Understanding the Principles of Earthquake Modeling from the 1999 Athens Earthquake Event

The 1999 Athens Earthquake occurred on September 7, 1999, registering a moment-magnitude of 6.0 (USGS). The tremor’s epicenter was located approximately 17km to the northwest of the city center. Its proximity to the Athens Metropolitan Area resulted in widespread structural damage.

More than 100 buildings including three major factories across the area collapsed. Overall, 143 people lost their lives and more than 2,000 were treated for injuries in what eventually became Greece’s deadliest natural disaster in almost half a century. In total the event caused total economic losses of $3.5 billion, while insured loss was $130 million (AXCO).

Losses from such events can often be difficult to predict; historical experience alone is inadequate to predict future losses. Earthquake models can assist in effectively managing this risk, but must take into account the unique features that the earthquake hazard presents, as the 1999 Athens Earthquake event highlights.

Background seismicity must be considered to capture all potential earthquake events

The 1999 event took Greek seismologists by surprise as it came from a previously unknown fault. Such events present a challenge to (re)insurers as they may not be aware of the risk to properties in the area, and have no historical basis for comparison. Effective earthquake models must not only incorporate events on known fault structures, but also capture the background seismicity. This allows potential events on unknown or complicated fault structures to be recorded, ensuring that the full spectrum of possible earthquake events is captured.

Hazard can vary greatly over a small geographical distance due to local site conditions

Soil type had significant implications in this event. Athens has grown tremendously with the expansion of the population into areas of poorer soil in the suburbs, with many industrial areas concentrated along the alluvial basins of the Kifissos and Ilisos rivers. This has increased the seismic hazard greatly with such soils amplifying the ground motions of an earthquake.

The non-uniform soil conditions across the Athens region resulted in an uneven distribution of severe damage in certain regions. The town of Adames in particular, located on the eastern side of the Kifissos river canyon, experienced unexpectedly heavy damage wheras other towns of equal distance to the epicenter, such as Kamatero, experienced slight damage. (Assimaki et al. 2005)

Earthquake models must take such site-specific effects into account in order to provide a local view of the hazard. In order to achieve this, high-resolution geotechnical data, including information on the soil type, is utilized to determine how ground motions are converted to ground shaking at a specific site, allowing for effective differentiation between risks on a location level basis.

Building properties have a large impact upon damageability

The 1999 Athens event resulted in the severe structural damage to, in some cases the partial or total collapse of, number of reinforced concrete frame structures. Most of these severely damaged structures were designed according to older seismic codes, only able to withstand significantly lower forces than those experienced during the earthquake. (Elenas, 2003)

Earthquake models must account for such differences in building construction and age. Variations in local seismic codes and construction practices the vulnerability of structures can change greatly between different countries and regions, with it important to factor these geographical contrasts in. It is important for earthquake models to capture these geographical differences of building codes and this can be done through the regionalization of vulnerability.

Additionally, the Athens earthquake predominantly affected both low and middle rise buildings of two to four stories. The measured spectral acceleration (a unit describing the maximum acceleration of a building during an earthquake) decreased rapidly for buildings with five stories or more, indicating that this particular event did not affect high rise buildings severely. (Anastasiadis et al. 1999)

Spectral response based methodology most accurately estimates damage, modeling a building’s actual response to ground motions. This response is highly dependent upon building height. Due to the smaller natural period at which low and middle rise buildings oscillate or sway, they respond greater to higher frequency seismic waves such as those generated by the 1999 Athens event; while the reaction of high rise buildings is the opposite, responding the most to long period seismic waves.

The key features of the RMS Europe Earthquake Models ensure the accurate modeling of events such as the 1999 Athens Earthquake, providing a tool to effectively underwrite and manage earthquake risk across the breadth of Europe.