San Francisco — ''When you get this big an earthquake, it changes all the rules.'' Bob Decker, scientist in charge of the United States Geological Survey's Volcano Observatory on Mauna Loa, is assessing the Nov. 16 Hawaiian earthquake centered just off the southeastern flank of Mauna Loa.
''This established a new upper limit on the Kaoiki (fault). A big spike of energy like this can change our minds'' about things like the relationship of volcanic eruptions to earthquakes, and just how much energy is pent up beneath these 14,000-foot volcanoes, Dr. Decker said in a telephone interview.
The big quake was no surprise to geologists who monitor constant rumblings from atop this active Hawaiian volcano. But its magnitude and intensity were unexpected, say those US Geological Survey scientists. They placed the magnitude of the quake at 6.7 on the Richter scale.
The quake was the second biggest ever recorded in the region of the Kaoiki fault, a fissure that connects the two giant volcanic mounds of Mauna Loa and Kilauea, which rise to the southwest of Hilo. In 1975 a 7.2 earthquake centered off the southern side of Kilauea, and closer to the sea than last week's quake 40 miles inland, caused a tidal wave, as a 30 kilometer by 10 kilometer chunk of the volcano's side was pushed horizontally toward the sea by a few meters.
Scientists who scrambled over the shaken island to reconnect their monitoring instruments were excited about the major quake, says Decker. The quake was unusual enough in its magnitude that it is believed to be the main event in this series of quakes, and not the precursor to a larger one, says Decker.
''We don't expect an imminent eruption (from Mauna Loa),'' says Decker. But he adds that based on the swelling up of magma in the summit and the small earthquakes monitored there, the USGS forecasted in September that chances of an eruption within the next two years were higher than average.
Decker and Jerry Eaton, a USGS seismologist in Menlo Park, Calif., explain that scientists have not yet documented if there is a cause-and-effect relationship between earthquakes and volcanic eruptions. Mr. Eaton explains that it could be some time before scientists are able to connect their speculation with evidence as to the nature of the source of last week's earthquake. But last week's quake, which Decker calls ''another brick in the castle'' of geological information, is the kind of event that can help prove or disprove the link between eruptions and earthquakes.
As it appears now on the Kaoiki fault, says Mr. Eaton, Mauna Loa is being lifted higher or Kilauea is sinking as the magma from deep below the surface continually rises through fissures causing swelling and stress that disrupts the earth's crust. Causes for earthquakes, like the one in 1975 and possibly like last week's, typically come from the increased pressure of volcanic buildup as sections of earth collapse and re-position themselves, he says.