Lisa Grant Ludwig, a principal investigator on the study, first visited the Carrizo Plain about 20 years ago, digging trenches in an area west of the Panorama Hills known as the Bidart Fan.
By looking at the pattern of soils and using radiocarbon dating on charcoal deposits, she found evidence of five large earthquakes dating back to the early 1200s. She found a gap of some 400 years between the 1857 earthquake and the one before, but only about 100 years separating the three preceding quakes.
Back then, the earthquake age estimates were very rough and the samples had to be fairly large, about the size of a jelly bean. Ludwig saved field notes and hundreds of soil samples in glass vials in her garage for more than 15 years, hoping that radiocarbon dating techniques would improve.
Once the technology improved, Ludwig and her colleagues could date samples with much higher precision and analyze charcoal flakes as small as the tip of a pencil.
They went back to her archive, and the redating effort, led by scholar Sinan Akciz, found that the four big earthquakes before the 1857 temblor probably occurred around 1310, 1393, 1585 and 1640.
"We were better able to constrain the dates and show that actually these five earthquakes were pretty evenly spaced," Ludwig said.
Because they are looking at only a handful of earthquakes, scientists can't be sure that the pattern will hold, Ludwig said.
"But we know it increases the probability of an earthquake," she said. "There's not any way I can look at the data and be comforted by it."
Ludwig's team has dug some new trenches in the area to supplement the redating project, hoping to find new soil samples that show the increased frequency of large earthquakes.
Results won't be finalized for a few months, Ludwig said, but preliminary analysis suggests that the time interval between earthquakes may be even shorter, something on the order of 100 years.
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jia-rui.chong@latimes.com
