Summary
On the morning of 16 August 2025, a significant magnitude 5.6MLa (Local Australian Magnitude – similiar to the Richter scale) earthquake occurred near Kilkivan, Queensland. The event’s most defining characteristic was its exceptionally shallow depth, estimated at 10 km below the surface. This report provides a detailed analysis of this earthquake, concluding that its shallow hypocentre was the primary factor in the widespread and intense shaking experienced across southeast Queensland. The analysis infers a reverse or thrust faulting mechanism, consistent with the compressive stress field affecting the Indo-Australian Plate.
The Kilkivan earthquake, while a rare event for Queensland in terms of magnitude, serves as a powerful reminder that the region is not seismically inert. It fits a predictable, albeit low-frequency, pattern of seismic activity along the eastern Australian coast. The widespread public reports of the event highlight the value of citizen science in monitoring intraplate seismicity, compensating for the sparser institutional network. Ultimately, the event reinforces the need for ongoing public education and preparedness, demonstrating that while large earthquakes are infrequent, they remain a genuine and persistent hazard within Australia’s tectonic landscape.
1. Introduction
1.1 Statement of Purpose and Scope
The objective of this report is to provide a comprehensive, expert-level analysis of the 16 August 2025 Kilkivan earthquake. This document synthesises public reports and available scientific data, 2 days after the event, to explain the event’s unique characteristics, place it within the broader context of Australian seismicity, and provide critical insights into its underlying causes. The analysis is structured to address key questions concerning the event’s seismological profile, its regional and national context, the phenomenology of shaking and sound, and essential community preparedness measures.
1.2 Background on the Kilkivan Earthquake
The earthquake occurred at 9:49 AM AEST on Saturday, 16 August 2025, with its epicentre located near the rural town of Kilkivan, Queensland, at longitude 152.17° and latitude -26.26°. Geoscience Australia confirmed the magnitude as 5.6, with a shallow depth of just 10 km. An aftershock of magnitude 3.9 was measured at 09:45am followed by a magnitude 3.4 at 11:59am AEST, with other smaller aftershocks expected in the coming days and weeks.
The impact of the tremor was felt over a vast area of southeast Queensland, reaching as far north as Bundaberg and extending south through the Sunshine Coast region, Gympie, and into Brisbane and the Gold Coast. The widespread nature of the shaking is evidenced by the sheer volume of public “felt reports” submitted to Geoscience Australia, which exceeded 2,800 within minutes and is now over 23,800. Reports from residents described a distinct rumbling sound and noticeable shaking, with rattling dishes, shaking walls, and even moving furniture. Despite the intensity of the shaking, initial reports indicated minimal damage, with no casualties and only scattered power outages in the region and some temporary disruption to transport services.
The following table from Geoscience Australia provides a concise summary of the key seismological data for the event:
Table 1: Key Seismological Data for the Kilkivan Earthquake
| Characteristic | Data |
|---|---|
| Magnitude | 5.6 (MLa) |
| Location | Near Kilkivan, Queensland (−26.26° 152.17°) |
| Depth | ~10 km |
| Time | 9:49 AM AEST, 16 August 2025 |
| Widespread Impact | Felt in Brisbane, Sunshine Coast, Gold Coast, and Bundaberg |
| Public Reports | Over 28,800 “felt reports” received by Geoscience Australia |
2. Seismological Profile of the Kilkivan Event
2.1 The Critical Significance of a 10 km Depth Hypocentre
The most critical factor in understanding the impact of the Kilkivan earthquake is its exceptionally shallow hypocentre at only 10 km below the surface. The intensity of an earthquake’s shaking at the surface is not solely determined by its magnitude. A crucial physical principle is the attenuation of seismic waves. As seismic energy radiates outwards from the hypocentre, it dissipates and weakens over distance.
A 10 km depth is considered a shallow tremor. A shallow quake tends to produce stronger surface shaking than a deeper one because the seismic waves have less distance to travel before reaching the surface. This extremely short travel path meant that very little of the earthquake’s energy was lost to attenuation. As a result, the shaking at the surface was significantly more intense than it would have been for a deeper earthquake of the same magnitude. This directly explains the paradox of an event that, at magnitude 5.6, is typically associated with only “slight damage to buildings,” but generated such strong and widely reported shaking, including rattling dishes and moving furniture, despite its rural location.
