RISK MANAGEMENT OF EARTHQUAKE HAZARD IN BANTUL REGENCY

By : Heru Setiawan, Oka Pratama, Dedy Humaidi, Selli Fidi Yani Wardani, Betti Yuniasih

I.                    BACKGROUND

An earthquake is defined as a vibration in the earth that is naturally occurring in specific location and is not sustainable. Vibrations on the earth can occur due to the sudden shift in the Earth's crust. Based on to the cause, earthquake is differentiated into two, tectonic and volcanic earthquakes. The cause of the occurrence of tectonic earthquakes originate from the movement in the Earth's interior (the style of mantle convection) that suppress the earth's crust (outer layer) which is fragile, so that when the crust is no longer strong in retaining force of the earth; it will generate earthquakes (Surono et al. 2006).

Geological hazards, like liquefaction, may occur in during a big earthquake. An occurrence of earthquake induced liquefaction can pose extensive damage on buildings and infrastructures. Earthquake disaster which occurred in Yogyakarta on 27 May 2006 has destroyed many infrastructures and killed thousand victims.

Bantul is one area in Yogyakarta which has the highest number. Source of the earthquake was centered at a depth of 33 km, precisely located 37 km from the coastline and epicenter in the Indian Ocean base at coordinates 8.26 ° S-110, 31 ° E (Hussein et al. 2006). Earthquake activity is thought to trigger the movement of faults in Bantul and around it. Active faults are believed to form a straight line starting from the epicenter in coordinates 8.007 ° S-110, 286 ° E (precisely in the estuary of the Opak River) to the northeast down to Prambanan and the surrounding (USGS on id.wikipedia.org).

The U.S. Geological Survey on Tsuji et. al (2009) estimated the magnitude of the earthquake as Mw 6.3. Subsequently, + 750 aftershocks have been reported, with the largest one measured at Mw 5.2. About 6000 people were killed, 50,000 were injured, and as many as 600,000 people were displaced in the Bantul-Yogyakarta area.

Bantul regency was chosen for the study area because of the active fault which is located on Opak River can be active again and trigger an earthquake. For the purpose of reduce the impact of earthquake; the risk assessment is needed by mapping the hospital which is located in Bantul regency. So that decision-making in the context of disaster mitigation can be done.

Bantul area is 514.33 square kilometers and it has 17 sub-district. This area is adjacent to Indian Ocean in the south, the city of Yogyakarta in the north, Gunung Kidul regency in the east and Kulon Progo regency in the west. The following figure shows the mapping area.

II.                  OBJECTIVES, PROCEDURES AND MATERIALS

1.         Objectives

The objective of this group assignment is to develop risk management for earthquake in Bantul regency.

2.         Procedures

The procedures of this assignment are :

a.     Select  of the hazard type (earthquake)

b.    Select of the element at risk

c.     Determine the probability of hazards (H)

d.    Determine area distribution of the H and then make a hazard map

e.    Estimate the vulnerability of the element at risk and the capacity

f.      Calculate the risk

g.     Determine the area distribution of the risk

h.    Develop an idea to manage the risk of the area concerned.

3.         Materials

The materials that is used for this assignment are :

a.     Bantul Regency administration map from RBI Map (Peta Rupa Bumi Indonesia / Indonesia Topographic Map)

b.    Bantul Regency landuse map from RBI Map (Peta Rupa Bumi Indonesia / Indonesia Topographic Map)

c.     Statistic data of Bantul Regency from Badan Pusat Statistik of Bantul Regency

III.                METHODOLOGY

In order to develop a good risk management for the earthquake on the Bantul regency, we need to determine two things. The first one is area distribution of the risk and the second one is potential lost. These two things will give us an illustration of Bantul regency’s situation that related to earthquake. We need a hazard map, vulnerability of the element at risk and the capacity to determine area distribution of the risk.

1.       Risk Assessment

Risk assessment is a process to analyze hazards’ characteristic within risk area for their potential mishap consequences and probabilities of occurrence, and combining the two estimations to reach a risk ranking. Risk assessment can be divided into two main activities, risk analysis and risks evaluation. 

Risk analysis is the use of available information to estimate the risk to individuals or populations, property, or the environment, from hazards. Risk analysis generally contains the following steps: scope definition, hazard identification, and risk estimation.

