There are few places in the world that more closely match the seismic hazard potential of the San Francisco Bay Area of California than the Marmara Sea area of Turkey. In both places a magnitude 7 earthquake threatens to shake a maritime megacity. The current population of Istanbul is approaching 15 million, with an annual tourist population of 3 million, and the probability of an M7.1 earthquake in the next 20 years estimated as 35-70%. In 1766 a previous earthquake, similar to the one anticipated, destroyed Istanbul when its population was less than 0.5M. Some believe that a repeat of this earthquake is imminent. GeoGONAF is an acronym for an international collaborative project to monitor the subterranean and submarine strain changes that will precede the earthquake. Its scientific team does not expect to predict the precise time of this future earthquake, but they are optimistic that the dense array of monitoring instruments now listening closely to the approaching rupture will capture a rich harvest of signals that will indicate which of the several branches of the North Anatolian fault is most likely to rupture. It has taken the better part of decade to install these instruments around the city and on offshore islands, but now a stream of data flows into laboratories in Turkey, Germany and the US, that in principle permit the scientific teams of these collaborating nations to identify signals characteristic of approaching rupture. Lessons learned in Turkey will have direct application in the interpretation of similar precursory processes occurring beneath the Hayward fault located in the San Francisco Bay.
Six strain-meters were installed in boreholes during the final phase of GeoGONAF (the Geodetic component of the Geophysical Observatory to study the North Anatolian Fault near Istanbul). The strain-meters can detect strain changes of 10e-12 (1 picostrain), equivalent to a change in the diameter of the Earth of one fifth of the diameter of a human hair. The stabilization phase following inserting these ultra-sensitive instruments at depths of 30 m to 100 m underground is almost over (1-3 years) and six streams of strain data at 100 Hz sampling rate (as of June 2016) now complement data from the borehole seismometers and GPS instruments installed by the German, US and Turkish collaborators of GeoGONAF. The current project plans first to calibrate the strain-meters using tides and inbound seismic waves, and then to monitor episodic and continuous signals occurring in the Marmara region. Specifically a search for slow slip events on the submarine faults south and southeast of Istanbul is planned. These slow creep events are invisible to seismometers but are seen as increments on GPS data and as distinctive time-evolving signals on strain-meters with durations of minutes to days. On 18 March 2016 a large slow slip events on the first of the six strain-meters to be commissioned was captured in remarkable detail, its importance being that such events incrementally advance rocks already poised for imminent rupture towards a future earthquake. Although each increment brings an earthquake closer, it is too early to know on which of three faults south of Istanbul this will occur. The proposed project will examine incoming strain and GPS data on a daily basis to search for coherent signals occurring synchronously on the array of sensors, in order to identify slipping patches of faults and the area and extent of their slip. Preliminary interpretations indicate that large creep events cannot occur more than once per year on any given patch of a fault but that creep events may be quite frequent throughout the interconnected network of faults beneath and near the Sea of Marmara. As part of a larger study of the fault system additional surface creep-meters will be installed specifically to identify the propagation characteristics of 2-16 mm amplitude creep-events along the North Anatolian fault to the east of Marmara.
