Extrapolation of microearthquake populations to predict consequences of
low-probability high impact events: the Pohang case study revisited
Abstract
The November 2017 magnitude (MW) 5.5 Pohang, Korea, earthquake, induced
by an Engineered 24 Geothermal Systems (EGS) project, caused one
fatality and ~US$300M of economic 25 consequences. It
has been proposed that a significant probability of such losses was
predictable 26 beforehand, from the small earthquakes caused by
well-stimulation, so the project should have 27 been suspended, implying
that its developer was remiss for not doing so. This argument depends 28
on the low (~0.61) estimated b-value of this earthquake
population. However, it is shown that 29 many of the magnitude
determinations are inaccurate (underestimated) and the true b-value is
30 higher (1.12 for one subset). The probability of any earthquake as
large as MW=5.5, predicted 31 beforehand by extrapolation, was thus much
lower than has been claimed. This analysis 32 highlights the necessity
of taking care over accuracy when reporting datasets like this,
especially 33 in situations where such analyses might influence criminal
trials of EGS developers.