Understanding Multi-Hazard Interactions and Impacts on Small-Island Communities: Insights from the Active Volcano Island of Ternate, Indonesia

Abstract

Drawing on a case study from Ternate Island, a densely populated volcanic island in Eastern Indonesia, this research illustrates how multi-hazards and extreme weather events are likely to compound and cascade, with serious consequences for sustainable development in small island context. At the heart of Ternate Island sits the active Gamalama volcano, posing a constant eruption threat. Its location within the Ring of Fire further exposes the island to the risks of tsunamis and earthquakes. Additionally, the island’s physical features make it highly susceptible to flooding, landslides, and windstorms. Rapid urbanization has led to significant coastal alterations, increasing exposure to hazards. Ternate’s small-island characteristics include limited resources, few evacuation options, vulnerable infrastructure, and inadequate resilience planning. Combining GIS multi-hazard mapping with a structured survey in 60 villages in Ternate, this case study investigates the multi-hazard exposure faced by the local population and land coverage. The findings suggest significant gaps between village chiefs’ perceptions of the types of hazards and the multi-hazard assessment in each village. Out of 60 village chiefs surveyed, 42 (70%) are aware of earthquake risks, 17 (28%) recognize tsunami threats, and 39 see volcanoes as a danger. GIS assessments show that earthquakes could impact all villages, tsunamis could affect 46 villages (77%), and volcanoes could threaten 39 villages. The hazard map indicates that 32 villages are at risk of flash floods and 37 are at risk of landslides, and extreme weather could affect all villages. Additionally, 42 coastal villages on Ternate Island face potential extreme wave and abrasion disasters, but only 18 chiefs acknowledge extreme weather as a threat. The paper argues that addressing the cognitive biases reflected in the perceptions of community leaders requires transdisciplinary dialogue and engagement.

1. Introduction

Current trends in disaster studies highlight a growing awareness of multi-hazard risks that interact, compound, and cascade, creating more systemic impacts on social, economic, and built-environment systems. These multi-hazard impacts often result in significant losses of human lives, livelihoods, and the built environment [1]. In light of this, the Sendai Framework emphasizes the need for multi-hazard approaches, recognizing that hazardous events can coincide, cascade, or accumulate over time, leading to interrelated effects [2]. It encompasses enhancing adaptive capacities and preparedness for adverse effects, reducing exposure to hazards, and reducing vulnerability to people and property.

Multi-hazard risk assessment (MHRA) is the primary tool for analyzing the consequences of multi-hazard disasters [3]. The objective of MHRA is to comprehensively analyze and chart the anticipated losses from different natural hazards on a region’s social, environmental, and economic assets [4,5,6]. The MHRA has a crucial function in preventing and minimizing hazards and exposure, explicitly emphasizing reducing risk in land development [7,8]. An appropriate framework is required to properly handle hazards in the construction of a zoning plan [9]. Hence, the MHRA methodology is founded on the notion that locations prone to natural disasters should avoid undergoing growth [10,11].

Scholars and policymakers have noted a significant increase in studies on multi-hazard risk assessment, impact, and systemic risk over the past 20 years [12,13]. However, multi-hazard assessments on small islands still need to be improved, and special attention is required. Multi-hazard impacts on small islands are likely to be less widespread but more intense, and the long-term effects of future climate change are more systemic. The main attributes causing compound vulnerabilities on small islands include settlement isolation, a significant rural presence, being close to the sea, limited size, reliance on natural resources, often inadequate adaptive capacity, prevalent poverty, income inequality, and poor educational attainment [14,15]. Furthermore, SICs face significant vulnerability and are at risk of food insecurity and enduring poverty because they rely on mainland agriculture and restricted access to market institutions and technologies [16].

A better understanding of the spatial characteristics of multi-hazard risk is crucial for implementing effective, evidence-based policies and programs for disaster risk reduction. As an archipelagic country, the Indonesian government has formulated policies as provided in the 2015–2045 Master Plan for Disaster Management. This plan mandates the regular preparation and updating of disaster risk studies to address changes at the village, district, city, provincial, and national levels. These studies serve as a foundation and input for preparing and reviewing spatial and land use plans, facilitating sustainable and disaster risk reduction-oriented development [17].

In line with this, the present paper aims to investigate multi-hazard risk levels and their interactions on Ternate, a small island in North Maluku, Indonesia. This aim involves analyzing population density and land use vulnerability factors and leaders’ perceptions of the hazard. This island has been chosen with consideration; it is known as an area with a multi-hazard threat. The National Disaster Management Agency (BNPB) has identified nine potential hazards on Ternate Island, including floods, flash floods, landslides, extreme weather, extreme waves, abrasion, earthquakes, tsunamis, volcanic eruptions, forest fires, and social conflicts. Moreover, Ternate Island is one of the most densely populated small volcanic islands in Eastern Indonesia. The population data indicate that Ternate Island experienced a population increase from 192.2 k in 2012 to 194.6 k people in 2023, or an annual rate of 0.9% [18], with an estimated population in 2032 of 386,353 [19]. In addition, Ternate is confronted with the problem of limited land use. The island is experiencing rapid growth in settlement coastal area development, from 15.19% in 2013 to 19.69% in 2023 [20].

Even though there has been research on this topic, the outcomes still need to be determined. Focusing on population and land use, this paper demonstrates the first comprehensive research on the multi-hazard risk in Indonesian small island context with a focus on Ternate Island. It also offers insights into how local experts can analyze and visualize results for multi-hazard exposure assessment on Ternate Island, Indonesia. Furthermore, the research results are expected to provide valuable input for policymakers in formulating spatial planning for islands that prioritize disaster risk reduction considering limited land. This outcome is crucial given the prevailing development trend in hazard-prone areas.

