Investigation of Ground Motion and Local Site Characteristics of the 2018 Lombok Earthquake Sequence

In 2018, Lombok Island was hit by a major earthquake sequence. The Indonesia Meteorological, Climatological, and Geophysics Agency (BMKG) reported that the Lombok Island earthquake sequence started with an M_w 6.4 foreshock, followed by an M_w 6.8 main shock, aftershocks of M_w 5.8 and Mw 6.2, and a second mainshock of M_w 6.9 in the eastern part of Lombok. This study presents an investigation of strong motion characteristics using the Indonesia National Strong Motion Network (INSMN) data from two accelerometer stations, the MASE station (at Praya Lombok International Airport, Lombok Island, V_s30 = 770 m/s, SB site class) and TWSI station (in Sumbawa Island, V_s30 = 1152 m/s, SB site class). Signal analysis techniques using a power spectrum via fast Fourier transform, wavelet transform and horizontal-to-vertical spectral ratio (HVSR) have been applied in this study. There are significant differences in the results (e.g., predominant frequencies, wavelets, H/V ratios, and frequencies at peak H/V ratio) for the MASE and TWSI stations, highlighting the importance of actual V_s30 profiles and the limitation of the site class system in providing necessary predictive information. The variation of the peak ground acceleration (PGA) values and the spectral amplitudes could only be explained by hypothesizing the effect of the volcanic structure of Mount Rinjani on the strong motion waveforms.

Fourier transform; HVSR; Strong motion; Vs30; Wavelet

The historical seismicity in Lombok shows that Lombok Island is an area of active seismicity. Historical destructive events with M >5 have occurred, e.g., May 30, 1979, M 6.1; January 2, 2004, M 6.2; June 22, 2013, M 5.4; and June 9, 2016, M 6.2. According to the Indonesia Meteorological, Climatological, and Geophysics Agency (BMKG) earthquake database, on July 1, 2018, the Lombok region was hit by an earthquake with a magnitude of 4.5. Then, on July 29 at 05:47:39 local time, Lombok Island was hit by an M_w 6.4 earthquake at a depth of 13 km. This was a destructive earthquake that caused many buildings to collapse and was felt by almost everyone on Lombok Island, significantly impacting the western Lombok area.  The source mechanism identified from the moment tensor indicated a thrust fault mechanism.

BMKG earlier believed that this earthquake was a mainshock, but then on August 5, 2018, an M_w 6.8 earthquake struck Lombok Island. It was followed by a sequence of earthquakes with magnitudes of M_w 5.8 and M_w 6.2, as well as an M_w 6.9 mainshock in the eastern part of Lombok on August 19, 2018. It is noted that these magnitudes have been recalculated and are somewhat different from those stated in earlier publications (e.g., Supendi et al. 2020).

Figure 1 shows the distribution of relocated earthquake epicenters and depths, including the five largest events in the Lombok earthquake sequence. The distribution of aftershocks reflects the back thrust fault model in northern Lombok Island. The aftershocks had a similar mechanism to the first and second mainshocks, as indicated by the model of focal mechanisms in Figure 1. The depths of the earthquakes were consistently shallow.

Figure 1 Seismicity of mainshocks and aftershocks of the Lombok earthquake sequence

This study presents an investigation into the characteristics of the strong ground motions of the Lombok earthquake sequence, with mainshocks of M_w 6.8 and M_w 6.9. These characteristics could be related to the potential damage to the buildings and other critical infrastructure on Lombok Island. The aim of the study is to examine the effects of local site characteristics on the strong motions of the Lombok earthquake sequence. The approach taken examines the strong motions recorded at BMKG stations with 1) the same site class and 2) similar distances from the sources. The site class considered is SB with an averaged shear-wave velocity to 30 m depth (V_s30) of 750 m/s to 1500 m/s (e.g., BSN 2019). The parameters considered include peak ground accelerations, energy amplitudes, wavelets, and predominant frequencies. The differences observed would lead to the hypothesis of the effect of Mount Rinjani on strong motions; the mountain is located south of the hypocenters. In this study, two stations were used to examine detailed characteristics of the ground motions. One station is the MASE station located in Lombok International Airport, while the other station is the TWSI station in Taliwang in the western part of Sumbawa Island, east of Lombok Island; the locations of these stations are shown in Figure 1.

1.1. Geological Setting

A geological map from the Center of Geological Survey of Indonesia reveals that the area around Mount Rinjani is within the Quaternary Period (Q_v), and that the other parts of Lombok Island are predominantly within the Tertiary Period (TI). The periods of these locations are shown in Figure 2a.

