Predicting the frequency of seismic events for subsurface engineering projects usingbackground microseismicity data
Authors: Dafnioti Eleni , Chatzopoulos Georgios , Pytharouli Stella , Vallianatos Filippos , Lunn Rebecca 
Published: February 2023

The general public is concerned about subsurface energy projects and their link to seismic activity.However, in order to meet the Net Zero requirements, low carbon technologies involving fluidinjection into the subsurface, such as geothermal, Carbon Capture and Storage (CCS) and gasstorage, must be developed and implemented. In an effort to enhance their confidence, it isimportant to comprehend the seismogenic effects and the triggering mechanism that suchtechnologies could have in a region.The frequency of occurrence of microearthquakes (earthquakes with magnitude &lt; ML0 in theGutenberg &#8211; Richter scale) in natural faults can provide crucial details about the evolution ofseismicity of a region and the geological structures that support this. The permanent seismicmonitoring networks, due to their detection threshold on small magnitude earthquakes (&lt; ML1), donot enable for the long &#8211; distance recording of all microearthquakes. These events have higherfrequencies that attenuate fast, making detection at stations 10s or more kilometres away difficult,if not impossible.In order to determine whether microearthquakes share the same characteristics as largerearthquakes and whether they can be used as a precursor to the occurrence of larger earthquakes,this research focuses on the study of the characteristics of natural microseismicity (frequency ofoccurrence, magnitude distribution, depth of hypocentres etc.). In July 2022, we deployed atemporary microseismic monitoring network in the southern Heraklion prefecture (Crete, Greece) incollaboration with the Hellenic Seismological Network of Crete with the aim of recording very smallin magnitude earthquakes (within the range of -0.5 &lt; M &lt; 3, i.e. two orders of magnitude below thecurrent completeness magnitude of M c 2.0). The network consisted of seven short periodseismometers, one of which was placed in the centre of a nearly circular geometry with theremaining six distributed around it at a radius of approximately 6.1km. This geometry provided agood azimuthal coverage for determining the hypocentres.In this work, we present analysis and results of the microseismic data collected from our localmonitoring network. We find a significantly larger number of microseismic events than that reportedfor the same time period within 50 km distance from the central station of our local network in thepublished seismic catalogue by the National Observatory of Athens. The number of detected eventswe report here refers to only those events that can be visually observed in the recordings. The realnumber of smaller in magnitude events is larger but obscured by noise. We use the detectedmicroseismic events to populate the Gutenberg Richter magnitude distribution for lower magnitudeevents and discuss the implications on the existing Gutenberg Richter magnitude distribution for theregion as derived from events recorded by the permanent seismic network in Crete.