The seismic hazard from the Lembang Fault, Indonesia, derived from InSAR and GNSS data
Abstract. A growing number of large cities are located near poorly understood faults that have not generated a significant earthquake in recent history. The Lembang Fault is one such fault located near the city of Bandung in West Java, Indonesia. The slip rate on this fault is debated with estimates ranging from 6 mm/yr to 1.95–3.45 mm/yr, derived from GNSS campaign and geological measurements respectively. In this paper we measure the surface deformation across the Bandung region and resolve the slip rate across the Lembang Fault using radar interferometry (InSAR) analysis of 6 years of Sentinel-1 satellite data and continuous GNSS measurements across the fault. Our slip rate estimate for the fault is 4.7 mm/yr, with the shallow portions of the fault creeping at 2.2 mm/yr. Previous studies have estimated the return period of large earthquakes on the fault to be between 170–670 years. Assuming simplified fault geometries and a reasonable estimate of the seismogenic depth we derive an estimated moment deficit of a magnitude 6.8–7.2 earthquakes; indicating that the fault poses a very real risk to the local population. Using the Global Earthquake Model OpenQuake-engine we calculate ground motions for these two earthquake scenarios and estimate that 2.5–3.3 million people within the Bandung Metropolitan region would be exposed to ground shaking greater than 0.3 g. This study highlights the importance of identifying active faults and understanding their past activity, and measuring the current strain rates of smaller crustal active faults located near large cities in seismic zones.
Ekbal Hussain et al.
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2022-1472', Anonymous Referee #1, 20 Apr 2023
- RC2: 'Comment on egusphere-2022-1472', Anonymous Referee #2, 30 May 2023
Ekbal Hussain et al.
Ekbal Hussain et al.
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This study investigates the slip rate of the Lembang fault, Indonesia, from Sentinel-1 images and GNSS measurements. The authors argue potential seismic hazards around the fault based on the estimated slip rate and previous recurrence of large earthquakes.
Because investigating seismic hazards posed by the Lembang fault is not only interesting from a scientific point of view but also important from a practical point of view, this study merits publication. However, the manuscript improves by addressing the following points.
1. The authors' definition of locking (d1) and creep depths (d2) needs to be clarified. The equation below Line 100 indicates that the fault creeps from the surface to the d2 with a rate C, locks between d2 and d1, and slips below d1 with a rate S. If that is the case, d1 must be greater than d2. However, Figure 6 indicates that d2 can be greater than d1. To resolve my confusion, the authors must clearly define d1 and d2 with a figure, if necessary.
2. Figure 5 shows no discontinuities in the observed velocity field, indicating that surface creep is unlikely. Nonetheless, the authors assume the surface creeps on the fault. Is there any evidence for surface creeps from surface measurements, for example?
3. The Lembang fault dips to the north by 75 degrees (Line 24), but the modeling assumes a vertical fault (Line 100). Does this discrepancy affect the modeling much?
4. The authors should say something about the subsidence of >50 mm/yr to the south of the Lambang fault because it is more visible than the displacement by faulting.
5. The scenario seismic hazard delineated in Section 5 depends on the obtained fault-slip parameters, which have a fair amount of uncertainties. Then how does this scenario change with different fault-slip parameters within uncertainty bounds?
6. I cannot understand how to look at Fig. 6. I understand that the black part at the center of a contour is where the probability is high. Then, how about the black part at the edge of and outside of the contour?
7. Related to the above comment, does Fig. 6 show that there are trade-offs between slip rate and creep depth, for example?
8. Line 103: What is emcee?