of 4
Northridge 20 Years After
Urban earthquakes such as the 1994
M
6.7 Northridge earth-
quake provide unique moments of clarity for those of us work-
ing on earthquake hazards. I recall being jolted awake at
4:31 a.m., 17 January 1994, in my Pasadena home. As I hurried
to my U.S. Geological Survey (
USGS
) office on the Caltech
campus, I remember wondering what the day would bring.
Was the epicentral region sparsely populated, or had we just
taken a direct hit to urbanized Los Angeles (LA)? We now
know that, although damaging shaking extended into the
LA basin, it was the San Fernando Valley that experienced truly
violent shaking; five strong-motion accelerometers recorded
peak ground velocities (PGV) of more
than
1m
=
s
. Although the attack was over
in 15 seconds, it left behind about $20 bil-
lion in damage and millions of terrified
Angelenos, especially those living in the
San Fernando Valley.
Despite the intense shaking, the offi-
cial death toll was only 57. We clearly
dodged a bullet that winter morning. Al-
most everyone in the epicentral region was
asleep in wood houses or low-rise wood
apartments. California wood structures
are resilient in earthquakes; they are nat-
urally lightweight and stiff enough that
the plasterboard walls do not crack as
the house adjusts to the gravity load from,
say, a large dinner party. This stiffness
brings the benefit that they are also strong. To be sure, the
violent shaking in the San Fernando Valley caused extensive
damage to wood houses, but none of them collapsed. Sixteen
people died when part of the ground story of a wood
Northridge Meadows apartment building collapsed, but many
other wood apartment buildings did not collapse.
Reinforced-concrete-frame (
RCF
) buildings did not fare as
well. Short reinforced-concrete-wall buildings (e.g., 7-Eleven
stores) survived, but frightening collapses struck other concrete
structures that would have been filled with people several hours
later. In a sense, Northridge was a replay of the 1971
M
6.7 San
Fernando earthquake, which also struck early in the morning
wood homes and short concrete-wall buildings did not col-
lapse, but a few
RCF
buildings did.
The collapse of
RCF
buildings in 1971 prompted impor-
tant changes to building codes. Unfortunately, more than 1000
concrete buildings had already been constructed in metropoli-
tan LA using the older techniques; these buildings are com-
monly referred to as nonductile concrete-frame buildings.
Figure
1
shows approximate contours of the PGV during
the Northridge earthquake and locations of pre-1994 buildings
at least four stories tall. Several colleagues have cautioned that
some buildings likely are misidentified, so do not take this
figure too literally
but the pattern is valid. The buildings are
separated into two categories: (1) Circles represent pre-1975
concrete buildings; many are nonductile and may not have
survived the type of shaking that happened in the northern
San Fernando Valley in 1994 and 1971. (2) Triangles represent
pre-1994 steel-frame buildings. Clearly, the urban parts of LA
with frame buildings of at least four stories were not shaken as
violently as the San Fernando Valley sub-
urbs were.
Probably the largest surprise in the
Northridge earthquake was the brittle frac-
ture of the key welds in steel moment-
resisting frame buildings (
Reis and Bono-
witz, 2000
). Most steel-frame buildings are
designed so that, at the largest deforma-
tions, plastic yielding of steel (plastic
hinges) should occur mostly in horizontal
beams; plastic hinges in vertical columns
are to be avoided because they tend to
form premature collapse mechanisms.
The welds are critical because they carry
the loads between the columns and
the beams.
Several steel-frame buildings in the
southern San Fernando Valley were very strongly shaken, so
inspectors expected to find postearthquake evidence of plasti-
cally bent beams. However, inspectors did not find any such
beams; instead, the welded moment-resisting connections frac-
tured. Subsequent full-scale laboratory tests confirmed that the
capacity of the welded connections was less than the forces re-
quired to plastically bend the beams. This was an enormous
surprise, and it meant that the steel-frame buildings were
far more brittle than the designers had intended. It would take
many pages to more completely describe this problem and the
measures taken to mitigate it for new construction. Let me just
say that, at present, most California steel-frame buildings con-
tinue to have pre-1994 brittle welds. Simulations of 20-story
steel-frame buildings suggest that they are three to five times
more likely to collapse than buildings with sound welds when
intensely shaken (
Olsen
et al.
