Older Faults Produce Larger Earthquakes, Seismologist Finds

New research by seismologists suggests that the magnitude of an earthquake scales with maturity of the fault line, meaning the older the fault is, the larger the earthquakes it can produce.

The results of the study were announced this week at the Seismological Society of America (SSA) 2014 Annual Meeting in Anchorage, Alaska.

Marco Bohnhoff, a professor of geophysics at the German Research Center for Geosciences in Potsdam, Germany, led the research, which focused on two well-known fault lines, the San Andreas Fault in North America, and the North Anatolian Fault Zone (NAFZ), a system of fault lines that extends across Turkey.

The San Andreas Fault and the NAFZ share similar properties; they are both known as "continental transforms" and have similar lengths and slip rates. Basins beneath the sea in both California and Turkey are being pulled apart by the strike-slip system that each of the fault zones creates.

Istanbul, with its population of 18 million, is at particular risk from a large earthquake along the NAFZ, therefore, identifying the maximum earthquake magnitude in the fault zone is critical for seismic hazard assessment, the researchers said.

"It has been argued for decades that fault systems evolving over geological time may unify smaller fault segments, forming mature rupture zones with a potential for larger earthquake,"Bohnhoff said in a statement form the SSA. "With the outcome of this study it would in principal be possible to improve the seismic hazard estimates for any transform fault near a population center, once its maturity can be quantified."

Istanbul has been populated for more than 2,000 years, and the city has a rich history of written records and data which catalog earthquakes in the NAFZ region.

"What we know of the fault zone is that it originated approximately 12 million years ago in the east and migrated to the west," Bohnhoff said. "In the eastern portion of the fault zone, individual fault segments are longer and the offsets are larger."

The researchers report that the largest earthquake to occur along the NAFZ were approximately magnitude 8 and were exclusively observed along eastern section of the fault zone, which is older. The younger sections of the fault zone, to the west, have historically produced earthquakes no larger than magnitude 7.4, Bohnhoff said.

"While a 7.4 earthquake is significant, this study puts a limit on the current seismic hazard to northwest Turkey and its largest regional population and economical center Istanbul," Bohnhoff explained.

When compared to the San Andreas and the Dead Sea Transform Fault systems, which are geologically similar, the analysis of the NAFZ still stood, Bohnhoff said.

Last year, Bohnhoff published a study that predicted the epicenter of the next big earthquake near Istanbul to be just south of the city in the Sea of Marmara.