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December 14, 1998
By Tim Stephens
On March 25, 1998, the largest oceanic intraplate earthquake ever recorded (magnitude 8.2) shook the seafloor beneath the Antarctic Ocean near the Balleny Islands. This unusual earthquake has stimulated a flurry of investigations by seismologists worldwide.
Susan Schwartz, an associate professor of earth sciences and director of the Keck Seismological Laboratory at UC Santa Cruz, analyzed the rupture characteristics of this earthquake using seismic recordings of the long-period surface waves it generated. Schwartz will present her findings in a special session of the AGU meeting devoted entirely to the 1998 Antarctic earthquake.
According to Schwartz, earthquakes of this magnitude happen every few years, but they typically occur on plate boundaries, where two plates of the Earth's lithosphere collide. This earthquake occurred within the Antarctic plate, far from its boundary and more than 60 miles (100 kilometers) from the nearest fault zone.
"There is seismicity within oceanic plates, but we've never seen anything of this magnitude before, so there is something unusual about this region," Schwartz says.
Schwartz's analysis indicates that the earthquake lasted about 100 seconds and consisted of two distinct rupture events occurring about 60 seconds apart. The location of the second event was about 120 miles (almost 200 kilometers) west of the first event, and the aftershocks were also aligned in an east-west direction.
This east-west alignment suggests the earthquake involved the rupture of an east-west fault plane, but that only adds to the quake's peculiarity because the nearest fault zone has a north-south orientation. "There are fracture zones north of the event that run north-south and, if extended, would intersect the source of this event," Schwartz says.
Because there was not a continuous rupture between the two subevents, Schwartz thinks there may have been some north-south rupturing within each subevent. "This event occurred south of the active zones, so my hypothesis is that it reactivated fossil north-south fracture zones," she says.
Extracting the details of this earthquake from the available data is not easy, however. One problem is that the earthquake occurred in a fairly remote region with few seismic recording stations located nearby. "We have a lot of recordings, but not in the key distance ranges," Schwartz says.
This is one reason Schwartz opted to focus her analysis on the surface waves generated by the earthquake rather than the deeper "body" waves. Surface waves travel many times around the Earth, generating useful data at recording stations all over the world. An unavoidable complication, however, is that seismic waves generated by the two subevents interfere with one another. "Pulling out the details of the subevents is the hard part," Schwartz notes.