The frequent earthquakes in Indonesia are related to the convergence of the Pacific, Indo-Australian, and Eurasian plates. The Lombok area is active, as revealed by many seismotectonic studies. Volcano activities have also been frequently observed. Tectonically, Lombok Island is located in the inner arc of the Nusa Tenggara islands, which was formed by the Indo-Australian plate subducting under the Eurasian slab. This subduction is nearly arc–normal, with a dip angle between 60-70^o and a crust thickness of about 20 km (Curray et al. 1977). The Wadati-Benioff zone extends to a depth of about 164 km (Rachmat et al. 2016).

Seismotectonically, Lombok Island is surrounded by four earthquake sources. South of Lombok Island, there is a subduction zone related with subduction activity from west of Sumatera and south of Java and Bali Island to the Timor Trough. In the northern part of Lombok Island, a back arc thrust source mechanism has been identified, the Bali Basin back arc thrust, and the Flores Thrust running from west to east. In the western and eastern parts of Lombok Island, faults (Lombok and Sumbawa faults) have also been identified (PUSGEN, 2017). According to the newly updated Indonesia Hazard Map, the maximum size of an earthquake on the back arc thrust is M_w 7.4.

Figure 2 (a) Geological map of Lombok Island; (b) Schematic seismic faults for Lombok (modified after PUSGEN, 2017); (c) V_s30 profiles of the MASE and TWSI stations

1.2. V_s30-Based Site Classification

We conducted a local site investigation using the HVSR method with single microtremor measurements to obtain the predominant periods and the MASW method to obtain V_s30 values for the MASE accelerometer station at Lombok International Airport and the TWSI accelerometer station in Taliwang, Sumbawa Island. Figure 2c shows the profiles of V_s (shear wave velocity) with depth. The weighted average of V_s over the top 30 meters (V_s30) is 770 m/s for the MASE station (Part of the MASW survey was reported in Krisnanto et al. (2018) and 1151 m/s for the TWSI station. Even though the actual V_s profiles are different, both were classified as site class SB (BSN, 2019). Therefore, other parameters are needed to explain the actual different responses when an earthquake occurs.  

We examined the strong motions of the Lombok earthquake sequence by considering two stations located in sites of the same site class. The stations have V_s30 values of 770 m/s (MASE) and 1151 m/s (TWSI). Five earthquakes in the sequence were considered. We performed a fast Fourier transform analysis, wavelet analysis, and HVSR analysis. In general, the predominant frequency of strong motions from the MASE station are lower than those from the TWSI station. For the MASE station, the maximum energy of horizontal signals typically occurred at higher frequencies compared to that of the vertical signals, and the time trend for the maximum energy of the horizontal and vertical signals is similar for all earthquakes. For the TWSI station, the maximum wavelet energy of the horizontal signals occurred at lower frequencies compared to that of the vertical signals, and the maximum wavelet energy of the horizontal signals tended to occur at earlier or similar times as that of the vertical signals. Moreover, the H/V ratio for the MASE station tended to be lower than the ratio for the TWSI station. It is highlighted therefore that differences in measured V_s30 can cause different earthquake strong motions and that the site class system might not provide all necessary predictive information. The variation of the peak ground acceleration (PGA) values and the spectral amplitudes could only be partially explained by the theoretical wave geometrical damping. This suggests that there is a major waveform modifier along the paths from the epicenter to both stations; it is hypothesized that the modifier is the volcanic structure of Mount Rinjani, a major geologic feature of Lombok Island.

We would like to gratefully thank the support for this study provided by the Indonesian Agency for Meteorological, Climatological and Geophysics (BMKG), as well as a Kemenristekdikti PTUPT Research Grant (Universitas Indonesia Contract No. 120/SP2H/PTNBH/DRPM/2018).

Badan Standarisasi Nasional (BSN), 2019. SNI 1726:2019. Tata Cara Perencanaan Ketahanan Gempa Untuk Bangunan Gedung dan Non Gedung, (Earthquake Resistant Design Codes for Buildings and Non-buildings), Jakarta

Bard, P.Y., 1999. Microtremors Measurements: A Tool for Site Effect Estimation? In: Proceedings the Effects of Surface Geology on Seismic Motion, Volume 3. Rotterdam, Balkema, pp. 1251–1279

Bonnefoy-Claudet, S., Cornou, C., Bard, P.Y., Cotton, F., Moczo, P., Kristek, J., Fäh, D., 2006. H/V Ratio: A Tool for Site Effects Evaluation. Results From 1-D Noise Simulations. Geophysical Journal International, Volume 167(2), pp. 827–837

