Characterization of the Moreras Fault Zone and related faults (Águilas Arc, Eastern Betic Cordillera)

Deep beneath the sun-drenched coast of southeastern Spain, the Earth's crust is slowly being squeezed. This region sits where two massive tectonic plates, the Nubian and Eurasian plates, are pushing against one another. When these giant slabs of rock collide, the land between them crumples, folds, and breaks, creating a landscape of mountains and valleys. In this specific area, known as the Eastern Betic Cordillera, the pressure has created a curved zone of deformation that geologists call a tectonic arc. For decades, scientists have mapped the major cracks, or faults, in this zone, trying to understand how the land moves and where the next earthquake might come from. Knowing which faults are active and how long they are is crucial for assessing the risk of future earthquakes, as longer faults can generally produce larger, more destructive quakes.

In the southern part of this tectonic arc lies a region called the Águilas Arc, home to a major geological feature known as the Moreras Fault. Previous maps and studies had suggested that this fault was part of a much larger, continuous system stretching over a hundred kilometers, connecting the land to the sea floor and potentially acting as a single, massive engine for earthquakes. However, a new study led by researchers from the University of Alicante and the University of Jaén has taken a fresh, close-up look at this area. By walking the terrain, mapping the rocks in high detail, and analyzing how the ground has shifted over millions of years, they have rewritten the story of this fault. Their work reveals that the Moreras Fault is not the giant, continuous structure it was once thought to be, but rather a shorter, distinct zone that has largely quieted down in recent geological times.

The researchers focused their efforts on the Moreras Fault Zone, a complex area of broken rock that runs roughly east-west across the landscape. Using detailed maps and high-resolution aerial photographs, they traced the exact path of the fault, measuring its length and width with a precision that was not possible in earlier, broader surveys. They found that the fault zone is about 23 kilometers long and up to 2.8 kilometers wide. It is not a single crack in the ground, but a complex zone made of two main parallel strands and many smaller, secondary cracks that twist and turn around each other. As the researchers examined the rocks, they looked for "slickenlines," which are polished grooves on the rock surfaces that act like fingerprints, showing the direction the ground moved when the fault slipped. These marks, along with the way different rock layers have been shifted apart, told a clear story: the fault has moved sideways, with the ground on one side sliding past the other in a right-lateral motion. Over millions of years, this movement has displaced the land by approximately 10 kilometers.

One of the most significant discoveries of this study concerns the fault's connection to the sea. For years, geologists believed the Moreras Fault continued eastward underwater, linking up with a series of faults along the Mazarrón Escarpment and the Cape Tiñoso fault system to form one massive, 100-kilometer-long structure. The new fieldwork, however, shows that this connection does not exist. The researchers found that the Moreras Fault actually stops abruptly when it hits a different set of faults running in a northeast-southwest direction, known as the Peñas Blancas fault system. These neighboring faults act like a wall, cutting off the Moreras Fault and shifting its eastern end away. Furthermore, the faults found underwater near Cape Tiñoso run in a different direction and behave differently than the Moreras Fault, proving they are separate structures entirely. This means the Moreras Fault is an isolated system, roughly 23 kilometers long, rather than a piece of a much larger, continuous earthquake machine.

The study also investigated how active these faults are today. While the Moreras Fault was a major player during the Miocene epoch, millions of years ago, the evidence for recent movement is surprisingly scarce. The researchers looked for signs of deformation in the youngest layers of rock, specifically the gravel and sand deposits left by ancient rivers and floods from the last few hundred thousand years. They found only subtle, minor disturbances in a few small areas, mostly in the eastern part of the fault. In contrast, the faults further north in the region show much stronger evidence of recent activity and produce more frequent, albeit small, earthquakes. This suggests that the center of tectonic activity in this part of Spain has slowly migrated northward over time. The Moreras Fault, once a primary driver of landscape change, appears to have slowed down significantly, with its movement largely arrested in the last few million years.

These findings have important implications for how we understand the seismic risk in the region. If the Moreras Fault were part of a 100-kilometer continuous system, it could theoretically generate a very large earthquake. By redefining it as a shorter, 23-kilometer structure that is no longer the primary focus of tectonic stress, the potential size of a future earthquake on this specific fault is reduced. The study does not say the fault is completely dead; it may still move slightly, perhaps as a reverse fault where one side pushes up over the other, but the massive sideways sliding that shaped the landscape in the past has largely ceased. The researchers conclude that the deformation caused by the collision of the tectonic plates has shifted away from this southern arc and toward the Eastern Betic Shear Zone to the north. This new understanding, built on careful observation of the rocks themselves, provides a clearer, more accurate picture of the geological forces at play beneath the Spanish coast.