Flores Sea Earthquake: Analysis of the Kalaotoa Fault Event
The Flores Sea region of Indonesia experienced a significant seismic event that challenged previous geological assumptions about the area. Initially recorded as a magnitude 7.7, the earthquake was later downgraded to a magnitude 7.3. This event was particularly surprising to seismologists, as the region is generally less seismically active and typically dominated by thrust faulting rather than the mechanism observed in this instance.
Key Facts
- Magnitude: Downgraded from 7.7 to 7.3.
- Fault Type: Right-lateral strike-slip (dextral).
- Fault Name: Identified as the previously unknown Kalaotoa Fault.
- Maximum Intensity: Modified Mercalli intensity VI (Strong) on the Selayar Islands.
- Tsunami Impact: Minor, measuring only 7 centimeters (2.8 inches).
- Historical Context: Largest event in the region since a deep-focus Mw 7.9 in 1996.
Geological Mechanism and Fault Dynamics
The earthquake was caused by shallow strike-slip faulting—a process where two blocks of the Earth's crust slide horizontally past one another—within the shallow crust beneath the Flores Sea. According to the USGS, the rupture occurred on an east northeast-west southwest fault with a right-lateral motion. Scientists from the Earth Observatory of Singapore noted that the fault acted as a boundary between two crustal blocks, with the northern block moving to the right relative to the southern block.
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Rupture Dimensions and Slip
Different models provide varying estimates of the rupture's scale. A finite fault model by the ANSS indicates a rupture measuring 90 km by 15 km, with a maximum coseismic slip (the sudden displacement of rocks during an earthquake) of 4.0 meters. Conversely, research from the Bandung Institute of Technology suggests the seismogenic fault may have spanned 80 km to 110 km in length and reached depths of up to 30 km, with a coseismic slip of approximately 2.3 meters across the rupture area.
The Discovery of the Kalaotoa Fault
One of the most significant scientific outcomes of this event was the identification of the Kalaotoa Fault. A 2022 research paper revealed that this was a previously unknown dextral strike-slip fault system. The system is composed of three distinct segments with lengths of approximately 100 km, 50 km, and 40 km.
Interestingly, the 2021 event did not occur on any of the eight known faults in the epicenter region. Furthermore, the earthquake triggered activity on the Selayar Fault, a prominent normal fault located to the west, likely due to stress transfer—where the movement of one fault increases the pressure on adjacent faults.
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Impact and Intensity
The shaking was felt across several regions of Indonesia, with varying degrees of intensity as measured by the Modified Mercalli scale.
| Location | Intensity (Modified Mercalli) | Description |
|---|---|---|
| Selayar Islands | VI | Strong |
| Ranakah | V | Moderate |
| Lembata, Labuan Bajo, Larantuka, Ruteng | III–IV | Weak |
Tsunami Response
Following the quake, tsunami warnings were issued for coastal communities in the Flores region. This led to widespread panic, with residents fleeing to higher ground. However, the resulting tsunami was minor, reaching a height of only 7 centimeters (2.8 inches). The warnings were officially cancelled at 13:20 local time, two hours after the initial alert.
Frequently Asked Questions
What was the final magnitude of the earthquake?
The earthquake was initially recorded as a magnitude 7.7 but was subsequently downgraded to a magnitude 7.3.
What is the Kalaotoa Fault?
The Kalaotoa Fault is a previously unknown dextral strike-slip fault system consisting of three segments (approximately 100 km, 50 km, and 40 km long) that was identified following the 2021 event.
Why was this earthquake unexpected by seismologists?
Experts did not expect a large quake because the region is not typically as seismically active, and the strike-slip mechanism is unusual for the area, where thrust faulting is the dominant mechanism.
Did the earthquake cause a significant tsunami?
No. While warnings caused panic and evacuations, the actual tsunami was minor, measuring only 7 centimeters (2.8 inches).
How does this event compare to previous earthquakes in the region?
It was the largest earthquake in the region since a deep-focus Mw 7.9 event that occurred in 1996.