What are the earthquake risks in Honolulu/Oahu?
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
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