2. Multi-Hazard Assessment: Concepts and Approaches

2.1. Basic Definition

There is a consensus shared by the United Nations that defines a ‘single hazard’ of natural or man-made origin as a process that can result in loss of life, injury, or other health consequences, damage to property, loss of livelihoods and services, disruption to social and economic activities, or harm to the environment [2,9]. In addition, ‘multi-hazards’ and compounding hazards (or compound events) are often used interchangeably to suggest a complex aggregation of hazards involving interactions and/or associations between multiple events and hazards [9].

When a society has to deal with more than one parallel hazard at a point in time in a locality, experts often label such a reality as a multi-hazard event. Artificial Intelligence suggests ‘multi-hazard disaster risk’ as ‘potential adverse impacts arising from multiple hazards occurring simultaneously or sequentially’ [21]. The key features include hazard interactions, cumulative impacts, vulnerability and exposure, preparedness, response, and resilience building [21].

Multi-hazards can be serial hazards, where one hazard compounds the impact of previous hazards on a society [22], or parallel hazards, where more than one hazard occurs simultaneously [3]. Each can also have compounding and cascading impacts. The terms such as multi-hazard and compounding and cascading hazards have many interlinked and overlapping definitions [12]. These interactions might result in a higher impact than expected from individual (parallel) hazards [23]. This is because multi-hazards have dynamic characteristics, involve several hazards, occur repeatedly throughout human time scales, and have sources that might be sequential or mixed, and their potential repercussions can vary significantly. Furthermore, a multi-hazard can be defined as a hazard that arises quickly when it occurs suddenly, such as the simultaneous occurrence of earthquakes, volcanic eruptions, or tsunamis [22]. Even though there is no widely agreed-upon concept of a natural hazard, its meaning has been extensively discussed and debated in several fields [5,24].

2.2. Interaction and Risk Assessment

Various approaches to assessing multi-hazard risk and interactions can be categorized into qualitative, semi-quantitative, and quantitative methods. Qualitative approaches rely on expert judgment and descriptive methods, while semi-quantitative methods use indices and classifications. Quantitative methods employ numerical data and statistical techniques for precise assessments [6]. Further, there are three main methods for visualizing and constraining hazard interactions: qualitative descriptions and classifications, hazard matrices and diagrams, and probability/scenario trees [25]. Meanwhile, hazard interrelations can be quantified using matrices, models, and classification methods, including stochastic, empirical, and mechanistic models [12]. These diverse approaches provide a range of tools for understanding and managing the complex interactions between multiple hazards. Tilloy’s idea of a mutual exclusion scenario is also interesting, known as negative dependence; it refers to a situation where two events cannot occur simultaneously and are mutually exclusive (see

Table 1. Different interrelation classifications for natural hazards from various sources.

Source	Interaction Type
Gill and Malamud 2014 [25]	Interactions where a hazard is triggered: One hazard triggers one (or more) other hazard(s). Interactions where the probability of a hazard is increased: One hazard changes environmental parameters that move toward a change in the likelihood of another hazard. Interactions where the probability of a hazard is decreased: One hazard alters the frequency or magnitude of another. Events involving the spatial and temporal coincidence of natural hazards: Two hazards are independent and occur simultaneously.
Liu et al., 2016 [5]	Independent relationship: Two hazards are independent. Mutex relationship: Two hazards cannot occur together; their trigger factors are mutually exclusive. Parallel relationship: Two hazards depend on the same trigger factors. Series relationship: One hazard triggers another hazard.
Van Westen and Greving 2017 [26]	Independent events: Two hazards are independent. Coupled events: Two hazards are triggered by the same triggering event. One hazard changes the conditions for the next. Domino or cascading hazard: One hazard causes the next.
Tilloy et al., 2019 [12]	Independence: Two hazards occur simultaneously without influencing each other. Triggering (Cascading): A primary hazard triggers a secondary and tertiary hazard. Compound Hazard: Multiple hazards occur together, arising from a common primary event. Changes in Circumstances: One hazard alters the conditions affecting another hazard. Mutual exclusion: One hazard excludes the possibility of another hazard happening simultaneously.
De Angeli et al., 2022 [3]	Parallel hazards, where the same trigger generates multiple hazards. Cascading hazards, where one adverse event triggers a series of sequential events. Disposition alteration, where the occurrence of one hazard influences the frequency or magnitude of a second hazard. Additional hazard potential, where different hazards occurring in the same space and time amplify each other. Coincident triggering, where two hazards coincide and trigger a third hazard. Cyclic triggering, where the triggering of a second hazard worsens the first hazard, leading to further episodes of the secondary hazard.

) [12].

Multi-hazard risk assessment and their interactions involve evaluating the specific attributes of each hazardous event, such as likelihood, occurrence rate, and intensity, as well as how these events influence and affect one another [6,27]. The initial stage of hazard risk assessment involves identifying hazards, determining their intensity, and assessing potential damage to evaluate disaster severity [24]. In line with this, Nejedlik and Delezios (2015) [28] emphasized that effective disaster risk management begins with identifying hazards and determining susceptibility, which is then displayed through hazard maps that show potential disasters and their interactions. This approach benefits users by organizing hazard risk information and offering visually informative depictions of possible hazards, which is crucial for disaster risk assessment and planning [29]. Hazard maps are also key references for infrastructure construction and urban planning [30]. Additionally, national and regional policymakers can use the multi-hazard map to develop strategies to mitigate multiple hazards. The strategies include managing land use, updating and enforcing building codes, and developing plans to reduce the risks associated with regional hazards [31].

