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The 9 February 2010 Siheung, Korea, Earthquake Sequence: Repeating Earthquakes in a Stable Continental Region

The 9 February 2010 M w 3.1 Siheung earthquake occurred close to the metropolitan Seoul City in the central Korean peninsula and was widely felt by residents in Seoul. The shock occurred at a depth of 12 km in Precambrian crystalline bedrock with predominantly strike‐slip faulting on a near vertical fault plane striking east‐southeast–west‐northwest. A rupture radius of ∼0.12±0.01  km is estimated for the M w 3.1 shock, with a stress drop of 15.2 MPa. Sixteen small repeating earthquakes, spread in time up to about five and a half years, are detected based on their waveform similarities in a stable continental region setting. These shocks cluster along a trend striking ∼110°, which suggests a nodal plane of the mainshock focal mechanism striking N109° is the likely fault plane. The repeating events around Siheung, Korea, show a recurrence interval of 0.77±0.37  year/event for shocks of magnitude range M w 0.6–3.1. The waveform cross‐correlation detector provided identification of small repeating earthquakes with similar waveforms on an expanded time window, which may suggest the existence of such repeating earthquakes in stable regions worldwide with relatively low seismicity. Such data may lead to improvements in delineating seismogenic features, understanding the earthquake source properties, and evaluating the earthquake hazards associated with infrequent occurrence of such earthquakes in stable regions.

Online Material: Figures showing locations from HYPOINVERSE single event location and double‐difference methods, and source time functions determined using empirical Green’s function analysis.