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Microtremor Analysis Using HVSR Method for

Microzonation Mapping of Earthquake Prone Areas in Klaten

District, Central Java

Rizqi Prastowo1

, Melfa Utari1

, Fitri Puspasari1

, Rita Desiasni1

, Rizka Anggraini1

,Achmad Nabil Zulfaqar2, Muhammad Rizki Fitraldi2

1Master Program of Geophysical, Universitas Gadjah Mada, Indonesia2Bachelor Program of Geological Engineering, Universitas Pembangunan Nasional Veteran

Yogyakarta, Indonesia

Email :achmadnabilz@gmail.comor murizfit@gmail.com

Abstract. Klaten District is one of districts in Central Java that on 27 May 2006 were affected

by powerful earthquake (also known as Bantul Earthquake) with a moment of magnitude 6,3

Richter scale based on (USGS, 2006). The shock occurred by the movement Opak Fault, where

located in Yogyakarta , with a distance of 40 km south-southeast from Klaten District. Based

on Indonesian Social Ministry, more than 18.000 people injured and 90.000 houses damaged.Therefore, mitigation steps are needed such as the distribution of earthquake prone

microzonation to reduce a risk that will occur in Klaten District. This microzonation is using

microtremor analysis then processed by HVSR method. Datas was collecting on 31 points

which cover the area of 67 km2. Based on analysis using HSVR method, value of quake

vulnerability obtained range 4,92-2489, the value of land amplification between 2,48113 -

21,4151, the value of natural frequency between 0,1842 1,7808 Hz and a maximum ground

acceleration values between 6,55 31,74 gal. From these result, Klaten District could be

divided into 2 earthquake prone microzonation. That is area with MMI VI-VII index in Central

Klaten Sub-district and Kalikotes Sub-district also the area with MMI

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There are several factors should be considered such as consisted lithology, hydrological condition, and

peak ground acceleration (Seed and Idris 1971). Generally, the lithologies in Klaten areas consisted by

volcanic deposits and alluvial deposits (soft sediment). These lithologies if passed by the earthquakewaves will tend to reinforce the wave, thus making the area a high risk of damage. Therefore, a study

on the potential hazards of earthquakes in a region is important.

One effort that could be done is to divide the earthquake-prone areas in detail. The measurement

microtremor using HSVR method could be done HVSR microtremor measurement method can be

done for these activities. The result is the value of natural frequency (fo), soil amplification (Ao), soil

vulnerability index (kg), and peak ground acceleration (PGA) microzonation . Study of this

microzonation is limited only to the coordinates x: 452000-462000 and y: 9143000 - 9152000.

Figure 1. Map location of study area

2. Methodology

The process of data collection is divided into two phases: survey design and data collection stage

microtremor. In the design phase of the survey, the study area includes Ngawen Sub-district,

Kebonarum sub-district, Northern Klaten Sub-district, Southern Klaten Sub-district, Wedi Sub-district,

Gantiwarno Sub-district and Kalikotes Sub-district. Data retrieval of microtremor measurements

obtained from 31 points of data retrieval locations by the distance between the point about 1-2 km.

Seismometer Short Period Tools Lennartz LE-3D is used in microtremor measurements with long time

70-75 minutes at each measurement point. Then data of Microtremor measurement is processed by

geopsy software with HVSR method to determine the value of natural frequency and amplification

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values could be calculated so that the value of land vulnerability and the acceleration of ground motion

could obtained.

Figure 2.Research flow diagram

3. Result and Discussion

Natural Frequency

Theoretically magnitude or frequency of the vibration period of soil or rock is a reflection of the

physical condition of the soil or rock. Soil or rock that is soft and loose rock will have a more compact

(Pusat Survey Geology, 2007). From the overall retrieval points obtained Fo value: 0.1842 with long

periods of vibration. The Softer rocks have a higher risk when shaken by earthquakes wave, because

of a greater amplification than 1.7808 Hz. Regions with high Fo value is in the south area of research.

Fo relatively uniform indicating the same rock lithology that consisted the area, which is material

deposit of Merapi.

Figure 3.Map of natural frequency (f0)

Soil Amplification

From whole retrieval points obtained values of Ao between 2.48113 - 21.4151. Great value ofamplification is located in the northeast area of research. Great value of amplification indicates that

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zone prone to earthquake vibrations. Because the greater value of Ao, the tremor with a certain

frequency that pass through the area will be amplified greater.

Figure 4. Map of soil amplification

Soil Vulnerability Index

Vulnerability of land in Klaten area has a value of 4.92 - 2489. The higher Kg, indicates the area more

vulnerable to earthquakes. Kg max value in the northeast area of research that indicates areas are

relatively more vulnerable if hit by an earthquake in compare airfares with other regions.

Figure 5. Map of Soil Vulnerability Index

Peak Ground Acceleration (PGA)

From the results of data processing in Klaten area values obtained Peak Ground Acceleration (PGA)

from 6.55 - 31.74 gal. Highest PGA located in the south of the research area which covers Central

Klaten Sub-districts and Kalikotes Sub-district.

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Figure 6. Map of Peak Ground Acceleration

Theoretically, the peak ground acceleration is the impact of earthquake waves at the measurement

location, so it could be a measure of earthquake intensity experienced. Based on PGA Klaten District

data could be divided into two categories the level of risk that the Central Klaten sub-district and

Kalikotes Sub-district included in the category of small risk to the value of the acceleration 25-50 gal

and intensity VI-VII whereas for the Ngawen sub-district, Kebonarum Sub-distict, Northern Klaten

Sub-district, Southern Klaten Sub-district, Wedi Sub-district, and the Gantiwarno Sub-district includedin very small category of risk with the acceleration values

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Acceleration (PGA). Simply microzonation illustrates grouping regions based on a risk level to the

earthquake region. Where in Klaten district, the Central Klaten Sub-district and Kalikotes Sub-district

included in the category of less risk with intensity VI-VII whereas for the Ngawen sub-district,Kebonarum Sub-distict, Northern Klaten Sub-district, Southern Klaten Sub-district, Wedi Sub-district,

and the Gantiwarno Sub-district in very small category of risk with intensity

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6. Acknowledgments

This microzonation in Klaten district is expected to help goverment and local community to mapping

the city from the possibility of earthquake and build awareness the importance of disaster mitigation.

But for now this microzonation not covered all of Klaten District. So it is expected to further research

can cover the whole of the district klaten and can complete the data from previous studies that still has

not been reached.