A document by Hilary Chang. Click on the document to view its contents.

The 2016 Amatrice-Norcia seismic sequence in central Italy activated a system of normal faults in the central Apennines and ruptured the surface along the Monte Vettore normal fault. Due to the complex rupture behavior, including antithetic faults and the proposed reactivation of an old thrust front, the Amatrice-Norcia seismic sequence offers a unique opportunity to study the relationship between fault complexity, surface ruptures, and earthquake source properties. Here, we focus on the first two months of the Amatrice-Norcia seismic sequence, including the 30 October 2016 Mw 6.5 mainshock near Norcia and more than 25000 aftershocks. Using continuous waveform data from 94 seismic stations with epicentral distances of up to ~100 km, we estimate source parameters of all cataloged earthquakes that exceed specific quality control criteria in a time period ranging from 24 October – 29 November 2016. Displacement spectral corner frequency and seismic moment values are fit using individual earthquake spectra, and corner frequency estimates are refined using spectral ratios. Constrained spectral parameters then provide input for static stress drop estimates based on a circular crack model. Preliminary results suggest the majority of earthquakes have static stress drop values between 1 and 10 MPa and self-similar scaling. Due to the high quality and quantity of available data, including precise earthquake locations, manually reviewed phase arrivals, and detailed mapping of surface ruptures, the Amatrice-Norcia earthquake sequence represents an opportunity to link earthquake source parameters to geological structures and surface rupture complexity. Preliminary results show correlations between high stress drop values and areas with increasing fault complexity, such as fault intersections at depth (inferred from precise earthquake hypocenters) or the mapped tip of the Monte Vettore normal fault, relative to other fault patches with fewer intersections or mapped surface trace terminations. Future work will examine whether the correlation of stress drop and fault complexity holds using refined stress drop estimates obtained using spectral ratio approaches.

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.

We investigate the influence of local site effects on earthquake source parameter estimates using the LArge-n Seismic Survey in Oklahoma (LASSO). The LASSO array consisted of 1825 stations in a 25 km x 32 km region with extensive wastewater injection and recorded more than 1500 local events (M < 3) during spring 2016. We analyze the site amplification dependence on earthquake corner frequency (fc), seismic moment (M0), and stress drop estimated by modeling individual spectra. We evaluate and correct these site effects and compare the effectiveness of the correction to results using the spectral ratio method. We estimate local site amplification at each station using the average Peak-Ground-Velocity (PGV) of 14 regional earthquakes (~130 km away). The fc from the single spectrum method negatively correlates with site amplification, whereas M0 from the single spectrum method positively correlates with site amplification. This suggests the source parameters calculated by modeling individual spectra are biased by the local site effects. The high amplifications are typically located on young alluvial sedimentary deposits. We correct site effects by removing the trend between PGV and these two parameters in the regression analysis, which reduces the standard deviation of these parameters across the array and makes the calculated stress drop less site dependent. We compare corrections using other site-effect proxies such as the Root-Mean-Square (RMS) amplitude, surface geological formation, P-arrival-delay, and topographic slope. The PGV and the RMS corrections provide the greatest reduction of the spatial deviation of source parameters. In comparison, the spectral ratio method effectively removes the site effects using the Empirical Green’s Function (EGF) approach. The trends being removed by EGF are close to the apparent trends between the single spectrum estimated parameters and the PGV, which suggests the consistency of these different correction approaches. Our results provide a potential way to remove the site effects when only the main event spectrum is available and demonstrates the effectiveness of using the EGF approach for removing site effects. The resulting inter-station variability provides an estimate of the likely uncertainty in source parameters estimated from smaller numbers of stations.

Source processes of injection induced earthquakes involve complex fluid-rock interaction often elusive to regional seismic monitoring. Here we combine observations from a local seismograph array in the Montney basin, northeast British Columbia, and stress modeling to examine the spatial and temporal evolution of the 30 November 2018 M 4.5 hydraulic fracturing induced earthquake sequence. The mainshock occurred at ~ 4.5 km in the crystalline basement two days following injection at ~ 2.5 km, suggesting direct triggering by rapid fluid pressure increase via a high-permeability conduit. Most of the aftershocks are located in the top 2 km sedimentary layers, with focal mechanisms indicating discrete slip along sub-vertical surfaces in a ~ 1 km wide deformation zone. Aftershock distribution is also consistent with static stress triggering from the M 4.5 coseismic slip. Our analysis suggests complex hydraulic and stress transfer between fracture/fault networks needs to be considered in induced seismic hazard assessment.

Crustal velocity variation within impact-related seismic zones is commonly attributed to mechanisms such as pore pressure changes, dense fracture network, and compositional variation. In this study, we combine seismic tomography, rock physics analysis, and potential field modeling to quantitatively investigate the mechanisms that influence crustal velocity variation in the Charlevoix Seismic Zone (CSZ), a meteorite impact-related seismic zone in eastern Canada. Earthquakes in the CSZ align along two broad NE-SW trending clusters related to reactivated paleo-rift faults. Within the impact structure, the earthquakes are diffusely distributed and lower velocity bodies are ubiquitous which can be attributed to crustal damage from tectonic inheritance exacerbated by the meteorite impact. The Bouguer gravity anomaly decreases southeastward across the St. Lawrence River due to density disparity between rocks in the Grenville Province and the Appalachians. We find a higher velocity body northeast of the impact structure that does not exhibit an observable gravity anomaly, which suggests the presence of a rock (e.g. anorthosite) of comparable density but a higher elastic moduli within another rock (e.g. charnockite). Outside the impact structure, compositional variations control velocity changes, whereas inside the impact structure, velocity variations can be explained by porosity enhancement of up to 10% by low (0.1) aspect ratio cracks. Our results suggest that intense fracturing and compositional alteration, rather than pore pressure, control velocity variations, hence earthquake processes in the CSZ.

Intraplate tectonic stress fields are complex due to the imprint of a long geological history. Here we use a new dataset of earthquake focal mechanism solutions and relocated events to investigate the relationship between regional stress, crustal strength, and seismicity in the Charlevoix Seismic Zone (CSZ), the most active seismic zone in eastern Canada. Our stress inversion shows that SHmax gradually rotates clockwise from approximately St. Lawrence River-parallel near the surface to river-perpendicular in the lower crust, as postglacial rebound stress becomes increasingly dominant at greater depth. The stress rotation occurs primarily between ~13 and ~26 km depth, where glacial rebound induced stress perturbation is further amplified by a “weaker” middle crust of an estimated apparent friction coefficient of ~0.5. Finally, depth-dependent b-values confirm the rheological difference between upper and middle crust in the CSZ.

Earthquakes resulting from hydraulic fracturing (HF) can have delayed triggering relative to injection commencement over a varied range of time scales, with many cases exhibiting the largest events near/after well completion. This poses serious challenges for risk mitigation and hazard assessment. Here, we document a high-resolution, three-dimensional source migration process with delayed mainshock triggering that is controlled by local hydrogeological conditions. Our results reveal that poroelastic effects might contribute to induced seismicity, but are insufficient to activate a non-critically stressed fault of sufficient size. The rapid pore-pressure build-up from HF can be very localized and capable of producing large, felt earthquakes on non-critically stressed fault segments. We interpret the delayed triggering as a manifestation of pore-pressure build-up along pre-existing faults needed to facilitate seismic failure. Our findings can deepen our understanding of the current stress state of crustal faults and also explain why so few injection operations are seismogenic.