Earthquake hazard mapping with seismic micro-zonation
To assess the hazard, earthquake hazard mapping with seismic micro-zonation necessitates a thorough field investigation. It's very good at identifying seismic hazard variations across space. They're also helpful for assessing risk scenarios in the research field. Seismic micro-zonation maps are extremely useful in urban planning because they aid in the prediction of possible earthquakes. (EASSER, 1954). In this study, seismic micro-zonation was used at a map detail scale of 1: 25,000.
Based on the results of seismic micro-zonation analysis, three parameters can be obtained for further calculations by giving the highest class in earthquake-prone areas, with parameters and classification as shown in Table 2.
Table 2
Parameters for the Preparation of the Palu City Earthquake Micro-zonation Map
No
|
Parameter
|
Class Division
|
Score
|
1
|
Dominant Period
|
< 0.4 (Alluvial/Sediment)
|
1
|
0.4–0.6 (Alluvial/Sediment)
|
2
|
> 0.6 (Granit/Metamorf)
|
3
|
2
|
Amplification
|
1.01–2.69 (Alluvial/Sediment)
|
1
|
2.69–4.86 (Alluvial/Sediment)
|
2
|
4.86–7.56 (Granit/Metamorf)
|
3
|
3
|
Peak Ground Accelaration (PGA)
|
0.59–0.691 (Medium High)
|
1
|
0.691–0.783 (High)
|
2
|
0.783–0.875 (Very High)
|
3
|
The value of the dominant period is in the range of 0.4 to 0.6 seconds; by referring to the National Earthquake Hazard Reduction Program classification, the research area shows that the dominant geological layer is alluvial to granite and metamorphic sediments, especially along the fault line. The value of the dominant period of land indicates the level of vulnerability. The greater the value of the dominant period of land in a location, the more vulnerable the location is to earthquake shocks. Almost all study areas are in a thick layer of sediment with a very high risk of damage at the time of the earthquake (> 0.6 seconds), especially in most of Mantikulore and Tawaeli Districts, areas with a dominant period ranging from 0.4–0.6 seconds. A medium soil layer has a high risk of damage, especially in most of the Ulujadi District, Mantikulore District, and Tawaeli District. Areas with a dominant period of < 0.4 seconds indicate a hard soil layer with a fairly low risk of damage, especially in parts of South Palu District, West Palu District, North Palu District, and Ulujadi District.
The amplification value ranges from 1.01 m/s to 7.56 m/s. The amplification value is a parameter of the soil gain factor related to the contrast ratio between the impedance of the surface layer and the layer below it. The stronger the shock strengthening, the smaller the S wave velocity and the softer the formation of the soil material. The harder the soil is, the greater the shear wave speed and the smaller the shock amplification factor. Soils are getting softer, the shear wave velocity is getting smaller, and the shock amplification factor will be bigger. Most of the amplification values of Palu City are 2.69–4.86 (Alluvial / Sediment), especially in Ulujadi District, Tatanga District, South Palu District, North Palu District, and Tawaeli District.
The PGA value obtained is the value of the wave acceleration on the rock report that occurs due to an earthquake. The PGA value will be smaller as the distance from the site to the epicenter is caused by the absorption of earthquake energy by the soil media. The distribution of values consists of: (1) a value of 0.59–0.60, which indicates the magnitude of the MMI strength on the X scale, which results in damaged strong buildings of wood, part of wooden buildings and frames and damaged foundations. Locations in Palu City are in most of the eastern regions of Mantikulore and Ulujadi Districts, with large cracks in the ground, curved rails, and landslides. PGA values range from 0.691 to 0.783, equivalent to the strength of the XI scale MMI. Only a few wooden buildings are still standing; there are broken bridges and wide cracks in the ground, where most of the central area of Palu City is in this area. PGA values from 0.783 to 0.875 or the equivalent strength of the XII scale MMI, resulted in total damage, with visible waves on the ground. The view darkens and objects are thrown away, and the whole area of Palu's terrace, especially in the middle and eastern part of Palu, is included in this category.
Figure 2 shows a micro-zonation map, which referred to the three parameters mentioned above, namely the value of the dominant period, amplification, and PGA, using the GIS overlay technique with the results as shown.
Based on the map of earthquake-prone areas, 48.9%, or 193.29 km2, of Palu City is dominated by areas with a moderate level of danger. Most of them are in Tawaeli District, West Palu District, Ulujadi District, North Palu District, Mantikulore District, and Tatanga District; and areas prone to high earthquake disasters 29.25% km2 or an area of 115.75 km, especially in most of Tawaeli District, East Palu District, South Palu, West Palu and Ulujadi Districts; while the low disaster-prone area of 21.77% or an area of 86.02 km2, spread across Mantikulore District, Tawaeli District and Ulujadi District.
