TY - JOUR A1 - Martinez-Garvon, Patricia A1 - Bohnhoff, Marco A1 - Mencin, David A1 - Kwiatek, Grzegorz A1 - Dresen, Georg A1 - Hodgkinson, Kathleen A1 - Nurlu, Murat A1 - Kadirioglu, Filiz Tuba A1 - Kartal, Recai Feyiz T1 - Slow strain release along the eastern Marmara region offshore Istanbul in conjunction with enhanced local seismic moment release JF - Earth & planetary science letters N2 - We analyze a large transient strainmeter signal recorded at 62.5 m depth along the southern shore of the eastern Sea of Marmara region in northwestern Turkey. This region represents a passage of stress transfer from the Izmit rupture to the Marmara seismic gap. The strain signal was recorded at the Esenkoy site by one of the ICDP-GONAF (International Continental Drilling Programme - Geophysical Observatory at the North Anatolian Fault) strainmeters on the Armutlu peninsula with a maximum amplitude of 5 microstrain and lasting about 50 days. The onset of the strain signal coincided with the origin time of a M-w 4.4 earthquake offshore Yalova, which occurred as part of a seismic sequence including eight M-w >= 3.5 earthquakes. The Mw 4.4 event occurred at a distance of about 30 km from Esenkoy on June 25th 2016 representing the largest earthquake in this region since 2008. Before the event, the maximum horizontal strain was subparallel to the regional maximum horizontal stress derived from stress inversion of local seismicity. During the strain transient, we observe a clockwise rotation in the local horizontal strain field of about 20 degrees. The strain signal does not correlate with known environmental parameters such as annual changes of sea level, rainfall or temperature. The strain signal could indicate local slow slip on the Cinarcik fault and thus a transfer of stress to the eastern Marmara seismic gap. KW - strain transient KW - slow slip KW - transform faults KW - seismic hazard KW - strainmeter data KW - Sea of Marmara Y1 - 2019 U6 - https://doi.org/10.1016/j.epsl.2019.01.001 SN - 0012-821X SN - 1385-013X VL - 510 SP - 209 EP - 218 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Kwiatek, Grzegorz A1 - Saarno, Tero A1 - Ader, Thomas A1 - Blümle, Felix A1 - Bohnhoff, Marco A1 - Chendorain, Michael A1 - Dresen, Georg A1 - Heikkinen, Pekka A1 - Kukkonen, Ilmo A1 - Leary, Peter A1 - Leonhardt, Maria A1 - Malin, Peter A1 - Martinez-Garzon, Patricia A1 - Passmore, Kevin A1 - Passmore, Paul A1 - Valenzuela, Sergio A1 - Wollin, Christopher T1 - Controlling fluid-induced seismicity during a 6.1-km-deep geothermal stimulation in Finland JF - Science Advances N2 - We show that near-real-time seismic monitoring of fluid injection allowed control of induced earthquakes during the stimulation of a 6.1-km-deep geothermal well near Helsinki, Finland. A total of 18,160 m(3) of fresh water was pumped into crystalline rocks over 49 days in June to July 2018. Seismic monitoring was performed with a 24-station borehole seismometer network. Using near-real-time information on induced-earthquake rates, locations, magnitudes, and evolution of seismic and hydraulic energy, pumping was either stopped or varied-in the latter case, between well-head pressures of 60 and 90 MPa and flow rates of 400 and 800 liters/min. This procedure avoided the nucleation of a project-stopping magnitude M-W 2.0 induced earthquake, a limit set by local authorities. Our results suggest a possible physics-based approach to controlling stimulation-induced seismicity in geothermal projects. Y1 - 2019 U6 - https://doi.org/10.1126/sciadv.aav7224 SN - 2375-2548 VL - 5 IS - 5 PB - American Association for the Advancement of Science CY - Washington ER - TY - JOUR A1 - Hofmann, Hannes A1 - Zimmermann, Günter A1 - Farkas, Márton Pál A1 - Huenges, Ernst A1 - Zang, Arno A1 - Leonhardt, Maria A1 - Kwiatek, Grzegorz A1 - Martinez-Garzon, Patricia A1 - Bohnhoff, Marco A1 - Min, Ki-Bok A1 - Fokker, Peter A1 - Westaway, Rob A1 - Bethmann, Falko A1 - Meier, Peter A1 - Yoon, Kern Shin A1 - Choi, JaiWon A1 - Lee, Tae Jong A1 - Kim, Kwang Yeom T1 - First field application of cyclic soft stimulation at the Pohang Enhanced Geothermal System site in Korea JF - Geophysical journal international N2 - Large-magnitude fluid-injection induced seismic