MORPHOTECTONIC ANALYSIS TO IDENTIFY OPAK FAULT ...

Proceedings Jogya Earthquake in Reflection (International Conference for Commemorating 10 years of Yogyakarta Earthquake at Inna Garuda Hotel on 24-26 May 2016), Published by UPN “Veteran” Yogyakarta, page 28-35 1

MORPHOTECTONIC ANALYSIS TO IDENTIFY OPAK FAULT

(PRELIMINARY RESULT)

By : Supartoyo*), Sri Hidayati*), and Subandriyo*)

*) Center for Volcanology and Geological Hazard Mitigation, Geological Agency of Indonesia

Abstract

The existence of Opak fault became more popular after Bantul earthquake on May 27th, 2006. This

earthquake events cause a great risk such as fatalities, loss of property and damage to the

environment. However, the fault line and geometry have not been clearly determined probably due to

the thick of Quaternary deposit covering the area. Morphotectonic and resistivity analysis is applied

to identify its fault line and zone.

The SRTM (Shuttle Radar Topography Mission) and topography map showed the mountain front zone

from Parang Tritis, Jetis, Plered and continue to Berbah area in SW-NE direction. Other linements

were found along Gunung Bangkel, Gunung Curu and Gunung Candi Abang in Berbah area, and

Gunung Kunden and Gunung Sentono in Plered area. The direction of both lineaments is also SW to

NE, it suggests an Opak Fault zone.

Resistivity survey was conducted in Parangtritis, Srimartani, Gunung Bangkel, and Gunung Candi

Abang areas. Morphology in Parangtritis line is recognized as a saddle, while in Srimartani, Gunung

Bangkel, and Gunung Candi Abang as a mountain front zone. The four profile resistivity lines showed

contrast resistivity is interpreted as a line of Opak Fault.

Key word : Opak fault, morphotectonic, lineament, resistivity

Introduction

Yogyakarta is an area in Indonesia that vulnerable to the earthquake. Earthquake sources in this area

not only from subduction zone located in Hindia Oceanic south of Yogyakarta, but also from active

fault in land. One of active fault in this area is called Opak Fault. This fault became more popular

after the Bantul earthquake on May 27th

, 2006. Yogyakarta area has experienced seven events of

destructively earthquake in 1840, 1867, 1943, 1981, 2006, 2010 and 2015 (Supartoyo et al., 2014).

The event in 1867, 1943 and 2006 estimated source from Opak Fault.

Mostly researchers suggest that Bantul earthquake on May 27th

caused by Opak fault due the epicenter

in land and shallow depth. This earthquake events cause a great risk such as fatalities, loss of property

and damage to the environment. The earthquake event on September 25th

, 2015 with epicenter in land

and shallow depth probable associated by Opak Fault. However, the fault line and geometry have not

been clearly determined probably due to the thick of Quaternary deposit covering the area. Opak Fault

direction is southwest - northeast which extends from Parangtritis Jetis, Plered, Piyungan, Berbah and

continue to Prambanan. Research of Opak fault has not been widely published. In the geological map

of Yogyakarta sheet, Rahardjo et al. (1995) have drew inferred fault in SW – NE direction in east and

southeast Yogyakarta. This fault probable as a Opak Fault, and mainly covered by young volcanic

deposits source from Merapi volcano.

This paper as a preliminary result to identify Opak Fault that conducted by Center for Volcanology

and Geological Hazard Mitigation (CVGHM), Geological Agency (GA). CVGHM will focus to

identify active fault in Indonesia region, include Opak Fault. Morphotectonic analysis used to identify


of active fault and supported by geophysical data such as resistivity, Ground Penetration Radar

(GPR), seismic refraction.

Data and Method

In this paper we apply morphotectonic analysis to identify Opak fault. Morphotectonic is study

relationship between morphology and tectonic, especially in Quaternary tectonic. Tectonic processes

in the morphotectonic study focused on active fault. Landforms produced by active faulting will

provide specific results including fault scarps, warped and tilted slope, subsidence features such as sag

ponds, and offset features in stream channels (Keller and Pinter, 1996 and 2002, Huzita et al., 1992).

This specific landform can can be seen from the satellite image with high resolution and also field

observations.

We use the Shuttle Radar Topography Mission (SRTM) and topography map with scale 1 : 25.000 to

morphotectonic analysis. In this paper we use seven sheets of topography map with scale 1 : 25.000

that published by Geospatial Agency, namely sheets of Yogyakarta, Timoho, Imogiri, Bantul, Dringo,

Jabung, and cawas.

