Most major earthquakes are on subduction zones, where one of the of the Earth’s tectonic plates descends beneath another. But large earthquakes occur on transform boundaries too, where the plates slide past one another. And while none of these has made the United States Geological Survey’s list of the largest earth tremors since 1900, there are noteworthy examples along, in particular, the San Andreas fault zone (SAFZ) where San Francisco’s infamous 1906 earthquake may have been as large as magnitude 8.
A large earthquake on one of the world’s major transform faults, which include the North Anatolian and Dead Sea fault zones as well as New Zealand’s Alpine fault, places large numbers of people at risk.
The numbers are huge: the population of San Francisco’s Bay area exceeds 7 million while Istanbul is home to over 13 million.
With so many people living in close proximity to transform margins such as these, it’s vital to understand the mechanisms of earthquakes in order to make a realistic assessment of seismic hazard – and attempt to mitigate risk. A study looking at earthquakes along the length of the North Anatolian Fault Zone (NAFZ) has enhanced our knowledge of what we might expect from such an earthquake
The North Anatolian Fault Zone (NAFZ)
The NAFZ is a major transform fault which runs through the north of Turkey, extending from eastern Anatolia, passing by the Bosphorus and terminating somewhere in the north Aegean Sea. Here the North Anatolian tectonic block is moving westwards at a rate of a few centimetres each year in relation to the Eurasian continent which lies to the north.
A key feature of this setting is a series of earthquakes which began in 1939 with an M~8 near the city of Erzincan and culminated in a devastating M7.4 event in Izmit in 1999. The earthquakes occurred at intervals varying from a few months up to 32 years; and they moved westwards, indicating that earthquakes on the fault were propagating in that direction.
Izmit, site of the most recent tremor, lies only around 100km to the east of central Istanbul. Though earthquakes remain unpredictable, there’s an unquestioned threat to Turkey’s capital and its population on the basis of what seismologist Robert Yeats describes as “evidence that is fairly strong within our state of knowledge.”
In research presented to the Seismological Society of America’s 2014 annual meeting, Professor Marco Bohnhoff of Germany’s Research Center for Geosciences (GFZ) concluded that the maximum size of an earthquake increases with the maturity of the fault. In other words, the older and more developed a fault system is, the larger an earthquake could be – a theory which has been previously discussed but not previously confirmed.
This has important implications for those who live on such fault zones, as Professor Bohnhoff explained to Decoded Science. “We have shown based on a historical earthquake catalogue for Anatolia that the maximum size of earthquakes – and thus the subsequent seismic hazard- scales with the maturity of transform fault zones. In other words the older the earthquake zone the larger the maximum possible earthquakes.”
How Old is the Fault?
Determining the maturity of a fault is a tricky business. Seismographs as we know them have been around only since the beginning of the twentieth century so that seismologists have to look elsewhere for information.
In looking at the NAFZ, Prof Bohnhoff and his team drew on two key sources – a rich written record which includes descriptions of historic but uncatalogued earthquakes extending back for around two millennia; and a geological field study which measured offsets in key landscape features such as river valleys. Integrating the two allowed them to determine that the fault is better developed (more mature) in the east than in the west – and that earthquake magnitude increased in the same direction.
As a result, the research team were able to conclude that there’s a probable maximum size for an earthquake in the Istanbul segment of the fault. Such an earthquake, he said, “will probably not be larger than 7.4 or 7.5.”
At one level this is good news (one order of magnitude difference is roughly a factor of 30 difference in released energy) but it doesn’t mean that Istanbul is safe. “The seismic hazard and risk in the Istanbul region very high given the small distance (~20km) between the fault and the population centre of Istanbul with its more than 13 million inhabitants,” he warned.
Applying the Research: Seismic Hazard
How can the research be applied, given that earthquakes cannot be predicted with any degree of certainly (except at the most general level)?
What it can do is help to refine estimates of seismic hazard. Professor Bohnhoff tells us: “We can give probabilities for the occurrence of earthquakes of a given magnitude at a given location within a certain time interval. In the case of Istanbul the probability is 35-70% for a M>7 earthquake in the next 30 years, a very high probability.”
Consideration of other faults is a little more difficult as nowhere but Anatolia offers the combination of written record, which offers dating and clues to magnitude, and on-the-ground studies.
The SAFZ has only a limited historical record, although it has been and continues to be among the most-studied regions of the world. And the Dead Sea Transform fault zone, which runs from the Red Sea into southern Anatolia, has the long historic record (including earthquakes as potential sources for Bible stories such as the fall of Jericho and the destruction of Sodom and Gomorrah) but so far has only limited instrumental studies.
Bigger Earthquakes: Older Faults
The confirmation of the theory that earthquake magnitude is related to the age of a fault won’t help to predict the timing of an earthquake, nor do anything to prevent it – but it does have the potential to aid civil authorities in combating this hazard in Istanbul and elsewhere.
“Available data from both fault zones are in accordance with our findings from the NAFZ,” concluded Professor Bohnhoff. “In that sense our results might as well serve for a reassessment of their seismic hazard.”