How long did the strong shaking last?
Even though it seemed like an eternity for some, the strong shaking lasted about 15-20 seconds. Most of HVO's seismometers, which are tuned to pick up smaller volcanic earthquakes, went off-scale during the first several minutes of each quake (HUA helicorder plot), so they didn't provide much detail to help answer this question.
Many years ago, the USGS installed a number of strong-motion seismometers specifically designed to stay on scale during even the strongest shaking. About a dozen of these automatically report their data, using telephone lines, immediately after an earthquake large enough to trigger them.
These instruments worked flawlessly, providing data with which the USGS produced preliminary maps of shaking intensity called a SHAKEMAP. This map can be used by emergency managers to see where they might anticipate the worst damage.
The data from the strong-motion instruments can show us the details of the early parts of the earthquakes. Each of the dozen or so instruments located around the island show that the strong shaking lasted between 15 and 20 seconds. Weaker shaking both preceded and followed the strong shaking for up to a minute in total duration.
Horizontal (east) accelerations measured at three locations on Hawai`i Island from the M6.7 earthquake. Note the difference in accelerations at the USDA facility and the Old Hospital in Hilo on different geologic deposits. For reference, the acceleration due to gravity is 981 cm/sec/sec - peak horizontal accelerations at Waimea slightly exceeded that value.
The intensity of shaking varied, depending on how far you were from the epicenter and what kind of material you were on. For example, thick soils might amplify the vibrations felt on nearby solid rock. Just 0.5 m (1.5 feet) of ash or soil can double the shaking. On the same type of ground, the shaking would be more intense nearer the quake's epicenter.
A good illustration of the effect of geology is the difference in shaking on either side of the Wailuku River in Hilo. Despite being about the same distance from the quake, USGS strong-motion seismometers on either side of the river show that the ground accelerations and velocities were two to three times larger on the Mauna Kea side of the river (Hilo USDA facility) than on the Mauna Loa side. The Mauna Kea side of the river has a good thickness of soil that would amplify the shaking while the Mauna Loa side has a little soil.
Next Question: Was the M6.0 earthquake at 7:14 am an aftershock of the M6.7 quake at 7:07 am?
Updated: 31 October 2006 (pnf)