Acre has not been considered an area of major seismic concern, yet recent work on deep earthquakes in the western Amazon shows a level of activity that conflicts with long held expectations about the region. The study that examined these events focused on earthquakes forming at depths near six hundred kilometers, far below the crust and well within the mantle. Deep earthquakes of this type are uncommon in continental interiors and are generally associated with regions affected by strong subduction forces. Acre does not fall within those zones, which is what brings attention to the recorded events. The data shows several high magnitude quakes in recent years, including a six point six in 2024, and the modeling reveals that the forces involved originate in a part of the interior where seismic failure is normally limited.
The simulations used in the study trace the rise of seismic motion from the lower mantle toward the crust. They show that movement begins as a compact disturbance before separating into two distinct pulses as it travels upward. The separation occurs because the deeper layers of the Earth do not transmit energy uniformly. As the pulses climb, boundaries within the mantle redirect part of the motion downward while allowing the remainder to continue toward the crust. This behavior does not erase the disturbance. Instead, it divides and reshapes it. When the model extends to the surface, the residual motion remains strong enough to produce a measurable vertical lift at distances that exceed two hundred kilometers from the source. The modeled uplift reaches more than a tenth of a meter, which is significant for a deep event because most energy from that depth would typically dissipate before reaching the crust. The fact that the modeled motion remains coherent indicates that Acre sits above a structure that does not fully absorb or scatter energy rising from the mantle.
The surface data from Acre supports the presence of deeper influence. Independent geodetic measurements show that the crust in the region has shifted within a short timeframe. These shifts are not isolated anomalies. They represent regional scale movement of the upper crust that is detectable through established surveying methods. The recorded movement occurred within a period of only a few years, which stands out because continental interiors normally change at slower rates unless influenced by strong tectonic processes. No large shallow earthquakes accompanied these changes, which leads attention toward the deep events that have been recorded.
The frequency of seismic activity in Acre has also changed. Records indicate an increase in the number of local earthquakes since 2010. Although many events remain small, the increase in frequency creates a pattern that was not present in earlier decades. When viewed alongside the deep events identified in the study, the shift in regional activity suggests a change in the mechanical state of the area. A region that once produced minimal seismic movement is now showing repeated disturbances at both deep and shallow levels, which marks a change in its overall behavior.
The Tarauacá Fault, which runs through the region, provides another point of evidence for the shift. This fault has historically shown limited activity. Recent measurements, however, document movement that does not match the behavior expected from typical continental fault systems. Instead of remaining stable, the fault displays subtle but consistent changes in position. These changes do not match shallow loading patterns and do not correlate with known stress pathways near the surface. They appear to respond to deeper forces that are not fully defined. The study notes indications that pressure may be moving into the region from the west, potentially linked to large scale geodynamic activity beneath the Andes. This influence is not confirmed, but the motion observed along the Tarauacá Fault is consistent with a system responding to deep pressure rather than surface stress.
The modeling used in the study provides a detailed view of how deep earthquakes propagate through the interior beneath Acre. The simulations incorporate variations in density, rigidity, and attenuation within the mantle. These variables change with depth and control how energy travels. When the seismic source is placed within the deep mantle, the model shows the initial disturbance rising through layers that transmit energy differently. Some boundaries slow the motion. Others bend it. Others reflect small portions of the wave back into the lower regions. The combined effect is a complex path of energy movement rather than a simple upward trajectory. Despite this complexity, the energy in the model remains strong enough to influence the crust after traveling through hundreds of kilometers of rock.
What makes the findings notable is that Acre is not located near the type of plate interface where deep earthquakes typically form. Deep seismicity is normally associated with subduction zones where one plate descends beneath another. The western Amazon is far from the active portion of the subduction front. For deep earthquakes to occur at the recorded depths in Acre, conditions within the mantle must allow localized failure. This requires specific temperature and pressure states along with material that is capable of brittle behavior even in the lower mantle. The presence of these conditions beneath Acre was not previously identified, which raises interest in how the region developed its current internal configuration.
The recorded deep earthquakes also display magnitudes that align with significant energy release. The six point six event in 2024 is large for a deep intraplate earthquake. Deep events often reach moderate magnitudes, but high magnitude deep earthquakes in continental interiors are not common. When such an event occurs, it indicates substantial stress accumulation at depth. The fact that Acre has produced multiple deep events in a short span of time places the region in contrast with earlier assumptions about its seismic character.
The surface response to these deep events adds another piece to the picture. The measured crustal displacement does not match signals produced by local shallow processes. Instead, it resembles the type of slow deformation that can follow repeated deep disturbances. When deep earthquakes occur in clusters or when they increase in frequency, the accumulated energy can translate into subtle but detectable surface changes. The measurements in Acre show that such changes have taken place. These observations align with the timeline of increased seismic frequency since 2010, confirming that the crust is responding to deeper processes even though the exact driver remains unresolved.
The study maintains a focus on measured behavior rather than interpretation. It records the properties of the deep earthquakes, simulates their upward propagation, and compares the model outputs to observed surface data. The match between deep activity, modeled surface motion, and recorded crustal displacement supports the conclusion that Acre is experiencing a period of unusual seismic change. The work avoids attributing the changes to a specific mechanism because the internal structure of the region has not been fully mapped and the forces involved have not been characterized in detail.
A notable part of the study is the demonstration that deep seismic energy in Acre can reach the surface in measurable form. The modeled uplift is an example of how signals from the mantle can cross the full depth of the interior and alter the surface without producing the large shallow earthquakes that normally accompany such changes. This type of behavior is important because it shows that a region can undergo internal deformation without obvious shallow seismic release. When the crust shifts without surface earthquakes, the cause must be sought at depth.
Acre is now documented as a location where deep seismic processes are acting within the continental interior. The region shows deep earthquakes at significant magnitudes, measurable crustal displacement over short intervals, increased local seismic frequency, and fault movement that does not align with expected patterns. The combination of these observations indicates that the interior beneath Acre is active in a way that has not been captured in earlier models of the western Amazon. Each new measurement adds clarity to the pattern forming across the region.
Source:
Based on data modeled in Modelling and Simulation of the Propagation of P-SV Seismic Waves from Earthquakes: Application to Deep Earthquakes in Acre, Brazil (arXiv:2601.03177v1).
https://arxiv.org/abs/2601.03177
