Engineering attributes of ground motions from the August 2026 San José del Palmar, Colombia, earthquake
This paper analyzes the dense strong-motion dataset from the August 2026 7.4 San José del Palmar earthquake in Colombia to characterize ground-motion attributes, evaluate regional and global prediction models, identify southwestward source directivity and prolonged durations, and generate event-specific ground-motion fields for engineering applications.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
When the earth shakes, the ground does not move in a single, uniform way. It pulses with different speeds, lasts for varying lengths of time, and carries energy that changes as it travels through rock and soil. For engineers designing buildings and bridges, knowing exactly how the ground moves is a matter of life and death. They need to understand not just how hard the ground accelerates, but how long that shaking lasts and how the energy is distributed across different frequencies. This knowledge helps them predict whether a structure will sway safely or collapse. While scientists have built models to predict these movements for many types of earthquakes, large earthquakes that happen deep within the Earth's crust, known as intraslab events, have been difficult to study in certain parts of the world because there simply haven't been enough recordings to go around.
In August 2026, a massive earthquake struck western Colombia, offering a rare and detailed look at this specific type of seismic event. The tremor, measuring 7.4 on the magnitude scale, occurred deep underground at a depth of 103 kilometers. Because it happened in a region where such large, deep earthquakes are uncommon and poorly recorded, the event provided a unique opportunity for researchers to gather a vast amount of data. Scientists from the Georgia Institute of Technology, working with local networks, collected recordings from dozens of stations across cities like Cali, Pereira, and Bogotá. Their goal was to measure the specific characteristics of the shaking and see how well existing global predictions matched what actually happened on the ground.
The researchers began by cleaning up the raw data from the recording stations, removing electronic noise and fixing errors to ensure the signals were accurate. They then measured several key aspects of the shaking: how fast the ground accelerated, how fast it moved, how long the shaking lasted, and the total energy it carried. When they compared these real-world measurements to the standard models used by engineers globally, they found that the global models tended to overestimate the intensity of the shaking for this specific region. However, a model developed specifically for northern South America matched the observations much more closely. This suggests that local geological conditions and the way seismic waves travel through this part of the world are distinct enough that global averages are not always precise.
One of the most striking discoveries was about the duration of the shaking. The earthquake produced ground motions that lasted significantly longer than the models predicted. In fact, the shaking lasted longer than recordings from similar deep earthquakes in Japan. This extended duration is critical because it means structures are subjected to stress for a longer period, which can lead to different types of damage than short, sharp jolts. The researchers also noticed that the shaking was not the same in every direction. The ground moved with greater intensity toward the southwest, which is the direction the earthquake's rupture traveled underground. At the same time, the shaking was shorter in duration in that same direction. This pattern, where the energy is focused in the direction of the rupture, is known as directivity, and the data confirmed that this deep earthquake behaved in a way that concentrated its force toward the southwest.
The study also highlighted how much the local ground conditions changed the shaking within cities. By comparing recordings from stations on solid rock against those on soft soil or within buildings, the researchers saw that the local soil acted like a filter, amplifying certain types of shaking while dampening others. In cities like Cali and Pereira, the difference between the shaking on rock and the shaking on soft soil was substantial, proving that knowing the exact location of a building is just as important as knowing the size of the earthquake. To help engineers assess damage in areas where no instruments were placed, the team created detailed maps that reconstructed the shaking across the entire region. These maps combined the best available prediction models with the actual data they collected, filling in the gaps to show a continuous picture of the earthquake's impact.
Ultimately, this research provides a clearer, more accurate picture of how deep earthquakes behave in northern South America. It confirms that while global models are useful starting points, they cannot fully capture the unique behavior of local events. The findings emphasize that for this region, the shaking lasts longer and is more directional than previously thought, and that local soil conditions play a massive role in how buildings experience the quake. By creating these detailed maps and refining the models, the researchers have given engineers better tools to design safer structures and to understand the true risk posed by future deep earthquakes in the region.
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