events are a potential risk for geothermal energy developments worldwide. One potential risk mitigation measure is the application of cyclic injection schemes. After validation at small (laboratory) and meso (mine) scale, the concept has now been applied for the first time at field scale at the Pohang Enhanced Geothermal System (EGS) site in Korea. From 7 August until 14 August 2017 a total of 1756 m(3) of surface water was injected into Pohang well PX-1 at flow rates between 1 and 10 l s(-1), with a maximum wellhead pressure (WHP) of 22.8 MPa, according to a site-specific cyclic soft stimulation schedule and traffic light system. A total of 52 induced microearthquakes were detected in real-time during and shortly after the injection, the largest of M-w 1.9. After that event a total of 1771 m(3) of water was produced back from the well over roughly 1 month, during which time no larger-magnitude seismic event was observed. The hydraulic data set exhibits pressure-dependent injectivity increase with fracture opening between 15 and 17 MPa WHP, but no significant permanent transmissivity increase was observed. The maximum magnitude of the induced seismicity during the stimulation period was below the target threshold of M-w 2.0 and additional knowledge about the stimulated reservoir was gained. Additionally, the technical feasibility of cyclic injection at field scale was evaluated. The major factors that limited the maximum earthquake magnitude are believed to be: limiting the injected net fluid volume, flowback after the occurrence of the largest induced seismic event, using a cyclic injection scheme, the application of a traffic light system, and including a priori information from previous investigations and operations in the treatment design. KW - Cyclic soft stimulation (CSS) KW - induced seismicity KW - risk mitigation KW - enhanced geothermal systems (EGS) KW - granite KW - Pohang (Korea) Y1 - 2019 U6 - https://doi.org/10.1093/gji/ggz058 SN - 0956-540X SN - 1365-246X VL - 217 IS - 2 SP - 926 EP - 949 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Bentz, Stephan A1 - Martinez-Garzon, Patricia A1 - Kwiatek, Grzegorz A1 - Dresen, Georg A1 - Bohnhoff, Marco T1 - Analysis of Microseismicity Framing M-L > 2.5 Earthquakes at The Geysers Geothermal Field, California JF - Journal of geophysical research : Solid earth N2 - Preparatory mechanisms accompanying or leading to nucleation of larger earthquakes have been observed at both laboratory and field scales, but conditions favoring the occurrence of observable preparatory processes are still largely unknown. In particular, it remains a matter of debate why some earthquakes occur spontaneously without noticeable precursors as opposed to events that are preceded by an extended failure process. In this study, we have generated new high-resolution seismicity catalogs framing the occurrence of 20 M-L > 2.5 earthquakes at The Geysers geothermal field in California. To this end, a seismicity catalog of the 11 days framing each large event was created. We selected 20 sequences sampling different hypocentral depths and hydraulic conditions within the field. Seismic activity and magnitude frequency distributions displayed by the different earthquake sequences are correlated with their location within the reservoir. Sequences located in the northwestern part of the reservoir show overall increased seismic activity and low b values, while the southeastern part is dominated by decreased seismic activity and higher b values. Periods of high injection coincide with high b values and vice versa. These observations potentially reflect varying differential and mean stresses and damage of the reservoir rocks across the field. About 50% of analyzed sequences exhibit no change in seismicity rate in response to the large main event. However, we find complex waveforms at the onset of the main earthquake, suggesting that small ruptures spontaneously grow into or trigger larger events. KW - induced seismicity KW - earthquake nucleation KW - The Geysers KW - earthquake sequences Y1 - 2019 U6 - https://doi.org/10.1029/2019JB017716 SN - 2169-9313 SN - 2169-9356 VL - 124 IS - 8 SP - 8823 EP - 8843 PB - American Geophysical Union CY - Washington ER -