Reconstructing the 2018 Lombok Earthquake Sequence: Insight from An Incident-Angle Corrected InSAR Analysis

The ground beneath our feet is rarely still. In many parts of the world, massive slabs of the Earth's crust are constantly pushing against one another, building up invisible pressure over decades. When this pressure finally releases, the result is an earthquake, a sudden jolt that reshapes the landscape. To understand these violent events, scientists often look at how the surface moves up, down, or sideways. One powerful tool for this is a technique that uses satellites to take pictures of the ground from space. These satellites send out radar signals that bounce off the Earth and return to the sensor. By comparing the timing of these signals over time, researchers can detect changes in the ground's position with incredible precision, measuring shifts as small as a few centimeters. However, this method has a quirk: because the satellites fly at an angle rather than straight down, the measurements they take can be distorted, especially in places with steep mountains or deep valleys. This distortion can make the ground appear to move less than it actually is, or hide the true direction of the shift. For scientists trying to assess the danger of future earthquakes or understand how the Earth's crust breaks, seeing the full, unblurred picture is essential.

In 2018, the island of Lombok in Indonesia experienced a terrifying sequence of earthquakes that killed hundreds of people and caused widespread destruction. The disaster began with a tremor on July 29, followed by a much larger quake on August 5, and another major shock on August 19. These events were caused by the rupture of a hidden fault line beneath the sea, known as the Flores Back-Arc Thrust, which runs just south of the island. Because the fault lies underground and the terrain is dominated by the steep slopes of the Mount Rinjani volcano, understanding exactly how the ground moved was difficult. A researcher named Guruh Sukarno Putra set out to reconstruct the precise movement of the Earth's surface during these three events. The goal was to see past the limitations of standard satellite data to reveal the true scale of the deformation. By applying a specific mathematical adjustment to the raw satellite data, the study aimed to correct for the angle at which the satellite viewed the ground, providing a clearer, more accurate map of how the land rose and fell.

The analysis focused on three distinct moments in the earthquake sequence, revealing a story of how the rupture traveled across the island. The first event, a foreshock on July 29, showed a complex and somewhat surprising pattern. While the coastal areas to the north experienced a noticeable rise in the ground, the area around the Mount Rinjani volcano and the Sembalun region to the east actually sank. The corrected data showed that the land near Mount Rinjani dropped by about 13.45 centimeters, while the nearby Sembalun area subsided by roughly 10 centimeters. In contrast, the coastal village of Anyar saw the ground lift by 13.16 centimeters. This simultaneous sinking of the volcanic highlands and rising of the coast suggests that the initial tremor did something more than just shift the tectonic plates; it likely caused the loose volcanic ash and rock within the mountain to compact or the underground water systems to lose pressure. This loss of structural integrity in the volcano may have acted as a trigger, shifting stress onto the deeper fault lines and setting the stage for the larger disasters to come.

The second event, the mainshock on August 5, was the most violent and caused the most dramatic changes to the landscape. This quake unleashed a massive upward surge across the northern coast of the island. The corrected measurements revealed that the ground in the village of Selengen rose by an astonishing 62.10 centimeters, the highest uplift recorded in the sequence. Other coastal locations, such as Kayangan and Montongpal, also saw the ground lift by nearly 59 centimeters. The standard satellite data had shown these numbers to be lower, but the correction process proved that the true vertical movement was significantly greater than initially thought. This massive uplift confirms that the fault rupture was shallow and steep, pushing the land upward with great force. The data indicates that this event was the primary driver of the destruction, as the sudden rise of the ground would have violently shaken everything built upon it.

The final major event, an aftershock on August 19, marked a clear shift in the location of the damage. Unlike the first two events which centered on the western and central parts of the island, this quake moved the zone of deformation eastward. While the western coastal areas saw very little movement, the eastern villages experienced their most significant shifts during this phase. The village of Belanting saw the ground rise by 31.21 centimeters, and Darakunci experienced a lift of 40.57 centimeters. This eastward migration suggests that the stress from the earlier earthquakes had transferred to a new, separate segment of the fault line further to the east, causing it to break. The pattern of movement across the three events paints a picture of a cascading failure, where one break in the crust led to the next, traveling from west to east across the island over three weeks.

The study highlights a critical lesson for understanding earthquakes in mountainous regions. Without correcting for the angle of the satellite view, the true scale of the ground movement can be underestimated, particularly in areas with steep topography like the slopes of Mount Rinjani. The corrected data revealed that the deformation was not a uniform lifting of the entire island, but a complex interaction where the volcanic mountain behaved differently from the sedimentary land to the north. The sinking of the volcano during the first tremor appears to have been a key precursor, altering the stress on the fault and leading to the catastrophic mainshock. By mapping these subtle and large-scale movements with greater accuracy, scientists can better understand how faults behave in volcanic environments and improve the models used to assess future risks. The findings serve as a reminder that in the dynamic and dangerous landscape of the Indonesian archipelago, the ground is constantly shifting, and seeing it clearly requires looking past the distortions of our view.