The island of Lombok in Indonesia is located between the Indo-Australian and Eurasian subduction trenches and the Flores back-arc thrust, making it vulnerable to earthquakes. On 29 July 2018, a significant earthquake Mw 6.4 shook this region and was followed by series of major earthquakes (Mw>5.8) on 5, 9, and 19 August, which led to severe damage in the northern Lombok area. In this study, we attempt to reveal the possible cause of the sequences of the 2018 Lombok earthquakes based on aftershock monitoring data. Twenty stations were deployed to record earthquake waveform data from 4 August to 9 September 2018. In total, 3259 events were identified using 28,728 P- and 20,713 S-wave arrival times during the monitoring. The aftershock hypocenters were determined using a nonlinear approach and relocated using double-difference method. The moment magnitude (Mw) of each event was determined by fitting the displacement spectrum amplitude using a Brune-type model. The magnitudes of the aftershocks range from Mw 1.7 to 6.7. The seismicity pattern reveals three clusters located in the Flores oceanic crust, which fit well with the occurrences of the four events with Mw>6. We interpret these events as the main rupture area of the 2018 Lombok earthquake sequence. Furthermore, an aseismic zone in the vicinity of Rinjani extending toward the northwestern part of Lombok was observed. We propose that the crust in this area has elevated temperatures and is highly fractured thus inhibiting the generation of large earthquakes. The aseismic nature is therefore an artifact of the detection threshold of our network (Mw 4.6).

We have successfully conducted the first ambient noise tomography on the island of Lombok, Indonesia using local waveform data observed at 20 temporary stations. Ambient noise tomography was used to delineate the seismic velocity structure in the upper crust. The waveform data were recorded from August 3rd to September 9th, 2018, using short-period and broadband sensors. There are 185 Rayleigh waves retrieved from cross-correlating the vertical components of the seismograms. We used frequency-time analysis (FTAN) to acquire the interstation group velocity from the dispersion curves. Group velocity was obtained for the period range of 1 s to 6 s. The group velocity maps were generated using the subspace inversion method and Fast Marching Method (FMM) to trace ray-paths of the surface waves through a heterogeneous medium. To extract the shear wave velocity (Vs) from the Rayleigh wave group velocity maps, we utilize the Neighborhood Algorithm (NA) method. The 2-D tomographic maps provide good resolution in the center and eastern parts of Lombok. The tomograms show prominent features with a low shear velocity that appears up to 4 km depth beneath Rinjani Volcano, Northern Lombok, and Eastern Lombok. We suggest these low velocity anomalies are associated with Quaternary volcanic products, including the Holocene pyroclastic deposits of Samalas Volcano (the ancient Rinjani Volcano) which erupted in 1257. The northeast of Rinjani Volcano is characterized by higher Vs, and we suggest this may be due to the presence of igneous intrusive rock at depth.

Because of its robustness and practicality, the Horizontal-to-Vertical Spectral Ratio (HVSR) method has been widely used to obtain subsurface structure, mainly the sediment thickness that resides over bedrock. The method uses Fourier Transform to obtain frequency spectrum and calculate the H/V ratio. However, the conventional Fourier Transform method has some limitations; e.g., the inability to remove local noises that are very common in microtremor recordings. In this study, we investigate the application of the Hilbert-Huang Transform (HHT), in obtaining the HVSR curve, and compare it to the conventional method in terms of its effectiveness in removing local noise through the inversion results of HVSR curves. Such a comparison has never been conducted before. We used data from a microtremor survey in Mataram City, Lombok Island, Indonesia which experienced a series of destructive earthquakes in 2018. The results show that the S-wave velocity structure derived from the inversion process of the HHT-HVSR curves is in better agreement with the previous study of Mataram City than the conventional method. Furthermore, the resulting S-wave velocity structure is also interpreted based on geological reference, giving new insights into the subsurface structure beneath Mataram City.

In 2018, Lombok Island was hit by a series of destructive earthquakes. According to Indonesian Meteo- rological, Climatological, and Geophysical Agency data, about 1,973 felt earthquakes (M > 3) which shaken Lombok were recorded during August 2018 with three earthquakes with the largest magnitude of 6.9 Mw, 6.8 Mw, and 6.2 Mw. National Board for Disaster Management reported about 555 deaths, 1,833 people injured, and 186,010 houses damaged as a result of Lombok earthquake on August 5th, 2018. A number of seismometers were placed on Lombok Island from August 3rd, 2018 to October 19th, 2018 to monitor the aftershock events. There are 17 stations that record seismicity in Lombok consisting of 10 broadband sensors and 7 short period sensors. In this study, we used the Horizontal-to-Vertical Spectral Ratio (HVSR) method to analyze the risk of earthquake in the Lombok region. The basic concept of this method is to do a comparison between the horizontal component spectrum and the vertical component spectrum of a wave, where theoretically the particle movement of the horizontal component is greater than the particle movement of the vertical component on soft ground, whereas on the hard ground both components (horizontal and vertical) will be similar. H/V curve obtained from earthquake record- ings (Earthquake Horizontal-to-Vertical Ratio/EHVR) and H/V curve obtained from microtremor recordings (Microtremor Horizontal-to-Vertical Ratio/MHVR) shows good agreement and high correlation. Empirical correction of EMR (Earthquake-to-Microtremor Ratio) managed to decrease the difference of estimation of predominant frequency and amplification factor between EHVR and MHVR. Predominant frequency, amplification, and seismic vulnerability map agree with the geological condition of Lombok Island, where high value of amplification and seismic vulnerability was found on soft and thick ground. This study conclude that the maximum ground acceleration and the construction of the building should also considered when one wants to investigate the effect of an earthquake to the damage occurred, beside the site effects.

Local earthquake data was used to determine a three-dimensional (3D) seismic attenuation structure around the aftershock source region of the 2018 Lombok earthquake in Indonesia. The aftershocks were recorded by 13 seismic stations from August 4 to September 9, 2018. The selected data consist of 6,281 P-wave t∗ values from 914 events, which had good t∗ quality in at least four stations. Our results show that the two aftershock clusters northwest and northeast of Lombok Island have different attenuation characteristics. A low P-wave quality factor (low-Qp), low P-wave velocity (Vp), and high ratio of P-wave velocity and S-wave velocity (Vp/Vs), which coincide with a shallower earthquake (<20 km) northwest of Lombok Island, might be associated with a brittle area of basal and imbricated faults influenced by high fluid content. At the same time, the high-Qp, low Vp, and low Vp/Vs, which coincide with a deeper earthquake (>20 km) northeast of Lombok Island, might be associated with an area that lacks fluid content. The difference in fluid content between the northwest and northeast regions might be the cause of the early generation of aftershocks in the northwest area. The significant earthquake that happened on August 5, 2018, took place in a region with moderate Qp, close to the contrast of high and low-Qp and high Vp, which suggests that the earthquake started in a strong material before triggering the shallower aftershocks occurring in an area affected by fluid content. We also identified an old intrusive body on the northeast flank of the Rinjani volcano, which was characterized by a high-Qp, high-velocity, and a high Bouguer anomaly.