossibility that an aftershock will causeadditional damage, life-threatening inju-ries, and even death. The more timespent inside the building, the greater therisk.

Limiting Entry Risk

Because the aftershock hazard dimin-ishes with time, it is possible to deter-mine in general when the risk of enteringan UNSAFE building, for a given periodof time, reaches an acceptable level. First,

however, a level of acceptable risk mustbe established that considers the urgencyof the need to enter. In this analysis, thechoice of risk level was guided by the riskto which a firefighter is exposed in fight-ing a structural fire, since both situationsinvolve the risk of injury consciouslyaccepted by knowledgeable individualswith a specific purpose. The mean rate ofinjuries sustained by firefighters engaged

Figure 5: Probability of an aftershock with a magnitude 5.0 or larger occurring some-where in the aftershock zone during a 7-day period starting at a specified time after a

mainshock. Curves shown are for mainshocks of magnitude 6, 6.5, and 7. For example(point A), the probability is 46% during the 7-day period beginning 10 days after anM7 mainshock. Curves are based on general trends from past earthquakes.

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istics that warrant a structure to be clas-sified as unstableinclude: an ongoing(progressive) lean; ongoing creep orstructural deterioration; or damage sosevere that the structures stability cannotreadily be determined. Evaluation of the

stability of a damaged building may be adifficult task and is best judged by astructural engineer. Guidelines for classi-fying a building as unstableare providedin Table 2 on page 2.

The importance of assessing the sta-bility of a structure is demonstrated inthe following example. In the 1989 LomaPrieta, California, earthquake, one seri-ously damaged residential buildingdeveloped a lean at the first level. Thebuilding was posted UNSAFE and was

judged to be unstable when the localbuilding department monitored the leanand determined that it was increasingdue to the weight of the building. Col-lapse occurred within 48 hours. Figure 8shows a collapsed building that had beenstanding after the Kobe, Japan, earth-quake, but was unstable.

Estimate for Re-entry

Table 3, shown on page 3, providesguidelines for when to allow limitedemergency reentry into damaged build-ings posted UNSAFE and considered sta-ble. Table 3 gives the waiting period (in

days after the mainshock) that must passbefore the estimated risk to an individualentering the building for a given lengthof time will drop to the acceptable level.

For example, following an earthquakewith a magnitude of 6.5 or greater, threedays must elapse before the risk is accept-able for an eight-hour entry period. Thetable recommends that an additional fivedays elapse before allowing occupancy fora full 24 hours. Table 3 may be usedimmediately after the mainshock to esti-

mate waiting times before entry can bepermitted.

Since the risk of injury or deathdepends on the length of exposure time,three durations of occupancy are shownin Table 3. The two-hour period of entryis intended to permit emergency shoringactivities, rapid retrieval of small per-sonal items, or maintenance of criticalequipment. The eight-hour duration isintended to permit the systematic reloca-tion of all building contents, to permit

the day-long emergency operation of acritical facility, or to allow short-termemergency or construction activities.The 24-hour period is intended to coverthe need for around-the-clock repairs. Itis not an indication to return to normaluse or occupancy. This normal userequires approval of the local buildingdepartment. Interpolation may be usedfor other durations of occupancy greaterthan two hours.

Several approximations and worstcase assumptions were used to derivethe waiting times in Table 3. For mostpurposes, it is generally advisable not topermit any access into UNSAFE build-ings for at least one day, regardless of themainshocks magnitude. However, insome situations, this restriction may notbe appropriate. For example, emergencyshoring of a building with severe wall

Figure 7: This reinforced concrete build-ing was determined to be stable eventhough leaning slightly, because of thepresence of substantial concrete shearwalls on all sides.

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damage, or shoring the walls and roof ofa tilt-up building with separationsbetween the walls and roof may need tobe done immediately to prevent further,more consequential, damage. The riskmust be weighed against benefits, and ahigher risk than that assumed in this

TechBrief may be appropriate. Table 3 isintended to provide general guidanceitshould not be used as a rigid rule.

It may appear to some engineers thatthe wait times for large magnitude earth-quakes are long, especially if the after-shocks are less in number and magnitudethan average. This may be due to thebroad use of UNSAFE postings in pastearthquakes and the lack of informationrelated to aftershocks from these events.Many buildings that receive UNSAFE

postings may only need re-inspection bya structural engineer to receive a lessrestrictive posting. Also, the addition oftemporary shoring may permit a lessrestrictive posting.

Search and Rescue Considerations

Search for the injured and rescue of thosetrapped are among the most importantand urgent postearthquake activities.

Those conducting these activitiescan themselves become victims. Search

and rescue personnel, by nature, takehigher risks. Those risks can be lessenedif time spent in dangerous situations iskept to a minimum and if those involvedtake precautions. These include aware-ness of falling hazards and, in protractedrescue situations, use of temporary shor-ing. Table 3 does not apply to search andrescue situations.

