Assessment of each hazard using a weighted overlay shows a varying spatial distribution (see Figure 3). This variation in spatial distribution is caused by differences in conditioning factors and by the weights used for each hazard. Around 56.72% of the Gunungkidul area has a high flood hazard classification (see Table 3). This high flood-class area has a flat to sloping topography (Figure 3). In addition, according to historical data, flood events in Gunungkidul are strongly influenced by extreme rainfall. Tropical Cyclone Cempaka that occurred in 2017 resulted in groundwater flooding in the Ngreneng Karst Window, Gunungsewu Karst Area (Cahyadi et al., 2019). Tropical Cyclone Savannah passed through the Special Region of Yogyakarta on 17^th March 2019 and also caused groundwater flooding in the Kalinongko Karst Window (Riyanto et al., 2020). Based on past events, the hydrology of the karst area is known greatly to contribute to the flood hazard even though the area looks arid.

In terms of the landslide hazard, 65.96% of the Gunungkidul area is in the low hazard class. Most areas are classified as low landslide hazards because the Gunungsewu karst has a thin soil layer with an average soil thickness of <50cm (Cahyadi et al., 2017). The characteristics of soil and rock materials in Gunungsewu tend to allow for subsurface landslides to occur and for collapses/sinkholes to form. This process is supported by the factor of the average annual rainfall in this area which is classified as moderate (2,000 m. a year). This rainfall feeds into a solutional process (Wijayanti & Noviani, 2020). The topography of Gunungkidul is undulating, with half of the area sloping >15%, namely the northern zone (Baturagung Hills), and the western, southern, and eastern zones sloping (Gunungsewu Karst Hills) (Bappeda Gunungkidul, 2019). The northern zone is strongly influenced by faults and fractures and is the location of the Nglanggeran Ancient Volcano. Most of the population in this area work as farmers (BPS-Statistics of Gunungkidul Regency, 2022b). Farmers in the Nglanggeran Agricultural Technology Park develop agroforestry plants (Saputro et al., 2021). These agricultural activities trigger intensive erosion and increase the potential for landslides in the Baturagung Zone.

The results of the drought hazard assessment show that Gunungkidul is dominated by moderate (30.92%) and high (48.86%) hazard classes. Spatially, the distribution of high drought hazards dominates the karst landscape—the southern part of the Wonosari Basin and the Gunungsewu hills. Even though this area has high rainfall (about 1,875 – 2,125 mm/year), Panggang District which is part of the Gunungsewu Karst Hills often experiences drought because rainwater cannot be stored as groundwater (Nugroho et al., 2020). The dry characteristics of the karst area, with the characteristics of subsurface flow make this area prone to drought in the dry season.

The Baturagung Hills area is influenced by fault activity from the Opak and Dengkeng Faults (Mulyasari et al., 2020). This condition is described bythe results of the seismic hazard assessment which shows that the northern part has a moderate (28.86%) and high (20.83%) hazard level. In addition to the influence of faults, this area’s geomorphology is a significant contributor to hazard levels. This area is at risk of  landslides because the northern part of Gunungkidul Regency is composed of loose material such as the Gunung Merapi Muda Formation in which it is easier for  seismic waves to be propagated, so the impact of landslides  is more severe (Husein et al., 2007).

The seismic hazard profile of Gunungkidul Regency is generally dominated by a low level of hazard (40.87%) which spreads from the centre to the southern part of the district (Figure 3). This is because the central and southern parts of this regency are relatively far from faults. On the other hand, the northern part has a high to very high level of seismic hazards due to the presence of faults and fractures. The effect of distance from the fault is evidenced by the distribution of damage to buildings by the earthquake measured at a magnitude of 6.3 by the US Geological Survey that occurred on May 27, 2006 in the Special Region of Yogyakarta. The earthquake had a pattern following the direction of the fault (Marsell, 2013). Earthquakes occurring in Gunungkidul generally originate from Opak fault activity or from fault activity around the Indian Ocean. Therefore, historical data on earthquake events originating in the northern area of Gunungkidul is hard to find. Another cause of the high level of seismic hazard in the northern part of Gunungkidul is that it has unconsolidated sediment rocks which are included in the earthquake-prone category (Gunawan & Khadiyanto, 2012). This contrasts with the karst hill zone (consolidated sediments).

Table 3. Areas of different classes of hazards.

Figure 3. Hazard maps of Gunungkidul Regency.

3.2. Integrated multi-hazard map

Figure 3 shows that all areas in Gunungkidul have more than one type of hazard. The distribution of the multi-hazard classes follows a pattern. The high to very high multi-hazard areas are mostly in the north, moderate multi-hazard areas are spread out in the north, west, and east, and very low to low multi-hazard areas are in the south and centre of the Gunungkidul Regency. The number of hazard types in an area and the class of each hazard affect the area’s multi-hazard class.

