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October 15, 2001

Geologists increase estimates of earthquake frequency

Study is based on age of marine terraces on Santa Cruz coast

By Tim Stephens and Becky Oskin

The marine terraces that rise above the Santa Cruz shoreline are surprisingly young, according to a new analysis by UCSC researchers. The study suggests that earthquakes, which incrementally uplift the local landscape, may hit Santa Cruz much more frequently than previously thought, said Lesley Perg, who conducted the study while earning a Ph.D. in Earth sciences.

Five ancient marine terraces rise from the shoreline in the Santa Cruz area.
Photo: Lesley Perg
Perg used a dating technique never before applied to marine terraces--flat expanses of former seafloor--to determine when they emerged from the sea. Since the tectonic forces that uplift the terraces also generate earthquakes, a faster uplift rate means more frequent earthquakes.

"I'm proposing that the uplift rate is three to four times faster than previously suspected, which implies that the earthquake recurrence interval is three to four times shorter," said Perg, who is currently doing postdoctoral research at the University of Hannover in Germany.

Santa Cruz sits on a portion of the Earth's crust that is slowly squeezed upwards between the San Andreas and San Gregorio Faults, said Robert Anderson, Perg's adviser and a professor of Earth sciences. "A slight kink in the San Andreas, about where Highway 17 crosses the mountain crest, creates particularly high stresses," he said.

When the crust shifts to release the stress, it generates earthquakes such as Loma Prieta and lifts the marine terraces. Perg noted she is not predicting more earthquakes, but is only revising estimates of earthquake frequency. Her results apply to local earthquakes, such as the 1989 Loma Prieta quake, she said. Perg's new estimate of the recurrence interval for similar major earthquakes (magnitude 6.9) is 160 to 270 years.

"That's assuming all of the strain is released through such major earthquakes, which is not necessarily the case," she said.

Perg, Anderson, and coauthor Robert Finkel of Lawrence Livermore National Laboratory published their findings in the October issue of the scientific journal Geology.

The Santa Cruz marine terraces formed over tens of thousands of years through a combination of tectonic uplift and cyclical changes in sea level. Sea level falls during an ice age, when huge ice sheets trap water on the continents, and rises when the ice sheets melt. Waves carve the broad terrace platform during the period between ice ages when sea level is high. As the next ice age begins, sea level drops, the platform is progressively exposed, and a thin blanket of beach sand is deposited on it.

"The trick is to raise the terrace before the next ice age ends," Perg said.

When the glaciers melt again, the waves cut a cliff into the uplifted terrace and create a new platform below it.

There are five of these marine terraces in the Santa Cruz area. Highway 1 traverses the smooth plain of the lowest and youngest terrace. An ancient sea cliff marks the seaward edge of the second terrace, and the third forms the base of the UCSC campus. The fourth and fifth terraces are highly eroded at the UCSC campus, but they remain intact in Wilder State Park and further up the coast.

Dating the terraces is essential to determine the frequency of earthquakes in Santa Cruz. Until now, geologists had no way to date them accurately, Anderson said. "We've been running blind in terms of absolute dates," he said.

The terraces are too old for dating techniques that rely on carbon-containing material. Instead, geologists have referred to corals in the South Pacific that record the same cycle of sea level rise and fall that created the terraces. But the correlations between the terraces and the corals are fuzzy at best, Anderson said.

Perg's method involves analyzing grains of quartz in the soil of the terraces to detect so-called "cosmogenic radionuclides"--atoms that have been created through the splitting of other, larger atoms in the quartz by the impact of cosmic rays. Cosmic rays are highly energetic particles that constantly bombard the Earth from space. Over long periods of time, the rate at which cosmic rays create radionuclides in quartz grains is relatively constant, like the ticking of a clock.

Geologists have used this radionuclide clock to date ice cores and lava flows, and Anderson and his graduate students have used it successfully to date river terraces in Wyoming and Utah. To analyze her samples, Perg used an accelerator mass spectrometer at Lawrence Livermore National Laboratory.

She began by dating the third terrace. The resulting age of 133,000 years was much younger than she had expected. Although Perg's results differed from the expected age by 100,000 years, she had done everything correctly in conducting the analysis, Anderson said. With his encouragement, she analyzed all five terraces. Perg soon had a consistent set of results showing that all the terraces are relatively young, ranging from 58,000 years for the youngest to 212,000 years for the fifth and oldest terrace.

Using the radionuclide dates, Perg calculated an uplift rate of about 1.1 millimeters per year for the Santa Cruz marine terraces. Previous estimates ranged from 0.20 to 0.46 millimeters per year.

Perg noted that the radionuclide dating technique can be used for a wide range of geologic features that would otherwise be undatable.

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