Detection and Characterization of Seismic Events from Twelve Months of Continuous Data Recorded at the Onitsha Seismic Station, Anambra State

The characterization of local and regional seismic activity in southeastern Nigeria depends critically on continuous, high-quality waveform recordings from strategically located monitoring stations. This study detects and characterizes seismic events recorded over a twelve-month period at the Onitsha Seismic Station operated within the Nnamdi Azikiwe University geophysical network in Anambra State. A continuous data stream recorded by a three-component broadband seismometer was archived in MiniSEED format and analyzed using SeisComP3 and MATLAB-based signal processing routines. An automated STA/LTA trigger algorithm was applied across the data stream, followed by manual analyst review to confirm seismic event classification. A total of 248 trigger events were detected, of which 163 were classified as tectonic earthquakes, 47 as quarry blasts, and 38 as unclassified. Local magnitude estimates for the tectonic events range from Ml 0.4 to Ml 3.2. Hypocentral locations computed from P and S arrival time differences place the majority of events within the Anambra Basin and along the southern Benue Trough. Focal mechanism solutions derived for 14 well-recorded events indicate predominantly normal and strike-slip faulting consistent with the regional extensional tectonic regime. Quarry blast events are identified by their characteristic waveform envelope, surface wave deficiency, and temporal correlation with daylight working hours. Spectral analysis reveals corner frequencies between 4 and 18 Hz for local events, providing estimates of source stress drop. The dataset constitutes a reference catalogue for seismicity in the Onitsha area and contributes to improved seismic hazard assessment for Anambra State.

Keywords: seismic monitoring, STA/LTA detection, focal mechanism, Anambra Basin, local magnitude