Detailed Analysis of a Multiplet Earthquake Sequence

The Mexican National Seismological Service reported a sequence of four small earthquakes (2.5 < M < 3.0) occurring in Morelia, a city of 1,000,000, which is the capital city of Michoacán State. A careful revision of the records from a three-component broad band station, located ~10 km far from the earthquakes, showed a sequence of 7 earthquakes in a period of about 36 hours. Waveforms are remarkably similar between them and they may be considered as a "multiplet". In this work, we use the records from the broad-band station and a coda wave interferometry based methodology to obtain the relative distance between pair of events. The 21 inter-event distances obtained are considered as over-determined system for the relative positions between events. A non-linear damped scheme is used to solve the over-determined system and to obtain the spatial distribution of the 7 earthquakes. Results show (1) distances between events are < 200 m, and (2) the sequence has an approximate linear distribution.