# Application of Acoustic Emission Technique in Landslide Monitoring and Early Warning: A Review
## 1 Introduction  
Landslides are a major global hazard, causing significant socioeconomic losses and over 50,000 fatalities worldwide in the past decade [1]. These destructive events occur in nearly every country, triggered by factors such as heavy rainfall, earthquakes, erosion, deforestation, and human activities like urbanization. Mountainous regions, characterizedby steep terrain and heavy precipitation during the wet season, face a notably higher frequency and risk of landslides [2]. For instance, the southwestern region of China, with its proximity to active seismic zones, experiences a higher prevalence of slope failures (Figure 1). These events endanger human lives and the environment by posing threats to engineering structures, disrupting critical infrastructure, causing structural damage, and contaminating water sources. Given the complex interplay of factors that contribute to landslides, effective monitoring and early warning systems are essential for mitigating their devastating impacts. The causes of landslides are multifaceted, and involve a combination of short-term triggering factors and underlying predisposing conditions [3; 4]. Research has identified a combination of short-term triggering factors and underlying predisposing conditions that contribute to the occurrence of landslides [5]. Major triggering factors include heavy or prolonged rainfall [6], rapid snowmelt [7], changes in groundwater level [8], and earthquakes [9]. Intense rainfall plays a critical role in triggering landslides by weakening the shear strength of soil and rocks through two primary mechanisms. Firstly, it increases the water content in soils and pore water pressure, leading to a decrease in effective stress [10]. This reduction in effective stress makes the soil and rocks more susceptible to failure. Secondly, heavy rainfall adds mass to the already unstable slope material, creating an additional driving force for landslides [11]. The combined effect of increased water content and added mass significantly increases the likelihood of slope failure [12]. In addition to rainfall, rapid snowmelt can also contribute to landslides by saturating the soil and increasing pore water pressure [13]. Changes in groundwater levels can further exacerbate landslide risk by altering the stability of slopes. Earthquakes, with their ground-shaking effects, can directly trigger landslides by destabilizing already vulnerable slopes. To address these challenges,  
Figure 1: Spatial distribution of slope disasters in China. the implementation of advanced monitoring techniques, such as acoustic emission (AE) monitoring, becomes increasingly necessary. Landslides often develop gradually, characterized by incremental deformations that culminate in sudden failures [14]. This gradual progression emphasizes the importance of continuous monitoring technologies to detect early signs of destabilization and provide timely warnings [15]. However, the geological heterogeneity of terrain complicates the identification of potential failure zones and the accurate quantification of slope movements. Various surface and subsurface monitoring techniques have been employed to investigate landslide characteristics [16]. Geodetic methods, such as synthetic aperture radar (SAR) [17], aerial/satellite imagery [18], and terrestrial laser scanning (TLS) [19], provide valuable spatial data but are limited in capturing subsurface information. In contrast, geotechnical instrumentation, including inclinometers [20], wire extensometers [21], and time domain reflectometry (TDR) [22], allow for the direct measurement of slope displacements but are restricted to specific monitored areas. Geophysical techniques, including electrical resistivity tomography (ERT) and ground penetrating radar (GPR), offer insights into subsurface structures but require meticulous data processing [23; 24]. Conventional measurement methods often detect landslides only after failure has occurred, limiting timely evacuation efforts [25; 26].