Final answer:
Moving lahars near Mount Rainier are primarily detected through seismic monitoring, which captures ground vibrations indicative of lahars' movement. While other techniques may offer geological insights, seismographs are key for real-time lahars detection.
Step-by-step explanation:
Moving lahars, which are destructive mudflows on the slopes of a volcano, can be detected in the upper reaches of valleys, such as those near Mount Rainier. One of the primary ways this detection is accomplished is through seismic monitoring. This method is used because lahars tend to generate ground vibrations that can be picked up by seismographs. These instruments are sensitive to the kinds of motions induced by lahars and provide essential, real-time data on their occurrence and movement.
Other methods such as weather radars, satellite imaging, and gravitational measurements can detect various geological phenomena, but in the context of this specific question about lahars near Mount Rainier, seismic monitoring is the correct answer. For example, seismic monitoring is also crucial in detecting earthquakes and volcanic eruptions, both of which can be indicators of forthcoming lahars.
Furthermore, LIDAR imagery can be used to produce detailed images of topography, including areas potentially affected by previous lahars, but it is more used for mapping than for real-time detection and monitoring.