Uncertainties and inaccuracies of earthquake hypocenter absolute locations: one geothermal application and one theoretical development

Emmanuel Gaucher


Date & Time
Building 3, Room 3240 (main USGS conference room)

Earthquake hypocenters constitute a unique source of information for understanding the physical processes at the origin of earthquakes, for describing the subsurface and for quantifying earthquake seismic hazard. However, location errors exist and need to be properly quantified because together with the earthquake hypocenter they determine the scale of meaningful investigation.

The earthquake location error can be described as the combination of two quantities: the location inaccuracy and the location precision. The latter is taken equivalent to the a posteriori location uncertainty resulting from the propagation in the inverse location problem of the a priori uncertainty. On the contrary, the location inaccuracy is defined as the wrong positioning of the hypocenter due to all effects that have been overlooked in the inverse problem.

In a first hand, we investigate the effect of velocity model errors on the determination of earthquake absolute hypocenters. This analysis is conducted for the Rittershoffen geothermal field which is located in the Upper Rhine Graben (France). To do so, we first generate a 3D synthetic cloud of seismic events in the geothermal reservoir and calculate the associated travel-times to the seismic monitoring network assuming several propagation media. The second step consists in relocating the events using a non-linear absolute location procedure, however within a reference 1D velocity model. Thus, we introduce controlled velocity model errors between the synthetic and the relocation phases, and the knowledge of the Rittershoffen field allows us simulating realistic subsurface scenarios. Result analysis shows neither constant nor aleatoric location errors at this scale and emphasizes the clear differences existing between the two entities, uncertainty and inaccuracy, with regards to their distributions and amplitudes. As a consequence, positioning and orientation of features delineated by seismicity may be strongly distorted and difficult to correctly interpret from uncertainties alone.

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