Eventually, a comprehensive approach that considers the spatial, demographic, and physical contexts and their interconnections and feedback can significantly reduce the human and economic losses caused by a disaster. This approach recognizes that the conditions before a disaster can be predicted so as not to intensify or lessen its impacts [32]. Although multi-hazard risk analysis offers numerous advantages, its development has various obstacles. Aksha (2020) and Carpignano et al. (2009) [32,33] highlighted that data availability, quality, and accuracy pose significant constraints for multi-hazard research.

3. Materials and Methods

3.1. Geographical Context and Hazard Risk on Ternate Island

Ternate Island, located in North Maluku Province, Indonesia, is characterized by its volcanic landscape and distinctive geomorphology (Figure 1). The island’s geological materials are predominantly volcanic, resulting from repeated eruptions that deposited pyroclastic, lava, lahars, and tephra over thousands of years [34,35]. The island’s landscape features steep slopes, some exceeding 25%. There are even inclines over 40% that lead to the summit of Mount Gamalama. The coastal areas’ slopes vary from 2% to 8% [35,36,37]. Ternate is the site of three beautiful lakes: Laguna, Tolire Besar, and Tolire Kecil. These lakes were formed by the volcanic activity of Gamalama, located in the island’s eastern and southwestern parts [38].

Administratively, Ternate Island is an island town (Figure 1) comprising 5 sub-districts and 60 villages [18]. Among the 60 villages, 38 are coastal villages (bordering the coastline). Ternate is a trading hub connecting 64 inhabited islands in the province. Various infrastructures on the island facilitate maritime and trading activities, including ports, airports, commercial centers, educational centers, and government centers [39]. The economy in the city of Ternate is seeing continuous growth [40]. According to the Central Bureau of Statistics data, the economic growth rate of Ternate in 2023 was 5.0% [18].

The National Disaster Management Agency (BNPB) has identified nine potential hazards on Ternate Island, including floods, flash floods, landslides, extreme weather, extreme waves, abrasion, earthquakes, tsunamis, volcanic eruptions, forest fires, and social conflicts [41]. Recent studies have focused on the impacts of single hazards, including tsunamis [42,43], coastal abrasion [44,45,46], volcanic eruptions [38,47], landslides [48,49], earthquakes [50,51], floods [52,53,54], and extreme weather [55].

3.1.1. Hydro-Meteorological Hazards on Ternate Island

Frequent flash floods occur due to the topographic characteristics and residual volcanic risk from Gamalama. These floods often occur when cold lava is carried by water down rivers, eventually reaching residential areas [41]. Flash floods resulted in fourteen deaths between December 2011 and May 2012, displaced 1040 individuals, and damaged 273 houses to varying degrees [56]. Landslides also commonly occur in Ternate. In 2009, six recorded flood-induced landslides resulted in 191 damaged houses, 6 injuries, and 265 displaced people [49]. The landslides affected several villages, including Salahuddin, Rua, Tabam, Takome, and Dorpedu.

Sultan Babullah Meteorological Station in Ternate reported extreme weather events on Ternate Island on 15–16 January 2021 [55]. This disaster had substantial impacts, leading to tidal waves, floods, landslides, and extensive damage to wave-protecting embankments, settlements, roads, and ports [55]. Moreover, extreme waves frequently hit this region, causing coastal abrasion [42,44,45].

3.1.2. Geological Hazards on Ternate Island

Mount Gamalama is situated right at the center of Ternate Island and is well known for its high level of volcanic activity [37]. Historical records indicate that Gamalama volcano has erupted 75 times from 1510 to 2015 [47]. These eruptions have often been catastrophic. For instance, the eruption in 2011 in Tubo Village resulted in the loss of three lives and the destruction of 78 dwellings [57]. The most recent eruption, observed in 2015, had a rather lengthy period, from 16 July to 8 September 2015 [47]. Earthquakes also often shake Ternate Island. According to statistics from the United States Geological Survey (USGS), there were 580 recorded earthquakes with a magnitude of 5 and a depth of 0–70 km in North Maluku Province from 2000 to 2020 [58,59,60]. Furthermore, it was estimated that at least 32 tsunami events were recorded in the Maluku Sea between 1600 and 1992, resulting in the deaths of 7576 individuals [61].

3.2. Multi-Hazard Interaction Assessment

Different methods have been developed to evaluate hazard interactions in a particular region. In the present study, we evaluate hazard interactions by employing two approaches. First, we assess multi-hazard risks [62,63,64]. The integration of interrelations between hazards is advised where they exist in a multi-hazard risk assessment. By implementing probabilistic approaches to infer connections between events, one can investigate the geographical and temporal overlay of several hazards [65]. Second, the hazard interaction matrix approach is utilized [12,65]. A review of the scientific literature on the potential interactions between hazards can be used to develop the interrelation matrices [12].