Risk evaluation is the stage at which values and judgements enter the decision process, explicitly or implicitly, by including consideration of the importance of the estimated risks and the associated social, environmental, and economic consequences, in order to identify arange of alternatives for managing the risks.

a.       Hazard

Hazard refers to a potential harm which threatens our social, economic, and natural capital on a community, region, or country scale. Hazards may refer to many types of natural (flood, hurricane, earthquake, wildfire, etc.), technological (hazardous materials spill, nuclear accident, power outage, etc.), or human-induced events (biochemical, bombing, weapons, mass destruction, terrorism, etc.). Hazard probability can be defined as the possibility of occurrence of a potentially damaging phenomenon within a specified period of time and within a given area.

To analyze a hazard, it must be determined exactly how that hazard exists within the specific community or country. Each hazard will be different in this respect, due to climate, geography, settlement patterns, regional and local political and stability, among many other factors. Disaster managers commonly create what is called a risk statement, which serves to summarize all of the necessary information into a succinct report for each identified hazard (Coppola, 2007).

Bantul areas have been struck several times with earthquake, the last one was happenned on May 27, 2006 of Bantul Earthquake with the magnitude of 5.9 Richter Scale, where more than 5000 people died and about hundred thousands of houses in Bantul, Sleman and Gunung Kidul Regencies were damaged or collapced.

There have been many researches to map seismic hazard for Indonesia, one of which is released by USGS in 2008. The research expressed seismic hazard as Peak Ground Acceleration (measured in % g). From the map, it can be seen that Yogyakarta area and its surrounding (including Bantul) have probability to earthquake with value range from 20 – 40 % g with a 10 % probability of exceedance in 50 years.

In this assignment, hazards probability values for Bantul area is taken from the microzonation map of Bantul area from the previous May 2006 earthquake (Karnawati et all. 2007). A microzonation map can be used to estimate the zonation of earthquake severity at various level of risk.

This map was created by  iInterpretation on the aerial photographs and satelite images, reviews on previous geological study at Yogyakarta Basin and the surrounding areas, and site investigations by drilling, well logging, geophysical surveys (georadar, magneto tuleric and resistivity surveys) as well as micro tremor survey have been conducting to establish the microzonation map, especially at the area surrounding Opak – Oya rivers.

Some results of this investigation confirm that distribution of faults, as well as the characteristics and thickness of overburden sediments above the bedrock dictate the severity of construction damages. In general, zonation of soil sensitivity to response seismic wave existed within depth less than 30 km can be devided into four sensitivity zones, they are:

1.          extremely high amplification area

2.          high amplification area

3.          moderate amplification area

4.          Low amplification area.

In this assignments, earthquake hazard probability of bantul area is divided into 5 (five) zones. This zonation is derived from above mentioned microzonation map. Since the map is not covering whole Bantul area, therefore it is added with one additional zone. It is presumed that earthquake hazard still has effect in this zone, but it is with very low level of damage severity.

 

Table 1. Hazard Probability

Zone	Hazard Probability Value	Earthquake Amplification level
1	0,1	Very low
2	0,25	Low.
3	0,5	moderate
4	0,75	high
5	1	extremely

Hazard probability values are ranged from 0.1 to 1. Minimum value of 0.1 shows that the area has very little probability to suffer severe damage from earthquake hazard that is considered dangerous, whereas area with probability value of 1 means that this area would be severely affected by earthquake hazard when it occurs.

b.      Vulnerability

Vulnerability is defined as: “the degree of loss to a given element at risk or a set of elements, resulting from the occurrence of a hazard” (Erdik 1994). Vulnerability can also be defined as the degree of loss to a given element at risk, or set of such elements, resulting from an earthquake of a given magnitude or intensity, which is usually expressed on a scale from 0 (no damage) to 1 (total loss). Earthquake vulnerability is thus a function of the potential losses from earthquakes (death and injury to people, damage and other physical structures) and the level of preparedness.

The assessment of vulnerability to seismic risk requires particular information on each one of the factors and elements at risk. All items, like geologic evolution, urban development, strength of structures that can be affected, and possible collateral effects, have to be considered carefully to assess as accurate as possible the vulnerability of a site to an earthquake.

Vulnerability of earthquake hazard can be measured by a range of indicators. In this assignment, the vulnerability of the Bantul area is analysed in a simple way. Vulnerability levels were measured by using the difference distance residential to the hospital. The assumption that is used: houses that were located far from the hospital have a higher level of vulnerability compared with the houses that were located close to the hospital. This assumpsion is based on, if the earthquake occured, victims who live close to the hospital will be get help more quickly than the victims who live far from the hospital. To create vulnerability map of each area in Bantul Regency, used the ArcGIS 9.3 software with multiple buffers analysis.

