Periodic Reporting for period 2 - FaultScan (Passive seismic scanning of the preparation phase of damaging earthquakes)
Période du rapport: 2020-12-01 au 2022-05-31
What is the problem/issue being addressed?
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The quest for earthquake precursors is among the most vivid and active field of research in seismology. Still, our understanding of the preparation phase of earthquakes is quite limited and we are far from being able to detect and interpret robust, systematic early signs of a forthcoming large and damaging earthquake. As an example, in 2019, the magnitude 7.1 earthquake hit the Ridgecrest area in California and even though it was the largest in the United States in 20 years, the background seismicity preceding the mainshock didn’t show any signs of anomalous behavior. On more fundamental aspects, it is still debated if large earthquakes are predictable and would, for example, follow a process where slow fracture mechanisms would connect anomalously large sections of the fault before brutal rupture. On the contrary, some studies argue that large earthquakes are unpredictable and that their final size results from a stochastic cascade of smaller events without any previous connections between them. In the FaultScan project, we attempt to provide new insights to this problem by looking for signs indicating that large earthquakes rupturing over tens of kilometers are not the only consequence of random processes, but that they are preceded by a preparation phase that we will strive to detect.
Detecting earthquake precursors is challenging. Only seismic waves have the ability to travel down to the region where earthquakes nucleate and to directly probe potential early silent processes associated with the preparation of large earthquakes. However, in order to travel down to depths of a few kilometers and to be recorded back to surface, these waves must be generated by powerful sources of tens of kilograms of equivalent TNT. In addition, it is practically very difficult to repeat these controlled sources for monitoring purposes. Recently, we discovered that freight trains and possibly other cultural sources (e.g. factories, mining operations) could also generate powerful repetitive sources. In FaultScan, we aim at turning these signals into useful probes to monitor active tectonic faults by using the technique of seismic interferometry.
Why is it important for society?
____________________________
After decades of progress led by fine seismicity analysis, satellite remote sensing (GPS, InSAR), and large-scale simulations, the quest for earthquake predictability has stalled. Here, we propose a disruptive approach to monitor active faults that is still in its early developments and thus capable of ground breaking discoveries. Even if this approach can only provide a small improvement in the diagnostic of a fault to produce a large earthquake, the relevance for society will be huge. Earthquakes are still among the deadliest natural disasters and mostly affect developing countries that do not have earthquake-resistant structures.
Moreover, the monitoring approach that we develop has the potential to be applied to other domains like volcano monitoring, C02 sequestration and geothermal production. In the future, it should also help to provide insights on the processes of induced earthquakes by pressurized fluid injection operations (waste-water, geothermal), by tracking the associated silent poro-elastic stress changes. Finally, our approach also finds application in the monitoring of the ground water in regions where these resources becomes strategic, following periods of extreme drought like that of Southern California from 2015 to 2018.
What are the overall objectives?
____________________________
The overall objective is to discover new signs of the preparation phase of earthquakes by using a groundbreaking seismic approach that relies on monitoring small stress perturbations in the vicinity of active faults. This technique uses seismic waves generated by powerful anthropogenic sources of noise like trains, car traffic or mining operations. We developed a special technique to extract seismic waves that dive deep down into fault zones at a few kilometers depth, making our approach highly sensitive to changes occurring on the fault. Apart from borehole instrumentation, this seismic approach is the only one that enables us to directly probe faults at depth.
We are focusing on the most seismically active region in South California, namely the San Jacinto Fault (see Figure 1), because it is believed to have accumulated enough stress to be ready to rupture, leading to a possible magnitude 7 earthquake that could induce damage up to Los Angeles, located 30 km from the fault. We are instrumenting this region with dense seismic arrays (red dots on Figure 1) that will allow us to monitor the fault with unprecedented accuracy for a period of at least 2 years of continuous recording.
Our objective is to study early signs of the rupture of large earthquakes. There is still a little chance that a large earthquake will occur on the San Jacinto Fault during our project. We have thus developed a complementary approach to use permanent, individual stations instead of arrays to perform passive ballistic monitoring. This approach still requires the deployment of a temporary dense array to calibrate the permanent station measurements. We aim at studying two large past earthquakes: the M7, 2019 Ridgecrest earthquake (Southern Cal.) and the 2014, M6 Napa earthquake (Northern Cal.). These are almost the sole large (M>6) earthquakes worldwide occurring inland, that were recorded by nearby permanent seismic stations, and where strong cultural noise is present for our method to be applied.
We also have the objective to extend our approach to Japan, which is instrumented with 800 evenly distributed seismic stations and where seismicity is broadly distributed. Finally, we also aim at extending our approach to a more applied domain that is C02 sequestration. We are involved in a collaboration with the University of Wyoming for the monitoring of a controlled fluid injection experiment (CarbonSafe).