3.2.1. Multi-Hazard Risk Assessment

This study uses the commonly accepted risk assessment framework for natural hazards, where risk is a function of hazard and vulnerability [26,62,66]. Hazards are risks to people’s lives, livelihoods, and the environment, and risk measures the likelihood or probability of injury, causalities, or loss of life due to an unsafe occurrence. Vulnerability is the condition created by physical, social, economic, and environmental elements or activities that raise a community’s susceptibility to hazards. Therefore, risk can be presented conceptually with the following basic equation [26,66]:

Risk = Hazard × Vulnerability

(1)

The initial stage of hazard risk assessment involves identifying hazards, determining their intensity, and assessing potential damage to evaluate disaster severity [24,64]. Therefore, we collected and analyzed the extent of the hazard area from the individual and integrated hazard maps. Meanwhile, vulnerability is approached through each village’s population density components and land use distribution [62]. Eventually, we applied three research stages that contributed to the risk assessment of multi-hazards in the study location, as shown in Figure 2.

Generating Individual Hazard and Integrated Hazard Maps

This stage focuses on analyzing and visualizing individual and integrated hazards in each village. Individual hazards will concentrate on seven disasters that have occurred and can potentially occur on Ternate Island, including hydro-meteorological hazards (flash floods, landslides, extreme weather, extreme waves, and abrasion) and geological hazards (earthquakes, tsunamis, and volcano eruptions). The individual hazards and integrated hazard map were extracted from the website https://inarisk.bnpb.go.id/ (URL accessed on 28 February 2024). Then, we employed geoprocessing and raster analysis in ArcGIS 10.8 software to reclassify and create individual hazards and integrated hazard maps. Finally, we categorized all hazard areas as low, moderate, and high [67].

Generating Population Density and Land Use Maps

Vulnerability analysis examines the spatial arrangement of individuals, infrastructure, or other valuable assets susceptible to hazard. There are many different areas that hazards might impact, such as population distribution, the natural environment (such as flora, fauna, and conservation areas), the built environment (such as residential and commercial buildings), and crucial public infrastructure (such as airports, ports, hospitals, and roads) [68]. In addition, vulnerability can be approached through each village’s population density components and land use distribution [62].

This study involved the simultaneous consideration of the potential exposure of populations and the assessment of land use maps. This seeks to comprehend that the concept of “potential population exposure” involves evaluating population density in various areas, which assesses the potential risks faced by residents [69]. The population data were obtained from the Ternate City Statistics Bureau in 2023 [18]. The population data are then transformed into a spatial map, which is divided into three distinct categories based on the population density in each village: low, moderate, and high. A land use map was obtained from the Ternate city regional planning, research, and development agency (BAPELITBANGDA). Further, the analysis and categorization of these two maps will create a vulnerability map. This map will be generated using a weighted ranking system, as shown in

Table 2. Weighted risk and ranking.

Hazard	Weighting	Index Class	Ranking
Population density (/km2)	0.5	<500	1
		500–2000	2
		>2000	3
Land use type	0.5	forest/shrub	1
		agriculture	2
		residential area	3

. This activity was carried out using the ArcGIS Map tools, as executed in [9].

Generating Multi-Hazard Risk Map

The vulnerability map and integrated hazard map are input for the map algebra command in ArcGIS 10.8, constructing a multi-hazard risk map. The hazard risk level in the outcome map attributes is determined by multiplying the vulnerability class (1, 2, and 3) with the hazard intensity class (1, 2, and 3). This calculation produces risk classes categorized as low (1), medium (2), and high (3). The multiplication operation assigns a weight of 0.5 to the vulnerability map and the integrated hazard map, as shown in Equation (2) below.

(Vulnerability map × 0.5) + (integrated risk map × 0.5)

(2)

3.3. Multi-Hazard Matrix Assessment

The interaction matrix method can be used to include hazard interaction in multi-hazard risk assessment, as recommended by Gill and Malamud (2016) [25,65] and implemented by De Pippo et al. (2008) [70]. Scholars systematically encode all potential connections between hazards into a matrix. The vertical axis of the matrix depicts the primary hazards, which are the initial hazards that either cause or modify the probability of other hazards transpiring. The interrelation matrices can be constructed by utilizing comprehensive literature information to identify and understand the potential relationships among hazards [12]. By analyzing a variety of significant hazards, the hazard matrix method identifies which hazards may precipitate or increase the probability of other hazards. It provides a structured, semi-quantitative method for analyzing and visualizing hazard interactions [12].

In analyzing hazard interactions, two criteria are used to facilitate the systemic understanding of the various classes of hazard combinations: (1) the number of events and hazards and (2) the relationships between the hazards and events [71]. With this approach, we will refer to data from the Regional Disaster Management Agency (BPBD) and previous research on disaster events on Ternate Island [72]. Thus, we identified multiple hazard interactions for each of the seven hazards chosen for this investigation. For guidance on completing the hazard interaction matrix, we follow the work of Tilloy et al. (2019) and Gill and Malamud (2014) [12,25].

3.4. Survey

In order to complement this investigation, we surveyed all villages with village officials (mainly chiefs or Lurah) as key respondents to determine and identify qualitative information on the types of hazards each village encountered in the last ten years. The fieldwork took place during May–August 2023. The critical question includes identifying Lurahs’ perception of natural hazards faced by the local communities in the last ten years.

4. Results

4.1. The Level of Individual Hazard Zones

4.1.1. Hydro-Meteorological Hazards

Figure 3a–d display maps derived from examining four maps that pertain to hydro-meteorology disasters such as flash floods, landslides, extreme weather, and extreme waves and abrasion.

Table 3. The area of hydro-meteorological hazards for each sub-district and village in low (L), moderate (M), and high (H) categories.