Most areas of Gunungkidul have low multi-hazard levels. The area around Playen is a low multi-hazard area because it consists of a combination of high flood hazards and high drought hazards. However, the area to the south of Saptosari which also consists of high flood hazard and high drought hazard has a very low multi-hazard classification. This is because, in addition to the composition of the high and very high hazard classes, the multi-hazard assessment also considers the very low, low, and medium hazard classes of each hazard. Thus, even though two areas are both at risk of two high-level hazards, it could be that the areas have different multi-hazard classifications. The composition of the weights of each hazard is very influential because it affects the results of the multi-hazard assessment. The results of the multi-hazard assessment in Gunungkidul Regency are very low 12.32% (177.98 km²), low 39.35% (568.51 km²), moderate 28.47% (411.38 km²), high 16, 44% (237.57 km²), and very high 3.42% (49.36 km²) (Table 4).

Figure 4. Multi-hazard map of Gunungkidul Regency.

The results of this multi-hazard mapping in Gunungkidul Regency also show the uniqueness of this area. The area around Wonosari and Semanu districts is a low-level multi-hazard area. What is highlighted is that this area has a very high level of flood hazards (see Figure 3). This very high flood hazard is due to the area’s sloping topography and proximity to the river. However, the vicinity of Semanu District also has a high level of drought hazard due to the factors of the average annual rainfall of 1500-2000 mm and soil composed of limestone material. Limestone is a good aquifer after volcanic rocks and alluvial deposits, nevertheless, the potential for groundwater in limestone areas is not evenly distributed and its potential is highly dependent on the dissolution holes (Kementrian PUPR, 2017). There is an intersection area with a very high level of flood hazard and a high level of drought hazard. This area has serious water problems because in the rainy season it experiences flooding and in the dry season it experiences drought.

Table 4. Areas of different classes of multi-hazard in Gunungkidul Regency.

3.3. Multi-hazard effects

Gunungkidul Regency has a multi-hazard potential consisting of landslides, earthquakes, droughts, and floods. Diverse potential disasters require good mitigation and adaptation planning. Multi-hazard mapping is one of the efforts required to reduce the potential for damage and losses when a disaster occurs, such as loss of life and damage to infrastructure, utilities, and public facilities (Khan et al., 2020 ; Wang et al., 2020). Multi-hazard mapping can show the spatial distribution of several hazards that can have regional interactions. These hazards are also related to the geomorphological conditions and characteristics of the landscape in Gunungkidul Regency.

One of the objectives of multi-hazard analysis is to determine the interactions, interdependencies, and cascading effects between hazards. Gill & Malamud (2016) describe five hazard relationships or interactions, including: Natural disasters triggering other natural disasters, human activities triggering natural disasters, human activities exacerbating natural hazards triggers, cascading effects, and the encounter of two or more hazards. The approach that can be used to analyze the relationship between disasters is to identify the triggering factors for each disaster, especially in the study area. According to Van Westen & Greiving (2017), several factors contribute to creating hazards, such as topography, geomorphology, geology, soil, and human activities.

Multi-hazards can have effects related to other disaster risks. The relationship can be triggering, amplifying, compounding, or consecutive. However, the general effect of interaction between hazards is triggering and amplifying. It can result in an increase in the probability of occurrence (Gill & Malamud, 2017 ; He & Weng, 2020), . Furthermore, several examples of interactions between hazards are explained, such as earthquakes as primary disasters that can trigger secondary disasters in the form of erosion or erosion and flooding which have a tendency to trigger and strengthen each other. Based on Table 5, we can see that there are several hazard factors including local landscape conditions in Gunungkidul Regency.

Table 5. The Interrelation of Multi-hazard Factors in Gunungkidul Regency.

Source: Liu et al., (2016)

Another type of interaction between hazards is the domino effect, also known as the cascading effect. This interaction explains how a disaster can trigger further disasters or other disasters. Based on the results of the multi-hazard mapping, we can say that several areas have either very high levels or high levels of hazard, especially in the northern region of Gunungkidul Regency. Referring to the northern region, especially in the geomorphological area of the Nglanggeran fault hills, we can see how seismic and landslide hazards are found with very high and high classifications. In line with Table 5, we can conclude that seismic waves can trigger landslides. This makes the area potentially prone to a hazard domino effect, that is the relationship between earthquakes and landslides is one of triggering. Good disaster mitigation, specifically measures controlling human activities in some hilly areas which tend to exacerbate hazard-triggering factors, is needed to minimize losses from hazards.