______________________________________
The quest for earthquake precursors is among the most vivid and active field of research in seismology. Still, our understanding of the preparation phase of earthquakes is quite limited and we are far from being able to detect and interpret robust, systematic early signs of a forthcoming large and damaging earthquake. As an example, in 2019, the magnitude 7.1 earthquake hit the Ridgecrest area in California and even though it was the largest in the United States in 20 years, the background seismicity preceding the mainshock didn’t show any signs of anomalous behavior. On more fundamental aspects, it is still debated if large earthquakes are predictable and would, for example, follow a process where slow fracture mechanisms would connect anomalously large sections of the fault before brutal rupture. On the contrary, some studies argue that large earthquakes are unpredictable and that their final size results from a stochastic cascade of smaller events without any previous connections between them. In the FaultScan project, we attempt to provide new insights to this problem by looking for signs indicating that large earthquakes rupturing over tens of kilometers are not the only consequence of random processes, but that they are preceded by a preparation phase that we will strive to detect.
Detecting earthquake precursors is challenging. Only seismic waves have the ability to travel down to the region where earthquakes nucleate and to directly probe potential early silent processes associated with the preparation of large earthquakes. However, in order to travel down to depths of a few kilometers and to be recorded back to surface, these waves must be generated by powerful sources of tens of kilograms of equivalent TNT. In addition, it is practically very difficult to repeat these controlled sources for monitoring purposes. Recently, we discovered that freight trains and possibly other cultural sources (e.g. factories, mining operations) could also generate powerful repetitive sources. In FaultScan, we aim at turning these signals into useful probes to monitor active tectonic faults by using the technique of seismic interferometry.
Why is it important for society?
____________________________
After decades of progress led by fine seismicity analysis, satellite remote sensing (GPS, InSAR), and large-scale simulations, the quest for earthquake predictability has stalled. Here, we propose a disruptive approach to monitor active faults that is still in its early developments and thus capable of ground breaking discoveries. Even if this approach can only provide a small improvement in the diagnostic of a fault to produce a large earthquake, the relevance for society will be huge. Earthquakes are still among the deadliest natural disasters and mostly affect developing countries that do not have earthquake-resistant structures.
Moreover, the monitoring approach that we develop has the potential to be applied to other domains like volcano monitoring, C02 sequestration and geothermal production. In the future, it should also help to provide insights on the processes of induced earthquakes by pressurized fluid injection operations (waste-water, geothermal), by tracking the associated silent poro-elastic stress changes. Finally, our approach also finds application in the monitoring of the ground water in regions where these resources becomes strategic, following periods of extreme drought like that of Southern California from 2015 to 2018.
What are the overall objectives?
____________________________
The overall objective is to discover new signs of the preparation phase of earthquakes by using a groundbreaking seismic approach that relies on monitoring small stress perturbations in the vicinity of active faults. This technique uses seismic waves generated by powerful anthropogenic sources of noise like trains, car traffic or mining operations. We developed a special technique to extract seismic waves that dive deep down into fault zones at a few kilometers depth, making our approach highly sensitive to changes occurring on the fault. Apart from borehole instrumentation, this seismic approach is the only one that enables us to directly probe faults at depth.
We are focusing on the most seismically active region in South California, namely the San Jacinto Fault (see Figure 1), because it is believed to have accumulated enough stress to be ready to rupture, leading to a possible magnitude 7 earthquake that could induce damage up to Los Angeles, located 30 km from the fault. We are instrumenting this region with dense seismic arrays (red dots on Figure 1) that will allow us to monitor the fault with unprecedented accuracy for a period of at least 2 years of continuous recording.
Our objective is to study early signs of the rupture of large earthquakes. There is still a little chance that a large earthquake will occur on the San Jacinto Fault during our project. We have thus developed a complementary approach to use permanent, individual stations instead of arrays to perform passive ballistic monitoring. This approach still requires the deployment of a temporary dense array to calibrate the permanent station measurements. We aim at studying two large past earthquakes: the M7, 2019 Ridgecrest earthquake (Southern Cal.) and the 2014, M6 Napa earthquake (Northern Cal.). These are almost the sole large (M>6) earthquakes worldwide occurring inland, that were recorded by nearby permanent seismic stations, and where strong cultural noise is present for our method to be applied.
We also have the objective to extend our approach to Japan, which is instrumented with 800 evenly distributed seismic stations and where seismicity is broadly distributed. Finally, we also aim at extending our approach to a more applied domain that is C02 sequestration. We are involved in a collaboration with the University of Wyoming for the monitoring of a controlled fluid injection experiment (CarbonSafe).