Sub-District	Flash Flood (ha)			Landslide (ha)			Extreme Weather (ha)			Extreme Wave and Abrasion (ha)
	L	M	H	L	M	H	L	M	H	L	M	H
North Ternate	88	44	66	62	379	347	273	135	531	0	0	95
Central Ternate	4	1	5	13	544	471	42	32	325	0	0	36
South Ternate	189	80	157	160	328	990	378	85	500	0	0	90
West Ternate	70	58	328	137	482	1816	701	487	120	6	3	139
Ternate Island	78	22	38	95	38	1508	318	26	49	2	6	64
Total Area	428	205	593	466	1771	5133	1711	764	1526	8	9	424

presents the calculated area, measured in hectares (ha), of the hydro-meteorological hazard zones for each sub-district in the research areas, categorized as low, moderate, and high. The details of the hydro-meteorological hazard areas of all villages are in the Supplementary Materials.

Examining the flash flood hazard map (Figure 3a), the regions susceptible to flash floods align with the course of river flow, originating from elevated terrains and extending towards coastal areas. Consequently, regions located in river basins are particularly vulnerable zones. West Ternate sub-district has a significant potential for flash flood hazards, with a high category hazard area of 327.6 ha. In contrast, the Central Ternate sub-district has a relatively lower potential for flash flood hazards, with an area of 18.1 ha (Table 3). Overall, the flash flood hazard area encompasses 12.6% of the entire land area of Ternate Island. The five villages with the most significant flash flood hazard areas are Sulamadaha, with 143 ha; Kulaba, 103 ha; Takome, 99 ha; Sasa, 99 ha; and Tabona, 98 ha.

Figure 3b depicts the results of the reclassified spatial map analysis of landslide hazards in the research area. The slopes leading to the mountain’s summit are most susceptible to landslide risk [48]. The Ternate Island sub-district, with a high category area of 1508.3 ha, comes in second place to the West Ternate sub-district as the area with the highest classification (1816 ha) (Table 3). The high-hazard category encompasses 52% of the study area. The five villages with the most significant areas susceptible to landslides are Loto, with 681 ha; Rua, 483 ha; Foramadiahi, 442 ha; Moya, 410 ha; and Fitu, 370 ha.

Extreme weather includes strong winds with speeds of more than 40 km/h and heavy rainfall that can occur simultaneously or separately [73]. The hazard zone of extreme weather indicates that all villages possess risk areas, with highly sensitive villages primarily on the east coast (Figure 3c). High-risk areas include the North Ternate sub-district (531 ha), South Ternate (500 ha), and Central Ternate (325 ha), covering 16.1% of the island (Table 3). Five villages with the most significant extreme weather area include Takome (568 ha), Sulamadaha (318 ha), Sango (147 ha), Tubo (134 ha), and Kastela (135 ha).

Extreme waves, characterized by significant height and generated by wind in shallow waters, can erode coastal areas [73]. Mapping high-risk locations for powerful waves and erosion is conducted in coastal regions. Figure 3d shows that all sub-districts on Ternate Island, especially coastal areas, are susceptible to severe waves and abrasion. High-hazard regions include the West Ternate sub-district (139 ha), North Ternate (95 ha), and Central Ternate (36 ha), comprising 4.3% of the coastal area. Villages with high extreme wave and abrasion threats are Takome (51 ha), Sulamadaha (29 ha), Tafure (22 ha), Rua (22 ha), and Jambula (19 ha).

4.1.2. Geological Hazards

Figure 4a–c depicts the research location’s geohazard mapping analysis for earthquakes, tsunamis, and volcanic eruptions.

Table 4. The area of geological hazards for each sub-district and village in low (L), moderate (M), and high (H) categories.

Sub-District	Earthquake (ha)			Tsunami (ha)			Volcano Eruption (ha)
	L	M	H	L	M	H	L	M	H
North Ternate	164	351	1029	0	0	169	440	111	191
Central Ternate	383	345	676	0	0	108	369	365	240
South Ternate	439	656	890	5	6	188	563	320	165
West Ternate	709	1093	1566	0	1	82	399	130	127
Ternate Island	1016	429	241	0	0	130	380	507	485
Total Area	2710	2874	4402	5	7	677	2151	1432	1208

displays the computed geological hazard area for each sub-district in the research areas, categorized as low, moderate, and high. The details of the geological hazard areas of all villages are put into the Supplementary Materials. The earthquake hazard mapping shows that all sub-district areas on Ternate Island are potentially affected by earthquakes in the low (2.710 ha), moderate (2.874 ha), and high (4.402 ha) categories (Figure 4a; Table 4). The five villages with the highest earthquake hazard level are Takome (792 ha), Loto (685 ha), Sulamadaha (587 ha), Rua (449 ha), and Foramadiahi (442 ha).

The tsunami hazard map (Figure 4b) indicates that all coastal areas on Ternate Island are at risk of tsunamis, while mountainous villages are comparatively safer. The hazard area in the sub-district shows South Ternate to have 199 ha at risk, North Ternate 169 ha, Ternate Island 130 ha, Central Ternate 108 h and West Ternate 83 ha. The villages most prone to tsunamis are Afe Taduma, 45 ha; Akehuda, 36 ha; Bula, 32 ha; Dorpedu, 29 ha; and Dufa-tufa, 29 ha. Moreover, volcanic hazard mapping in Figure 4c shows that zones within 1–2 km from the volcano’s crater and along river flows are most at risk. There is 742 ha of volcano risk areas in North Ternate, Central Ternate has 974 ha, South Ternate 1048 ha, West Ternate 655 ha, and Ternate Island sub-district 1372 ha (Table 4). The five high-category villages include Afe Taduma (566 ha), Akehuda (326 ha), Bula (319 ha), Dorpedo (303 ha), and Dufa-dufa (273 ha).

