Multiplets as a tool for identification of seismogenic structures at various geothermal fields

Identification of seismogenic zones in geothermal fields undergoing active fluid injection is an important issue for seismic hazard assessment in such destinations. It is well known that subsurface discontinuities can be successfully imaged using multiplets, i.e. seismic events with very similar waveforms. Very promising results of this method were obtained using the dataset from Prati-9 and Prati-29 injection wells at The Geysers geothermal field. With the use of multiplet analysis followed by double-difference relocation we imaged three fractures and one fault within the reservoir and described their different seismic response to injection.

In this work we present the current results of multiplet identification followed by double-difference relocation in two other geothermal sites: (1) Helsinki geothermal site (Finland), and (2) Coso geothermal field (California). The mentioned geothermal sites exhibit very different geological conditions. Helsinki site is a typical example of geothermal stimulation of crystalline Precambrian basement rocks in the area of very low background seismicity. On the contrary, Coso geothermal field is located in tectonically active volcanic area cut with a complex system of faults. Moreover, we extend the multiplet analysis performed in the area of Prati-9 and Prati-29 injection wells at The Geysers geothermal field by searching for multiplets in various frequency ranges. In this way seismic events from broader magnitude range can be included in the following relocation procedure. The multiplets identified within each frequency range are relocated separately. At the end, obtained images are stacked together using common reference events.

Our results confirm that relative relocation of similar seismic events with double-difference method can be successfully applied for the identification of seismogenic structures in geothermal areas exhibiting very different geological and tectonic complexity.

This research was supported by research project no. 2022/45/N/ST10/02172, funded by the National Science Centre, Poland, under agreement no. UMO-2022/45/N/ST10/02172. This work was also partially supported by a subsidy from the Polish Ministry of Education and Science for the Institute of Geophysics, Polish Academy of Sciences. This research was supported in part by PLGrid Infrastructure.