Preliminary Results from a Field Experiment on Volcanic Tremor at

Ijen Volcano Using an Array of Digital Seismographs

Arin Wildani

1,2,* a

, Sukir Maryanto

, Didik R. Santoso

and Hetty Triastuty

Department of Physics, Brawijaya University, Veteran, Malang, Indonesia

Physic Education Study Program, Universitas Islam Madura, Bettet, Pamekasan, Indonesia

Center for Volcanology and Geological Hazard Mitigation, Bandung, West Java, Indonesia

Keywords: Eartquakes, Volcanic Tremor, Ijen Volcano, Source Location.

Abstract: Spectral is a basic characteristic that needs to be understood from a volcanic event. This study aims to analyze

the spectral and direction of orientation of the source of the volcanic tremor of Ijen volcano. Volcanic tremors

are the volcanic events that usually appear before, during and after volcanic eruptions. In 2012 the Ijen volcano

showed abnormal conditions, namely the recording of many volcanic tremors with a dissertation on changes

in the condition of the lake. This study used secondary data, digital seismic recordings of the Ijen volcano in

January-February 2012 (the conditions at Ijen were not normal). Spectral analysis was performed using the

Fast Fourier Transform method. Analysis of the direction of source orientation is carried out using the

convariance matrix method. The volcanic tremor spectral of Ijen volcano has the characteristics of two

frequency peaks with one dominant frequency peak in the range of 1.3-1.5 Hz. The direction of the orientation

of the epicenter is towards the Ijen crater and the hypocenter is towards the bottom of the Ijen crater, ranging

from a depth of 400-1000 meters. The most appropriate source model to explain the source process of volcanic

tremors of Ijen volcano is fluid fill crack.

1 INTRODUCTION

Indonesia is surrounded by 129 active volcanoes

spread throughout Indonesia. Volcanic eruption

disasters can disrupt the socio-economic life of

communities around volcanoes, and can even be life

threatening (Wildani, Maryanto, Gunawan, Triastuty,

& Hendrasto, 2013). Ijen volcano is an active volcano

in Indonesia, which is included in the Banyuwangi

Regency, East Java. It is one of the volcanoes that is

used as a tourist destination. The eruption can be

dangerous for tourists or residents around it

(Virgiawan, 2020). In 2012, the seismic activity of the

Ijen volcano experienced a significant change in

activity which made the authorities (PVMBG) raise

the status of the Ijen volcano to standby, namely level

3 (on a scale of 1 to 4). Unusual volcanic earthquake

activity and changes in the condition of the crater lake

https://orcid.org/0000-0002-1138-3890

https://orcid.org/0000-0002-1882-6818

https://orcid.org/0000-0003-3430-1744

https://orcid.org/0000-0002-8910-146X

were recorded at this time. Volcanic tremor activity

was recorded in large numbers and dominated the

seismic activity of Ijen Volcano at that time. Volcanic

tremor activity recorded at that time was followed by

changes in crater lake conditions such as the

appearance of gas bubbles and an increase in crater

lake temperature (Caudron et al., 2012). Even though

at that time there was no phreatic eruption,

monitoring and understanding (from a research

perspective) this volcanic tremor activity was

important to do. Understanding this can be done by

understanding everything related to this volcanic

tremor such as the nature of the signal, the location of

the source and the mechanism of the source.

The nature of a volcanic seismic signal (in this

case a volcanic tremor) can be identified from its

frequency characteristics. Determination of the

frequency of seismic signals has become one of the

main tools for investigating the nature of volcanic

146

Wildani, A., Maryanto, S., Santoso, D. and Tr iastuty, H.

Preliminary Results from a Field Experiment on Volcanic Tremor at Ijen Volcano Using an Array of Digital Seismographs.

