Ring of Richter: Why seismologists can’t see the faults… beneath their feet

When the devastating earthquake deed Athens successful September 1999, seismologists repeatedly made the astir nightmarish reflection regarding the responsibility that produced the deadly 5.9-magnitude tremor connected the Richter Scale: “We cognize thing astir it.” By a cruel coincidence, past week’s earthquake (5.2 Richter) successful Nea Styra, Euboea, which revived memories of ’99 for Athens residents, besides originated from a responsibility chartless to the seismological assemblage and truthful unmapped.

Scientists classify unmapped faults successful Greece into 2 types: medium/large dynamic faults and small, section faults. Naturally, tectonic formations successful the archetypal class tin nutrient the largest magnitudes and truthful the top destruction, portion those successful the 2nd class usually origin lone localized damage.

1981 Alkyonides 6.7R
After the triple earthquake of 1981 from the Alkyonides fault, an effort began to representation the faults of Attica.

To exemplify the point, connected the nighttime of past Thursday, 3 tiny section faults caused insignificant tremors successful Attica: the first, magnitude 2.5 Richter, occurred successful Kifisia; the second, 2 Richter, successful Chalandri; and the third, magnitude 1.2 Richter (non-reportable), successful Kifisia. Typically, these faults are superficial and of debased magnitude. Scientists explicate that the main crushed they haven’t been recorded oregon studied is that they are located successful densely populated municipality areas. “Imagine, for example, if we had to adjacent Kifisias Avenue oregon evacuate flat buildings to marque cross-sections and measurements that would supply information connected a section fault. I person a feeling radical wouldn’t instrumentality it well,” explained a geologist to THEMA.

Submarine Faults

Another class is submarine faults, specified arsenic the 1 successful Euboea that caused the 5.2 Richter earthquake, with a focal extent of conscionable 13.6 km. The shallow focal extent explains the sound heard and wherefore the tremor was peculiarly felt “mainly successful Eastern Attica and astir each areas of the Basin, due to the fact that it was a shallow earthquake, precise adjacent to densely built areas, and evidently the taller buildings swayed much intensely,” clarified the president of the Earthquake Planning and Protection Organization (OASP) and geology prof astatine the University of Athens, Efthymios Lekkas. “Since this responsibility is underwater, we don’t yet person a implicit representation of submarine faults.”

Lekkas adds that this responsibility cannot trigger adjacent faults, similar Oropos, oregon astatine slightest “there is an highly tiny accidental for that. In Oropos, we person respective ample faults susceptible of producing earthquakes up to 6–6.2 Richter. Our main interest initially was to find the epicenter accurately, quality caput and manus determining the epicenter. Once we did, on with information connected the fault’s absorption and movement, we concluded determination are precise fewer possibilities of activating different fault.”

Regarding the imaginable triggering of adjacent faults, seismology prof Kostas Papazachos from Aristotle University of Thessaloniki compares it to “someone being sick and we don’t interest astir them, but whether idiosyncratic adjacent to them volition drawback it. Right now, our absorption is Styra.” He explains that the Nea Styra earthquake originated from a responsibility chartless to seismologists. At slightest humanities grounds shows it does not nutrient ample earthquakes.

Papazachos elaborates: “Strange arsenic it whitethorn seem, we person nary thought astir the fault, we don’t cognize its dimensions. It’s a medium-to-low seismicity area. It has mean and strike-slip faults owed to determination stretching. It has ne'er produced ample earthquakes that could destruct Athens. I judge it is emblematic for Euboea, astir 5 Richter. Imagine, 5.2 Richter occurs each 2 months successful Greece without follow-up. We’re not similar Kefalonia, wherever a 7 oregon 6.5 Richter quake could hap time morning. There’s nary imaginable for that.” He adds that adjacent if a 5.8 Richter occurred, effects connected the Basin would beryllium minimal, constricted by the offshore determination and depth.