One of the recent huge disaster events in Indonesia that caused cascading effects was the earthquake that occurred on September 28, 2018, with a magnitude of Mw 7.4, in Palu, Central Sulawesi (Mason et al., 2021; Triyanti et al., 2022). The earthquake has triggered cascading secondary hazards (tsunami, liquefaction, and landslides) and tertiary hazards (flash floods in several areas in the Sigi Regency). Gunungkidul Regency has also experienced cascading hazards. An example of such cascading effects in this area is the 2017 Cempaka hurricane. According to Gunungkidul Bappeda (2020), the Cempaka storm was one of the triggers for the worst flooding in Gunungkidul, which also damaged several forms of infrastructure due to its triggering of landslides. The Cempaka storm acted as a primary hazard that increased the probability of secondary hazard events as a form of cascading.

Referring to the multi-hazard map (Figure 4), we can see how some areas, especially Nglipar and Semin districts, have a high risk of flooding and landslides. The results of the analysis explain that the area has the potential for cascading effects in particular due to meteorological tiggering factors for floods – landslides. However, the cascading effect analysis is quite difficult to analyze, considering that there are differences in the pattern and magnitude of disasters and vulnerabilities in any given area (Van Westen & Greiving, 2017 ; Qie & Rong, 2022). This also explains why some areas which are classified as low multi-hazard can experience disasters because they have vulnerabilities and are hit by disasters on a large scale. Furthermore, Van Westen & Greiving’s explanation can also mean that a disaster in an area that has several types of hazards does not always cause a cascading effect.

4. Conclusion

Gunungkidul has a variety of landscapes, including the Baturagung Fault Hills in the north, the Wonosari Karst Basin in the middle, and the Gunungsewu Karst Hills in the south. This variation raises the threat of several hazards such as floods, landslides, earthquakes, and droughts that may interact with each other and have the potential to cause extreme events called disasters. All areas in Gunungkidul Regency have the potential to be exposed to multi-hazards. The distribution of multi-hazard levels varies depending on the characteristics of the conditioning factors for each disaster. Although most areas of Gunungkidul Regency have low multi-hazard levels (568.51 km² or 39.35% of the total area), the interactions between hazards cannot be ignored. The interaction between hazards can trigger other disasters called cascading effects. The cascading effects in the study area have the most potential to occur in the north of Gunungkidul Regency with a very high seismic and landslide hazard due to the geological and geomorphological conditions of the Baturagung Structural Hills. It is important to study the interactions between these hazards because the interaction of more than one hazard can result in extreme events called disasters. Further research in Gunungkidul Regency is needed to plan disaster mitigation methods based on existing multi-hazard analysis and further multi-hazard analysis has the potential to increase capacity and create resilience to disasters.

Acknowledgements

We would like to thank the entire team of supervisors for Field Work Lecture 3 as well as the Regional Disaster Management Agency of Gunungkidul Regency for their support in providing data for this research. All spatial data used in this study were processed using ArcGIS software.

Author Contributions

Conceptualization: Melati Mustikaningrum, Adrianus Farrel Widhatama, Khrisna Wasista Widantara, Mirza Ibrohim, Muhammad Fikri Hibatullah, Rinanda Amdalista Prastia Larasati, Sri Utami, Danang Sri Hadmoko; methodology: Melati Mustikaningrum, Adrianus Farrel Widhatama, Khrisna Wasista Widantara, Mirza Ibrohim, Muhammad Fikri Hibatullah, Rinanda Amdalista Prastia Larasati, Sri Utami, Danang Sri Hadmoko; investigation:  Melati Mustikaningrum, Adrianus Farrel Widhatama, Khrisna Wasista Widantara, Mirza Ibrohim, Muhammad Fikri Hibatullah, Rinanda Amdalista Prastia Larasati, Sri Utami, Danang Sri Hadmoko; writing—original draft preparation: Melati Mustikaningrum, Adrianus Farrel Widhatama, Khrisna Wasista Widantara, Mirza Ibrohim, Muhammad Fikri Hibatullah, Rinanda Amdalista Prastia Larasati, Sri Utami, Danang Sri Hadmoko; writing—review and editing:  Melati Mustikaningrum, Adrianus Farrel Widhatama, Khrisna Wasista Widantara, Mirza Ibrohim, Muhammad Fikri Hibatullah, Rinanda Amdalista Prastia Larasati, Sri Utami, Danang Sri Hadmoko; visualization: Melati Mustikaningrum, Adrianus Farrel Widhatama, Khrisna Wasista Widantara, Mirza Ibrohim, Muhammad Fikri Hibatullah, Rinanda Amdalista Prastia Larasati, Sri Utami, Danang Sri Hadmoko. All authors have read and agreed to the published version of the manuscript.

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