This paper presents spatial distribution, temporal seismotectonics, and return period of earthquake in East Nusa Tenggara Province, Indonesia, using earthquake data and Maximum Likelihood methods. The data used are ISC, USGS/NEIC, and Indonesian Meteorology, Climatology and Geophysics Agency (IMCGA) earthquake catalog data for the period of 1918 to 2015. The results show that the a-value ranges from 5.0 to 8.5 and b-value ranges from 0.6 to 1.3. The pattern of spatial distribution of b-value is relatively low corresponding to the low of a-value, which means the high level of stress of rock in the area. The fractal dimension shows that the D value ranges from 1.384 to 1.874. The earthquake that occurred in East Nusa Tenggara Province was dominated by a small magnitude with great seismicity and the fastest return period is in Alor and Timor islands which is 44 days.
East Nusa Tenggara Province (ENTP) is one of the areas where the frequency of earthquake occurrences is high. There are about 4,162 tectonic earthquakes occurring in this area in the period of 1918 to 2015 (Figure
Earthquake distribution map in ENTP region from 1918 to 2015.
Natural hazards such as tectonics earthquake are often devastating in terms of loss of life and environmental damage. The occurrence of the earthquake is not preventable, but its effect can be minimized through effective prevention and reduction of vulnerability. An earthquake can be predicted through approaches by using various methods with a number of pieces of sophisticated earthquake detection equipment, for example, the calculation of seismicity with the calculation of b-value and the return period of earthquake. The earthquake distribution can be indirectly considered as a fractal (D) that describes seismicity with tectonic parameters (b-value).
One effort to find out the disaster-vulnerable areas from earthquake is by understanding the fault pattern by analyzing the previous earthquakes and calculating the earthquake precursor based on the results of spatial and temporal analysis using b-value and the return period of earthquakes.
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Tectonic earthquake data were obtained from the catalog of International Seismological Center (ISC), United States Geological Survey (USGS), and Indonesian Meteorology, Climatology and Geophysics Agency (IMCGA) with depth of ≤ 600 km and magnitude of mb ≥ 3 in the study area 118° E-128° E and 6° S-12° S. The number of data obtained from the catalog is 4,162 events. Based on the existence of fault, the study area is divided into 3 clusters (Figure Cluster 1: 6° S–9° S and 118° E–122,5° E (Flores island, Labuan Bajo, and surrounding areas) Cluster 2: 6° S–12° S and 122,5° E–128° E (Alor island, Timor island, and surrounding areas) Cluster 3: 9° S–12° S and 118° E–122,5° E (Sumba island and surrounding areas)
Clusters distribution of ENTP and surrounding areas used to find the differences of b-value for each region.
The data are processed with Microsoft Excel, ArcGis 9.3, and ZMAP 6.0 software using declustering according to Gardner and Knopoff method to obtain the main earthquakes. The main earthquakes were then processed to obtain the seismicity map (b-value and a-value) using Maximum Likelihood method and map variation of b-value spatially and temporally in ENTP and surrounding areas for each cluster. The flowchart of research can be seen in Figure
Flowchart of the research.
The earthquake in ENTP generally occurred at shallow depth (< 50 km) around the Timor island, moderate depth (50-70 km) around the ocean of Savu or between the Timor island and the Flores island, and deep (> 70 km) around the Flores island (Figure
Earthquake seismicity map in ENTP showing depth and magnitude using ZMAP 6.0.
The 3-dimension view of the earthquake distribution toward depth is presented in Figure
The 3-dimension map of earthquake distribution in ENTP using ZMAP 6.0.
Magnitude distribution graph of seismicity in ENTP.
Spatial variation map of (a) a-values and (b) b-values for all earthquakes in ENTP.
Figure
Based on temporal variation of b-values (Figure
Temporal variation of b-value in ENTP region.
Cluster 1 (Flores island, Labuan Bajo, and surrounding areas) can be depicted as shown in Figure
Magnitude distribution graph of seismicity for cluster 1 (Flores, Labuan Bajo, and surrounding areas): (a) for mb > 3 SR, (b) for mb= 3-5 SR, and (c) for mb > 5 SR.
Based on spatial variation of seismicity, cluster 1 shows the b-values varied from 0.7 in the eastern Flores island to 1.7 in the western Labuan Bajo and surrounding areas. This implies that the Flores island to the east is categorized as high stress concentration area where weak earthquakes frequently occurred. Moreover, active volcanoes that are located in this area can trigger the occurrence of weak earthquakes. On the other hand, the west part of Flores island which has the b-value tending to be high or is called the creeping area [
Spatial variation of seismicity of cluster 1 (Flores island, Labuan Bajo, and surrounding areas): (a) a-value and (b) b-value.
