Knowing the groundwater dynamics in landslide areas is very important for numerically analyzing slope stability and for planning countermeasure works. This review summarized the observation methods of groundwater within the landslide mass and groundwater inflow into it and classified them into direct and indirect methods. Direct methods such as groundwater level observation and groundwater logging can reveal the temporal and spatial fluctuations of the groundwater level and the depths of groundwater flow layers. The direct methods are still central for understanding groundwater dynamics in landslide areas. However, the information obtained with the methods is limited to the location surveyed. Indirect methods have been remarkably developed in recent years. Airborne electromagnetic surveys rapidly provide wide-area groundwater information in steep mountainous areas where workers are difficult to enter, or large landslides occur. In addition, a method of quantifying the sound of groundwater flow to determine the flow paths of groundwater has been put to practical use. Although the indirect methods have the advantage of providing information in wide areas, they have the risk to misidentify or overlook groundwater. Direct methods and indirect methods can complement each other's shortcomings. The combination of direct and indirect methods can provide detailed information on spatiotemporal groundwater distribution over wide areas efficiently in term of cost and labor. This enables a more accurate analysis of landslide sliding and more effective construction of landslide countermeasure works.
On August 10, 2021, a total of 454 landslides and debris flows in the northern part of the Shimokita Peninsula, Aomori, Japan, resulted in one of the major natural disasters in the region in recent years. Large amounts of rainfall of 385 mm fell on 9 and 10 August, due to low atmospheric pressure and the consequent Typhoon Lupit (No. 9) passing over the Sea of Japan. The event resulted in landslides, flooding, and damage to houses, roads, bridges, and carried large wood and debris into the rivers, which caused blockages at dam locations. Landslides occurred mainly in the fluvial dissected Koakakawa and Ohakakawa River Basins and on the steep slopes of costal terrace scarps nearby Yakeyamasaki. Landslides were primarily associated with weathered tuffaceous mudstone and weathered andesitic green-colored tuff in the Koakakawa and Ohakakawa Basins and weathered pyroclastic flow, colluvium, and gravel layers nearby the Yakeyamasaki. Pipping holes were observed within or at the bottom of weathered tuffaceous mudstone, weathered pyroclastic flow, and colluvium, suggesting the landslide occurrence may have been caused by the amount of groundwater present.