Field observations conducted to verify the results analysis from SRTM and topography maps. Field

observations focused along the Opak Fault zone in Parang Tritis, Jetis, Plered, Berbah and Prambanan

area. This survey was also supported by data resistivity. We use resistivity equipment with 24 and 48

channel. Totally there was four resistivity line along Opak Fault line during survey. The fault line is

analyzed in resistivity profile based on contrast of resistivity value.

Result and Discussion

Morphotectonic analysis focused on the lineament of mountain front zone, hills and valleys.

Mountain front is the transition zone between mountains and plains, and this landscape

assemblage includes the escarpment, the streams that dissect it, and the adjacent piedmont

landforms (Bull, 2007). In general the fault zone is located in the mountain front zone.

Based on SRTM and topography map it can see main lineament the mountain front zone from

Parangtritis, Jetis, Plered and continue to Berbah area. The main lineament in SW-NE

direction. In the western part of the lineament is plain composed by by quaternary volcanic

and alluvial deposits, while in the eastern is Semilir Formation composed of interbedded tuff

and breccia, and Nglanggran Formation composed of volcanic breccia.

Other linements were found along Gunung Bangkel, Gunung Curu and Gunung Candi Abang

in Berbah area, and Gunung Kunden and Gunung Sentono in Plered area. The direction of

both lineaments is also SW to NE, it suggests an Opak Fault zone. Based on the topographic

map showed offset indications in the Opak River in Srimartani village. Indications of this

offset is needed further analyzed to determine regarding the fault activity or by another

process. Based on morphotectonic analysis it appears that the existence of Opak Fault

estimated in the western part of the mountain front zone, and covered by volcanic deposit.

In the other area it can see lineament the mountain front zone from Prambanan, Ganti Warno,

Wedi and Bayat area in relatively W-E direction. The earthquakes in 2006 to form ground

fracturing trending east - west in Gantiwarno area in the mountain front zone with length

approximately 2950 m (Supartoyo, 2006). The direction linemaments probably it suggests a

Dengkeng Fault zone.


Resistivity survey was conducted in four area selected, namely Parangtritis, Srimartani,

Gunung Bangkel, and Gunung Candi Abang areas. This location is estimated as Opak Fault

lines that have been morphotectonic analyzed based on topographic maps and field

observations.

Parangtritis Line

Resistivity line of Parangtritis located in Parangtritis village, Bantul regency. Direction of

resistivity line is north west – south east. Morphology in Parangtritis line is recognized as a

saddle. Morphology saddle formed by the bulge hill depressed and is one of the

characteristics for strike slip fault. During resistivity measurement, the situation is fine

weather. There were many houses damaged and collapse during the earthquake in 2006.

Figure 1. Map of resistivity line measurement of Parangtritis (blue dashed line). Red dashed

line as an Opak Fault.

Resistivity length this line about 120 m and the spacing between the electrodes is 5 m.

Regarding this condition, the depth resistivity values on this line about 15 meters. Resistivity

value of Parangtritis line range from 0.228 hingga 59.3 ohm meter. Low resistivity values are

shown by blue colour, while the high resistivity values by a dark red colour. Low resistivity

value in the northwest part of this line as a human burial (in the peak of hill). Differences

high and low resistivity values seen in the northwest part of this line, and in the field as

saddle morphology. It is estimated as Opak Fault line with dipping plane to the southeast.


Figure 2. Resistivity profile in Parangtritis line. Black line estimated as an Opak Fault line.

Srimartani Line

Resistivity line of Srimartani located in Srimartani village, Piyungan district, Bantul regency.

Direction of resistivity line is north west – south east. Morphology in Srimartani line is

recognized as a mountain front. During resistivity measurement the weather is good. Based

on information from local societies during the earthquake in 2006 there were 23 peoples were

died and many houses collapse and damaged in this area.

Figure 3. Map of resistivity line measurement of Srimartani (blue line). Red line as an Opak

Fault.

Southeast Northwest


Figure 4. Resistivity profile in Srimartani line. Black line estimated as an Opak Fault line.

Resistivity length this line about 120 m and the spacing between the electrodes is 5 m.


value in this line range from 0,59 to 15,5 ohm meter. Low resistivity values are shown by

blue colour, while the high resistivity values by a dark red colour. Circular high resistivity

value in this line probable as a volcanic boulder. Differences high and low resistivity values seen in the middle this line, it is estimated as a fault line with dipping plane to the southeast.

This fault is not Opak main fault.

Gunung Bangkel Line

Resistivity line of Gunung Bangkel located in Klenggotan sub village, Sri Mulyo village,

Piyungan district, Bantul regency. Direction of resistivity line is east – west. Resistivity line

lies to the west of the mountain Bangkel and as a mountain front zone. During resistivity

measurement, the weather is cloudy, then rain before finish the work. There were many

houses damaged and several were collapse during the earthquake in 2006.