Bonnefoy-Claudet, S., Baize, S., Bonilla, L.F., Berge-Thierry, C., Pasten, C., Campos, J., Volant, P., Verdugo, R., 2009. Site Effect Evaluation in the Basin of Santiago de Chile using Ambient Noise Measurements. Geophysical Journal International, Volume 176(3), pp. 925–937

Borcherdt, R.D., 2015. NGA-West2 GMPE Average Site Coefficients for Use in Earthquake-Resistant Design, USGS Open-File Report, pp. 2015–1116

Curray, J.R., Shor, G.G., Raitt, R.W., Henry, M., 1977. Seismic Refraction and Reflection Studies of Crustal Structure of the Eastern Sunda and Western Banda Arcs. Journal of Geophysics Research, Volume 82(17), pp. 2479–2489

Ghofrani, H., Atkinson, G.M., 2011. Forearc versus Backarc Attenuation of Earthquake Ground Motion. Bulletin of the Seismological Society of America, Volume 101(6), pp. 3032–3045

Ji, K., Ren, Y., Wen, R., 2017. Site Classification for National Strong Motion Observation Network System (NSMONS) Stations in China using an Empirical H/V Spectral Ratio Method. Journal of Asian Earth Sciences, Volume 147, pp. 79–94

Konno, K., Ohmachi, T., 1998. Ground-motion Characteristics Estimated from Spectral Ratio between Horizontal and Vertical Components of Microtremor. Bulletin of the Seismological Society of America, Volume 88(1), pp. 228–241

Krisnanto, S., Irsyam, M, Asrurifak, M., Alatas, M.I., Adi, A.D., Darjanto, H., Prakoso, W.A., Pramono, S., 2018. Aspek Geoteknik dan Seismic Hazard. Kajian Rangkaian Gempa Lombok, Provinsi Nusa Tenggara Barat, Indonesia (Geotechnical Aspects and Seismic Hazard. Study of the Series of Earthquakes in Lombok, West Nusa Tenggara Province, Indonesia). Research Institute for Housing and Human Settlements, Ministry of Public Works and Housing, Republic of Indonesia

Mase, L.Z., 2018, Reliability Study of Spectral Acceleration Designs Against Earthquakes in Bengkulu City, Indonesia, 2018.  International Journal of Technology, Volume 9(5), pp. 910–924

National Center for Earthquake Studies of Indonesia (PUSGEN), 2017. Seismic Sources and Hazard Maps for Indonesia. Research Institute for Housing and Human Settlements, Ministry of Public Works and Housing, Republic of Indonesia

Prakoso, W.A., Rahayu, A., Sadisun, I.A., Muntohar, A.S., Muzli, M., Rudyanto, A., 2017. Comparing Shear-wave Velocity Determined by MASW with Borehole Measurement at Merapi Sediment in Yogyakarta. International Journal of Technology, Volume 8(6), pp. 993–1000

Pramono, S., Prakoso, W.A., Cummins, P., Rahayu, A., Rudyanto, A., Syukur, F., Sofian, 2017.  Investigation of Subsurface Characteristics by using a Vs30 Parameter and a Combination of the HVSR and SPAC Methods for Microtremor Arrays. International Journal of Technology, Volume 8(6), pp. 983–992

Rachmat, H., Rosana, M.F., Wirakusumah, A.D., Jabbar, G.A., 2016. Petrogenesis  of  Rinjani  Post-1257-Caldera-Forming-Eruption  Lava  Flows. Indonesian Journal on Geoscience, Volume 3(2), pp. 107–126

Seyhan, E., Steward, J.P., 2014. Semi-empirical Nonlinear Site Amplification from NGA-West2 Data and Simulations. Earthquake Spectra, Volume 30(3), pp. 1241–1256

Supendi, P., Nugraha, A.D., Widiyantoro, S., Pesicek, J.D., Thurber, C.H., Abdullah, C.I., Daryono, D., Wiyono, S.H., Shiddiqi, H.A., Rosalia, S., 2020. Relocated Aftershocks and Background Seismicity in Eastern Indonesia Shed Light on the 2018 Lombok and Palu Earthquake Sequences. Geophysical Journal International, Volume 221(3), pp. 1845–1855

Woolery, E.W., Street, R., 2002. 3D Near-surface Soil Response From H/V Ambient-noise Ratios. Soil Dynamics and Earthquake Engineering, Volume 22(9-12), pp. 865–876