, 2008
). Furthermore, simulations
of earthquakes on the Hollywood fault, the Santa Monica fault,
Subsequent full-scale
laboratory tests confirmed
that the capacity of the
welded connections was
less than the forces
required to plastically
bend the beams. This was
an enormous surprise, and
it meant that the steel-
frame buildings were far
more brittle than the
designers had intended.
doi: 10.1785/0220130194
Seismological Research Letters Volume 85, Number 1 January/February 2014 1
Figure 1.
Contours of peak ground velocity in the 1994 M 6.7 Northridge earthquake. Circles represent pre-1975 concrete buildings;
many of these buildings are nonductile and they may not have survived the type of shaking that happened in the northern San Fernando
Valley in 1994 and 1971. Triangles represent pre-1994 steel-frame buildings. Building data are from the LA County Tax Assessor
s files and
have not been checked for accuracy. Adapted from
Wald
etal.
(1996)
.
2 Seismological Research Letters Volume 85, Number 1 January/February 2014
the Newport
Inglewood fault, and the Puente Hills fault all
show that violent shaking could someday occur in regions with
these older, brittle frame buildings (both concrete and steel)
(
Day
et al.
, 2005
;
Olsen, 2008
).
It is now almost 20 years since we produced Figure
1
,
which supports the idea that we did dodge a bullet in 1994.
Unfortunately, the gun is still loaded and cocked, because very
few of the fragile frame buildings have been retrofitted. The
earthquake research community is well aware of the brittle-
frame issue, but the general community is not. A recent
Los
Angeles Times
article on nonductile concrete frames (
Lin
et al.
,
2013
) surprised most Angelenos. Why don
t residents of these
buildings know that they live in buildings that earthquake
professionals have known for decades to be of dubious safety?
Pre-1975 concrete frames seem to be the most alarming prob-
lem, but pre-1994 brittle steel frames also present a genuine
concern. The occupants have no clue, and the building owners
have little motivation to pursue this problem.
Furthermore, why isn
t there a program in place to at least
tell people that they are in harm
s way? There is lots of blame
to go around.
1. The City of Los Angeles should have
compiled a list of potentially hazard-
ous buildings, and the occupants of
these buildings should have been
made aware of the list. Although law-
suits would have inevitably followed, I
doubt any would have been successful;
public safety is the highest priority.
Los Angeles is not alone here; many
other cities have not tackled this issue.
2. The California Seismic Safety Com-
mission should have made this issue
plain and clear to all four California governors since
1994, who should have acted on the commission
s recom-
mendations.
3. The National Institute of Standards and Technology
(NIST) should be systematically compiling quantitative,
easily accessible information on the shaking resilience of
different types of buildings and developing a national
database of the resilience of individual buildings. The Na-
tional HighwayTransportation Safety Authority publishes
a list of the safety of all automobiles
why are buildings
different? NIST is the lead agency in the National Earth-
quake Hazards Reduction Program (
NEHRP
), and it ulti-
mately bears the brunt of the responsibility for ensuring
that these problems are effectively addressed.
4. The
USGS
has focused its ever-diminishing resources on
probabilistic seismic-hazard assessments, but it is also criti-
cally important for the
USGS
to collaborate with NISTto
simulate the impact of specific earthquakes on different
urban environments. The ShakeOut (see
Porter
et al.
,
2011
) is a fine example of how Earth scientists and earth-
quake engineers can collaborate to highlight specific prob-
lems. Unfortunately, the ShakeOut
s impact was all too
fleeting.
5. The President and Congress need to rediscover the earth-
quake problem. NIST has never had a meaningful
NEHRP
budget (despite its advertised key role). The
USGS
budget
has steadily declined to less than one-third its size of 30
years ago. To add insult to injury,
NEHRP
has not been
reauthorized since 2004.
Of course, there are some bright spots in this story. For
example, the California Department of Transportation
(Caltrans) began systematically retrofitting all nonductile con-
crete bridge columns more than 20 years ago. The California
Office of Statewide Health Planning and Development
(
OSHPD
) has been systematically reviewing the structural
capacity of hospitals. When appropriate, structural retrofits
are required with the intention that the hospitals can remain
functional during earthquakes. These Caltrans and
OSHPD
programs have been expensive and difficult. The bridge retrofit
program is essentially finished, whereas the hospital retrofit
programs are an ongoing struggle.