Table 2. Vulnerability Degree

Nr.	Radius from hospital (km)	Vulnerability Degree	The speed level in getting first-aid from hospital
1.	3	0.2	Very fast
2.	6	0.4	Fast
3.	9	0.6	Medium
4.	12	0.8	Slow
5.	15	0.9	Very slow

c.       Capacity

Capacities are those positive condition or abilities which increase a community’s ability to deal with hazards. It also means by which people or organizations use available resources and abilities to face adverse consequences that could lead to a disaster. In general, this involves managing resources, both in normal times as well as during crises or adverse conditions.

The strengthening of coping capacities usually builds resilience to withstand the effects of natural and human-induced hazards. Capacity building also includes development of institutional, financial, political and other resources, such as technology at different levels and sectors of the society. Therefore the capacity often interpreted as preparedness.

If there is an earthquake occurs in Bantul regency, medical personnel who work at each sub-district Puskesmas are the fastest aid that the victims can get. Therefore, the number of medical personnel that work there can be interpreted as preparedness or capacity of each regency. The higher the number of medical personnel in a sub-district the higher the level of readiness for earthquake. The number of medical  personnel vary from 26 to 75 person. Hence, we divide it into 10 class of capacity value. Each class has five person range. The following table shows capacity value classification.

Table 3. Capacity Value Classification

Nr.	Number of Medical Personnel	Capacity Value
1.	26   - 30	0,1
2.	31 - 35	0,2
3.	36 - 40	0,3
4.	41 - 45	0,4
5.	46 - 50	0,5
6.	51- 55	0,6
7.	56- 60	0,7
8.	61- 65	0,8
9.	66- 70	0,9
10.	71- 75	1,0

Using the capacity value classification, each sub-district of Bantul regency can be classify. The following table shows capacity level of each sub-district on Bantul regency based on the number of medical personnel who work there.

Table 4. Capacity Value

No.	Sub-district	Number of Medical Personnel	Vulnerability Degree
1.	Pundong	26	0.1
2.	Bambanglipuro	28	0.1
3.	Srandakan	28	0.1
4.	Pajangan	29	0.1
5.	Pleret	30	0.1
6.	Kretek	31	0.2
7.	Sanden	32	0.2
8.	Piyungan	33	0.2
9.	Bantul	44	0.4
10.	Jetis	44	0.4
11.	Sedayu	47	0.5
12.	Pandak	48	0.5
13	Dlingo	53	0.6
14.	Imogiri	53	0.6
15.	Kasihan	57	0.7
16.	Sewon	61	0.8
17.	Banguntapan	75	1

IV.                RESULTS

For this assignment, risk map of earthquake hazard in Bantul is built using a hazard map, a vulnerability map and a capacity map. Those map is made using the methodology that is explained in the preceeding part of this report.

1.         Hazard Map

This Hazard Map shows that the central part of Bantul regency has the highes hazard value. There is also a small area in the west part of Bantul regency that has the highest hazard value. The following table shows each hazard value area.

Table 5. Area of Hazard Probability Value

Zone	Hazard Probability Value	Earthquake Amplification level	Area ( Hectares)	% of area
1	0,1	Very low	22.867,97	44,13
2	0,25	Low.	6.831,57	13,18
3	0,5	moderate	10.534,93	20,33
4	0,75	high	8.090,88	15,61
5	1	extremely	3.493,70	6,74
Total			51.819,05	100,00

2.         Vulnerability Map

This vulnerability map shows that most of the Bantul regency’s hospital located on the center of Bantul regency. it makes the area that distant from the center of Bantul regency get the highest vulnerability value.

3.         Capacity Map

This capacity map shows that the capacity value in the north part of Bantul regency is more than the south.

4.         Risk Map

This risk map shows that there are two area that has very high risk to earthquake in Bantul regency. The first one located on Pundong sub-district and the second one located on the north part of Srandakan sub-district.

These are the area that need more concern, because if a major earthquake happend, the resident of this area may have trouble of getting help. But as we can see on the map, most of the area has low risk or very low risk to earthquake hazard. This means that generally Bantul regency is well prepared for earthquake.

5.         Risk Management of Bantul Regency

Mitigation efforts are attempts to prevent hazards from developing into disasters altogether or to reduce the effects of disasters. Earthquake mitigation is the effort to reduce loss of life and property by lessening the impact of earthquake. This is achieved through risk analysis, which results in information that provides a foundation for mitigation activities that reduce risk.

Earthquake mitigation activities can be carried out before the earthquake, during the earthquake and the aftermath of earthquake. Based on the risk map that we produced, there several mitigation activities that we can suggest.

a.      Before the earthquake

1)     Avoid Establishing Buildings in the Fault Area

The areas that have an active fault are prone to earthquakes. This area should not be used as residential areas because when the earthquake is occurred, loss of life and property will be high. Areas close to the fault only permitted for the open areas, such as paddy fields, crops, or forests which can be used as an evacuation spaces in case of an earthquake.

