What patterns of damage were controlled during the Australia earthquake?

Victoria’s largest earthquake in at least two centuries shook buildings across Melbourne, but some damaged more than others.

Written by Mark Leonard, Ph.D., Leonard Seismic Consulting

Citation: Leonard, M., 2021, What patterns of damage were controlled during the Australia earthquake?, Temblor, http://doi.org/10.32858/temblor.210

A 5.9-magnitude earthquake struck southeast Australia on September 22, 2021 at 9:15 a.m. local time. This is the largest earthquake in Victoria in the two centuries of European settlement. The Australian aborigines lived in eastern Victoria about 40,000 years ago.

The earthquake occurred in the Victorian Alps, a remote mountainous region in eastern Victoria. The region has hosted four major earthquakes in the past half century: the 1968 Hotham earthquake of magnitude 5.4m, the Sara earthquake of magnitude 5.4m of magnitude in 1982, and of the magnitude of the Paw Paw/Thompson earthquake of magnitude of 5.0 of magnitude for Tuesday. -5.9 Woods Point earthquake.

Epicenter and distribution of felt reports for the eastern magnitude 5.9 earthquake of Victoria on September 22, 2021. Warmer colors indicate reports of stronger shaking. Credit: Geoscience Australia

Vibration felt over a wide area

In the 24 hours following the earthquake, Geoscience Australia received 41,000 felt reports, in which people recounted how much shaking occurred at their site. Of the reports submitted, only 30 responders were within 25 miles (40 kilometers) of the epicenter. This is consistent with the small population of the area. Reports from a distance of 12 miles (20 kilometers) indicate a strong shaking intensity (MMI VI).

The earthquake was felt across most of southeastern Australia: from Adelaide (750 km west) to Sydney (600 km northeast), and even as far as Brisbane (1,200 km north).

Ground motion models (GMMs), which estimate the level of ground shaking observed at a site of a given magnitude and distance, are used to assess earthquake hazards in any given area. The level of local ground shaking depends on factors including the location and depth of the earthquake and the type of ground.

Based on both data consistent with the available strong ground motion data and geological symmetry, for eastern Australia most seismic risk modelers use GMM systems developed in the western US, where the shaking occurs over a relatively narrow area compared to the eastern US. The tremor felt in the Woods Point earthquake is similar to the area felt by the 5.8-magnitude mineral earthquake in 2011 in Virginia, on the east coast of the United States. This raises the question of whether western American models are best suited to eastern Australia. Robust motion data from this earthquake will be invaluable in developing new models and better evaluating the GMM for the region.

The Victorian Alps and the Latrobe Valley in the south are among the most seismically active areas in Australia. As such, the seismic hazard in these areas is relatively high, for both the 2012 and 2018 national seismic hazard maps from Geoscience Australia.

Shaky data from the US Geological Survey shows that earthquakes on the East Coast shake a wider area than those on the West Coast. The dots indicate the locations that recorded shaking in each earthquake. Credit: USGS, Public Domain

Melbourne shook, but relatively little damage was reported

Melbourne, with a population of five million, is 80 miles (130 kilometers) from the epicenter. The eastern edge of the city is only about 60 miles (100 km). For an earthquake of this size so close to a large city, the effect was relatively insignificant. There were no reports of deaths or serious injuries. No serious structural damage occurred.

One of the worst-hit buildings in Melbourne was on the corner of Green and Chapel Streets in Windsor, where a section of a loose brick wall that rose at least 10 feet (three metres) over the second-floor ceiling collapsed on the street below. . This type of structure is known to be susceptible to earthquake damage.

The distribution of felt reports across greater Melbourne indicates that the most prevalent experience is light vibration (MMI IV, light blue). 30 miles (50 km) from the epicenter, a small strong shaking intensity (MMI VI, yellow) was reported. Credit: Geoscience Australia

Initial insurance claim data indicates that at least 1,500 buildings sustained minor damage such as building falls (such as chimneys) and cracks in plaster, paint and walls.

An explosion in Melbourne’s population from 23,000 in 1850 to 490,000 in 1890 created unsupported multi-storey buildings throughout the area. Initial reports indicate that these two- to three-story late Victorian buildings suffered the most severe damage in the Woods Point earthquake. Most of the damage occurred in suburbs three miles (five kilometres) southeast of Melbourne city

Earthquake damage

Building damage from an earthquake may be related to the strength of the ground beneath it. Soft soils amplify transient seismic waves, while hard rocks do not. When the waves are amplified, the vibration is more intense.

On Melbourne’s eastern edge, the land is mostly made of rocks. None of the unsupported Victorian buildings in this part of the area sustained any damage. However, suburbs such as Brahan and Windsor were damaged by the quake, even though they are 20 miles (30 km) from the epicenter. These areas lie on hard soil, which is softer than rock and would have amplified seismic waves. Not surprisingly, the areas where the shaking was more intense saw more damage.

Initial reports from field crews indicate that no buildings within 12 miles (20 kilometers) of the epicenter were damaged. Until about 1940, the two-story, two-story building blocks only seven miles (12 kilometers) from the epicenter were unaffected. More research is needed to determine why these buildings have been preserved. Detailed building damage assessments, along with subsoil studies, will provide invaluable limitations for the assessment of future earthquake risk in Melbourne.

earthquake doubts

Some details of the location and mechanism of this earthquake are still unresolved. Geoscience Australia, the Earthquake Research Center, and the USGS estimate that the quake occurred about seven miles (11 kilometers) below the surface. However, Geoscience Australia and USGS have also calculated torque fault plane solutions – another means of estimating earthquake depth – which indicate depths of about 11-13 miles (18-21 kilometres). Depth resolution will be important in understanding the Earth’s vibration distribution.

Researchers use earthquake magnitude and the scaling relationship to estimate the size of the ruptured fault (Wells and Coppersmith 1994). Using the relationship from Leonard (2014), the fault associated with this earthquake is expected to be about 5.5 miles (nine kilometres) in length and 3.5 miles (5.5 kilometres) long.

Given how deep the earthquake was, it is unlikely that the surface would have ruptured. The area around the epicenter is rugged and densely forested. So it will be difficult to identify any superficial crack. Satellite-based imaging, which is used to determine the exact surface displacement near faults, unfortunately performs poorly in forested areas. The lack of roads, steep terrain, and dense forest cover will make fieldwork a challenge.

references

Clark, Dr.; , and Leonard, M. (2003). Key stress trends from multiple focal plane solutions: new insight into the stress field within the Australian plate. Special Papers – Geological Society of America, 91-106.

Leonard, M.; (2014). Self-consistent relationships to earthquake fault: update and expansion to stable continental-slip faults. Bulletin of the American Seismological Society, 104(6), 2953-2965.

Wells, D. L., & Coppersmith, K. J. (1994). New empirical relationships between volume, rupture length, rupture width, rupture area, and surface displacement. Bulletin of the American Seismological Society, 84(4), 974-1002.

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