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"Harrington, R. M."
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Megathrust Stress Drop as Trigger of Aftershock Seismicity: Insights From the 2011 Tohoku Earthquake, Japan
2023
Numerous normal‐faulting aftershocks in subduction forearcs commonly follow large megathrust earthquakes. Postseismic normal faulting has been explained by stress changes induced by the stress drop along the megathrust. However, details of forearc stress changes and aftershock triggering mechanisms remain poorly understood. Here, we use numerical force‐balance models combined with Coulomb failure analysis to show that the megathrust stress drop supports normal faulting, but that forearc‐wide aftershock triggering is feasible within a narrow range of megathrust stress drop values and preseismic stress states only. We determine this range for the 2011 Tohoku earthquake (Japan) and show that the associated stress changes explain the aftershock seismicity in unprecedented detail and are consistent with the stress released by forearc seismicity before and after the earthquake. Plain Language Summary Earthquakes release stresses that build up in the Earth due to the motion of tectonic plates. The stress release can cause additional earthquakes called aftershocks. Several thousand onshore and offshore aftershocks followed the great Tohoku subduction earthquake in March 2011. Whether the stress release of the Tohoku earthquake triggered most of the aftershocks is not well understood, because it is largely unknown how the stress field changed following the earthquake. We therefore use a computer model to estimate the stress release and resulting stress change required to explain the aftershock distribution. We find that 78% of the aftershocks occurred in areas where the Tohoku earthquake caused a subsequent stress increase. Our model results are further consistent with the stress release of smaller earthquakes that occurred in Japan before and after the Tohoku earthquake. Our findings provide new insights into aftershock triggering and help to understand where aftershocks occur after great earthquakes at subduction zones. Key Points We show using force‐balance modeling that a megathrust earthquake stress drop can trigger forearc‐wide aftershock seismicity Model results explain the Tohoku earthquake aftershock distribution and reveal spatial variability in forearc stress and strength Most aftershocks occurred in areas that experienced an increase in deviatoric stress
Journal Article
The Weisweiler passive seismological network: optimised for state-of-the-art location and imaging methods
by
Knapmeyer-Endrun, Brigitte
,
Oswald, Thomas
,
Harrington, Rebecca M.
in
Anthropogenic factors
,
Background noise
,
Datasets
2023
Passive seismic analyses are a key technology for the exploration and monitoring of subsurface reservoirs. Searching for alternative resources in the framework of the energy transition is creating a surge for identifying as many potential sites as possible suitable for geothermal exploitation. The Lower Rhine Embayment, at the western border of North Rhine-Westphalia in Germany, is an extensional system with a very high potential for geothermal exploitation. The area experiences moderate but continuous natural seismicity. Here, we report on a passive seismic dataset recorded with 48 seismic stations centred at and around Eschweiler–Weisweiler (https://doi.org/10.14470/MO7576467356, Finger et al., 2022). Background seismic noise levels are high at this site due to high levels of anthropogenic noise and thick unconsolidated sedimentary layers. The final station layout is a compromise between targeted network design and suitably quiet locations. We show that the network design allows for the application of state-of-the-art methods including waveform-based source location methods and ambient-noise velocity imaging methods.
Journal Article
Preliminary evidence of prolonged timing effects of theta-burst stimulation in the reading system
2023
Theta-burst stimulation (TBS) is a repetitive transcranial magnetic stimulation technique that can be used to upregulate or downregulate different brain regions. However, the timing of its effects and the differing effects of continuous TBS (cTBS) versus intermittent TBS (iTBS) in the reading system have not been explored. This study assessed how stimulation type and post-stimulation timing affected change in performance during a phonological discrimination and sight word recognition task after stimulation of supramarginal gyrus (SMG). Fourteen right-handed young adults (age 18–27 years; 44% male) were block-randomized to receive either iTBS or cTBS to the supramarginal gyrus. Participants then performed a pseudoword discrimination task and an orthographic awareness task (behavioral control) at four different time points and change in reaction time compared to baseline was measured from each time point. There was no effect of stimulation type on change in reaction time [ t (16) = −0.2, p = 0.9], suggesting that both types of TBS caused similar effects. Percent change in reaction time decreased over time in the pseudoword task [ t (50) = −5.9, p < 0.001], indicating faster pseudoword processing speed with better performance 60–70 min after stimulation. In contrast, no change was demonstrated over time for the behavioral control task [ t (43) = −0.6, p = 0.6], suggesting that the change over time seen in the test condition was not a learning effect. These findings provide insight into the effects of TBS on the reading system and can guide future study designs.
Journal Article
Functional Activation following Transcranial Magnetic Stimulation in Neurotypical Adult Readers
2025
Background: Transcranial magnetic stimulation (TMS) is considered a promising technique to noninvasively modulate cortical excitability and enhance cognitive functions. Despite the growing interest in using TMS to facilitate reading performance in learning disabilities, the immediate TMS-induced effects on brain activity during reading and language tasks in adults with typically developed reading skills remain to be further investigated. In the current study, we explored how a single offline session of intermittent theta burst stimulation (iTBS) delivered to core left-hemisphere nodes of the dorsal and ventral reading network changes brain activity during a spoken and written reading task. Methods: A total of 25 adults with typically developed reading skills participated in a sandwich design TMS-functional magnetic resonance imaging (fMRI) study, which was comprised of a baseline fMRI picture-word identification task that involved matching written or spoken words to picture cues, a single transcranial magnetic stimulation (TMS) session to either the left supramarginal gyrus (SMG) or the left middle temporal gyrus (MTG), followed by a post-stimulation fMRI session. A whole-brain analysis based on the general lineal model (GLM) was used to identify overall activated regions during the processing of spoken and written words. To identify differences between pre-and post-stimulation fMRI sessions, a paired sample t-test was conducted for each group separately (SMG and MTG groups). Results: Significant differences were found between pre-and post-stimulation fMRI sessions, with higher functional activation (post > pre) for spoken words only following SMG stimulation, and for both spoken and written words following MTG stimulation, in regions associated with the reading network and additional cognitive and executive control regions. Conclusions: Our results showed how a single-offline TMS session can modulate brain activity at ~20 minutes post-stimulation during spoken and written word processing. The selective contribution of the SMG stimulation for auditory (spoken) word processing provides further evidence of the distinct role of the dorsal and ventral streams within the reading network. These findings could contribute to the development of neuromodulatory interventions for individuals with reading and language impairments. Clinical Trial Registration: No: NCT04041960. Registered 29 July, 2019, https://clinicaltrials.gov/study/NCT04041960?cond=NCT04041960&rank=1 .
Journal Article