Close-to-shore Oregon earthquake deemed 'extraordinarily rare event'

By Terry Dillman Of the News-Times

Offshore earthquakes are a common occurrence. But the one that gently rocked Lincoln County Monday morning provided a rare event, along with a reminder that in the Pacific Northwest, the "Big One" is a matter of when, not if.

"This was an extraordinarily rare event," said Robert Dziak, a researcher and seismologist at Hatfield Marine Science Center in South Beach. "It was a pretty big earthquake (magnitude 4.9) for being so close to shore (33 miles out)."

Dziak could find no record of any earthquake of similar magnitude that close to the Oregon shoreline in the past 30 years.

Dziak, who works both with Oregon State University and the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory, is among the scientists from OSU and NOAA who use a network of underwater hydrophones to listen for the sounds of ocean floor earthquakes and other phenomena from their labs at HMSC. They recorded all three earthquakes that occurred Monday - a 2.7 magnitude about 9 a.m., the 4.9 about an hour later, then a 3.3 aftershock about 9 p.m.

HMSC researchers have recorded more than 30,000 in the Pacific Ocean 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 monitoring project falls under the auspices of NOAA's Vents Program, which investigates undersea volcanoes and hydrothermal vents. The hydrophones monitor a section of the Juan de Fuca Ridge about 200 miles off the Washington and Oregon coasts, always listening for seismic activity.

SOSUS originally monitored submarine activity in the northern Pacific during the Cold War. As that stalemate ebbed, Navy officials offered the network and "other unique military assets" to civilian researchers involved in environmental studies.

"This is the only real-time hydrophone system in the world, at least for civilians," said Dziak. "It allows us to listen to earthquakes as they occur. When something unusual happens, we can send out a group of scientists to study the events as they unfold."

According to Dziak, the hydroacoustic method allows detection of low-magnitude seismicity and volcanic activity that provides more accurate source locations than land-based seismic networks. In fact, few of the 30,000-plus earthquakes detected by the system ever showed up on land-based seismic equipment.

A two-dimensional underwater layer - located about 1,000 feet below the ocean's surface, with a certain water pressure, salinity, and temperature - creates a "sound channel" that focuses sound energy horizontally, much like a wire.

During one test, researchers exploded a stick of dynamite below the surface. The explosion failed to register at all on land. "Yet hydrophones recorded it as a magnitude 1 event 6,000 miles away," said Dziak. "That's why the hydrophone array is so important to our research."

From a marine science standpoint, the results so far have been astounding.

Oceanographers often respond to the telltale rumblings detected by the underwater network, allowing them to observe seafloor spreading, undersea volcanoes, and related activities as they occur.

Researchers surveying the water column and seafloor spreading at one site discovered a large colony of microorganisms that were lying dormant in the shallow ocean crust until the heat of the volcanic spreading "activated" them. "Those bacteria could have been down there literally for centuries," Dziak said. "It gives us another little clue to how life may have formed on Earth, and may be lying dormant on other planets."

The hyrdophones have also recorded marine mammal vocalizations, allowing one research team to detect regional differences in the sounds made by blue whales and fin whales. It revealed that whales from different places have their own "accents."

The key value of the hydroacoustic set-up for most people lies in its ability to detect and record earthquakes.

Initially, the number of offshore earthquakes startled researchers, because even the most sensitive land-based seismometers failed to detect them. Scientists also discovered that quakes occur daily, interspersed with occasional "swarms" of as many as 1,000 quakes during a three-week period.

"The (tectonic) plate doesn't move in a continuous manner, and some parts move faster than others," said Dziak, noting that he has seen seven earthquake swarms during the past decade. "When it gets caught and meets resistance, these swarms occur, and when they do, lava breaks through onto the seafloor."

Normal plate movement is less than half an inch per year. Earthquake swarms are the spawn of much quicker movement - perhaps as much as two feet or more in two weeks.

Innovations and refinements have enhanced the system, making it even more effective at earthquake detection.

In November 2003, the Navy finished a series of repairs on the hydrophone arrays used to monitor the Juan de Fuca Ridge, and the OSU/NOAA team installed a new data acquisition center at the naval air station on Whidbey Island near Seattle. The center encrypts the hydrophone data, then sends it to HMSC for decoding and analysis.

OSU/NOAA researchers are also working to refine a portable hydrophone system - developed by HMSC researcher Haru Matsumoto - to deploy at hot spots around the world.

The original system featured instruments - with battery-powered recording equipment - housed in a titanium casing, and attached to upright moorings anchored on the ocean floor. While serviceable, its use was limited because data wasn't available in real time.

"The portables weren't powerful enough to transmit data, so it has to be recorded," said Dziak. "We'd have to haul them up every few months to see what happened. If there was an earthquake swarm, we wouldn't know it until a year later."

A new prototype features a buoyancy system that ascends to the surface whenever the equipment records an earthquake. It then transmits a satellite signal to alert researchers to unusual seismic activity. "The portable hydrophones will give us the ability to study and compare different areas around the world," Dziak concluded. "Most of the models on seafloor spreading and plate tectonics are based on magnetic anomalies recorded every million years or so. Now, for the first time, we are able to determine exactly how these tectonic plates are moving."

Public education about undersea earthquakes and their potential hazards is a key component of such on-going earthquake research.

From January to March 2003, the HMSC Visitor Center featured a display called "The Big One: Earthquakes in the Pacific Northwest." Created by the Burke Museum of Natural History and Culture in collaboration with the region's leading earthquake experts, the exhibit focused on the geological processes that cause earthquakes, earthquake detection and measurement, the hazards they can create, and the steps everyone can take to protect their homes and families.

It also featured research that proves major earthquakes can, do, and will happen here.