Finite-fault slip inversion for the Mw 6 Mutata earthquake (2016), Colombia

Abstract

Even though the amount and quality of data now available for seismologists is increasing in Colombia, seismic or GPS stations near the event are usually unavailable. For this reason, the usage of far-field stations is necessary especially for procedures such as slip inversion. In the present study, we investigate the slip distribution for the Mutata earthquake, Colombia. This is one of the strongest and shallowest events that had occurred in Colombia in the past few decades. On September 14, 2016, a Mw 6.0 earthquake occurred in Northwestern Colombia on the Uramita fault very close to a small town called Mutata, nearby Medellín. The event occurred at a depth of 18km in the proximity of both, the Nazca - South America subduction zone (to the west) and the Caribbean - South America subduction zone (to the North). The USGS moment tensor given for this event shows a thrust fault focal mechanism with a strike of 121º and dipping at 35º.

To calculate the slip distribution, we merged different functions from ObsPy, MudPy and Syngine in order to retrieve, process and invert the broadband seismic data provided by the USGS and the RSNC (Red Sismológica Nacional de Colombia) including near- and far-field stations. First, we defined the fault plane for inversion using the moment tensor and regional geologic information. We then divided the fault plane into 300 rectangular subfaults in order to calculate the slip amplitude for each patch in the source region. The green functions and synthetics are calculated using different codes for the far-field (Syngine) and the near field (MudPy), both applying a triangle source time function with different durations to identify the correct rise time and fall time for this event. The synthetics and observed seismic signals are finally processed using ObsPy and the inversion is done using MudPy. After calculating the slip distribution, we find two main asperities at depths of approximately 20km with a maximum slip of about 20 cm. The two asperities are located on the left and right sides of the mainshock hypocenter. The aftershocks are first relocated and then compared with the slip distribution in order to investigate coseismic triggering by the mainshock. This kind of study is one of the first carried out for a Colombian earthquake and will help to better define fault systems and seismic hazard in this area.