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October 21, 1999

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


Is the world shaking more now than before?

For the past two months, reports of large earthquakes and the havoc they cause seem to be constantly in the news. Earthquakes in Turkey, Taiwan, Mexico, and California have made the front pages and the nightly television newscasts. Is world-wide earthquake activity increasing?

The four earthquakes mentioned above had magnitudes greater than 7.0. Earthquakes with magnitudes between 7.0 and 7.9 are classified as major earthquakes. Since 1900, the average number of major earthquakes annually has been 18. This year, the major earthquake count stands at 13, with two months to go.

The annual number of major earthquakes does not vary widely, because most major earthquakes are the result of tectonic plate interactions, and these interactions occur at a steady rate. Plates slide past, collide with, and subduct beneath one another.

Each plate moves at a constant speed - some as fast as the growth of your fingernail - but the movement is usually impeded at the boundaries where two plates are in contact. Stresses accumulate along the boundary until the impediment can be overcome, and the locked section of the plate moves to catch up with the rest of the plate.

The deadly earthquake in Turkey on August 17 was along the North Anatolian fault, where the Turkish microplate is sliding past the Eurasian plate. The Turkish microplate is moving about 24 mm (1 inch) per year to the west relative to the Eurasian plate. The magnitude-7.4 earthquake produced 5 m (16 ft) of sideways slip between the plates. There were over 15,000 fatalities, primarily caused by the collapse of nearly 76,000 buildings.

The magnitude-7.6 Taiwan earthquake occurred on September 20 along the Shundong fault, where the westward-moving Philippine Sea plate collides with and overrides the Eurasian plate. Up to 8 m (26 ft) of uplift was measured from the thrust motion of the fault. The earthquake caused the collapse of over 38,000 structures and killed more than 2,300 people.

On September 30, a magnitude-7.5 earthquake shook the Oxaca region of Mexico and caused 18 deaths. The earthquake was deep within the northeastward-moving Cocos plate, which subducts beneath the North American plate. The 53 km (32 mi) depth of the earthquake and the sparse population of the region curtailed a higher death toll. No surface fractures were reported.

A week ago, on Sunday, October 16, a magnitude-7.1 earthquake undulated through the Mojave desert in southern California. The earthquake was centered on the Lavic Lake fault, a strike-slip fault that takes up some of the slip between the northwestward-moving Pacific plate and the westward-moving North American plate. Up to 4.7 m (15 ft) of right-lateral offset was measured along a 40-km-long (24-mi-long) surface rupture. Except for a cracked highway overpass and broken items that fell, damage was minimal in the lightly populated area.

A common report from Turkey and Taiwan following their deadly earthquakes was the call for the arrest of architects and contractors responsible for their shoddy design and construction of buildings that fell. Public officials are well aware of the earthquake hazard in those countries, and proper building codes are in place to mitigate the hazard for the people at risk. It is known that most fatalities from an earthquake are from collapsing structures and tsunamis.

Here in Hawai`i County, we also have large earthquakes, although we are in the middle, and not at the edge, of a tectonic plate. The growth and weight of our volcanoes produce the stresses that cause the rocks to break. Our County Council recently adopted a new building code that includes provisions for building in a zone with the highest level of earthquake shaking. So when you have to use 12-penny nails, instead of 10-penny nails, consider it an investment in keeping your home from collapsing in the next major earthquake.

Eruption Update

Eruptive activity of Kilauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing through a tube to the southeast in the direction of the sea coast. Breakouts from the tube feed flows that travel short distances before stopping. Lava is also ponding at the 530-m (1750-ft) elevation. A broad `a`a flow made it halfway down the pali before stopping. The lava pond within Pu`u `O`o remains very active, and a bright red glow, visible throughout Puna, is cast upon the fume clouds overhead.

There were no earthquakes reported felt during the week ending on October 21.

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Updated: 15 Nov 1999