By analyzing the outcomes of individual hazard mapping, specific village areas at risk of various disasters can be identified. Hydro-meteorological disasters, particularly flash floods, are possible in 32 villages within the study location. Landslides have the potential to occur in 37 villages, extreme weather disasters can happen in all villages, and extreme wave and abrasion disasters are possible in 42 villages on Ternate Island. For geological disasters, earthquakes could affect all villages within the research locations, tsunamis are projected to impact 46 villages, and volcanic disasters can potentially affect 39 villages.

4.2. The Level of Integrated Hazard Zone

A Ternate Island integrated hazard map was obtained by processing the available data in GIS software packages (Figure 5). The high-risk area is predominantly located in the West Ternate sub-district, covering an area of 3270 hectares. The 1749-hectare South Ternate sub-district follows it. The smallest high-risk area is found in the Central Ternate sub-district, covering 1104 hectares (

Table 5. The areas exposed to multi-hazards for each sub-district in low (L), moderate (M), and high (H) categories.

Sub-District	Multi-Hazard Exposure Areas (ha)			Total (ha)
	L	M	H
North Ternate	22	245	1306	1573
Central Ternate	4	297	1104	1404
South Ternate	25	274	1749	2047
West Ternate	21	85	3270	3376
Ternate Island	25	38	1654	1717
Total area	97	938	9083	10,118

).

4.3. Population Density, Land Use, and Vulnerability Maps

The correlation between population density and land use provides a comprehensive perspective on the convergence of population densities and land use with hazard risks. This enables a thorough analysis of local hazard exposure for every village and sub-district (Figure 6a).

Table 6. Number of males, females, and population density (people/km²) on Ternate Island in 2023.

Sub-District	Male	Female	Population	Household	Density
North Ternate	24,535	24,093	48,628	14,744	3493
Central Ternate	26,999	27,179	54,178	16,603	4086
South Ternate	35,613	35,466	71,079	21,265	3515
West Ternate	4560	4540	9100	2567	269
Ternate Island	4424	4303	8727	2578	502
Total	96,131	95,581	191,712	57,757

Source: [18].

shows the population density for each sub-district on Ternate Island. It is interesting to note that Ternate Island’s 2023 population was 191,712, with 96,131 males, 95,581 females, and 57,757 households [18]. Figure 6a maps the distribution of population density, revealing a significant concentration of people along the eastern coastline in the North, Central, and South Ternate sub-districts. Ten villages with significant population density are Kalumata, Maliaro, Jati, Sangaji, Marikurubu, Makkasar Barat, Toboleu, Salahuddin, and Makassar Timur. The details of population density for all villages are put into the Supplementary Material.

Furthermore, Ternate Island has a diverse range of land uses, encompassing dense vegetation, expansive forests, productive agricultural and plantation areas, and open ground formed by the volcanic activity of Mount Gamalama. The land use area reveals that the forest covers the largest land area, spanning 4.104 ha (40%), located on Mount Gamalama slopes. Agricultural land, on the other hand, occupies 3.646 ha (36%). The residential land is typically situated in flat coastal areas, covering an area of 1.841 ha (18%). Other use areas account for 636 ha (6%) (Figure 6b).

Specific locations’ vulnerability can be assessed based on their environmental hazard exposure (land usage) and human damage potential (measured by the area’s population density) [64]. Vulnerability maps depict the eastern part of Ternate Island as a highly vulnerable area (Figure 7a). The vulnerability area is categorized into the low, moderate, and high categories, showing that the South Ternate sub-district has a highly vulnerable area of 1505 ha, North Ternate has 1095 ha, Central Ternate 605 ha, West Ternate 398 ha, and Ternate Island 252 ha (

Table 7. Vulnerability areas of Ternate Island in low (L), moderate (M), and high (H) categories.

Sub-District	Area (ha)			Total (ha)
	L	M	H
North Ternate	1	485	1095	1581
Central Ternate	0	801	605	1405
South Ternate	0	517	1505	2022
West Ternate	198	2759	398	3355
Ternate Island	6	1447	252	1704
total	204	6008	3854	10,067

). At the village level, five villages recorded the highest areas of vulnerability, including Tekome with 772 ha, Loto with 686 ha, Sulamadaha with 593 ha, Rua with 482 ha, and Foramadiahi with 442 ha. Details of the vulnerable areas for all villages are include in the Supplementary Material.

4.4. Multi-Hazard Risk Assessment

The multi-hazard and multi-vulnerability pillars provide the foundation for understanding the multi-risk concept [23]. Therefore, as shown in Figure 2, the multi-hazard risk assessments result from vulnerability and integrated hazard combinations and interactions. Multi-hazard risk analysis shows that around 95% of Ternate Island is in the high-hazard risk category (Figure 7b). The hazard risk area for each sub-district indicates that the West Ternate sub-district covers 3338 ha (highest), and Central Ternate covers 1405 ha (lowest) (

Table 8. Multi-hazard risk areas of Ternate Island in low (L), moderate (M), and high (H) categories.

Sub-District	Area (ha)			Total (ha)
	L	M	H
North Ternate	0	57	1516	1572
Central Ternate	0	164	1241	1405
South Ternate	0	28	1991	2019
West Ternate	9	239	3091	3338
Ternate Island	0	35	1675	1710
Total	9	522	9513	10,044

). Five villages recorded the highest risk areas at the village level, including Tekome at 761 ha, Loto at 682 ha, Sulamadaha at 588 ha, Rua at 482 ha, and Foramadiahi at 442 ha. Details of the multi-hazard risk areas for all villages are put into the Supplementary Material.

