Creep of the Main Marmara Fault at the west of the Kumburgaz Basin observed by acoustic extensometers

The Main Marmara Fault (MMF) keeps unruptured since the last Marmara earthquake in 1766, while the most part of its extension along the North Anatolian Fault has released its strain through M7 class earthquakes progressively occurred during the last century. Revealing the coupling condition MMF is quite important because it controls magnitude of the next earthquake expected to be occurred.

Since the MMF is covered with sea water, acoustic ranging technique is required to monitor the fault behavior in geodetic measure. So far, two experiments were carried out to measure the creep rate of the MMF using acoustic extensometers; no significant creep at the west of Central High (Sakic et al., 2016), while nearly a half of the strain is released by creep at the Western High (Yamamoto et al., 2019). We further conducted the experiment at the west edge of Kumburgaz Basin to fill the spatial gap of the two experiments.

The experiment started from May 2017 installing five extensometers across a valley formed by MMF activity. Acoustic ranging was successfully operated between nine pairs out of ten combinations. The longest one is up to 3 km and the shortest one is just 0.5 km. The observed data were extracted remotely from a ship via acoustic communication while the measurement continues. We already extracted the data for nearly two years from the beginning to April 2019, just before the interruption due to COVID-19. Change rates in baseline length are evaluated using roundtrip times, which are converted into distance using sound speed corrected for in situ temperature of sea water. Combining change rates and crossing angle of their baselines with MMF, nearly 10-15 mm/yr of right-lateral creep is expected at the site.

Obtained result taken together the past experiments, the west half of MMF is partially creeping and the east half is rocked, which indicates that MMF still has a potential for over M7 earthquake. This distribution creep is consistent with electromagnetic imaging beneath the Sea of Marmara (Kaya-Eken et al., 2025). Our extensometers were still working at least the time of the Mw6.2 Marmara earthquake in April 2025, which occurred just beneath our site as a series of eastward progressive intermediate ruptures (Martinez-Garzon et al., 2025). We are expecting further data analysis after 2019.