Analysis of susceptible disaster region based on the peak ground acceleration and earthquake intensity in Mamasa 2018

This study aims to know the susceptible disaster region based on peak ground acceleration and earthquake intensity on earthquake study cases in Mamasa 2018. Data that is used in this research is data on earthquake cases in Mamasa 2018 that is gained from the Indonesian Agency for Meteorology, Climatology, and Geophysics (BMKG) district IV Makassar, it included data of the time when the earthquake happened, latitude, longitudinal (, depth (< 60 km) and magnitude (≥ 5) SR. The calculation of on peak ground acceleration was conducted by using Donovan and Mc.Guirre methods, whereas to calculate the value of intensity, it is using the Wald method (1999). The result from the calculation of on peak ground acceleration by Donovan is 95.903 gal to 377.094 gals with the intensity VI to VIII MMI. Whereas the calculation result of on peak ground acceleration by Mc.Guirre method is 58.981 gal to 227.39 gals with the intensity V to VII MMI. MMI intensity of the Wald method is obtained by using the PGA value conversation Mc.Guirre method that is closer to the intensity of the result of the macroseismic survey in Mamasa District and relates with the damage that happened on location. The mapping of earthquake intensity based on PGA Mc.Guirre method shows that 3 (three) sub-districts that have an intensity of VII MMI, namely Tawalian Sub-district, Mamasa Sub-district, and Sesean Padang Sub-district. The area is the most vulnerable. Mamasa has low-risk level from the earthquake, although sometimes an earthquake happened until now. This result is supported by a geological map in the Mamasa regions that showed source rock in that area is a type of hard rock that consis of granitic materials.


Introduction
West Sulawesi region is an earthquake zone from the expansion of the seabed in Makassar Strait. Seismotectonic activity in this area is also caused by faults on land including the Saddang Fault, the Kalukku Fault, and in the eastern part of this area, there is the Palu Koro fault. Seismicity activity in this area is dominated by shallow earthquakes (depth less than 60 km) and medium earthquakes (depths between 60-300 km) [1]. An earthquake rocked Mamasa Regency and its surroundings on November 6, 2018, at 02:35:53 WITA. The earthquake mechanism based on BMKG calculations is a strike-slip fault. Until November 21, 2018, at 06:00:00 WITA, BMKG has recorded 721 earthquakes of various magnitudes and 240 earthquakes of which were felt by the people [2].
The Saddang fault line appears to cross from the coast of Mamuju, West Sulawesi, cutting diagonally across the central part of South Sulawesi. To the southern part of South Sulawesi, it is continued by the Walanae fault. In Mamasa, the crossing of the Saddang Fault runs northwestsoutheast. in this segment, successive earthquakes occurred. Based on the mechanism, in this segment, the Saddang Fault is a horizontal strike-slip fault. The results of the analysis of the source mechanism of several significant earthquakes that occurred in Mamasa show that there is a similarity in the instrument, namely a horizontal fault (strike-slip) with a left movement. So it stands to reason that the current increase in earthquake activity in Mamasa is related to the Saddang Fault activity [3].
One of the essential things in seismological research is knowing the damage caused by earthquake vibrations to buildings. It is necessary to adjust the strength of the building to be built in the area [4]. The peak ground acceleration value is required to make the building design earthquake resistant. The amount of peak ground acceleration (PGA) is the most prone area to earthquake hazards [5]. The peak ground acceleration value obtained can be used to determine the magnitude of the earthquake intensity value (level of vulnerability) experienced by the place. Damage to buildings and facilities caused by the earthquake is determined by the intensity parameter (MMI), which correlates with the thickness of the sediment [6]. Although strong earthquakes did not occur frequently, they are very endangered human life [7]. There is a concentration of the population here. Hence earthquakes often cause loss of property and social life [8]. Therefore, a study is needed to find the peak ground acceleration value and the intensity caused by the earthquake.

Materials
The data used in this study is the 2018 Mamasa earthquake data obtained from Makassar Region IV Meteorology, Climatology and Geophysics Agency including data on earthquake event time, latitude (S), longitude (E), depth (<60 km), and magnitude (≥ 5 SR). (2)

Data
Where a is the ground acceleration (gal), M is the magnitude (SR), and r is the hypocenter distance in km.

Where
= eventuality of acceleration (gal), alpha (α)is a constant of 472, b is 0.278, and c is 1.301. While M = magnitude (SR), and R = distance of hypocentre (km). The depth of the earthquake being input from 9 and 70 km but was greater at 10 km. This empiric is more efficient for an earthquake with a strength of 6.5, and hypocentre not more than 50 km.

(4)
Where I MM is the MMI intensity, and PGA (gal) is the acceleration value that had been obtained in the research area.

Results of research
Earthquake-prone areas based on the map of the peak ground acceleration (PGA) and earthquake intensity in Mamasa Regency are described as follows:

Earthquake intensity in Mamasa Regency.
The MMI intensity is based on the PGA conversion of Donovan and Mc. Guirre will be compared with the results of the macroseismic survey in Table 3.    Figure 1 shows that based on the Donovan method the areas that have the maximum peak ground acceleration value are Tawalian District, Mamasa District, and Sesean Padang District.      Figure 3 shows the intensity value which was obtained from the PGA conversion using the Donovan method, which is VI to VIII MMI marked with the colour on the map, namely yellow to dark orange. The area that has the maximum intensity, namely VIII MMI, is the Tawalian District.  Figure 4 shows the intensity value of the PGA conversion method MC. Guirre (1977), which is V to VII with green to orange colour. The areas that have the maximum intensity of VII MMI are Tawalian, Mamasa, and Sesean Padang District.