DOI: 10.5220/0012115600003680

In Proceedings of the 4th International Conference on Advanced Engineering and Technology (ICATECH 2023), pages 146-151

ISBN: 978-989-758-663-7; ISSN: 2975-948X

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

seismic signals (K. I. Konstantinou & Schlindwein,

2003). In determining the frequency of volcanic

seismic tremor signals, special treatment is required

that is not the same as determining the frequency of

other volcanic seismic signals (Båth, 2012). This is

because several volcanic tremor conditions were

found which were not found in other seismic signals.

among others, Volcanic tremors are recorded over a

period of more than one minute or even up to several

days so that there is an accumulation of large amounts

of data. Second, there are temporal changes in

amplitude and frequency that must be considered

because they are related to source modeling and

predictions of future eruptions. Third, the resulting

spectrum may have some sharp peaks around narrow

frequency bands, in which case high resolution is

required to resolve individual frequencies. One

method to overcome this is the direct segment method

(Zobin, 2012). The long tremor time series is then cut

into small parts that do not overlap and the spectrum

is calculated for each part through the Fast Fourier

Transform (FFT). The final spectral estimate is the

average of all the individual spectra. This method has

been used in volcanic tremor recordings from Mt.

Etna, Italy (Nunnari, 2021; Sciotto, Cannata, Di

Grazia, & Montalto, 2022; Zuccarello, Burton,

Saccorotti, Bean, & Patanè, 2013) Hakone volcano,

Japan (Mannen et al., 2021; Yukutake et al., 2017;

Yukutake, Honda, Ukawa, & Kurita, 2022), Semeru

Volcano, Indonesia (K. Konstantinou, Perwita,

Maryanto, Budianto, & Hendrasto, 2013; Maryanto

& Mulyana, 2008). And this method is still being used

to analyze seismic signals of volcanic tremors from

various volcanoes in the world.

The characteristics of the seismic signal spectra

can also provide information on whether the signal is

caused by a site, path, or source effect. This can be

done by comparing the recorded spectra from many

stations or from one 3-component station. The

general method for examining possible site effects is

to compare the volcanic tremor spectra recorded at

the same station. Meanwhile, to examine the path

effect is more difficult than the site effect because it

requires knowledge of the geological structure in the

area around the volcano. but if the stations are spread

azimuth around the source it can be done by

comparing the volcanic tremor spectra at different

stations (Schlindwein, Wassermann, & Scherbaum,

1995).

A realistic visualization of vibrational sources

should not only account for the temporal evolution of

their properties, but should also extend spatially over

several volumes beneath the volcano, representing

possible channel geometries over a magma reservoir

or hydrothermal fluid circulation through a crack

system(K. I. Konstantinou & Schlindwein, 2003).

The methods discussed so far can determine the

origin of the tremor. One of the methods that can be

used for limited digital recording data is the

convariance matrix or polarization method. This

method can provide the orientation of the location of

the volcanic tremor source. The method has been

applied to the Tungurahua volcano, Ecuador (Zobin,

2012).

In this paper we investigate the site, path, and

source effect as well as the direction of the orientation

of the source of the Ijen Volcanic tremor during

unrest in 2012. We analyzed the spectral at 3 different

stations and at two 3-component stations. We also

analyze particle motion to determine the orientation

of the source of the Ijen volcano volcanic tremor. we

then combined the results of both analyzes to discuss

the possible source mechanisms of triggering

volcanic tremors.

Figure 1: Kawah Ijen seismic network (KWUI, IJEN,POSI

dan RAUNG : short-period sensors ; MLLR and TRWI

:Broadband seismometer), the top left corner of the map

shows the location of mountains in Indonesia, more

precisely in the province of East Java which is marked with

a white triangle.

2 METHODOLOGY

This study uses secondary data obtained from the

Center for Volcanology and Geological Hazard

Mitigation (CVGHM) Indonesia. The data used is

seismic data from the Ijen volcano from January-

February 2012. The data includes all records from six

stations around the Ijen volcano, namely IJEN,

KWUI, MLLR, POSI, RAUNG and TRWI stations.

Figure 1 shows the Ijen volcanic activity monitoring

network for all stations.

