Research Trends and Results

Research on assessing the risk of landslide in storms
following major seismic disturbances
NOMURA Yasuhiro, Researcher
OKAMOTO Atsushi, HeadOKUYAMA Yuki, Researcher
Research Center for Disaster Management, Erosion and Sediment Control Division（Key words）

Earthquake, Landslide, Risk assessment, Sediment disaster warning information

1. Introduction
It is said that once a large magnitude earthquake occurs,
it losens portions in slopes such as a crack, which makes
it more likely to cause landslides compared to before theearthquake (Photo 1). The sediment disaster warning
information issued by the prefectural governments andlocal meteorogical observatories in collaboration is madepublic when the 60 minutes cumulative rainfall andsoil-water index exceeds a certain criterion, but after thelarge magnitude earthquake, an interim criterion that is 50
to 80 percent of normal is actually applied to issue theinformation. (Fig. 1 generally only the soil-water index islowered.) Here, we would like to introduce the results of
the research on the adequacy of this interim criterion.
2. Evaluation of the interim criterion of the Sedimentdisaster warning information after earthquakes
We have calculated the disaster capture rate which isthe percentage of the numbers of the issued warning
information before sediment disaster occurrence based on
the rainfall data and sediment disaster occurrence data ina half year in the Tohoku and Kanto region that haveobserved earthquakes of the intensity 5 or more due to theGreat East Japan Earthquake, 2011 and applied theinterim criterion. At that occasion, we have classified
three areas such as (1) Disaster capture area by thenormal criterion, (2) Capture area by the interim criterionand (3) No capture area (Fig. 1) and classified thenumbers of disasters in the respective area and largest
earthquake intensity in the Table 1 and Fig. 2. By that
result, we have found that introduction of the interim
criterion has successfully raised the capture rate by 18.8%,
which was 68.8% by the normal criterion just goingunder the nationwide average value of 75% at normal
time, so it can be called an improvement. Therefore theinterim criterion was somewhat effective. Further, wehave reviewed the required minimum lowering rate basedon the consideration that lowering the criterion might
increase the warning information issuance frequency andincrease the air shot, and obtained the result that there
was no change in the capture rate between 70 to 80% and
50 to 80% of the criterion value.
3. To finish
From now, we are to implement a further review
through the slope stability analysis to have a quantitativeevaluation on loosened ground due to earthquakes.

Before collapse (Justafter the earthquake)

Photo 1 Example of the slope that has collapsed by therainfall after to the Great East Japan Earthquake,2011(provided by Tochigi Pref.)Fig. 1 Provisional reference and disaster capture
image after large magnitude earthquake
Table 1 Relation between the disaster capture stateand largest earthquake intensity
Area
(1)
(2)
(3)
Total

Number of landslide occurrence
Level5+Level6Level6+
Level7
58
90
10
5000
40
68
180

Total

Share

226432

68.8%
18.8%
12.5%
-Fig. 2 Relation between the disaster occurrence
state and largest earthquake intensity