Comparison of scenario seismic hazard and probabilistic seismic hazard analysis with Single Source, Lembang Fault, Indonesia: a preliminary study

. Seismicity analysis can be carried out using deterministic and probabilistic methods. One approach that can be done with the help of OpenQuake is the Scenario Seismic Hazard Analysis (SSHA) and Probabilistic Seismic Hazard Analysis (PSHA) methods. The question arises regarding the equivalence between the two methods in producing earthquake acceleration values. In this study, it was tested using a single earthquake source, namely the Lembang Fault. The observed area is in the Bandung Basin area, bounded by mountains on the north and south sides. The method compares the acceleration values (PGA/Peak Ground Acceleration) between PSHA return periods of 500, 1000, 2500, 5000 and 10000 years with SSHA. The results obtained are a value agreement between PSHA with a return period of 5000 years and SSHA. The resulting acceleration (PGA) near the Lembang Fault is 0.5-0.6g.


Introduction
In the last few decades, seismic analysis has progressed and become a discussion topic.The current analysis can be divided into two [1] major groups, namely Probabilistic Seismic Hazard Analysis (PSHA) and Deterministic Seismic Hazard Analysis (DSHA) [1][2][3].One of the striking differences between the above methods is the assumed location of the earthquake event.
In PSHA, all points at the source of the earthquake are calculated to have the potential for an earthquake to occur.On the other hand, DSHA only assumes a certain point is the epicenter [4,5].
Questions arise regarding the relationship between PSHA and DSHA results, namely, at what return period can PSHA results match DSHA results [6,7].This study attempts to answer this question by taking a strategic location in the West Java region, namely the Bandung Basin (Figure 1).The Bandung Basin is an area bounded by mountains on the north and south sides.This area includes Bandung City, Bandung Regency, West Bandung Regency, Cimahi Regency and parts of Sumedang Regency, with a total area of around 3497.5 km 2 [8,9].This study will consider the values of acceleration in bedrock (PGA) analyzed based on the DSHA and PSHA return periods of 500, 1000, 2500, 5000 and 10,000.By comparing the spatial and statistical results, an equivalent comparison will be obtained between the two methods.

Data
The data used in this study are the seismic parameters of the Lembang Fault, as presented in Table 1.After the PSHA and DSHA/PSHA steps have been carried out, the next step is visual observation and comparison of the trendline regression from several coordinate points.All steps can be seen in Figure 2.

Result and discussion
The results of running OpenQuake for both methods are presented in Figure 3.The calculation process uses the help of a server that has OpenQuake software installed.Based on the results shown in Fig 3, it can be seen that the PSHA method and the higher the return period, the greater the acceleration value (PGA).The pattern formed is gradually getting bigger and closer to the source of the earthquake.The same thing was found with the SSHA method.The difference that arises from the two methods is the formed pattern.For SSHA, the pattern looks not so smooth and oval.On the other hand, PSHA has a smoother shape and occasionally appears in larger areas of its surroundings, as seen on the 10,000 years return period.The pattern that emerges from the PSHA and SSHA methods is allegedly due to the interpolation effect.To clarify the differences that exist, it is necessary to review several points that can provide information about the values arising from multiple return periods and SSHA.For that, predefined locations are compared in graphical form.Figure 4 shows us the five test points' acceleration values (PGA).The point itself as depicted in Figure 1.It can be seen that the trendline line adjacent to the SSHA method is PSHA with a return period of 5000 years.Therefore, the two methods can be compared with the provisions that if using PSHA, the return period is 5000 years.

Conclusion
A comparative study of seismic analysis using the PSHA and SSHA methods has been carried out with the Lembang Fault earthquake source and the help of the OpenQuake software.It can be seen that there are similarities in results in the form of maximum acceleration values and patterns between the PSHA 5000 years return period and SSHA methods.The greatest acceleration value (PGA max) is in the range of 0.5-0.6g.In the future, reviewing it with alternative interpolations and deviation levels is necessary.

Fig. 1 .
Fig. 1.The Bandung Basin area is divided into eastern (brown), central (yellow) and western (light green) areas.Five (5) study points were selected to compare the results of PGA calculations between PSHA and DSHA.

Fig. 4 .
Fig. 4. Linear histogram and trendline plot of multiple review points.RP means Return Period (Years).

Table 1 .
Parameters of the Lembang Fault seismicity.Seismic analysis testing steps for both PSHA and DSHA were carried out with the help of the OpenQuake software.As a note, because the OpenQuake software does not clearly state the use of DSHA, this study is approached with Scenario Seismic Hazard Analysis (SSHA).SSHA [3, DSHA have the same input parameters, including maximum magnitude, earthquake source geometry, and hypocenter being analyzed[3,10].In addition to this, the GMPE (Ground Motion Prediction Equation) used is the same as that used for making the Indonesian Earthquake Hazard Map and Sources, namely Boore et al., 2014, Campbell Bozorgnia 2014 and Chiou Youngs 2014[11- 13].