2.2 Assessment of the Likely Fault Mechanism
Australia is situated in an intraplate region, far from the boundaries where the major tectonic plates collide. The Indo-Australian Plate is unique in being the fastest-moving continental landmass, travelling north-northeast at approximately 7 cm per year. This rapid movement generates a significant build-up of compressive stress within the interior of the Australian continent.
Earthquakes in intraplate settings are caused by the sudden release of this slowly accumulating stress when rocks break and move along a pre-existing fault line deep underground. Given the dominant compressive forces acting on the plate, the most probable fault movement is one that shortens the crust, such as a reverse or thrust fault. In this type of faulting, one block of rock is pushed up and over another. This mechanism is consistent with other major Australian intraplate events, such as the 1988 Tennant Creek earthquake sequence, which ruptured along a thrust fault. Therefore, while the specific fault has not been identified, it is highly probable that the Kilkivan earthquake was caused by a reverse or thrust faulting mechanism in response to the regional compressive stress field.
2.3 Geological Features and Potential Fault Lines in the Kilkivan Area
The Kilkivan area is geologically complex, situated near the Blackall Range, a landscape shaped by volcanic extrusion that occurred between 27 and 31 million years ago. The local geology includes extensive red basaltic soil and, in the Kilkivan area itself, formations such as the Mount Mia Serpentinite and the Amamoor beds, separated by a fault.
The type of rock in a region significantly influences how seismic waves travel. Hard, dense volcanic rock, such as the basalts of the Blackall Range and the serpentinite in the Kilkivan area, can act as an efficient “waveguide” for seismic energy. This enables the waves to travel with very little attenuation, potentially explaining why the earthquake was felt over such a wide area and why residents reported a distinct rumbling sound.
The rumbling sensation is a key phenomenon. The primary waves, or P-waves, travel faster than the more destructive secondary waves (S-waves). P-waves are compressional waves that can travel through air as sound waves. The dense rock structures in the region would have efficiently transmitted these P-waves, allowing them to be heard as a low-frequency rumble before the arrival of the slower, more intense S-waves that caused the ground to shake. While a specific, active fault line has not been identified, and the provided information does not link the Mount Mia Serpentinite fault to seismic activity, the local geology’s role in wave transmission is a critical part of the earthquake’s observable effects.
3. The Kilkivan Event in a Regional and National Context
3.1 Significance in the Australian Seismic Record
Australia is widely, though mistakenly, perceived as a seismically inert continent. The Kilkivan earthquake challenges this misconception directly. On average, the Australian continent experiences only one earthquake of magnitude 5 or greater each year. The 16 August 2025 magnitude 5.6 event therefore represents the most significant seismic event of the year, underscoring the reality of the ongoing, albeit infrequent, seismic hazard.
Major earthquakes, while rare, are a documented part of Australia’s geological history. The 1988 Tennant Creek earthquake sequence, which included a magnitude 6.6 event, was the largest recorded in mainland Australia and caused surface ruptures stretching for 32 km. In Queensland’s own history, the two largest recorded earthquakes were the 1918 Gladstone event (magnitude 6.3) and the 1935 Gayndah event (magnitude 6.1), both occurring in the state’s eastern coastal region.
The Kilkivan earthquake joins this select list of significant seismic events and reinforces the scientific message that despite long periods of quiet, a major earthquake is a real possibility in Australia’s intraplate environment. The event is a powerful reminder that an area with no recent history of seismic activity can still be susceptible to a significant earthquake when the right combination of geological and stress conditions is met, as demonstrated by the 1968 Meckering earthquake in Western Australia.
The table below places the Kilkivan event in a broader historical context of significant Australian earthquakes.
Table 3: Significant Earthquakes in Australian History (M≥5.0)
| Year | Location | Magnitude (M) | Key Details |
|---|---|---|---|
| 1918 | Off Lady Elliot Island (near Gladstone), Qld | 6.3 | The largest recorded Queensland earthquake |
| 1935 | Gayndah, Qld | 6.1 | One of the largest recorded Queensland earthquakes |
| 1988 | Tennant Creek, NT | 6.2, 6.3, 6.6 | The largest recorded mainland Australian earthquake |
| ~2015 | Rainbow Beach, Qld | 5.5 | Part of a series of offshore magnitude 5+ events |
| 2025 | Kilkivan, Qld | 5.6 | A significant and rare intraplate earthquake with an exceptionally shallow depth |
3.2 Historical Seismicity of Southeast Queensland
The Kilkivan earthquake is not a random anomaly but a recurring event within a geologically defined seismic zone. Research from the University of Queensland indicates that the state’s highest earthquake hazard areas are concentrated along the populated eastern coast and near offshore regions. This region is subject to a constant build-up of compressive stress, which is periodically released in seismic events.
The historical record confirms this pattern. Significant earthquakes have occurred near Gladstone and Gayndah in the past century, and more recently a series of magnitude 5+ earthquakes struck offshore near Rainbow Beach approximately 10 years ago. The Kilkivan event, located further inland but still within this general corridor, is a contemporary manifestation of the same underlying tectonic processes. It reinforces the conclusion that the eastern Australian coastal region is a predictable and long-term zone of stress release.
3.3 Dams and Induced Seismicity
One of the questions that naturally arises following an earthquake in a populated area is whether human activity, such as the presence of nearby large dams, could have played a role. It is a documented phenomenon that the filling of large reservoirs can induce seismicity, primarily by increasing pore pressure in the underlying crust, which can reduce the strength of existing fractures. Microearthquake swarms have, in fact, been detected in the vicinity of Wivenhoe Dam, with magnitudes ranging from M0 to M1.
However, the Kilkivan earthquake does not fit the typical profile of a dam-induced event. The Kilkivan earthquake was a powerful M5.6 event, which is several orders of magnitude larger than the microearthquakes associated with dams. Induced seismicity is typically characterised by clusters of small events, rather than a single, powerful tremor resulting from the release of long-term regional stress. Therefore, based on the magnitude and nature of the event, it is highly likely that the Kilkivan earthquake was a natural tectonic occurrence driven by the regional stress field and was unrelated to nearby water storage facilities.
4. Technical Insights and Lived Experience
4.1 The Physics of the Distinctive Rumbling Sound
Many residents reported a “loud rumbling sound” preceding the shaking. This is not an unusual phenomenon for a shallow earthquake and is a direct result of the physics of seismic waves. An earthquake releases energy in two primary forms of body waves: Primary (P) waves and Secondary (S) waves. P-waves are compressional waves, meaning they push and pull the ground in the direction of travel, similar to sound waves in air. They are the fastest of the seismic waves, arriving first at a location.
S-waves are shear waves that move the ground from side to side, perpendicular to the direction of travel, and are responsible for the most intense and damaging shaking. Because P-waves can travel through air, they can be heard as a low-frequency rumble before the arrival of the slower S-waves. This phenomenon, often described as a “rumble before the shake,” is a classic sign of a nearby, shallow earthquake, providing a few seconds of auditory warning before the more violent shaking begins.
4.2 Variations in Perceived Shaking Duration
Reports from residents varied regarding the duration of the shaking, with some feeling it for a few seconds and others for up to 15 seconds. This is not a contradiction but a predictable consequence of the relative speeds of the seismic waves.
The duration of the perceived shaking is the time between the arrival of the fastest P-waves and the slower S-waves and surface waves. For residents located very close to the epicentre, the different waves arrive in rapid succession, resulting in a single, short, and intense burst of shaking. For those further away, such as in Brisbane, the time lag between the arrival of the waves is more pronounced. This separation in arrival times creates a longer-lasting, rolling sensation, as different types of waves arrive sequentially over a longer period. The perception of duration is therefore directly related to the distance from the epicentre and the staggered arrival of the various seismic waves.
4.3 Aftershock Forecast and Geophysical Monitoring
The low frequency of significant earthquakes in Australia means that the national seismic monitoring network, while robust, is less dense than those in tectonically active plate boundary regions. In this context, the role of public reporting becomes invaluable.
Geoscience Australia relies on thousands of “felt reports” submitted by the public to create what it calls a “FeltGrid.” This system of citizen science provides a near real-time map of publicly reported shaking intensity from an earthquake, which is used by emergency managers to increase their awareness of the situation and coordinate a response. This data is a critical complement to the instrumental data, providing a detailed picture of the earthquake’s impact that might otherwise be missed by a sparser monitoring network. While an official aftershock forecast for the Kilkivan event was not available in the provided materials, the general scientific principle is that aftershocks are a common occurrence, with the largest aftershock typically being at least one magnitude lower than the mainshock.
5. Community Impact and Safety Preparedness
5.1 Assigning a Modified Mercalli Intensity Rating (MMI)
The magnitude of an earthquake is a measure of the energy released at the source, while the Modified Mercalli Intensity (MMI) scale is a subjective measure of the observed effects of shaking at a specific location. By correlating public reports with the MMI scale, it is possible to provide a more meaningful measure of the earthquake’s severity for the general public.
Based on the descriptions provided by residents across southeast Queensland, the shaking corresponds to several levels on the MMI scale. In more distant areas, such as the suburbs of Brisbane, the shaking was likely MMI III (“Weak”) or MMI IV (“Light”), described as “vibrations similar to the passing of a truck.” Reports of “rattling of dishes, windows, doors,” and “walls and house shook, furniture moved,” align with MMI V (“Moderate”) to MMI VI (“Strong”) shaking. The absence of widespread structural damage suggests that the shaking did not reach the more severe levels of VII or VIII. The following table provides a breakdown of the estimated MMI ratings based on public reports:
Table 2: Estimated Modified Mercalli Intensity (MMI) Ratings for Southeast Queensland
| MMI Level | Description / Damage | Corresponding Resident Accounts |
|---|---|---|
| III (Weak) | Felt indoors by several; vibration like passing of a truck | “Thought it might have been the washing machine or my dad” |
| IV (Light) | Felt indoors by many; dishes, windows, doors disturbed | “Glasses and plates rattling,” “windows were rattling” |
| V (Moderate) | Felt by nearly everyone; unstable objects overturned | “Unstable objects overturned” |
| VI (Strong) | Felt by all; some heavy furniture moved; slight damage | “Furniture moved,” “walls and house shook” |
5.2 Essential Safety Actions for Residents
The Kilkivan earthquake, being a rare event in a populated area, underscores the importance of public awareness and preparedness. The Queensland government has established clear guidelines for earthquake safety, which are an essential resource for residents.
The core protocol during an earthquake is to DROP, COVER, and HOLD ON:
- DROP to the ground and get under a sturdy table, desk, or bench.
- COVER your head and neck with your arms.
- HOLD ON to your shelter and be prepared to move with it until the shaking stops.
Specific advice depends on a person’s location at the time of the event:
- If indoors, stay inside and move away from windows, chimneys, and overhead fittings.
- If outdoors, stay outside and move to an open area, away from buildings, streetlights, and power lines.
- If in a vehicle, pull over to an open area and wait for the shaking to stop.
After the shaking stops, residents should check for injuries, turn off utilities if safe, and be prepared for potential aftershocks. They should avoid using the telephone unless there is a life-threatening emergency, listen to local radio stations for updates from authorities, and avoid entering damaged buildings. The fact that many residents expressed surprise at the event and a lack of preparedness reinforces the need for authorities to conduct ongoing public education campaigns to ensure the community is ready for future, infrequent seismic events.
6. Conclusions and Recommendations
The 16 August 2025 Kilkivan earthquake, a magnitude 5.6 event, stands as a critical case study in Australian intraplate seismology. The analysis concludes that its defining feature, a shallow 10 km depth, was a primary cause of the surprisingly intense and widespread shaking experienced across southeast Queensland. This shallow hypocentre minimised seismic wave attenuation, delivering more concentrated energy to the surface. The event’s most probable cause is the release of long-term compressive stress within the Indo-Australian Plate, manifesting as a reverse or thrust faulting mechanism.
While the event was a rare occurrence for the region, it is not an anomaly. It fits into a historical pattern of significant seismic activity along the eastern Australian coast, reinforcing that this corridor is a genuine zone of ongoing, albeit low-frequency, seismic hazard. The event’s lived experience, from the auditory warning of P-waves to the varying duration of shaking, can be directly explained by the fundamental physics of seismic wave propagation.
- QFES: State Earthquake Risk Assessment
- GeoScience Australia
- Queensland University, Seismelogical Observations
- Queensland Government, State of the Environment Report 2020
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