The Importance of Tracing Old Earthquakes

Studying earthquakes of the past is necessary to understand their mechanisms, but also to better prevent future earthquakes.

1886 Charleston earthquake

Exploring historical seismic occurrences proves essential in comprehending their underlying mechanisms and enhancing the accuracy of future earthquake prognostication. Yet, the challenge remains: how can we detect the remnants of these age-old tectonic episodes?

Paleoseismology involves the examination of historical seismic activity, concentrating on antiquated earthquakes. This scientific field exists at the crossroads of seismology and tectonics, centered on the visible imprints on the Earth’s surface that were caused by prior earthquakes. This emphasis is particularly on the era preceding modern instruments (prior to the most recent century), during which precise documentation is absent. The primary goal encompasses measuring these occurrences, gauging their repetition, and subsequently endeavoring to deduce the potential locations and timings of forthcoming seismic events.

Paleoseismology, or the Study of Ancient Earthquakes

The city of Scythopolis (Beit She'an), a city destroyed by this earthquake.
The city of Scythopolis (Beit She’an), a city destroyed by this earthquake. Image: Wikimedia.

The role of a paleoseismologist involves a range of tasks, including conducting on-site observations and measurements, analyzing satellite imagery, and delving into historical records and data stored in global seismology repositories. Close collaboration transpires between the researcher and various experts such as geodesists, rock physicists, archaeologists, geographers, and historians.

It’s important to note that not all faults exhibit consistent activity, and the duration between two instances of earthquakes can be notably extended. The seismic cycle encompasses a span of approximately 10 to 1,000 years. As a result, there is often a need to journey far back in history across multiple centuries when viable historical paleoseismic data permits.

Traces Visible on the Surface

San Andreas Fault in California
San Andreas fault in California. Image: USGS.

Earthquakes emanate from the depths of the Earth’s crust; however, they have the capacity to imprint surface indications. The actions transpiring along geological faults possess the potential to be integrated into the topography, eternally inscribed in diverse forms, and potentially accessible to forthcoming generations.

Certain manifestations of these seismic occurrences can exhibit remarkable scenes: contorted railway tracks, collapsed bridge structures, altered water channels, and deviations within walls. On occasion, seismic events have the capability to initiate occurrences like landslides or tsunamis, with the imprints of these events enduringly preserved within sedimentary archives, facilitating chronological determination.

These manifestations assume paramount importance in retracing earthquakes in epochs prior to instrumental and historical documentation, wherein scripted annals are absent. Other indicators exist with subtlety, necessitating meticulous geodetic measurements for discernment. Nevertheless, they merit consideration, as substantial seismic activity is typically heralded by a phase of incremental minor shifts that could be construed as harbingers.

The effects of 15 years of fault creep on a curb in Fremont.
The effects of 15 years of fault creep on a curb in Fremont. Image: Wikimedia.

The motion of a geological fault finds notable representation in sediment layers, which encounter displacement along the fault plane. Scholars subsequently excavate trenches to unveil a vertical cross-sectional view, thereby scrutinizing the extent of dislocation transpiring during the seismic event, thereby enabling an approximation of its magnitude. Through dating procedures, especially leveraging carbon-14, the oldest strata devoid of deformation are dated, thus providing an estimation of the earthquake’s occurrence. This technique permits the consecutive unfolding of numerous events transpiring along the same geological fault line.

Prevention of New Earthquakes

The North Anatolian and neighbouring faults covering most of Turkey
The North Anatolian and neighbouring faults covering most of Turkey.

Through an analysis of the recurrence interval, the ability to approximate the impending date of a novel seismic occurrence and evaluate the seismic peril of a particular locale is attained. Furthermore, the spatial progression of seismic incidents can be observed. In truth, faults frequently do not undergo rupture at identical locations, and there exists a systematic shift in the epicenter’s position. The North Anatolian Fault, known for its powerful seismic activities (frequently exceeding a magnitude of 7), is a prime example of this phenomenon. Through paleoseismological investigations, it has been posited that the periodicity of a fault segment’s activity spans approximately 250 to 300 years. As per projections, the forthcoming earthquake is anticipated to transpire between 2030 and 2040 along the Istanbul coastline.

This example highlights the importance of paleoseismological studies in order to prevent the occurrence of major earthquakes in densely populated areas. The data from paleoseismological studies can be used to implement prevention measures to anticipate an earthquake and mitigate damage and its impact on human lives. Major earthquakes can indeed have significant repercussions on a country’s economic, political, and social life.

However, similar to any natural event, earthquake prediction remains approximate, and no date can ever be provided with certainty. Studying the traces of past earthquakes allows for probabilities to be estimated regarding the seismic risk of a region, which should be taken into account for local prevention efforts.