Microearthquakes preceding a M4.2 Earthquake Offshore Istanbul

A primary hurdle in observing small foreshocks is the detection-limit of most seismic networks, which is typically about magnitude M1-1.5. We show that a start-up test of a borehole-based seismic network with a much lower detection limit overcame this problem for an M_w4.2 earthquake. This earthquake occurred offshore of Istanbul, Turkey, on a fault system that is likely to rupture in an M > 7 event in the coming decades. In the three days before and two after, a total of 62 or more earthquakes, including at least 18 foreshocks, came from the mainshock source area. The signal similarity of the foreshocks shows a clear increase during the hours before the M_w4.2 mainshock. Similar foreshock sequences have recently been reported for a few well monitored M > 7 plate-boundary earthquakes. The sequence surrounding the M_w4.2 gives the impression of stochastic failures that ended up interactively unloading stress concentrations. The M_w4.2 mainshock then resulted from the accumulated release of significantly smaller events, as suggested by other field and laboratory studies.