The data recorded by the six stations is a

collection of seismic signals in the form of digital

data. Data reading was carried out using Seisan

software. This software reads data for 10 minutes in

one reading window from several stations on Mount

Preliminary Results from a Field Experiment on Volcanic Tremor at Ijen Volcano Using an Array of Digital Seismographs

147

Ijen as shown in Figure 2. The sampling time is 100

sample per second. So, in the event of 1 second, as

many as 100 points are recorded in digital recording.

The signal recordings in Figure 2 are volcanic tremors

at IJEN, KWUI, MLLR, POSI, RUANG and TRWI

stations respectively.

The recording at POSI and RAUNG stations was

not good, so that the data selection only used four

stations, namely IJEN, KWUI, MLLR, and TRWI.

The volcanic tremor signal from the recording results

is selected or selected based on the clear waveform at

least at 3 seismic stations.

Figure 2: Record section of velocity waveforms of volcanic

tremor recorded at all Ijen volcano monitoring station that

occurred in 12.20-12.30 WIB on January 2, 2012 (from top

to bottom: IJEN, KWUI, MLLR, POSI, RAUNG

dan

TRWI).

Spectral is determined by applying the Fast

Fourier Transform (FFT) method. In this study using

the help of Geopsy software. Before implementing

FFT. data is filtered first using a band pass filter in the

range between 0.05 Hz to 25 Hz. Then, data sampling

is carried out for 1 minute on the signal with the

maximum amplitude and finally the application of

FFT to the sampled data.

The direction of the orientation of the epicenter of

the volcanic tremor in this study was carried out by

plotting the amplitudes of the North-South and West-

East (NS-EW) components of the seismic recording

of the volcanic tremor of Ijen volcano. Meanwhile,

the direction of the hypocenter orientation in this

study was carried out by plotting the amplitude of the

radial and vertical components.

3 RESULT AND DISCUSSION

Figure 2 shows the recordings of volcanic tremors

from all seismic stations scattered around the Ijen

volcano at 12.10-12.20 WIB on January 1 2012.

Volcanic tremors were very clearly recorded at Ijen,

TRWI and KWUI stations, whose station positions

are around the top of Mount Ijen. Volcanic tremors

were also recorded at stations far from the Ijen crater,

namely MLLR and RAUNG.

Figure 3 shows the spectra of volcanic tremor at

all seismic stations scattered around Ijen volcano. The

peak at 1.35 Hz in the spectrum was observed at all

stations. The exact same spectrum peaks were seen at

stations close to the Ijen crater, namely IJEN, TRWI

and KWUI stations, which means that the volcanic

tremor signal in this incident is a source effect.

Meanwhile, at stations far from the Ijen crater,

namely RAUNG, MLLR and POSI, they also show

peaks of 1.35 HZ but are not the dominant frequency,

this is thought to be due to the path and site effect.

The effect of path and site is also seen in the

component spectra (TRWI and MLLR). The 1.35 Hz

peak is still recorded in the 3-component data, but the

vertical component is not the maximum frequency

peak.

The spectral volcanic tremor of the Ijen volcano

shows that the volcanic tremor signal originates from

a source effect, so in the next section we will discuss

the most suitable source model to explain the

appearance of the volcanic tremor of the Ijen volcano.

Models of volcanic tremor sources have been

developed to explain the occurrence of volcanic

tremor from a volcano, among others 1) fluid flow

induced oscillations (Julian, 1994) which has been

used in several volcanoes, including Mt. Etna

(Godano, Cardaci, & Privitera, 1996), Stromboli

(Godano & Capuano, 1999), Vatnajokull (K. I.

Konstantinou, 2002). 2) Excitation of fluid fill crack

(Chouet, 1988, 2003) which has been used in Mt. aso

(Sassa, 1935) , 3) hydotermal boiling (Leet, 1988) dan

4) Resonance of large magma budies (Bame &

Fehler, 1986). All these models have their own

characteristics both in terms of seismogram

characteristics, spectral or geometric parameters. The

approach by comparing the results of analysis from

field data with the results of mathematical modeling

is the most likely way to discuss the source model that

best fits the Ijen volcanic tremor source model.

