**2. Detecting geological disasters using optical technology for Earth observation**

Optical technology for Earth observation can provide visual images for disaster target interpretation and disaster information extraction. Airborne optical technology is one of the main instruments for Earth observation, with its mobility and flexibility to provide real-time disaster remote sensing and surface images. With disaster mitigation work done to remotely sense secondary disasters after the Wenchuan earthquake, including barrier lake breaches, road damage, and landslides and debris flows, we analyze and discuss technical methods and applications of optical technology for Earth observation in monitoring secondary geological disasters.

#### **2.1 Extracting background information from the disaster area in Wenchuan**

The Wenchuan earthquake occurred in the Longmen mountain fault zone. Longmen mountain runs in the general northeast to southeast direction, about 500km from Guangyuan to Ya'an. Longmen mountain is one of China's typical nappe structures, a tectonic rock sheet along a imbricated thrust to the basin mainly formed in the Mesozoic and early Cenozoic (Wang et al, 2001). The Wenchuan earthquake occurred in the crustal brittleductile transition zone, and was a shallow earthquake with a focal depth of 10 km to 20 km and longer duration, so its destructiveness was huge (Bi et al, 2008).

After the Wenchuan earthquakes, CAS urgently arranged airborne and satellite data coordinate acquisition plans and obtained 41 scenes of post-disaster, high-resolution satellite data, and 105 scenes of pre-disaster and concurrent high-resolution archive satellite data. An optical remote sensing airplane carrying an advanced ADS40 aviation camera obtained high-resolution (0.5 - 0.8 m) optical pictures of the disaster area totaling 5 TB with a coverage area of 23,000 km2.

#### **2.2 Remote sensing monitoring and analysis of barrier lakes after the Wenchuan earthquake**

High-resolution ADS40 optical images of the disaster area were used to analyze the barrier lake for the first time. In the coverage area, 51 barrier lakes were detected, some with a beadlike distribution. The location, area, water level and height, and area of the dam body were detected according to a monitoring algorithm of barrier lake risk factors and 1:50,000 DEM data. The risk conditions, geology, and distribution of the 51 major barrier lakes were evaluated to support urgent relief work. The research indicated that the distribution of barrier lakes and spatial features of the earthquake fracture zone were identical.

#### **2.2.1 Barrier lake volume detection algorithm**

294 Earthquake Research and Analysis – Statistical Studies, Observations and Planning

The Chinese Academy of Sciences (CAS) immediately arranged a cooperative data acquisition program of airborne and satellite remote sensing data after Wenchuan and Yushu earthquakes and obtained 17 categories of more than 500 scenes of satellite images and high-resolution optical and microwave airborne remote sensing data. 8.7 TB of high-resolution data were freely provided initially to 16 ministries and 28 units, and an additional 3.5 TB were later downloaded from the network. At the same time, a study on remote sensing monitoring methods for post-earthquake secondary geological disasters was carried out, which played an important role in the disaster response. This paper focuses on three aspects, including optical Earth observation technology for monitoring secondary geological disasters, multi-mode radar Earth observation for post-earthquake deformation analysis, and an earthquake disaster

simulation evaluation system using the results of seismic disaster remote sensing.

**2. Detecting geological disasters using optical technology for Earth** 

**2.1 Extracting background information from the disaster area in Wenchuan** 

and longer duration, so its destructiveness was huge (Bi et al, 2008).

Optical technology for Earth observation can provide visual images for disaster target interpretation and disaster information extraction. Airborne optical technology is one of the main instruments for Earth observation, with its mobility and flexibility to provide real-time disaster remote sensing and surface images. With disaster mitigation work done to remotely sense secondary disasters after the Wenchuan earthquake, including barrier lake breaches, road damage, and landslides and debris flows, we analyze and discuss technical methods and applications of optical technology for Earth observation in monitoring secondary

The Wenchuan earthquake occurred in the Longmen mountain fault zone. Longmen mountain runs in the general northeast to southeast direction, about 500km from Guangyuan to Ya'an. Longmen mountain is one of China's typical nappe structures, a tectonic rock sheet along a imbricated thrust to the basin mainly formed in the Mesozoic and early Cenozoic (Wang et al, 2001). The Wenchuan earthquake occurred in the crustal brittleductile transition zone, and was a shallow earthquake with a focal depth of 10 km to 20 km

After the Wenchuan earthquakes, CAS urgently arranged airborne and satellite data coordinate acquisition plans and obtained 41 scenes of post-disaster, high-resolution satellite data, and 105 scenes of pre-disaster and concurrent high-resolution archive satellite data. An optical remote sensing airplane carrying an advanced ADS40 aviation camera obtained high-resolution (0.5 - 0.8 m) optical pictures of the disaster area totaling 5 TB with a

**2.2 Remote sensing monitoring and analysis of barrier lakes after the Wenchuan** 

barrier lakes and spatial features of the earthquake fracture zone were identical.

High-resolution ADS40 optical images of the disaster area were used to analyze the barrier lake for the first time. In the coverage area, 51 barrier lakes were detected, some with a beadlike distribution. The location, area, water level and height, and area of the dam body were detected according to a monitoring algorithm of barrier lake risk factors and 1:50,000 DEM data. The risk conditions, geology, and distribution of the 51 major barrier lakes were evaluated to support urgent relief work. The research indicated that the distribution of

**observation** 

geological disasters.

coverage area of 23,000 km2.

**earthquake** 

The water level and area of the barrier lakes were first estimated using high-resolution airborne images. The capacity of the barrier lake was then calculated using elevation contours, and the calculation was based on DEM data with a resolution of 25 m, which were interpolated from a 1:50,000-scale topographical map.

The method to calculate the reservoir capacity involves the following steps:

The water surface area was derived from high-resolution airborne images. Then the water surface elevation *(*ℎ�*)* was acquired by overlapping the water surface with the DEM data, since the elevation of the water surface is a constant. If there is some small shift between the orthorectified ADS40 image and the DEM data, the interpreted water surface should be adjusted slightly to ensure all interpreted water surfaces' borderline is located at the same altitude. Meanwhile, the elevation of the midline of the river *(*ℎ�*)* was directly recorded from the 1:50,000-scale topographic map. Therefore, the elevation difference and the water level could be calculated.

The capacity of the barrier lake was calculated by an integral approach. The capacity V at the water elevation of H is:

$$V(H) = \sum\_{i=1}^{n} S\_i \times \Delta h \tag{1}$$

where ∆ℎ is the integration interval, *n* is the equally parted cells number of the elevation drop from the water surface elevation *(*ℎ�*)* to the elevation of the midline of the river, *(*ℎ�*)*, ∆ℎ = (ℎ� − ℎ�)�� is the integration interval of each cell, and��� is the water surface area at the elevation of ℎ� − (� − �)∆ℎ , which can be automatically derived from the DEM data. The capacity and area of all the 51 barrier lakes were calculated by this method. According to their capacities, the barrier lakes were clustered into three types: Type I (large-sized) with a capacity over 3,000,000 m³; Type II (medium-sized) with a capacity between 1,000,000 and 3,000,000 m³; and Type III (small-sized) with a capacity less than 1,000,000 m³

#### **2.2.2 Risk assessment of the barrier lakes**

Barrier lakes formed in an earthquake will result in extreme flooding when they burst. Therefore, the risk assessment of barrier lakes becomes very important. The dimensionless blockage index (DBI) was introduced by Casagli and Ermini (Ermini et al, 2003; Liu et al., 2009) to evaluate the stability of a dam:

$$\text{DBI} = \log(\frac{A\_B \times H\_d}{V\_d}) \tag{2}$$

where �� is the volume of the dam, which is the dominant parameter of stability since it determines the gravity of the dam; �� is the area of the basin, which is the primary parameter of instability since it determines the runoff in the basin; and �� is the height of the dam, which is an important parameter for evaluating the stability of the barrier lake when confronted with overflow. The smaller the DBI value, the more stable the barrier lake. It is difficult to calculate the dam volume with the remote sensing image without in-situ measurement. An approximate estimation of dam volume is to multiply the dam area with its height, and thus Eq. (2) can be written as:

$$DBI \approx \log\left(\frac{A\_b \times H\_d}{H\_d \times S\_d}\right) = \log\left(\frac{A\_b}{S\_d}\right) \tag{3}$$

Earth Observation for Earthquake Disaster Monitoring and Assessment 297

The Wenchuan earthquake triggered many geological hazards, including collapses and landslides along river valleys. Some of the large masses of land fell into the river valleys and formed a number of barrier lakes. Figure 1 shows the distribution of 51 barrier lakes through the interpretation of remote sensing images. From the distribution map, the barrier lakes were apparently along the Yingxiu-Beichuan fault, and the distribution was consistent with the direction of the fault zone. There were a series of high-risk barrier lakes distributed along the rivers such as the Jianjiang River's upstream in Beichuan County, the Mianyuan

**2.2.4 Geological conditions and spatial distribution of the barrier lakes** 

River's upstream in Mianzhu City, and the Pingshui River in Shifang City.

results may give transportation department powerful information support.

**2.3.1 Road blockage and damage conditions in badly stricken areas** 

Fig. 2. Map of damaged roads after the Wenchuan earthquake

**earthquake** 

**2.3 Remote sensing monitoring and analysis of roads damaged by Wenchuan** 

High-resolution ADS40 optical airborne remote sensing images and other data were used to analyze and locate some national and provincial highways in seriously damaged areas. The process included analyzing qualitatively, orientatively and quantitatively different factors and classes of blocked and damaged roads from landslip, debris flows, river bank collapse, barrier lakes, earthquake disruption and ground fissuring. These monitoring and analysis

The main remote sensing road blockage and damage condition detection focuses on national and provincial highways. There are 5 national and provincial highways in the badly stricken

where ܵௗ is the dam area.

#### **2.2.3 Detection results of quake lakes in the Wenchuan earthquake**

We interpreted the barrier lake surface and the dam area from the high spatial resolution ADS40 airborne images and located the position of the landslide forming the barrier lake. The basin area of the barrier lake was then extracted with the DEM and hydrological data. Therefore, the reservoir capacity and DBI of the barrier lake could be calculated according to Eqs. (1) and (3). The monitoring information of barrier lakes within airborne remote sensing data coverage shows that:


Fig. 1. Distribution map of barrier lakes caused by the Wenchuan earthquake (Liu et al., 2009)

*<sup>A</sup> H A DBI*

We interpreted the barrier lake surface and the dam area from the high spatial resolution ADS40 airborne images and located the position of the landslide forming the barrier lake. The basin area of the barrier lake was then extracted with the DEM and hydrological data. Therefore, the reservoir capacity and DBI of the barrier lake could be calculated according to Eqs. (1) and (3). The monitoring information of barrier lakes within airborne remote sensing

i. Generally, the slope of the landslided are steep, and most of them are over 20 degrees.

ii. DBI values can reliably reflect the stability of the barrier lakes. A lower DBI value indicates a more stable barrier lake, but the risk of a secondary disaster is higher if breaches and overflows occur. According to the overflow time of the two barrier lakes located at Xiaojia Bridge, Anxian County, and Tangjiashan, Beichuan County, the barrier lakes in the Wenchuan earthquake could survive for more than 30 days if the

iii. The Wenchuan earthquake generated 10 large-sized and 14 medium-sized barrier lakes. Therefore, immediate attention should be paid to those barrier lakes with serious and

Fig. 1. Distribution map of barrier lakes caused by the Wenchuan earthquake (Liu et al., 2009)

**2.2.3 Detection results of quake lakes in the Wenchuan earthquake** 

This condition causes the formation of barrier lakes.

where ܵௗ is the dam area.

data coverage shows that:

DBI were smaller than 4.0.

continuous breaches.

log log *bd b dd d*

*HS S* 

(3)