Land capability assessment
One approach in land use planning is capability assessment (Singer, 2014). Subsequent assessments can be used as a guide toward optimizing land use. This assessment reveals the constraints that certain land has (Mokarram et al., 2010), guiding land use decisions based on its capabilities and potentials (Amiri et al., 2012). By analyzing land capability, classification of land capability will be obtained, which can be developed according to the function and carrying capacity of the area and the final land suitability for the development of urban areas. Based on the analysis of the physical aspects of land capability-based on the LCU, namely: Morphological LCU, Slope Stability LCU, Foundation Stability LCU, Water Availability LCU, to obtain an overview of the LCU in Palu City, as shown in Fig. 3, as follows:
a. LCU morphology: morphology is a landscape. LCU morphology is carried out to sort out the natural landscape in the area and/or planning area that can be developed according to its function. The land capability of high morphology means that the area's morphological conditions are complex. High morphology means the landscape is hilly and wavy. Low morphology is not complex; the land is relatively flat and easy to develop for settlements and cultivation activities. Most of the Palu City area has a high morphology; 43% is hilly areas in the western and eastern parts, with limited development potential. Meanwhile, the morphology is low, at 27%, and is generally located along the bay of Palu and spread over almost all districts.
b. LCU Slope Stability: if an area has low slope stability, then the condition of the area is unstable because it is prone to landslides, is easy to move, so it is not safe to develop it for buildings, settlements, especially the development of built-in cultivation activities. This area can be used for forest areas, plantations, and water absorption. Slope stability in Palu City is dominated by low (31%) and moderate (31%) slope stability. In general, it still has a good chance of development into an established cultivation area, especially in Mantikulore District.
c. LCU foundation stability: to determine the level of land capability to support heavy structures in urban development and the type of foundation suitable for each level of land capability. West Palu District has high foundation stability, while low foundation stability is in Mantikulore District. However, when viewed as a whole, 51% of Palu City is dominated by a low level of stability, and only 10% shows a high level of foundation stability.
d. Water Supply LCU: conducted to determine the level of water availability and water supply capacity for urban development. If water availability is very high, it means that the availability of deep and shallow groundwater is quite a lot. Meanwhile, moderate water availability means that there is insufficient shallow groundwater, but there is a lot of deep groundwater. The level of water availability in Palu City 39% has low availability, mostly in Mantikulore and West Palu Districts, while the high water availability of 24% is mostly in Tawaeli District.
Based on the analysis carried out in each LCU, through the stacking of maps and assigning values and weighting to each parameter, land capability in Palu City is divided into 5 classes, namely class A, class B, class C, class D, and class E, as shown in Table 3.
Table 3
Value and Class of Land Capability
Class of Land Capability
|
Total Area
|
Classification
|
(km2)
|
(%)
|
A
|
124.48
|
31.51%
|
Capability Land of Very Low
|
B
|
94.48
|
23.92%
|
Capability Land of Low
|
C
|
37.11
|
9.39%
|
Capability Land of Medium
|
D
|
60.86
|
15.41%
|
Capability Land of High
|
E
|
78.13
|
19.78%
|
Capability Land of Very High
|
When viewed from the land capability map, most of Palu City is dominated by low (23.92%) to very low (31.51%) land capability, indicating that Palu City has the physical and environmental freedom to develop into an urban area, as shown in Fig. 4.
Integration of land sustainability and spatial planning of Palu City 2030
An integrated approach to sustainable land use planning is explicitly stated in Agenda 21 (Singer, 2014). It is very important to prepare land use planning that allows the legacy of good land resources for future generations. This can be done by integrating planned and sustainable land use and in a way that is appropriate to its capabilities and potential. The next study is integrating land suitability with the Palu City spatial plan by comparing an area's spatial plan with the land's ability to determine the suitability between the two. The results will show that the area currently under development can be classified as suitable, conditional, and unsuitable land, as shown in Table 4.
Table 4
Compatibility of the Palu Spatial Plan and ULCC in Palu City
Parameter
|
Area (%)
|
Classification
|
Class A with High Earthquake Prones with Protected Areas
|
9.12%
|
Suitable
|
Class B with Medium Earthquake Prones with Protected Areas
|
26.52%
|
Suitable
|
Class C with Low Earthquake Prones with Protected Areas
|
20.43%
|
Suitable
|
Class D with High Earthquake Prones with Built up Areas
|
21.21
|
Not Suitable
|
Class E with Medium Earthquake Prones with Built up Areas
|
21.57%
|
Conditional
|
Most (56.07%) of the 2030 Palu City's spatial use plan complied with the land's carrying capacity, which is dominated by spatial use plans for protected areas and is in class A in areas with a high-medium earthquake threat and cultivation area utilization plans, while only 1.15 percent of the built-up area is acceptable, notably those in Class F.. Cities in areas prone to earthquakes are low. Conditional land suitability is given to the spatial use plan in cultivation areas with the threat of an earthquake but is in class C land capability class covering 20.43% of the total area of Palu City. Land development is not suitable for urban development because it is in class C and is a high disaster-prone area covering 21.21% of the total area of Palu City. It requires special attention in the future development plan of Palu City. The quality of good city spatial planning avoids allocation of spatial use that is inconsistent with the capacity of the land. Figure 5 shows the land suitability distribution in Palu City.