2:45 PM - 3:00 PM
[HDS10-05] Study on the landslide mechanism of the Iya area where the deep-seated landslide occurred by Typhoon Talas in 2011 using Drone Airborne TEM
Keywords:Drone Airborne TEM, fault crush zone, groundwater
In 2011 Typhoon Talas, the total precipitation on the Kii Peninsula exceeded 1,000 mm in a wide area. As a result, deep-seated landslides occurred at 72 slopes around the Peninsula. Immediately after the disaster, researches on the mechanisms of the deep-seated landslides have been conducted from various viewpoints. In this study, taking the Iya district in Tanabe City, Wakayama Prefecture as an example, the mechanism of deep-seated landslides has been investigated from the viewpoint of guiding and blocking of the flow of groundwater by the fault crush zones that cross the slope. In particular, the behavior of groundwater around the fault crush zones caused by the heavy rain is clarified by using Drone Airborne Transient Electromagnetics (Drone Airborne TEM) technology that has rapidly become to be used in recent years.
For the purpose of the survey was grasping the rough geological characteristics, the distribution of the height of the groundwater and the distribution of the springs, and the presence of fault crush zones. We investigated the resistivity distributions of about 0-200 m underground was investigated. The date of the survey was two days after the total rainfall of 98.5 mm, and it is probable that the effects of the rainfall remained. Next, the electrical prospecting (about 700 m length) about 0-200 m underground was carried out (November 25-27, 2020) for the purpose of investigating the location of the fault crush zones and the underground crush structure in detail. The location of the electrical prospecting was located on the upper right side of the landslide slope viewed from the bottom of the landslide slope in which the results of the field survey reveal that fault crush zones are concentrated. At the start of the measurement, it was no rainfall for 4 days. And so, the resistivity distribution during the dry period was obtained. Drone Airborne TEM was conducted two times around the time of the electrical prospecting and at the same location as the electrical prospecting. The surveys were conducted in the days when was 3 days after the end of rainfall of 217 mm in total rainfall of Typhoon CHAN-HOM (October 13, 2020) and the day when was in the dry period and without rainfall for 11 days (December 2, 2020). In the Drone Airborne TEM, the resistivity distributions about 0-200 m underground and about 700 m length. Then, by taking these differences, the behavior of groundwater around the fault crush zone at the time of Typhoon CHAN-HOM in 2020 was clarified.
It was found that there are four fault crush zones around the landslide slope in the Iya area. In addition, it was found that three of these fault crush zones are of the type that blocks the flow of the groundwater. Also, it was found that the other one is a type that induces the groundwater from the surroundings. It is probable that at the time of the disaster in 2011, groundwater was induced by these faults, and the groundwater was blocked, causing the deep-seated landslide.
For the purpose of the survey was grasping the rough geological characteristics, the distribution of the height of the groundwater and the distribution of the springs, and the presence of fault crush zones. We investigated the resistivity distributions of about 0-200 m underground was investigated. The date of the survey was two days after the total rainfall of 98.5 mm, and it is probable that the effects of the rainfall remained. Next, the electrical prospecting (about 700 m length) about 0-200 m underground was carried out (November 25-27, 2020) for the purpose of investigating the location of the fault crush zones and the underground crush structure in detail. The location of the electrical prospecting was located on the upper right side of the landslide slope viewed from the bottom of the landslide slope in which the results of the field survey reveal that fault crush zones are concentrated. At the start of the measurement, it was no rainfall for 4 days. And so, the resistivity distribution during the dry period was obtained. Drone Airborne TEM was conducted two times around the time of the electrical prospecting and at the same location as the electrical prospecting. The surveys were conducted in the days when was 3 days after the end of rainfall of 217 mm in total rainfall of Typhoon CHAN-HOM (October 13, 2020) and the day when was in the dry period and without rainfall for 11 days (December 2, 2020). In the Drone Airborne TEM, the resistivity distributions about 0-200 m underground and about 700 m length. Then, by taking these differences, the behavior of groundwater around the fault crush zone at the time of Typhoon CHAN-HOM in 2020 was clarified.
It was found that there are four fault crush zones around the landslide slope in the Iya area. In addition, it was found that three of these fault crush zones are of the type that blocks the flow of the groundwater. Also, it was found that the other one is a type that induces the groundwater from the surroundings. It is probable that at the time of the disaster in 2011, groundwater was induced by these faults, and the groundwater was blocked, causing the deep-seated landslide.