Foreshocks and the emergence of an Eastern Marmara M4.2 earthquake
Abstract
We report on an apparently-emergent failure process leading to an Mw4.2 earthquake. This event occurred along the offshore Marmara section of the North Anatolian Fault Zone in Turkey. In this area, close to the 15-million population center of Istanbul, an M>7 event is believed to be likely in the next several decades.
By chance, a 61-event sequence was seen framing the M4.2 during a 5-day start-up test of the TESV borehole seismic array. TESV is part of the 7-station Geophysical Observatory at the North Anatolian Fault (GONAF) network in the eastern Marmara. Each GONAF site includes vertical and 3-C seismic sensors at 75 m intervals along 300-m deep boreholes. These sensors form an array with a local detection threshold of M 0. At this level it was found that at least 18 foreshocks occurred in the 48 hours before the M4.2. To assess whether this sequence represents a change from background activity, an ANalysis Of Variance (ANOVA) test was done on longer-term AFAD catalogue data. The results indicate that the time and distance relations of the sequence are unique for the year centered on the M4.2. Travel times, particle motions and cross-correlation of waveforms suggest spatial clustering of the entire sequence within 1 km² - about the source size of the M4.2. Running averages of the waveform cross-correlation coefficients significantly increased during the hours before the M4.2. This type of clustering of small events before a notably larger one is commonly seen in lab- and mine-scale studies. Such emergent failure processes have not commonly been observed for moderate earthquakes. As the data shown here suggests, it is possible that past network detection thresholds have not been low enough for seeing these foreshocks. If so, then real-time waveform correlation of borehole seismic recordings may help improve short-term earthquake forecasting and risk reduction.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.S51B..01M
- Keywords:
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- 7215 Earthquake source observations;
- SEISMOLOGYDE: 7223 Earthquake interaction;
- forecasting;
- and prediction;
- SEISMOLOGYDE: 7230 Seismicity and tectonics;
- SEISMOLOGYDE: 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICS