The severity of earthquake damage depends on several factors. The earthquake’s characteristics, such as its magnitude and depth, are one thing. Critically, however, the local geological setting introduces a significant, localised factor: the site effect. This phenomenon, driven by the specific properties of the near-surface soil and bedrock, can dramatically modify or amplify seismic waves as they reach the surface.
For urban planning and risk mitigation, acquiring a high-resolution spatial map of this site effect is paramount. It provides the essential data layer necessary for civil engineers and policymakers to accurately assess which urban areas and structures face an elevated risk or will undergo the greatest mechanical stress during future seismic episodes. The rapid estimation of earthquake effects and the assessment of future earthquake scenarios are directly supported by this data, culminating in the generation of ground motion maps known as ShakeMaps.
According to research, accurately mapping and quantifying the seismic site effect—especially with the high spatial resolution needed for urban planning—is challenging and is a complex, data-intensive area of ongoing study. To simultaneously improve the spatial resolution of ShakeMaps, while reducing their uncertainty, dense measurements of ground accelerations, the speed and direction of an earthquake, are necessary.
But a study published in Nature Communications by several leading European professors has elicited a breakthrough. They’ve discovered that detailed site amplification maps, tools that show how much the local soil is expected to amplify (increase the intensity of) earthquake ground shaking, can be created using citizens’ smartphones.
Accelerometers have been in our phones since 2007
Accelerometers are ubiquitous in almost every smartphone. With the release of the iPhone in 2007, they were installed to manage the automatic orientation of the screen. However, in the years since then, they’ve taken on many new roles, including analysing our physical activity in our health apps, in games, and to orient the phone in our map apps. So, they’re pretty useful anyway. However, the study reveals that, by combining thousands of measurements from the Italian region of Campi Flegrei with statistical models, it is possible to map the ground shaking using data from accelerometers, too.
In fact, the accelerometers’ measurements achieve a level of detail far beyond that which seismic stations can provide. This information is crucial for assessing earthquake risk (seismic hazard) and can actually help to guide faster emergency response following a seismic event.
A citizen science approach to earthquake mapping
The pursuit of higher-resolution ShakeMaps, particularly for mapping Peak Ground Acceleration (PGA) in cities, is now being advanced by a global effort to utilise consumer technology. This involves integrating the spatially dense accelerometer measurements gathered from smartphones participating in the Earthquake Network (EQN), a crucial development in real-time seismic monitoring and early warning systems. The Earthquake Network (EQN), a citizen science project launched in 2013 by Francesco Finazzi, has involved over 20 million participants, demonstrating the vast, untapped potential of crowdsourced seismic data.
The system works by having participating phones detect shaking, data which is then sent to a central server. This triggers an alert to users in the surrounding area within seconds, hopefully providing recipients with crucial time to find safety before the strongest shaking hits.
Although smartphones are effective for seismic monitoring, this approach seeks to determine if they can also be used for mapping ground shaking. This is a challenge because smartphone data is inconsistent: accelerometer readings are influenced by building characteristics, device location and placement. All of these factors can lead to measurements that differ significantly from professional seismic instruments.
To utilise the correlation structure between smartphone and station recordings, the authors applied a spatial statistical model. This method compensates for device- and building-specific effects and allows the underlying amplification pattern to be identified. While individual smartphone recordings are noisy, the aggregation of thousands of measurements and their fusion with classic seismological data provide reliable high-resolution amplification maps.
Test Case: Campi Flegrei, Italy
The Campi Flegrei (Phlegraean Fields) region near Naples, Italy, offered the perfect, critical test case. This area is a highly volcanic and seismically hazardous zone, home to approximately 500,000 people.
Between April and June 2024, the region experienced increased seismic activity. During this period, 7,000 to 9,000 residents per event in the 130 km² “red zone” actively provided data via the EQN network. This crowdsourced data was incredibly dense, contrasting sharply with the “only” 29 traditional seismic stations covering the same region.
Analysis of this data enabled the researchers to create the first high-resolution amplification map of the “red zone.” Lead author Francesco Finazzi explains:
“The EQN smartphone network is very dense and covers areas of the red zone where no stations are installed. This allows the variability of ground shaking to be captured with a higher spatial resolution across the entire red zone.”
The resulting amplification map revealed significant variations over a distance of only 10 kilometres. Ground shaking amplification varied from a damping factor of 0.25–0.5 in the eastern part of the zone to a strong amplification factor of 2–3 in the southwest.
Crucial for emergency teams
These new amplification maps, based on historical smartphone data, can be quickly combined with new, event-specific accelerometer measurements to create high-resolution “ShakeMaps” following any new earthquake.
ShakeMaps are an indispensable tool for impact assessment and future scenario studies. They are crucial for guiding rescue teams, damage assessment and organising emergency measures in the critical hours following a seismic event.
Co-author Prof. Dr. Fabrice Cotton summarises the significance:
“Given the world’s growing urban population and the urgent need for high-resolution ShakeMaps, this study demonstrates that combining citizens’ smartphone accelerometer data with seismic network observations allows for the creation of site-specific, high-resolution ShakeMaps in densely populated urban areas.”