4.5. Multi-Hazard Interaction Matrix

The development of the multi-hazard interaction matrix originates from a comprehensive analysis of the previous disasters on Ternate Island and data provided by the Ternate City Regional Disaster Management Agency. The data on disaster events highlight that the interconnectedness between hydro-meteorological and geological hazards, individually or in combination, can lead to a domino effect. This effect means that one hazard can sequentially trigger another, as shown in

Table 9. The hazards and their interactions on Ternate Island.

Primary Hazard	Secondary Hazard	Type of Interaction	Occurrence	Source
Flash Flood	Landslide	Triggering/Cascading	28 February 2020, at Bula village; 27 July 2020, at Tobololo village; 25 January 2021, at Fitu village; 12 April 2021, at Salahuddin village	[72]
Landslide	Flash flood	Triggering/Cascading	occurred in 2011 and 2012 at Tubo village	[56,74,75]
Extreme weather (heavy rain)	Flood/Landside	Compound hazard	19 June 2020, at Takome village; 16 September 2020, at Rua village; 15 October 2020, at Tabam village; 14 November 2020, at Bastiong Talangame village; 30 November 2020, at Makassar Barat village; 19 January 2021, at Jati village; 15 June 2021, at Soa village	[72]
Extreme weather (strong wind)	High wave	Triggering/Cascading	occurred in 2021	[55]
Extreme weather (strong wind)	High wave/Abrasion	Compound hazard	occurred in coastal area of Ternate Island; 5 August 2020, at Sulamadaha village; 1 February 2021, at Tafure village	[44,45,46,72]
Extreme Wave and Abrasion	Landslide	Triggering/Cascading	24 August 2020, at Sulamadaha village; 8 March 2021, at Togafo village	[48,72]
Earthquake	Tsunami	Triggering/Cascading	North Maluku area; Ternate Island	[60,61,76]
Earthquake	Tsunami/Landslide	Triggering/Cascading	Ternate island	[76]
Earthquake	Volcano eruption	Triggering/Cascading	Ternate island	[47]
Volcano eruption	Flash Flood	Triggering/Cascading	occurred in 2011 and 2012 at Tubo village	[56,74,77]
Volcano eruption	Tsunami	Triggering/Cascading	Ternate island	[76]

.

Further analysis generates the hazard interaction matrix, identifying 18 interactions within the 7 × 7 matrix (Figure 8). Sixteen interactions (32.6%) show a primary hazard increasing the likelihood of a secondary hazard, two interactions (4%) show a primary hazard triggering a secondary hazard without increasing its likelihood, and no interactions show an increased likelihood without triggering. Light gray shading indicates that a primary hazard might trigger a small number of secondary hazards, such as a landslide causing a single flash flood. Dark gray shading indicates that a primary hazard could trigger multiple secondary hazards, like extreme weather causing flash floods, landslides, extreme waves, and abrasion. The thin vertical strip shows that specific hazards have occurred on Ternate Island but have not been recorded by BPBD or previous studies.

4.6. Village Chiefs’ Perception of Hazards

The survey (Figure 9) shows the village chiefs’ perceptions of the types of hazards (framed as dangers or threats to the local communities) in each village. Data from the survey suggest that 42 out of 60 village chiefs (70%) know that their villages are at risk of earthquakes, 17 chiefs (28%) perceive tsunamis as a threat, and 39 chiefs view volcanoes as a threat. The hazard exposure assessment (Section 4.1.2) suggests that, in terms of geological hazards, earthquakes could affect all villages within the research locations, tsunamis are projected to impact 46 villages, and volcanic disasters can potentially affect 39 villages.

The survey also indicates that 52% of the village chiefs view floods as a danger, 30 chiefs (50%) perceive the threat of landslide hazards, and 30% (18 chiefs) view extreme weather and extreme wave hazards as a threat. In contrast, the hazard analysis map suggests that 32 villages are at risk of flash floods, and 37 villages are at risk of landslides. At the same time, the mapping indicates that extreme weather disasters can occur in all villages, and extreme wave and abrasion disasters are possible in 42 coastal villages on Ternate Island. Only 18 village chiefs, however, recognize extreme weather as a threat to their local communities.

5. Discussion

The combination of geomorphological conditions, land use changes, and rainfall on Ternate Island increases its landslide susceptibility [48,49]. Landslides are more likely on sloping landforms such as hills or mountains or in areas where the land shape can be altered to form steep slopes. The landslide hazard level on Ternate Island is characterized by slopes ranging from 15% to 45%. Furthermore, ref. [78] stated that mountainous or hilly regions are complex and vulnerable ecosystems frequently impacted by climate change. These areas face primary risk events such as floods, landslides, and debris flows, often accompanied by geological hazards like earthquakes. Due to these conditions, Ternate Island periodically experiences flash floods, landslides, severe weather, strong waves, and erosion.

Situated in the Ring of Fire, Ternate’s geological features shape its natural hazards [78]. Its geological complexity arises from the interaction of three major tectonic plates: the Pacific, Eurasian, and Indo-Australian plates [79]. The convergence of multiple tectonic plates results in the exertion of forces between adjacent plates, leading to significant seismic risk in North Maluku Province [51]. In addition, the many microplates in North Maluku, particularly the Halmahera, Sangihe, and Maluku Sea microplates, significantly impact seismic activity in the region. The collision between these plates generates intricate seismic patterns and exerts considerable pressure [59,79]. As a result, Ternate Island is susceptible to tsunamis [42], as evidenced by 109 tsunamis recorded in the region between 1600 and 2004 [80,81].

