August 26, 1999

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Why was the recent earthquake in Turkey so destructive?

The August 17, 1999, magnitude 7.4 earthquake in Turkey leveled thousands of buildings and resulted in over 10,000 deaths. This was the largest earthquake in western Turkey in this century, but it was just the latest in a series of destructive earthquakes from the North Anatolian fault.

The 1000-km-long (600 miles) North Anatolian fault has produced seven large (greater than magnitude 7.0) earthquakes in the period from 1939 through 1999. These earthquakes have ruptured the fault progressively from east to west. Seismologists recognized that the westernmost end of the fault, ruptured by the recent earthquake, was particularly hazardous.

The North Anatolian fault is similar in many ways to California's San Andreas fault. They are both right-lateral, strike-slip faults with similar lengths and similar long-term rates of movement. The ground across a strike-slip fault is displaced laterally, and right-lateral slip means that the opposite side of the fault moves to the right. Offsets of up to 3 meters (10 feet) were observed across the 100-km-long (60 miles) surface rupture of the recent Turkey earthquake.

The tectonic setting in Turkey is more complex than that in California. The San Andreas fault marks the transform boundary between the North American and the Pacific plates, two of the dozen major mobile plates making up the mosaic of the Earth's crust. The North Anatolian fault is the northern boundary of the small Turkish microplate, which is wedged between the Eurasian plate to the north and the Arabian plate to the south. The Turkish microplate is being squeezed westward as the Arabian and Eurasian plate converge.

Hawai`i experienced a magnitude 7.2 earthquake in 1975. Why does a magnitude 7.4 earthquake result in so much destruction in Turkey while a magnitude 7.2 earthquake produces relatively little damage in Hawai`i? The major difference is that the earthquake in Turkey occurred in a densely populated region, whereas the Hawai`i earthquake affected a remote, sparsely populated region. Another important factor is the difference in construction practices. Most of the buildings in Hawaii are small wood-frame structures, which are less likely to topple than the bulky masonry buildings typical of Turkey. Most larger buildings in Hawai`i are relatively new and built to withstand the shaking from large earthquakes. We speculate that most of the buildings in Turkey were not built to such exacting standards.

The recent earthquake in Turkey is a testament to the destructive potential of large earthquakes. Though this earthquake was not predicted, its occurrence was anticipated by scientists. Earthquakes cannot be prevented, but damage to structures can be mitigated through earthquake-resistant design and good construction practices. Translating science into public policy can be difficult, particularly in developing countries.

Eruption Update

There was a short pause in the eruption of Kilauea Volcano during the past week. On the evening of August 21, lava from the Pu`u `O`o vent stopped flowing through the network of tubes to the ocean at the Kamokuna site in Hawaii Volcanoes National Park. Eruptive activity resumed on Monday morning, August 23, and lava entered the ocean again at Kamokuna. The public is reminded that the ocean-entry area is extremely hazardous, with explosions accompanying unpredictable collapses of the new land. The steam clouds are highly acidic and laced with glass particles.

A magnitude-4.0 earthquake at 4:00 p.m. on August 24 was reported felt in Pahala and the Kahuku Ranch area. The temblor was located 2.5 km (1.5 mi) northeast of Pahala at a depth of 6.8 km (4.1 mi). Ninety minutes later that same afternoon, a resident of Pahala felt another earthquake. The magnitude-3.2 earthquake was located 1.5 km (0.9 mi) east of Pahala at a depth of 6.7 km (4.0 mi).