9th International Workshop on Statistical Seismology, 14 -18 June, Potsdam, Germany

Spatial Analysis of Earthquake Frequency-Magnitude Distribution at Geothermal Region in the South of Bandung, West Java, Indonesia

Dicky Ahmad Zaky(1), Andri Dian Nugraha(1), Rachmat Sule(1), Philippe Jousset(2)

(1)Faculty of Mining and Petroleum Engineering, Institute Technology of Bandung, Indonesia.

(2)Helmholtz Centre Potsdam GFZ German Research Center for Geoscience, Germany.

d.ahmad.zaky@gmail.com

High geothermal potential lies on the Quaternary volcanic complex in the South of Bandung. A seismicnetwork with 48 stations in total was setup at these geothermal regions. Wayang Windu and Darajat aretwo exploited geothermal field that are located within and near the network. The production activities andhydrothermal mechanism in the reservoir could trigger earthquakes. We determined the magnitude ofearthquakes. Furthermore, we did frequency-magnitude distribution (FMD) analysis to know the spatialvariation of b-value in this geothermal settings. We observed earthquakes located in 3 different cluster andrelate those with the geothermal exploited system, we expected that the results could give moreinformation about the geothermal field in the different aspect.

Figure 4. Compilation map, b-value spatial variation over gray-shaded relieftopographic map. Geological structures and thermal manifestations datafrom Alzwar et al. (1992).

As seen at Figure 2, earthquakes are dominantly distributed between 0 to 5 km depth. This suggest thatearthquakes are from the geothermal reservoir. The 𝑀𝐿 calculation results show that the values are in therange of 0.1 to 2.9. We performed an analysis of earthquake frequency-magnitude distribution (FMD) forthe whole data set using maximum-likelihood method (Figure 3). High b-values ~1.1+/- 0.09 and ~1.02 +/-0.07 are associated with the Darajat and Wayang Windu seismic clusters respectively. Lower b-values(~0.838 +/- 0.08) are associated with Mt. Kencana cluster. We used 0.5 x 0.5 km2 square grid to develop b-value map. Each grid will compute b-value using earthquakes within 3 km radius, with minimum number ofearthquakes that have magnitude more than Mc is 20. Spatial analysis of FMD (Figure 4) also reveal thatDarajat and Wayang Windu are relatively have higher b-value than Kencana cluster.

We interpret the low b-values as a result of higher stress condition due to tectonic stresses, whereas higherb-values observed in Darajat and Wayang Windu clusters are interpreted as resulting from lower stresscondition. Hot fluids in the geothermal production areas could be another factor for increasing b-value.Those results also suitable with Wyss (1973) study that suggest there are correlation between high porepressure with high b-value, also Warren and Latham (1970) experiment that found high temperature couldincreasing b-value.

Figure 1. Example of typical earthquakes waveform on the researcharea. Waveform after instrumental response correction (top),ground displacement (mid), result from integration calculation.Result from simulation Wood-Anderson instrument (bottom).

We gratefully acknowledge The Geophysical Instrumental Pool of Potsdam provided the instrument. We ThankMakky Jaya, Kemal Erbas for discussion and their support in the deployment of the stations. This project wasfunded by Federal Ministery of Education and Research of Germany. We thank to Near Surface GeophysicsLaboratory, Faculty of Petroleum and Mining Engineering ITB for supporting this research. DAZ would like tothank USAID – Star Energy for supporting his master degree study, including this research. Thank to Hasbi AshShiddiqi for permit using his code to calculate the magnitude of earthquakes, also thank to Bilqis, Fadli, and Nevifor their cooperation in picking arrival time. Topographic map was taken from SRTM data V4 (Jarvis et al., 2008)

During 8 months of recording, from October 2013 to August 2014 (Except December 2013, January andMay 2014), we manually identified nearly 600 local earthquakes with 4752 P-wave and 4589 S-wave arrivaltime phases. The local earthquake usually has less than 4 second difference between S and P arrival time,the earthquake signal duration is around 10 second.

Figure 3. Frequency-Magnitude Distribution (FMD) curve for all data set and each seismic cluster.

Joint hypocenter and 1D velocityinversion are performed to get highprecision location using VELEST(Kissling, 1988). Local magnitude(𝑀𝐿 ) was determined by generalinverse calculation (Pujol, 2003),involving simultaneous determi-nation of magnitude, stationscorrection and two constantexpress variation of amplitude asfunction of distance.

Instrumental response correction,integration to ground displa-cement, and simulation to Wood-Anderson instrument are routinessignal processing (Figure 2) thathave to conducted prior to pickingthe maximum amplitude ofearthquake signal.

The empirical equation by Gutenberg and Richter (1944), 𝐥𝐨𝐠𝟏𝟎𝑵 = 𝒂 − 𝒃𝑴, is the principle equation todo FMD analysis. 𝑵 is the cumulative number of events with magnitude more than 𝑴. Constant b or b-value is often associated with stress condition. We utilized MapSeis (Eberhard, 2014) to analyze the spatialb-value variation at research area.

Figure 2. Earthquakes distributions of research area over gray-shaded relief topographic map. Projectedearthquakes location of N-S and W-E cross section are laid on the left and bottom side of topographic maprespectively. Result of 1D Velocity inversion is shown on the bottom right corner. Epicenters are shown asfilled dots, with different color and size to distinguish the magnitudes. Mountain and station are shown asblack triangles and blue reverse triangles respectively. Three seismic clusters ([1] Wayang Windu, [2] Darajat,and [3] Kencana cluster ) are shown by numbered green rectangles.

-2,5

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(km

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Velocity (km/s)

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Result from Joint hypocenter and 1D velocity inversion shows three seismic clusters corresponding withtwo exploited geothermal fields (Wayang Windu and Darajat) and newly discovered clusters in Mt.Kencana. We eliminated earthquakes that have RMS residuals more than 0.3 second, number ofearthquakes were reduced to 501. Earthquake distribution and 1D velocity model are shown by Figure 2. • Results from hypocenters

determination indicate that theearthquakes are mostly fromgeothermal reservoir except inthe Mt. Kencana.

• The Local Magnitude deter-mination reveals earthquakesthat occurred are less thanmagnitude 3.

• Low b-value anomaly at Mt.Kencana are interpreted asresults from higher stresscondition due to tectonicstresses, while high b-value atWayang Windu and Darajat areinterpreted as results fromlower stress condition.

• Hot fluids presence at bothgeothermal field, WayangWindu and Darajat, aresuspected as another factor toincrease b-value. Since, hotfluid could lead on increasingpore pressure and tempera-ture. In contrast, there are nothermal manifestation at Mt.Kencna.

INTRODUCTION

DATA AND METHODS

RESULTS AND DISCUSSION

CONCLUSIONS

ACKNOWLEDGMENTps