Observations of Repeating Earthquakes at a Single Seismic Station Near Lake Sapanca

Abstract

The North Anatolian Fault Zone (NAFZ), which forms the plate boundary between Anatolian Plate and Eurasian Plate, is one of the most active transform fault zones in the world. Following two consecutive magnitude M>7 earthquakes in 1999, an intensified monitoring of western portion of NAF is commenced. Dense networks of onshore/offshore seismic, acoustic, geodetic sensors and surface creep and strain sensors were installed. A single seismic sensor among these, which is located at the midpoint of 1999 ruptures, near Lake Sapanca, exhibits some unusual seismic activity. On a fault segment where creep is known to be present, a series of minor seismic events was observed with identical locations and a recurrence time of three years. These events are quite short in duration and highly similar in their waveforms. Using a single station approach, their angle of incidence and back azimuth were found to coincide with the location of two M2.3 and M2.1 events. At this stage, it is not clear whether these events reflect fault creep at seismogenic depth. Nevertheless, these initial observations emphasize the necessity of monitoring this segment more densely, where recurrent minor earthquakes are likely to be observed.