2)     Build Earthquake-Resistant Structures

Buildings that are established in earthquake-prone areas should have a strong construction which is resistant to the threat of earthquakes. Several criteria for earthquake-resistant structures are:

a.   Have a strong and lightweight materials

b.  Building components must be bonded to each other

c.   Have good quality materials

d.  Do not use bricks for the walls thoroughly, just 1 meter from the foundation.

3)     Established a hospital, clinic, or health facilities in earthquake-prone area surroundings

The establishment of hospital, clinic, or health facilities in earthquake-prone area surroundings should be easily accessible from the earthquake-prone areas will reduce the number of earthquake victims. Earthquake victims will get help quickly so that the lives of these victims can be rescued.

Health buildings should have earthquake-resistant structures and they should also be provided emergency services, ambulance, and if possible be provided helipad.

4)     Making good road access and evacuation routes.

Good road access will reduce the vulnerability of earthquake-prone areas. With good road access, the people who live in areas prone to be out of the area to a safer place and can reach other emergency facilities such as hospitals.

5)     Socialization, training, and simulation of earthquake disasters.

Socialization, training, and simulation should be given routinely to people living in earthquake prone areas. This can increase the public's understanding and awareness of the earthquake so that they will know what should be done, before, during and after the earthquake. It can improve community preparedness so as to enhance community capacity.

6)     Create an early warning system about the tsunami caused by earthquakes.

Early warning system (EWS) should be established in coastal areas. So that it will provide information whether a potential tsunami will occur or not after the earthquake. Establishment of early warning system is expected to reduce casualties in the event of an earthquake followed by tsunami.

7)     Prepare the area gathered in the open area which is equipped with first aid equipment.

Gathering place at emergency sites should be established in a secure location and be equipped with emergency equipment such as first aid box, radio, and emergency lights. This place must be easily accessible when an earthquake happens.

8)     Create an organization that has responsibility to coordinate the community in face of the earthquake. The organization should be made in the lowest levels of society such as the village level so that the emergency information and instructions can be delivered quickly.

9)     Incorporating natural earthquake lessons in education curriculum.

Incorporation the lessons into the curriculum in the starting level of education is expected to introduce the threat of earthquake hazard as early as possible so as to increase community capacity.

b.  During the earthquake

1)     It is very important to stay calm.

If you are indoors, stay indoors. If outdoors, stay outdoors. Many injuries occur as people enter or leave buildings. During the earthquake, panic causes a person’s mind is not focused and feel scared. This can cause injury or death of victims.

2)     If you are indoors, go the center of room, do not close to the wall, get under a desk or sturdy furniture, and stay away from window to protect yourself from ruins of building and broken glass that arise due to the earthquake. Use stairs, do not use an elevator.

3)     If you are outdoors, stay in the open. Keep away from overhead electric wires or anything that might fall (such as cornices on buildings). Immediately go to the nearest through the evacuation route that has been created. Gathering will facilitate coordination in emergency conditions.

4)     If you are in a moving car, stop and remain inside until the shaking is over.

Driving during the earthquake can cause an accident, because during the earthquake the roads are moves as a result ground motion. Stay inside the car until the shaking over will protect you from construction debris.

c.   Aftermath the earthquake :

1)     Immediately check the condition of yourself and people around you.

If anyone is injured, give the first aid. Ensuring the safety of yourself and those around you is very important to reduce the number of casualties. If there are injured immediately give first aid, if the wound is severe immediately brought to the hospital or clinic. Handling victims quickly and precisely to reduce vulnerability of earthquake victims.

2)     Turn off water valves, gas, and electricity.

Immediately turn off the flow of gas, electricity, and water so that no fire or explosion due to electrical surge caused by a leaking gas.

3)     Turn on the radio to listen to emergency instructions.

Listen the information from the radio will deliver the information about the emergency conditions.

4)     Remained outside the building because there are the possibilities of the aftershocks.

Usually the main earthquake followed by aftershocks with smaller magnitude. Therefore if major earthquake vibrations have done, if you are indoors, exit the building immediately or remain in an open area if you are outdoors to anticipate the occurrence of aftershocks.

5)     Use footwear to protect feet from broken glass or trash.

 6)   If being around the beach, stay away from the coast and go to the high area to anticipate the occurrence of tsunami. A large earthquake could cause a tsunami. Lag time between the earthquake and tsunami waves just 15 minutes, then immediately evacuate to high places to reduce the casualties.