Southeast Northwest


Figure 5. Map of resistivity line measurement of Gunung Bangkel and Gunung Candi Abang

(blue line). Red line and dashed line as an Opak Fault.

Resistivity length of this line about 200 m and the spacing between the electrodes is 5 m.


value of Gunung Bangkel line range from 2 to 50 ohm meter. Low resistivity values are

shown by blue colour, while the high resistivity values by a dark red colour. High resistivity

values are present in the center of the resistivity line which interpreted as volcanic lava rock

with depth about 25 m. Differences high and low resistivity values seen in the eastern part of

this line, and in the field form of transition zone of Gunung Bangkel and plains (mountain

front zone). It is estimated as Opak Fault line with dipping plane to the west.


Figure 6. Resistivity profile in Gunung Bangkel line. Black line estimated as an Opak Fault

line.

Gunung Candi Abang Line

Resistivity line of Gunung Candi Abang located in Blambangan sub village, Jogotirto village,

Berbah district, Sleman regency. Direction of resistivity line is east – west. Resistivity line

lies to the west of the Gunung Candi Abang and as a mountain front zone. During resistivity

measurement, the weather is good. There were many houses damaged and several were

collapse during the earthquake in 2006.

Figure 7. Resistivity profile in Gunung Candi Abang line. Black line estimated as an Opak

Fault line.

Resistivity length of this line about 490 m and the spacing between the electrodes is 10 m.

Regarding this condition, the depth resistivity values on this line about 100 meters that

located in the center of this line. In the east of this line as a Gunung Candi Abang, and in the

middle to west as a paddy field. Resistivity value of Gunung Candi Abang line range from 2

to 60 ohm meter. Low resistivity values are shown by blue colour, while the high resistivity

East West

East West


values by a dark red colour. High resistivity in the surface as a paddy field. Medium

resistivity values are at the center of this line which interpreted as volcanic lava rock with

depth about 70 m. In the east this line, the resistivity value under surface is low. This

condition estimated as saturated water zone. Differences high and low resistivity values seen

in the east part of this line. It is estimated as Opak Fault line with dipping plane to the west.

The four profile resistivity lines showed contrast resistivity is interpreted as a line of Opak

Fault. There was saturated water zone in the east part of Gunung Candi Abang line and

potential for high liquefaction. Opak fault showed in this figure below.

Figure 8. Distribution of Opak Fault and Dengkeng Fault based on morphotectonic analysis

and support by resistivity data. Base map is SRTM with resolution of 30 m.

Conclusions

Morphotectonic analysis and support by resistivity data can be identify distribution of Opak Fault

Morphotectonic features of Opak Fault zone forming the mountains front, lineament of

the hills and valleys with dominantly trending of SW - NE.

Based on resistivity data showed the value resistivity difference that indicated as Opak Fault.


References

1) Bull,W.B., 2007, Tectonic Geomorhology of Mountain : a New Approach for

Paleoseimology, Blackwell Publishing: 316 pp.

2) Huzita, K., Imaizumi, T., Kaizuka, S., Matsuda, T., Nakada, T., Okada, A., Ota, Y., Utsu,

T., Yonekura, N., dan Yoshii, K., 1992, Maps of Active Faults in Japan, University of

Tokyo Press: 73 pp.

3) Keller, E.A. dan Pinter, N., 1996, Active Tectonic Earthquake, Uplift and Landscape,

Prentice hall, Upper Saddle River, New Jersey 07458: 338 pp.

4) Keller, E.A. dan Pinter, N., 2002, Active Tectonic Earthquake, Uplift and Landscape,

Prentice hall, Upper Saddle River, New Jersey 07458: 362 pp.

5) Supartoyo, 2006, Gempabumi Yogyakarta Tanggal 27 Mei 2006, Bulletin Berkala Merapi

volume 3 nomor 2, Edisi Agustus 2006, BPPTK, 36 - 55.

6) Supartoyo, Surono dan Putranto, E.T., 2014, Katalog Gempabumi Merusak di Indonesia

Tahun 1629 – 2014 (edisi kelima), Pusat Vulkanologi dan Mitigasi Bencana Geologi,

Bandung.

7) Wartono Rahardjo, Sukendarrumidi dan H.M.D. Rosidi, 1995, Peta Geologi Lembar

Yogyakarta, Pusat Penelitian dan Pengembangan Geologi, Bandung.

Acknowledgements

The authors thank to Imam Priambodo, Ilham, Wieke Pratiwi and Deden Djunaedi who

helping during the resistivity measurement in field.

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