Periodic Reporting for period 4 - PRESEISMIC (Exploring the nucleation of large earthquakes: cascading and unpredictable or slowly driven and forecastable)
Periodo di rendicontazione: 2023-07-01 al 2024-12-31
One of the key questions in fault mechanics is how earthquakes begin. This is central to our understanding of earthquakes, including the long and controversial issue of their predictability. Much of what we know about earthquake nucleation comes from the observation of foreshock events before large earthquakes. Yet, one major challenge is the inherent difficulty of identifying earthquakes as foreshocks before the mainshock occurs.
Two competing models have been proposed to explain foreshock sequences. The first is the “cascading model”, where successive stress changes from foreshocks lead to a cascade of random failures, ultimately triggering the mainshock. The second is the “nucleation model”, where foreshocks are driven by an aseismic nucleation phase involving gradual fault slip that accelerates into a dynamic rupture. These models have crucial implications for seismic hazard assessment. In the cascading model, foreshocks occur randomly and cannot be used for prediction. Conversely, the nucleation model suggests a causal relationship between aseismic slip and foreshocks, offering potential for forecasting large earthquakes.
The PRESEISMIC project aimed to address key limitations in our understanding of earthquake nucleation, namely the difficulty in quantifying the proportion of seismic versus aseismic slip during rupture initiation and the challenges of detecting and characterizing small microearthquakes (M<2). By leveraging the explosion of near-fault observations and combining seismic and geodetic datasets, the project aimed to clarify the contributions of seismic and aseismic processes during foreshock sequences.
The main conclusion of the project is that the “nucleation model” cannot explain most foreshock observations. Instead, the “cascading model” appears to play a dominant role, with foreshocks often triggering each other randomly. However, our research also revealed that some foreshock sequences deviate from this pattern, likely driven by slow-slip events that load nearby fault areas, enhancing seismic activity and increasing the likelihood of a larger rupture. While this “slow-slip event loading model” does not offer strong predictive power, the rapid detection of transient aseismic slip remains valuable for anticipating an increased level of seismic activity.
This project has advanced our understanding of the processes leading to earthquakes and highlighted the limitations of using foreshock sequences for short-term earthquake prediction. Its findings underscore the need for seismic hazard preparedness through robust engineering standards and long-term risk mitigation strategies. Another key outcome of the project is the development of methods to produce and analyze high-resolution seismicity catalogs. These tools can be applied in environments beyond tectonic faults and are particularly valuable for gaining new insights into magma migration, improving eruption forecasting on active volcanoes.
Two competing models have been proposed to explain foreshock sequences. The first is the “cascading model”, where successive stress changes from foreshocks lead to a cascade of random failures, ultimately triggering the mainshock. The second is the “nucleation model”, where foreshocks are driven by an aseismic nucleation phase involving gradual fault slip that accelerates into a dynamic rupture. These models have crucial implications for seismic hazard assessment. In the cascading model, foreshocks occur randomly and cannot be used for prediction. Conversely, the nucleation model suggests a causal relationship between aseismic slip and foreshocks, offering potential for forecasting large earthquakes.
The PRESEISMIC project aimed to address key limitations in our understanding of earthquake nucleation, namely the difficulty in quantifying the proportion of seismic versus aseismic slip during rupture initiation and the challenges of detecting and characterizing small microearthquakes (M<2). By leveraging the explosion of near-fault observations and combining seismic and geodetic datasets, the project aimed to clarify the contributions of seismic and aseismic processes during foreshock sequences.
The main conclusion of the project is that the “nucleation model” cannot explain most foreshock observations. Instead, the “cascading model” appears to play a dominant role, with foreshocks often triggering each other randomly. However, our research also revealed that some foreshock sequences deviate from this pattern, likely driven by slow-slip events that load nearby fault areas, enhancing seismic activity and increasing the likelihood of a larger rupture. While this “slow-slip event loading model” does not offer strong predictive power, the rapid detection of transient aseismic slip remains valuable for anticipating an increased level of seismic activity.
This project has advanced our understanding of the processes leading to earthquakes and highlighted the limitations of using foreshock sequences for short-term earthquake prediction. Its findings underscore the need for seismic hazard preparedness through robust engineering standards and long-term risk mitigation strategies. Another key outcome of the project is the development of methods to produce and analyze high-resolution seismicity catalogs. These tools can be applied in environments beyond tectonic faults and are particularly valuable for gaining new insights into magma migration, improving eruption forecasting on active volcanoes.
In the framework of the PRESEISMIC project, we developed new tools to distinguish between random cascades of foreshocks and behaviors driven by aseismic pre-slip.
We produced novel time-dependent models that assimilate multiple datasets to assess the relative contributions of seismic and aseismic slip. In addition, we are also developing approaches to improve seismicity catalogs and analyze earthquake interactions preceding mainshocks.
