Current Methods of Identifying Paleoearthquakes
Although major historic earthquakes in some areas are recorded in a written or oral tradition (an example being the notable Lisbon earthquake of 1755), many earthquake-prone areas have either not been long settled (in earthquake terms, at least) or have left no written descriptions – with California being a case in point. Until recently, therefore, the identification of historic seismic events relied heavily upon a lot of legwork.
Barrett Salisbury of Arizona State University, who led the research along the San Jacinto fault zone, explained to Decoded Science how the damage caused by major fault ruptures, which leaves its mark on the landscape for long periods of time, can be interpreted. “Fault ruptures can be preserved in a landscape for thousands of years (sometimes very subtly), so recognizing these fault scarps was initially a purely field-based endeavour” he explained.
Traditional methods have previously relied on mapping the fault traces and digging trenches from which organic material could be removed and dated. This approach is not only time-consuming but also difficult, especially in remote and inaccessible areas, or those where the area of study is obscured by vegetation.
Use of LiDAR and Other Technology
LiDAR (which stands for Light Detection and Ranging) isn’t the first use of technology in mapping paleoearthquakes – Salisbury points to aerial photography and remote sensing as its predecessors. But LiDAR is an advance upon these techniques and, he explains, “provides scientists with landscape models of nearly impeccable precision and accuracy in even the harshest of terrains” – though he also makes the point that interpretation of the data remains the responsibility of the human researcher!
The San Jacinto study used both traditional methods and LiDAR technology to examine the terrain and concluded that “results suggest that LiDAR technology is an exemplary supplement to traditional field methods in slip-per-event studies.” The study results allowed researchers to conclude that the section of the fault which they had studied has experienced three significant seismic events.
The importance of understanding past earthquakes lies in the fact that the patterns of the past increase seismologists’ understanding of earthquake mechanisms. Advances in technology, with the more accurate data and increased coverage which they bring to the canon of seismological knowledge, may perhaps move scientists a little closer towards their tantalising final goal. As Salisbury puts it: “While we hesitate to attempt earthquake prediction, we are moving towards more accurate earthquake forecasting with the help of LiDAR data.”
Salisbury, J. B. et al LiDAR and Field Observations of Slip Distribution for the Most Recent Surface Ruptures along the Central San Jacinto Fault. (2012). Bulletin of the Seismological Society of America. Accessed April 8, 2012.
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