A document by Grzegorz Kwiatek. Click on the document to view its contents.

On November 23rd 2022, a MW 6.0 earthquake occurred in direct vicinity of the MW 7.1 Düzce earthquake that ruptured a portion of the North Anatolian Fault in 1999. The Mw 6.0 event was attributed to a small fault portion of the Karadere segment that did not rupture during the 1999 sequence. We analyze the spatio-temporal evolution of the MW 6.0 Gölyaka-Düzce seismic sequence at various scales and resolve the source properties of the mainshock. Modelling the decade-long evolution of background seismicity of the Karadere Fault employing an Epistemic Type Aftershock Sequence model shows that this fault was almost seismically inactive before 1999, while a progressive increase in seismic activity is observed from 2000 onwards. A newly generated high-resolution seismicity catalog from 1 month before the mainshock until six days after created using Artificial Intelligence-aided techniques shows only few events occurring within the rupture area within the previous month, no spatio-temporal localization process and a lack of immediate foreshocks preceding the rupture. The aftershock hypocenter distribution suggests the activation of both the Karadere fault which ruptured in this earthquake as well as the Düzce fault that ruptured in 1999. First results on source parameters and the duration of the first P-wave pulse from the mainshock suggest that the mainshock propagated eastwards in agreement with predictions from a bimaterial interface model. The MW 6.0 Gölyaka-Düzce represents a good example of an earthquake rupture with damaging potential within a fault zone that is in a relatively early stage of the seismic cycle.

Non-volcanic tremor has been observed at the roots of many fault systems around the Pacific rim, including convergent and transform plate boundaries. The extent to which deep tremor signals are prevalent along plate boundaries elsewhere, including the Mediterranean basin, has not yet been documented in detail. A body of evidence suggests that tremor triggered during the surface waves of teleseismic events may commonly occur where ambient tremor during Episodic Tremor and Slip episodes occur, suggesting triggered tremor provides a useful tool to identify regions with ambient tremor. We perform a systematic search of triggered tremor associated with large teleseismic events between 2010 and 2020, at four major fault systems within the central-eastern Mediterranean basin namely the Hellenic and Calabrian subduction zones, and the North Anatolian and Kefalonia transform faults. In addition, we search for ambient tremor during a ~50-daylong slow slip event in the eastern Sea of Marmara along a secondary branch of the North Anatolian Fault, and two ~4-month long slow slip events beneath western Peloponnese. We find no unambiguous evidence for deep triggered tremor nor for ambient tremor. The absence of triggered tremor at the Hellenic and Calabrian subduction zones supports the less favorable conditions for tremorgenesis in the presence of old and cold slabs. The absence of tremor along the transform faults may be due to an absence of the conditions commonly promoting tremorgenesis in such settings, including high fluid pressures and low differential stresses between the down-dip limit of the seismogenic layer and the Moho.

Small stress changes such as those from tidal loading can be enough to trigger earthquakes. If small and large earthquakes initiate similarly, high resolution catalogs with low detection thresholds are best suited to illuminate such processes. Below the Sea of Marmara section of the North Anatolian Fault, a segment of 150 km is late in its seismic cycle. We generated high-resolution seismicity catalogs for a hydrothermal region in the eastern Sea of Marmara employing both AI-based and template matching techniques to investigate a complex long-lasting sequence including seismicity up to MW 4.5. We document a strong effect of the Sea of Marmara level changes on the local seismicity. Both high resolution catalogs show that local seismicity rates are significantly larger during time periods shortly after local minima on sea level. Local strainmeters indicate that the associated strain changes, on the order of 30-300 nstrain, are sufficient to promote seismicity.

We investigate induced seismicity associated with a hydraulic stimulation campaign performed in 2020 in the 5.8 km deep geothermal OTN-2 well near Helsinki, Finland as part of the St1 Deep Heat project. A total of 2,875 m3 of fresh water was injected during 16 days at well-head pressures <70 MPa and with flow rates between 400-1000 l/min. The seismicity was monitored using a high-resolution seismic network composed of 10 borehole geophones surrounding the project site and a borehole array of 10 geophones located in adjacent OTN-3 well. A total of 6,121 induced earthquakes with local magnitudes were recorded during and after the stimulation campaign. The analyzed statistical parameters include magnitude-frequency b-value, interevent time and interevent time ratio, as well as magnitude correlations. We find that the b-value remained stationary for the entire injection period suggesting limited stress build-up or limited fracture network coalescence in the reservoir. The seismicity during the stimulation neither shows signatures of magnitude correlations, nor temporal clustering or anticlustering beyond those arising from varying injection rates. The interevent time statistics are characterized by a Poissonian time-varying distribution. The calculated parameters indicate no earthquake interaction. Focal mechanisms suggest that the injection activated a spatially distributed network of similarly oriented fractures. The seismicity passively responded to the hydraulic energy input rate, with the cumulative seismic moment proportional to the cumulative hydraulic energy and maximum magnitude controlled by injection rate. The performed study provides a base for implementation of time-dependent probabilistic seismic hazard assessment for the project site.