As initially planned, we purchased a pool of 470 seismic stations meant to be deployed on the field along the San Jacinto Fault right when the pandemic broke in spring 2020. Fortunately, we already had dense array data acquired in 2018 and 2019 in that region that we reprocessed together with permanent station data for the last 10 years in that same region of Anza in Southern California (see Image 1). We developed a new approach that consists in learning from the temporary dense array data and applying the result backward in time to the permanent stations. The results were unexpected and stunning: The high quality of our observations based on the seismic waves generated by the powerful freight train passages in the nearby Coachella valley allowed us to monitor the stress-state of the fault over 10 years. We found a clear anomalous transient in the beginning of 2014 that was not detected before by any other means. We interpreted this perturbation as the effect of a slow fault movement in a region that was thought to be completely locked. We further found that this type of silent movement would typically have the potential to trigger a large-scale earthquake (Sheng et al. 2022, submitted to Nature Geoscience).
This monitoring method that we developed is referred to as passive ballistic-wave passive monitoring (Takano et al. 2020, Brenguier et al. 2020) (see Figure 2). It is based on detecting and characterizing the space and time distribution of specific strong anthropogenic sources that have the power to generate body-waves that travel down to a few kilometers depth close to the earthquake nucleation region. This method requires to deploy temporary dense seismic arrays in order to characterize these sources and assess the properties of the seismic wavefield that will be used for further monitoring. Following this stage, it is then possible to trace back stress perturbations on the fault using permanent seismic stations with the hypothesis that the sources of noise are stationary in time and this even before the installation of the dense array, allowing to study past earthquakes in a way that has never been done before (Sheng et al., submitted).
We also developed a method called seismic stereometry (Mordret et al. 2020). While the first method monitors changes in the seismic properties of the regions near the fault, seismic stereometry is a method to study the source of earthquakes. It uses slight differences in the shape of the seismograms of a specific earthquake at two nearby stations to infer how the rupture migrated in space. By using nearly colocated permanent stations, this method has also the potential to trace back in time the seismic connections between different fault patches and to monitor their activity especially before a large earthquake. In the Anza region, we have at least two magnitude 5 earthquake candidates for which we can apply this method and study in detail if there were early signs of anomalous seismicity on the portions of this fault before rupture.
This monitoring method that we developed is referred to as passive ballistic-wave passive monitoring (Takano et al. 2020, Brenguier et al. 2020) (see Figure 2). It is based on detecting and characterizing the space and time distribution of specific strong anthropogenic sources that have the power to generate body-waves that travel down to a few kilometers depth close to the earthquake nucleation region. This method requires to deploy temporary dense seismic arrays in order to characterize these sources and assess the properties of the seismic wavefield that will be used for further monitoring. Following this stage, it is then possible to trace back stress perturbations on the fault using permanent seismic stations with the hypothesis that the sources of noise are stationary in time and this even before the installation of the dense array, allowing to study past earthquakes in a way that has never been done before (Sheng et al., submitted).
We also developed a method called seismic stereometry (Mordret et al. 2020). While the first method monitors changes in the seismic properties of the regions near the fault, seismic stereometry is a method to study the source of earthquakes. It uses slight differences in the shape of the seismograms of a specific earthquake at two nearby stations to infer how the rupture migrated in space. By using nearly colocated permanent stations, this method has also the potential to trace back in time the seismic connections between different fault patches and to monitor their activity especially before a large earthquake. In the Anza region, we have at least two magnitude 5 earthquake candidates for which we can apply this method and study in detail if there were early signs of anomalous seismicity on the portions of this fault before rupture.
The monitoring method that we develop is considered groundbreaking by the community and was referenced to as an emerging topic by the Seismological Society of America (Pinzon-Rincon et al. 2021). It has the potential to make significant discoveries not only in earthquake studies, but also in volcanology and in the monitoring of C02 sequestration and geothermal production. Before the end of the project, we plan to obtain unprecedented monitoring observations along the San Jacinto Fault by deploying the first long-term (2 years) dense array (300 stations). By deploying such a high number of stations for such a long time, we plan to detect smaller silent fault movements that would unveil the hidden mechanisms that prepare fault rupture. We also plan to study the Ridgecrest region where a M7.1 earthquake stroke in 2019. This will be a unique opportunity to monitor the preparation phase of a large earthquake that already happened thanks to the availability of permanent seismic stations in the area. Finally, we also plan to learn from a controlled fluid injection experiment in Wyoming where we deployed 150 stations. By controlling the stress perturbation generated by fluid injection, we plan to calibrate our stress-seismic velocity model which is still largely unknown.