ICATECH 2023 - International Conference on Advanced Engineering and Technology

148

Figure 3: spectral of volcanic tremor of Ijen volcano which

occurred at 12.20-12.30 WIB on 2 January 2012 at all

stations (far left from top to bottom: IJEN, KWUI and

POSI; middle from top to bottom: MLLR vertical, NS and

EW components; most right from top to bottom: vertical

component TRWI, NS and EW).

The spectral characteristics of the volcanic tremor

of the Ijen volcano are one dominant frequency peak

with a low value, namely at 1.12-1.3 Hz and there is

another very small frequency peak with a value of

around 2.3 Hz. These spectral characteristics are

closer to or match the results from the fluid fill crack

modeling. So, in this study we used the fluid fill crack

model approach to explain the process of the source

of volcanic tremor at Ijen volcano. The fluid fill crack

model explains that volcanic tremors are caused by

resonances that occur in fluid driven cracks caused by

impulsive stress transients. Where the frequency

value depends on the crack stiffness value of the

crack. The greater the stiffness value of this crack, the

frequency will be lower (K. I. Konstantinou &

Schlindwein, 2003). This means that the low

frequency of volcanic tremor of Ijen volcano

indicates a large crack stiffness value. The crack

stiffness is proportional to the length of the crack and

inversely proportional to the thickness of the crack.

Meanwhile, the duration of the signal depends on the

viscosity of the fluid, the greater the viscosity value,

the longer the duration of the signal. The duration of

volcanic tremors was found to be less than days, but

a few minutes or hours, proving that the fluid that is

the source of volcanic tremors is not magma, but

steam or gas. This condition is supported by

monitoring at the Ijen crater that at the time of the

2012 crisis no magma was found flowing through the

crater.

Figure 4: Orientation direction of the epicenter of the

volcanic tremor of Ijen volcano (top) and orientation

direction of the hypocenter (bottom).

Figure 4 shows the orientation of the epicenter and

hypocenter of the Ijen volcano volcanic tremor using

the particle motion approach. Figure 5a Particle

motion is shown by a black line and the orientation

direction of the epicenter is shown by the blue dotted

line from the two 3-component data, namely from the

MLLR station and TRWI station. The green dotted

line indicates that the direction of the orientation of

the epicenter of the volcanic tremor is towards the

crater of Ijen volcano. Figure 5b shows the orientation

of the hypocenter which is also determined based on

the particle motion between the amplitude values of

the vertical and radial components. In Figure 5b, the

direction of the hypocenter orientation of the volcanic

tremor of Ijen volcano is obtained, which is under the

crater of Mount Ijen or around 400–1000 meters

below the Ijen volcano. at this depth it is stated that

there is a crack (Ayu & Jufriadi, 2014; Jufriadi,

Maryanto, Susilo, Purwanto, & Hendrasto, 2013) and

is thought to be a place for fluid to flow towards the

surface and to be the source of the volcanic tremors

of the Ijen volcano.

Preliminary Results from a Field Experiment on Volcanic Tremor at Ijen Volcano Using an Array of Digital Seismographs

149

4 CONCLUSIONS

The conclusion of this study is that the spectral

characteristics of the volcanic tremor of Ijen volcano

have two frequency peaks with one dominant

frequency peak in the range 1.3-1.5 Hz. Volcanic

tremor signals are caused by source effects except for

stations far from the peak which are affected by site

and path effects. The direction of the orientation of

the epicenter of the volcanic tremor of Ijen volcano

leads to the Ijen crater lake while the direction of the

orientation of the hypocenter leads to a depth of 400-

1000 meters below the Ijen crater. Based on the

spectral characteristics and direction of the source

orientation, the source model is suitable for

explaining the source process of the volcanic tremor

of the Ijen volcano, namely fluid fill crack.

ACKNOWLEDGEMENTS

We thank the Center for Volcanology and Geological

Hazard Mitigation (CVGHM) for allowing the use of

seismic data recorded on Ijen volcano.

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