Future Research

The entry guidelines given are based onreasonably conservative assumptions andprofessional judgment. Future develop-ments are expected to incorporate ongo-ing research in probabilistic riskmanagement. Additional informationsuch as the number of people allowed toenter at one time, more detailed after-shock sequence characteristics, the seis-mic performance of damaged structures,

Figure 8: This Kobe office building was unstable after the mainshock and collapsedunder its own weight in less than 24 hours.

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other possible posting levels, and theprobability of significant ground motionat a specific site will be considered.

A Word of Caution

Use of judgment is essential in

postearthquake building safety evalua-tion. The guidelines given above are fortypical situations. There may be situa-tions when this guidance is not appropri-ate or must be modified. An aftershockcan occur at any time, and can lead toinjuries to persons in the building.

Entry into an apparently stablebuilding should not be made until theinterior of the building has beeninspected by a small team of structuralengineers.

It is strongly recommended that per-sons entering severely damaged build-

ings do so only for emergency reasonsand take safety precautions, includingwearing a hard hat and strong shoes, car-rying a flashlight, and exercising extremecare.

Entry into seriously damaged build-ings is never risk-free.

How to Get Aftershock Forecasts On the Web

The U.S. Geological Survey began fore-

casting aftershocks after the 1989 LomaPrieta earthquake. After an earthquake inCalifornia of magnitude 5 or larger, the

USGS posts the probability of strong

aftershocks at its web site: http://quake.wr.usgs.gov/

Sources of Additional Information

ATC has developed a series of documentsdealing with the postearthquake safetyevaluation of buildings. These are listedbelow.

ATC, 1989, Procedures and Postearth-quake Safety Evaluation of Buildings,ATC-20 Report, Applied TechnologyCouncil, Redwood City, California.(This document presents the origi-

nal, complete ATC-20 methodol-ogy.)

ATC, 1989, Field Manual: PostearthquakeSafety Evaluation of Buildings, ATC-20-1 Report, Applied TechnologyCouncil, Redwood City, California.

ATC, 1995,Addendum to the ATC-20Postearthqake Building Safety Evalua-

tion Procedures, ATC-20-2 Report,Applied Technology Council, Red-wood City, California.

ATC, 1996, Case Studies in RapidPostearthquake Safety Evaluation ofBuildings, ATC-20-3 Report, AppliedTechnology Council, Redwood City,California.

ATC, 1993, Postearthquake Safety Evalua-tion of Buildings, Training Manual,ATC-20-T Report and slides, AppliedTechnology Council, Redwood City,California.

Additional information is available at theATC website:

http://www.atcouncil.org

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USGS researchers have written a numberof papers and articles on aftershocks andaftershock hazards. Some of these arelisted below.

Jones, Lucile M. and Paul A. Reasenberg,1996, Some Facts About Aftershocks inLarge Earthquakes in California,

USGS Open File Report 96-266.

Reasenberg, Paul A. and Lucile M. Jones,1989, Earthquake Hazard After aMainshock in California, Science,Volume 243, pp. 1173-1176.

Reasenberg, Paul A. and Lucile M. Jones,1994, Earthquake Aftershocks:Update, Science, Volume 265,pp. 1251-1252.

For additional information, see the web-sites:

http://quake.wr.usgs.gov/

http://www-socal.wr.usgs.gov/

http://quake.wr.usgs.gov/QUAKES/FactSheets/QuakeForecasts

http://www.scecdc.scec.org/lifewafter.html

Further References:

U.S. Fire Administration, 1997, Fire inthe United States 1985-1994,NinthEdition, Federal Emergency Man-agement Agency, Washington, D.C.

Acknowledgements

The Applied Technology Council and theauthors appreciate the reviews by AllinCornell, Stanford University; RichardHolguin, City of Los Angeles; LaurenceKornfield, City of San Francisco; Richard

Ranous, EQE International; and EugeneZeller, City of Long Beach. The photocredit for Figure 8 is unknown, but thephoto is greatly appreciated.

Copyright 1999 Applied Technology Council

About This SeriesATC TechBriefs are short documents on subjects of interest to earthquake engineering

professionals. TechBriefs summarize research that is relevant to design practice. Copies of ATC

TechBriefs can be downloaded from ATCs World Wide Web site (www.atcouncil.org), or are

available from:

Applied Technology Council

555 Twin Dolphin Drive #550

Redwood City, CA 94065

ATC DisclaimerWhile the information presented in this report is believed to be correct, ATC assumes no respon-

sibility for its accuracy or for the opinions expressed herein. The material presented in this

publication should not be used or relied upon for any specific application without competent

examination and verification of its accuracy, suitability, and applicability by qualified profes-

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