In September 2013, Caltrans opened the new East Span of
the San Francisco
Oakland Bay bridge, and Californians were
told that it is designed for the 1500-year
earthquake. Fifteen-hundred years is a long
time, and lots of horrible nonearthquake
things are likely to visit our society in that
period. Policy makers and the general pub-
lic seem to have interpreted this 1500-year
design target as evidence that the problem
of building for earthquakes is mostly
solved. The recent
LA Times
story,
though, presents a jarring contrast. It
showed that many Californians were very
surprised to learn that they might be
crushed between the floors of a modern
multistory California building. More concern and confusion
will follow when the public learns that some steel-frame high
rises may collapse in the near-source region of large earth-
quakes.
Personally, I have been around this business long enough
to have seen previous generations of earthquake professionals
reassure the public that, while the older buildings are possible
hazards, the newest buildings will perform well. At one time
this was said for the nonductile concrete-frame buildings that
we now fear. Perhaps it is possible that we have now achieved
an appropriate resistance strategy, but few of our buildings have
really been tested. It is going to be a giant experiment when we
get a direct strike to one of our cities. I somehow doubt we will
be talking much about 1500-year failure recurrence times after
such an event.
I consider myself an earthquake professional; I am a seis-
mologist and an earthquake engineer. I have often claimed that
our research provides the information necessary to take actions
that will decrease future tragedies and that we need not wait for
a future disaster to acquire actionable knowledge. However,
this optimistic view has been challenged by our collective fail-
ure to respond strongly to the near miss that occurred with the
Northridge earthquake. At the end of the day, this collective
It is a sure thing that if tens
of thousands of Americans
perish in a future urban
earthquake, we will focus
our attention on how we
can better protect the
public. Wouldn
titbenice
to do it before the disaster?
Seismological Research Letters Volume 85, Number 1 January/February 2014 3
failure is the responsibility of the entire community of earth-
quake professionals, including Earth scientists, earthquake en-
gineers, emergency responders, and public policy makers. It is a
sure thing that if tens of thousands of Americans perish in a
future urban earthquake, we will focus our attention on how
we can better protect the public. Wouldn
t it be nice to do it
before the disaster?
REFERENCES
Day, S. M., J. Bielak, D. Dreger, S. Larsen, R. Graves, A. Pitarka, and K. B.
Olsen (2005). Lifelines program task 1A03: 3D ground motion sim-
ulation in basins, Technical Report, Paci
fi
c Earthquake Engineering
Research Center.
Lin, R.-G., II, R. Xia, and D. Smith (2013).
Concrete Risks
, Los Angeles
Times, 13 October 2013, front page.
Olsen, A. H. (2008). Steel Moment-Resisting Frame Responses in
Simulated Strong Ground Motions: or How I Learned to Stop
Worrying and Love the Big One,
Ph.D. Thesis
, Department of
Civil Engineering and Applied Mechanics, California Institute of
Technology, Pasadena, California.
Olsen, A., B. Aagaard, and T. Heaton (2008). Long-period building re-
sponse to earthquakes in the San Francisco Bay area,
Bull. Seismol.
Soc. Am.
98,
1047
1065.
Porter, K., K. Hudnut, S. Perry, M. Reichle, C. Scawthorn, and A. Wein
(2011). Forward to Special Issue: The ShakeOut Scenario,
Earthq.
Spectra
27,
235
237.
Reis, E., and D. Bonowitz (2000).
State of the Art Report on Past
Performance of Steel Moment-Frame Buildings in Earthquakes
,
Report No. FEMA-355E, SAC Joint Venture.
Wald, D., T. Heaton, and K. Hudnut (1996). The slip history of the 1994
Northridge, California, earthquake determined from strong-
motions, GPS, and leveling-line data,
Bull. Seismol. Soc. Am.
1B,
S49
S70.
Thomas Heaton
Earthquake Engineering Research Laboratory
California Institute of Technology
heaton@caltech.edu
4 Seismological Research Letters Volume 85, Number 1 January/February 2014