Throughout the project we analyzed various seismic sequences in different areas of the world. Our findings have been published in several peer-reviewed articles, and the methods developed we developed are freely available online.
Our study in southern California indicates that non-cascading foreshock seismicity is relatively rare. However, a non-negligible portion of these foreshock sequences cannot be explained by the natural clustering of seismicity resulting from earthquake interactions. Other triggering mechanisms, such as aseismic pre-slip, are required to account for these anomalous sequences.
We also examined several slow-slip and earthquakes sequences in Chile – including the 2014 Iquique earthquake, the 2017 Valparaiso earthquake, and the 2020 Atacama sequence. For these events, we found that a significant portion of the fault slip was aseismic and that the observed seismicity could not be fully explained by earthquake interactions alone. Moreover, a self-accelerating nucleation cannot explain the GNSS displacements and foreshock observations. In the case of the 2014 Iquique earthquake, aseismic slip ceased several days before the mainshock. For the Valparaiso and Atacama earthquake sequences, clear slow-slip events were observed before the mainshock. However, this aseismic motion did not culminate in the mainshock and persisted afterward.
Overall, these observations suggest that pre-seismic aseismic slip does not correspond to self-accelerating nucleation phases but rather to slow-slip events that load nearby fault areas, thereby enhancing seismic activity and increasing the likelihood of a larger rupture.
We produced novel time-dependent models that assimilate multiple datasets to assess the relative contributions of seismic and aseismic slip. In addition, we are also developing approaches to improve seismicity catalogs and analyze earthquake interactions preceding mainshocks.
Throughout the project we analyzed various seismic sequences in different areas of the world. Our findings have been published in several peer-reviewed articles, and the methods developed we developed are freely available online.
Our study in southern California indicates that non-cascading foreshock seismicity is relatively rare. However, a non-negligible portion of these foreshock sequences cannot be explained by the natural clustering of seismicity resulting from earthquake interactions. Other triggering mechanisms, such as aseismic pre-slip, are required to account for these anomalous sequences.
We also examined several slow-slip and earthquakes sequences in Chile – including the 2014 Iquique earthquake, the 2017 Valparaiso earthquake, and the 2020 Atacama sequence. For these events, we found that a significant portion of the fault slip was aseismic and that the observed seismicity could not be fully explained by earthquake interactions alone. Moreover, a self-accelerating nucleation cannot explain the GNSS displacements and foreshock observations. In the case of the 2014 Iquique earthquake, aseismic slip ceased several days before the mainshock. For the Valparaiso and Atacama earthquake sequences, clear slow-slip events were observed before the mainshock. However, this aseismic motion did not culminate in the mainshock and persisted afterward.
Overall, these observations suggest that pre-seismic aseismic slip does not correspond to self-accelerating nucleation phases but rather to slow-slip events that load nearby fault areas, thereby enhancing seismic activity and increasing the likelihood of a larger rupture.
We are now working on the final developments of probabilistic time-dependent slip models, allowing us to study the spatiotemporal evolution of earthquake preparation. This approach is currently implemented to analyze the preparatory phase preceding the 2014 Iquique earthquake (North Chile). We are also working on obtaining high-resolution seismicity catalogs in areas where aseismic pre-slip has been reported before large mainshock events. Our goal is to identify if the corresponding foreshock sequences have properties that are different from usual seismicity patterns (temporal clustering, event location, waveform properties, etc.).
While the approaches developed in this project are necessary to understand the initiation phase of earthquakes, they can also be useful in other contexts. We are planning to use our new time-dependent inversion tool to investigate slow slip events that are observed on various active faults around the world. More specifically, we aim to target the Cascadia subduction zone, where many of these slow-slip events have been reported. This region has experienced large earthquakes in the past, getting some insights on the slip behavior of this region can therefore prove to be of significant interest. Along with the study of tectonic faults, our tools are also valuable to monitor active volcanoes. Our recent applications at Piton de la Fournaise and Kilauea volcanoes demonstrates how enhanced seismicity detections can improve the forecasting of volcanic eruptions and allow us to identify early signs of impending large caldera collapses.
While the approaches developed in this project are necessary to understand the initiation phase of earthquakes, they can also be useful in other contexts. We are planning to use our new time-dependent inversion tool to investigate slow slip events that are observed on various active faults around the world. More specifically, we aim to target the Cascadia subduction zone, where many of these slow-slip events have been reported. This region has experienced large earthquakes in the past, getting some insights on the slip behavior of this region can therefore prove to be of significant interest. Along with the study of tectonic faults, our tools are also valuable to monitor active volcanoes. Our recent applications at Piton de la Fournaise and Kilauea volcanoes demonstrates how enhanced seismicity detections can improve the forecasting of volcanic eruptions and allow us to identify early signs of impending large caldera collapses.