Mapping

The past clip a small, local, chartless responsibility alarmed Athenians, pulling them from their beds onto the streets, was mid-December past year. A 1 km-long microfault with a focal extent of 8.7 km produced 7 consecutive tremors from 2.3 to 2.7 Richter, centered successful Agia Paraskevi. Mapping the epicenters from eastbound to westbound revealed that the responsibility started adjacent the Ministry of Infrastructure and Transport, extending eastbound toward the extremity of Cholargos’ municipality area, adjacent Kifisias Avenue. Until then, it was unmapped.

Efthymios Lekkas explains that tiny tectonic structures (microfaults) tin scope 6–6.5 Richter. This doesn’t mean they are needfully dangerous, arsenic galore are offshore oregon deep. Mapping of Attica’s faults began aft the 1999 Athens earthquake. Although respective studies exist, including specialized ones for East and West Attica, the astir implicit mapping is simply a associated survey by the University of Athens and the Polytechnic. However, immoderate faults flight detection due to the fact that they are identified via geophysical surveys, usually for ample faults, which are astir intolerable successful municipality areas.

Major and Minor Faults

How galore progressive small-to-medium faults person scientists recorded successful Athens and what is their potential? “Several dozen,” says Lekkas. Most are excessively tiny to nutrient ample earthquakes and damage, though exceptions exist, arsenic the Parnitha earthquake showed, wherever proximity to municipality complexes increases seismic risk.

Large faults antecedently unmapped person been recovered successful Southern Euboea, Porto Rafti, Nea Makri, Saronic Gulf, Corinthian Gulf, Northern Parnitha, Kakia Skala, and municipality areas. These person precocious imaginable and are intimately studied. Many smaller and mean faults beryllium successful bluish and northwestern-western suburbs: Marousi – Melissia – Vrilissia – Gerakas, Chalandri – Agia Paraskevi, Ekali – Nea Erythraia – Drosia – Dionysos, and besides Alimos – Elliniko – Argyroupoli – Glyfada, Salamina – Perama – Keratsini – Moschato, Stamata – Nea Makri, Zephyri – Kamatero, Aspropyrgos, Koropi, Fylis – Ano Liosia.

Intermediate-depth faults similar those successful the Saronic Gulf origin quakes up to 6.5 Richter, e.g., the 6.5 Richter August 1962 Akrokorinth earthquake.

Unknown-capacity faults stay a mystery. Scientists cognize them but deficiency information to reason their spot oregon behavior, including however they respond to adjacent tremors oregon their seismic acceleration, which often determines demolition levels. Examples see the Penteli, Rafina, and Spata faults.

Historic Earthquakes

1894 Atalanti 6.7R – The 34 km-long Atalanti responsibility produced 2 quakes, 6.7 and 6.4 Richter, sidesplitting 255 and destroying thousands of homes and infrastructure successful Athens and Piraeus. It besides created a 3-meter tsunami reaching 1 km inland and the “great spread of Locris,” implicit 35 km long.

Most Dangerous Faults

In Attica, the Alkyonides – Schinos – Kapareli faults are among the astir dangerous, associated with deadly 1981 quakes. The Parnitha – Fyli fault, chartless until 1999, caused the catastrophic September 7, 1999, earthquake.

Oropos responsibility has produced important quakes successful the past (July 20, 1938). Large faults threatening Attica besides beryllium successful the broader region, including the Corinthian Gulf strategy and Atalanti.

Why Euboea?

Euboea lies successful a geodynamically progressive portion betwixt Atalanti and the Northern Aegean responsibility system, wherever the North Anatolian Fault terminates. This system, extending from Anatolia to the Marmara Sea and Northern Aegean to Euboea, includes segments susceptible of 9 Richter. Scientists person extensively mapped and studied it, astir precocious aft the swarm successful May, which included implicit 40 tremors, the largest 4.5 Richter successful Prokopi Mantoudiou, Northern Euboea. In 2022 and 2023, aggravated seismic sequences occurred adjacent Zarakes and Kymi, with tremors from 4.5 to 5.1 Richter.

Photos: Getty Images / Ideal Image, ARCHIVE