What are the earthquake risks in Honolulu/Oahu?
and
How many "recorded and reported" earthquakes have there been on Oahu?
When was the most recent and what size was it?
Hawaii's biggest earthquakes, up to magnitude 7.8, are
associated with dike intrusions into the active volcanoes and
expansion of the volcanoes across the old seafloor. While we can
feel such events in Honolulu, they occur too far away to cause
any damage here. There are, however, other earthquakes that we
have to consider. Potentially the most damaging are earthquakes
caused by the load of the Hawaiian Islands on the Pacific
lithosphere.
Since Hawaiian volcanoes are so large they are an immense burden
on the lithosphere, and it will sag beneath their weight (the
phenomenon of isostasy). Sometimes, in addition to just sagging,
the lithosphere will creak or groan; those creaks and groans are
earthquakes. The last such earthquake of any size was the
magnitude 6.2 Honomu event of 1973. That was beneath the Hamakua
Coast of the Big Island. Although the earthquake was 200 miles
from Honolulu it was felt very well because it was so deep.
Similar events have occurred closer to Oahu: the Maui earthquake
of 1938 had its epicenter somewhere to the north of Maui and was
about magnitude 6.5; the Lanai earthquake of 1871 had a
magnitude of perhaps 6.8 and probably had its epicenter near
Palaoa Point (65 miles SE of Honolulu). The 1871 event did
extensive but minor damage in Honolulu - no unreinforced masonry
building was left undamaged. Every building on the Punahou campus
required repair.
Any repeat of the 1871 Lanai earthquake would cause problems in
Honolulu. Damage would be limited just to little pockets where
there is intense ground motion (areas of fill, hilltops), but
within these areas the damage might be considerable (consider
the damage caused in San Francisco and Oakland by the Loma
Prieta earthquake of 1989 - that was about the same size and
about the same distance away). The shaking could conceivably
cause post-and-pier houses (the old plantation-style single-wall
houses so common in Kaimuki, Manoa, and Palolo) to "walk" off
their foundations. In areas of deep fill (e.g., a few places in
Waikiki) the ground might liquify, causing buildings to sink
into the ground. But the scariest damage of all would likely be
to highway bridges. The freeways H-1 and H-2 each have several
bridges and overpasses made using short-span box-girder
construction. In these overpasses, a section of roadway is held
up simply by a six-inch wide ledge at each end of the section.
Even quite modest shaking might cause these sections to
collapse, possibly with loss of life. The State is now
developing a plan to retrofit the questionable bridges.
Modern buildings in Honolulu are probably quite safe. Six years
ago the Uniform Building Code was upgraded from seismic zone 1
to seismic zone 2A; everything built to the new code should
survive fine. Older buildings might suffer minor damage but
should not cause loss of life. The bridges are the big worry,
but the State says it's working on that problem.
Other earthquakes that might affect Honolulu? We sometimes feel
very small local events within the Koolau mountains. We don't
know what these are (they could be minor gravitational
adjustments or they might result from the crystallizing of a
magma body), but they are always too small to cause any damage.
There are also sometimes small events offshore. These are almost
certainly caused by small-scale submarine landslides. Again,
they are too small to cause any damage. Landslides, however, can
cause local tsunamis, so if ever you feel the earth move when you
are at the beach, get away from the ocean.
So what is the earthquake risk in Honolulu? It's very hard to
judge because the history of the city is so short. Until more
statistics can be accumulated, or until we learn more about how
the islands grow and age, it is prudent to build structures so
that they will at least survive a repeat of the 1871 Lanai
event.
According to the 1986 catalog "Earthquakes felt on Oahu,
Hawaii, and their intensities," by Doak Cox, University of Hawaii
Environmental Center, August 1986. The compilation lists 113 felt and
reported earthquakes from 1859 to 1986; so there were between one and two
reported events a year. My guess is that that rate has continued to the
present day. But those are just events that can be located. There are quite
a few small shocks which don't even show up on the Pacific Tsunami Warning
Center network and therefore which cannot be located. For example, eighteen
months ago, a small shock shook my house on a ridge of the Koolau Mountains
above east Honolulu. The same event was felt over the mountains in
Waimanalo, but it never made it into the catalog (my guess is that it had a
Mercalli intensity of about II).
The 113 earthquakes listed in the 1986 catalog are about equally divided
between Big Island events, events local to Oahu, and events from the
Molokai-Maui area.
If your question is "When was the last earthquake which caused any damage
on Oahu?" the answer is 26 April 1973. That was the Honomu Earthquake
beneath the Hamakua Coast of the Big Island. It was only magnitude 6.2, but
it was a deep earthquake (about 40 km depth) and was felt well even on
Kauai. The only damage on Oahu was cosmetic (two broken windows, a few
minor cracks), but shaking was sufficient to alarm people and several
buildings were evacuated. On the Mercalli scale, the intensity in Honolulu
was about V.
But the Honomu Earthquake was more than 250 km from Honolulu. When was the
last damaging earthquake close to Oahu? That was the Oahu Earthquake of 28
June 1948. We don't really know where the epicenter was, but most
seismologists put it within 70 km of downtown Honolulu. The earthquake
magnitude was about 5.0, but shaking in Honolulu was surprisingly severe
for so small an event. There were widespread reports of cracked masonry and
other minor damage, phone service was disrupted, one water main was broken,
and books and dishes were thrown from shelves all over town. That
translates to a Mercalli intensity of VI, though shaking was more severe at
certain locations (Tantalus, Iwilei, and Tripler all reported damage
consistent with a Mercalli intensity of VII; i.e., enough to put cracks
into reinforced concrete and to move a grand piano across a room).
The most severe earthquake damage on record in Honolulu was in the Lanai
Erathquake of 19 February 1871. A lot of construction in Honolulu then was
unreinforced masonry, just about the least resistant to seismic damage.
and books and dishes were thrown from shelves all over town. That
translates to a Mercalli intensity of VI, though shaking was more severe at
certain locations (Tantalus, Iwilei, and Tripler all reported damage
consistent with a Mercalli intensity of VII; i.e., enough to put cracks
into reinforced concrete and to move a grand piano across a room).
The most severe earthquake damage on record in Honolulu was in the Lanai
Erathquake of 19 February 1871. A lot of construction in Honolulu then was
unreinforced masonry, just about the least resistant to seismic damage.
Many of those buildings were damaged, though none collapsed. *Every*
building on the Punahou School campus was damaged in one way or another.
Mercalli intensities for Honolulu were between VI and VII, with a few
patches of more severe shaking. The earthquake itself was about magnitude
6.8, but the epicenter was about 150 km from Honolulu.
Another event like the Lanai earthquake would be interesting...
Dr. Gerard Fryer
Hawaii Institue of Geophysics and Planetology
University of Hawaii, Honolulu, HI 96822
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