The warming sea surface temperatures and the recurrent El Nino-Southern Oscillation (ENSO) phenomenon in the Pacific Ocean can lead to more rainfall in the Maluku region [82,83]. However, historically, Ternate is not directly in the path of tropical cyclones, or at least due to the lack of records, its proximity to cyclone areas in the East Pacific occasionally results in cyclone winds affecting the region [73].

This study also indicates that Ternate Island is susceptible to multi-hazard interaction in the future, as shown in Figure 8. Meanwhile, Table 9 depicts the occurrence of hazards and their interaction on Ternate Island based on the available data. It is essential to understand that using the concurrent relationship as an example for both multi-event single hazards and multi-hazards is difficult because concurrent hazards suggest a shared source event [71].

These interactions can be broken down into triggering/cascading and compound hazards. For example, tsunamis trigger extreme waves and abrasion [76]. Landslides, severe weather, and volcanic eruptions are significant causes of flash floods, such as those on Ternate Island in 2011 and 2012 [56,74,75]. Extreme waves and abrasion are linked to severe weather conditions [55]. Tsunamis are associated with earthquakes and volcanic eruptions impacting Ternate Island [76]. These relationships are also evident in the combinations of hazards contributing to compound catastrophes. For instance, the combined occurrence of landslides, tsunamis, and earthquakes has been studied in eastern Indonesia by Kurnio et al. (2019) [84].

This interaction of primary and secondary hazards is essential in generating techniques to minimize and manage subsequent hazard events. An appropriate understanding of the hazard interactions might contribute to a hazard mitigation strategy by decreasing the probability of an event occurring [25]. Additional consideration must be given to integrating hazards into scenario planning and risk assessments for emergency managers [85]. From a long-term perspective, the hazard interaction examined in this study offers evidence that can be used to inform the development of more realistic multi-hazard disaster scenarios.

Rising population density increases vulnerability, worsened by urbanization and relocation to high-risk areas [86]. Growing demand for development expansion leads to significant alterations in coastal regions [20]. By 2035, Ternate will need 124k housing units, an increase from 70k in 2020. The need for land will increase from 750 ha today to 1245 ha by 2035 [87]. Between 2013 and 2023, residential development in coastal areas rose from 15.7% to 19.7%, especially in South, Central, and North Ternate [20]. This development has led to significant land use changes on Ternate Island, including coastal land reclamation for business centers [86,88,89] and deforestation near Mount Gamalama’s summit. The forest area decreased from 6937.3 ha in 2010 to 5592.8 ha within a decade [86]. Small islands like Ternate face environmental issues such as land degradation and biodiversity loss due to their limited size, isolation, and becoming more susceptible to natural hazards, worsened by population growth, urbanization, infrastructure development, and poor urban planning [30].

While a few hazards are adequately perceived as a threat, the data suggest gaps between perception and a more objective measure. Governments should consider this multi-hazard assessment when developing better spatial plans. Using a spatial approach to risk assessment helps with disaster risk reduction (DRR) by giving essential information about where hazards are coming from, where they might hit, and how people and things are spread out geographically, especially in high-risk areas [90]. Developing multi-hazard-sensitive spatial planning to achieve the Sustainable Development Goals (SDGs) must include mitigating multi-hazards, managing cascading risks, and addressing residual risks [91]. Therefore, multi-risk assessments should examine how well different adaptation options and strategies work by considering how various sectors are linked and the following effects [23].

6. Final Remarks

6.1. Conclusions

This article showcases the practicality of using a multi-hazard map at the local level to detect the population’s exposure and existing infrastructure. The purpose is to utilize this information for sustainability planning, practical demonstration of land use exposure assessment, and the proposal of a database for risk assessment. Therefore, the main findings are as follows:

• Given the island’s limited land area and the increase in population density and concentration of infrastructure, spatial planning must consider the risks posed by many hazards, including potential losses in future extreme climate events.
• More risk awareness and risk communication are imperative in areas where local communities, especially the village leaders, undermine the risk of natural hazards. The multi-hazard mapping results indicate that the impact of multi-hazards is becoming more extensive and encompasses the entire territory of Ternate Island, including both hydro-meteorological and all other hazards. This map is demonstrated by the substantial proportion of land classified as high-risk, which accounts for up to 61% of the entire area of Ternate Island.
• Multi-hazard zonation in small-island communities (SICs) is a fundamental step in managing disaster risk. It provides information about the location and severity of individual hazards. It allows the government and all stakeholders to prioritize and address the most critical hazards, allocate resources effectively, and develop a more resilient system to handle multi-hazard scenarios.

6.2. Limitation and Future Work

The authors acknowledge the limitations of this research. The approach must be completed to thoroughly understand disaster risk since it only focuses on creating a multi-hazard portfolio. The article does not cover the analysis of community vulnerability, adaptive capacity, community preparedness, and estimates of all risk components. Another possible drawback is that the availability and accessibility of data might be troublesome in some instance. This research exclusively utilized data that the researcher could obtain from national and other sources, as the local government lacks a centralized disaster database. We recommend implementing a resilience-enhancing strategy to address multi-hazard situations by examining the consequences of many systems, including their physical, technological, environmental, and social aspects. It is extremely beneficial for further, more detailed, or thorough catastrophe risk assessments. Furthermore, utilizing existing open-access risk assessment software such as RiskScape and RiskChanges can help create a more automatic multi-hazard process.