Wednesday night's rattler occurred just 10 miles north of its July predecessor, and 17 miles beneath the ocean floor. Even more mystify- ing than its occurrence was its perceived strength. Most folks queried by the News-Times said this quake shook them up much more than the previous one, yet scientific readings revealed a slightly lower magnitude.

     "I thought it felt a lot stronger than it was," Dziak agreed, noting that he estimated the strength at 5.5 or even 6.0.

     The key issue for him is the need to boost public awareness of earth- quakes and how to prepare for and survive the havoc bigger ones can werak.

     Dziak and his team use a network of underwater hydrophones - both fixed and portable - to listen for the sounds of ocean floor earthquakes and related undersea phenomena from their labs at HMSC. They have monitored earthquakes off the Northwest coast since August 1991, when they launched the Acoustic Monitoring Project using the U.S. Navy's Sound Surveillance System (SOSUS). The project falls under the auspices of NOAA's Vents Pro- gram, which investigates undersea volcanoes and hydrothemial vents.

     Moored to the seafloor, hy- drophones listen for activity via seismic (through the ground) and acoustic (sound) waves traveling through the water at many locations worldwide, including a section of the Juan de Fuca Ridge about 200 to 300 miles off the Washington and Oregon coasts. The hydrophone method allows detection of low- magnitude seismic and volcanic ac- tivity that provides more accurate source locations than land-based* seismic networks.

     In fact, few of the literally thou- sands of earthquakes detected by the underwater system ever show up on land-based seismic equipment.