Cluster 2 (Timor, Alor, and surrounding areas) has 2,435 earthquake events; 1,155 of them were main earthquakes. Using Maximum Likelihood method, it was found that the b-value is 0.712 and the a-value is 6.06 for mb > 3 SR, the b-value is 2.34 and the a-value is 13.50 for mb = 3–5 SR, and the b-value is 0.749 and the a-value is 6.20 for mb > 5 SR, indicating the high stress condition in this area causing a lot of weak earthquakes (Figure
Magnitude distribution graph of seismicity for cluster 2 (Timor island, Alor island, and surrounding areas): (a) for mb > 3 SR, (b) for mb= 3-5 SR, and (c) for mb > 5 SR.
Based on spatial variation of seismicity, the b-value and the a-value variation in cluster 2 (Figure
Spatial variation of seismicity of cluster 2 (Timor island, Alor island, and surrounding areas): (a) a-value and (b) b-value.
Cluster 3 (Sumba island and surrounding areas) has 651 earthquake events, where 390 of them were the main earthquake. Using Maximum Likelihood method, it was found that the b-value is 0.87 and the a-value is 6.33 for mb > 3 SR, the b-value is 1.79 and the a-value is 10.40 for mb = 3–5 SR, and the b-value is 0.921 and the a-value is 6.57 for mb > 5 SR. The low b-values indicate that the stress condition in this area is high that caused many weak earthquakes to occur (Figure
Magnitude distribution graph of seismicity for cluster 3 (Sumba island and surrounding areas): (a) for mb > 3 SR, (b) for mb = 3-5 SR, and (c) for mb > 5 SR.
Based on spatial variation of seismicity, the low b-value (0.8-0.9) was observed in the western part of Sumba island, indicating the large number of small-scale earthquakes. Meanwhile, the relatively higher b-value was found in the eastern part of Sumba island, indicating the small number of small-scale earthquakes (Figure
Spatial variation of seismicity of cluster 3 (Sumba island and surrounding areas): (a) a-value and (b) b-value.
The a-value variation in cluster 3 shows that Sumba island and its surrounding areas have variation of value with range of 6,2-8,2. This shows a lower level of seismic activity compared to two other clusters over a period of 75 years.
The a-value, b-value, and fractal dimensions for each cluster areas are presented in Table
The a-value and b-value and fractal dimension (D) of each cluster.
No | Cluster | b-value | a-value | Fractal Dimension |
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1 | Cluster 1 | 0.837-0.937 | 6.54 | 1.674-1.874 |
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2 | Cluster 2 | 0.692-0.732 | 6.06 | 1.384-1.464 |
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3 | Cluster 3 | 0.852-0.932 | 6.69 | 1.704-1.864 |
To illustrate the earthquake repetition that has been calculated based on the fractal value, it is calculated also based on the earthquake return period as in (
Seismicity and return period of earthquake for each cluster.
No | Magnitude (Mb) (SR) | Cluster 1 | Cluster 2 | Cluster 3 | |||
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N1 (year−1) | N1 (year−1) | N1 (year−1) | |||||
1 | 3-3,9 | 4.27339 | 0.23401 | 8.20681 | 0.12185 | 4.91215 | 0.20358 |
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2 | 4-4,9 | 0.55433 | 1.80396 | 1.59285 | 0.62781 | 0.62990 | 1.58755 |
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3 | 5-5,9 | 0.07191 | 13.90682 | 0.30915 | 3.23464 | 0.08077 | 12.38022 |
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4 | 6-6,9 | 0.00933 | 107.20814 | 0.06000 | 16.66578 | 0.01036 | 96.54470 |
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5 | 7-7,9 | 0.00121 | 826.47131 | 0.01165 | 85.86689 | 0.00133 | 752.88461 |
Table
It can be concluded that the b-value in ENTP region is relatively low with high stress condition so that there are many weak earthquakes. This result is supported by the variation of b-value and a-value in the ENTP region, respectively, ranging from 0.6 to 1.3 and from 5.0 to 8.5. The temporal variation in ENTP indicates that before the occurrence of big scale earthquakes, i.e., mb 7.5 SR in 1996 and 2004, there was a significant decrease in b-value. The average of the b-values ranges from 0.837 to 0.937 and the a-value is 6.54 in cluster 1, the average of b-values ranges from 0.692 to 0.732 and a-value is 6.06 in cluster 2, and the average of the b-value ranges from 0.852 to 0.932 and a-value is 6.69 in cluster 3. It implies that the b-value and a-value of the clusters are not significantly different.
The result of the fractal dimension calculation shows that clusters 1, 2, and 3 have D values of 1.674-1.874, 1.384-1.464, and 1.704-1.864, respectively. Meanwhile, based on the calculation of the earthquake return period, it is found that each cluster is still dominated by small-scale earthquake with the highest seismicity index and the fastest earthquake return period is in cluster 2 which is 44 days.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.