This work is providing new insights into how faults like the San Andreas move as well as new tools to help mitigate earthquake risk in California.
The magnitude 7.4 earthquake that struck the Izmit area of Turkey on August 17, 1999, killed at least 20,000 people. This tragic event provided further support for a new technique that involves analyzing the stress transfer along faults to determine where earthquakes are most likely to occur in the future. Because an earthquake event releases part of the stress that accumulates on fault segments over time, an earthquake usually will not recur on that segment until the stress rebuilds. Typically, such a reaccumulation takes hundreds to thousands of years. However, because an earthquake also transfers and raises the stress elsewhere along the fault, a newly ãloadedä segment is likely to be the site of the next earthquakes to occur.
Turkey's 1,400-kilometer-long (870-mile-long) east-west-trending North Anatolia fault has exhibited a systematic east-to-west progression of major earthquake events. These observations, which are consistent with the USGS stress transfer model, were used by USGS scientists to estimate a 12% probability that an earthquake of magnitude 6.7 or larger would occur in the Izmit area within the next 30 years -- a prediction that was made shortly before the 1999 Izmit earthquake. The USGS has also examined the seismic history of several regions of California and has found a marked tendency for earthquakes to occur selectively on those faults that had experienced a stress increase as a result of an earlier earthquake nearby.
The USGS continues to analyze the North Anatolian fault. According to preliminary calculations, the Izmit earthquake has increased stresses on a fault segment within the Marmara area directly southeast of Istanbul. Consistent with this finding, the rate of small earthquakes under the Sea of Marmara has increased markedly since the Izmit quake. These findings, in part, contributed to USGS estimates of a 62±15% probability (one standard deviation) of strong shaking in Istanbul during the next 30 years and 32±12% during the next decade.
As part of these hazard-related studies, the USGS has been developing new hazard assessment tools. One of these is the ãCoulombä computer program, a public domain software package that can be used to analyze stress transfer on faults. The USGS is actively transferring this technology to earthquake experts in Turkey. With the support of OFDA, it recently held a ãCoulombä training course in Turkey.
USGS Mission Tie In:
This work directly contributes to USGS efforts to understand the earthquake hazards associated with the San Andreas Fault and efforts to mitigate those hazards. Further, it also contributes to the USGS commitment to provide technical support to other Federal Agencies. This project has been supported by the Office of Foreign Disaster Assistance of USAID.
Partners:
Kandilli Observatory & Earthquake Research Institute
Istanbul Technical Univeristy
SwissRE (through a USGS CRADA)
Office of Foreign Disaster Assistance (OFDA)
