What we know about the past seismicity on the Horda Platform region

In order to obtain some estimates of the seismic hazard potential in the Horda Platform region, we consider the seismicity database of the National Norwegian Seismic Network with entries of the past 35 years. The general statistical distribution of earthquakes occurring in a certain region is often described in terms of the Gutenberg-Richter law and shows a log-linear distribution between earthquake magnitude and frequency of occurrence. For the global average of naturally occurring tectonic seismicity Gutenberg-Richter found that the b-value is equal or close to 1.

Log10 (N) = a – bM

N: number of earthquakes having a magnitude > M

a- and b-value:  constants depending on the occurrence and size distribution of the earthquakes in the catalogue. a represents the activity level, while b describes the scaling relation between larger and small events.

Any local deviation from the b-value may provide insights to the local tectonics and the way stresses are built up and released over time. For b-values larger than one, there will occur relatively more of the small magnitude events compare to larger magnitude events. For b-values smaller than 1, there will be a relative lack of small magnitude events compared to larger magnitude events, again compared to the general observation that there are ten times more earthquakes for decrease in the order of magnitude. 

In the below figure we can also see the current detection level, defined by the magnitude of completeness, Mc. All events that are larger than Mc (here about 1.4) will be detected with the existing NNSN network. Smaller events will also be detected, however, due to noise level variations, not all of the smaller events can be detected, identified and located. The observed slope of the linear fit to the distribution, the b-value, is close to 1, suggesting a normal tectonic regime in this region. The larger the event magnitude, the lower is the probability that it may occur, often described by long recurrence intervals. Within the past 30 years about ten earthquakes with magnitude larger than M3 occurred in this region, and these were occasionally felt by humans in the coastal communities along the Westcoast of Norway.

Figure: Relationship between magnitude of earthquakes and their occurrence within the Horda platform region.