Global Earthquakes of 1999
Updated 01/14/2000
1999 has drawn to a close and it seemed like a good time to review the year's earthquakes. In the last few months, earthquakes have dominated the news - events in Turkey, Taiwan, Mexico, and California have captured headlines.
It has been an exciting year for earthquakes. The figure below shows the 22 earthquakes of magnitude 7.0 and higher recorded globally. The size of the symbol reflects its magnitude, while the color and shape indicate its depth below the surface. Seismologists generally classify earthquakes as shallow (events with depths less than 70 km), intermediate (events between 70 and 300 km) and deep (events with depths greater than 300 km).
Several of these earthquakes caused significant damage and loss of life. The string of damaging earthquakes has led people to wonder whether the earthquakes are related - and whether this is an unusual year in terms of large shocks. Here are some sample headlines from CNN
While it may seem as if Mother Nature is conspiring against us this year, the number of large events (22) is comparable to the average number. The plot below shows the number of earthquakes of magnitude 7 or higher each year for the past century. It ranges from a mininum of 6 (1986) and a maximum of 41 (1943). These data are taken from the USGS National Earthquake Information Center (NEIC).
What we have seen this year is more events occurring in populated areas. Of these 22 events, most of them were not damaging. For further musings on large earthquakes, we recommend Big Earthquakes, Predictions, and the New Millennium by Susan Hough of the USGS. A nice introduction to earthquakes which discusses the Mexico City, Taiwan, and Turkey events is the Why Files write-up on earthquakes. Other useful resources on large and destructive earthquakes include these Web pages from the USGS NEIC.
As a new feature of our Web pages, we will present summaries of large and interesting events. For 1999, we are focusing on earthquakes of magnitude 7 and higher. These pages are under construction, as we put together figures and other supporting materials. At the moment, we have generated plots of seismograms recorded the Berkeley Digital Seismic Network for each event.
For additional information on earthquakes in 1999, the USGS NEIC has an a number of additional earthquake resources.
2) 1999/02/06 21:47:59.47 -12.853 166.697 90.10 7.40 Me SANTA CRUZ ISLANDS
4) 1999/03/08 12:25:48.99 52.397 159.520 56.60 7.10 Me OFF EAST COAST OF KAMCHATKA
7) 1999/04/08 13:10:34.08 43.607 130.350 565.70 7.10 Mw E. USSR-N.E. CHINA BORDER REGION
8) 1999/05/10 20:33:02.09 -5.159 150.880 138.00 7.10 Mw NEW BRITAIN REGION
This disastrous earthquake ruptured a 130-km segment of the North Anatolian Fault, killing over 15,000 people, injuring over 24,000, and destroying nearly 21,000 buildings. This event was the most damaging earthquake in 1999.
This earthquake occurred along the North Anatolian fault, which has the distinction of being the most active strike-slip fault in the world. As a strike-slip fault, it is similar to the San Andreas Fault System in northern California. This map from the USGS compares the San Andreas and North Anatolian fault and one can see that they are similar in the length and straightness. They also are similar in slip rate. However, the San Andreas fault has produced only 2 earthquakes of M >= 6.7 in this century (in 1906 and 1989), the North Anatolian Fault has produced 12!
Turkey is part of the Anatolia plate and this figure from CNRS shows how the Anatolia plate is being "squeezed" out to the west by the northward migration of the Arabian plate. Just as the San Andreas fault forms the main boundary between the North American and the Pacific plate, the North Anatolian fault forms the main boundary between the Anatolian and Eurasian plates. Turkey has a long earthquake history and this figure from the USGS shows that the August 17 event was the latest in a sequence that initiated in 1939. This sequence has largely propagated westward, that is, the earthquakes have occurred as if the fault were "unzipping" to the west. Seismologists have analyzed this progressive failure by modeling the change in stress following each earthquake. The August 17, 1999 earthquake occurred in a region of increased stress, as indicated by the warm colors in this plot from CNRS.
This major earthquake killed over 2,000 people and injured 10,000. A report by the USGS describes the tectonic setting of this event, which occurred at the boundary between the Eurasian and Philippine Sea Plates. A preliminary report on the earthquake is available online and describes the seismological observations. This event was extremely well recorded, as the Central Weather Bureau had completed an extensive program in seismic instrumentation (an investment of over $40 million). These data should be extremely valuable for seismological and engineering research. The EERI sent an engineering reconnaissance team to Taiwan following the earthquake and their report is available online.
This earthquake is the largest event to occur in California since the magnitude 7.3 Landers event on 06/28/1992. It ruptured the Lavic Lake and Bullion faults in the Mojave desert, just 40 km west of the Landers earthquake. This event had a right-lateral strike-slip mechanism and ruptured approximately 40 km in length. This figure from Doug Dreger of UC Berkeley shows a preliminary rupture model for the earthquake.
This earthquake followed closely on the heels of the August 17, 1999 earthquake. The rupture initiated at the east end of the zone that failed in August and closed a small "gap" on the North Anatolian fault. This event also occurred in an area where the stress was increased by the August earthquake, as shown in this figure from the USGS.
18) 1999/11/17 03:27:42.08 -5.978 148.820 47.50 7.00 Mw NEW BRITAIN REGION
19) 1999/11/19 13:56:46.45 -6.351 148.763 33.00 7.00 Mw NEW BRITAIN REGION
20) 1999/11/26 13:21:15.42 -16.434 168.227 33.00 7.50 Ms VANUATU ISLANDS
