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Scientific files |
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Large earthquakes of summer 1999 |
 Taiwan
22/09/1999 5:00 pm report
On September 20, 1999 at 5:47 pm UT (i.e. 7:47 pm in France), an earthquake of magnitude MS = 7.7 occurred in Taiwan 150 kilometers south-west of Taipei, the capital city. This earthquake occurred at shallow depth and was felt in China. The number of casualties is estimated at over 2000 (AFP dispatch) and is not surprising considering the magnitude and shallow depth of the earthquake, and the densely populated areas in the vicinity.
The earthquake triggered an EMSC alert, and its result was sent out one hour after the earthquake and included all available data. Earthquake characteristics are as follows:
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 ORIGIN TIME: 5:47:16 pm |
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LATITUDE: 23.87 North |
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LONGITUDE: 121.10 East |
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DEPTH: 10 km |
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MAGNITUDE: Mm = 7.7 (LDG), MS = 7.6 (USGS) |
Its location was determined using 8 networks providing 161 arrival times and good coverage of the epicentral area (gap = 120°). The arrival of data from the Papeete station significantly improved location quality by constraining the south-west azimuth.
The main shock was followed by numerous aftershocks of magnitude above 4.0 (over 30 as seen on the LDG network). To date, the strongest aftershock occurred on 22/09 at 00:14 am (UT). Its characteristics are as follows:
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ORIGIN TIME: 8:44:55.2 pm |
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LATITUDE: 23.7 North |
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LONGITUDE: 121.1 East |
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MAGNITUDE: MS = 6.5 (GSSC) |
Characteristics of the main shock source
The focal mechanism and the scalar seismic moment were calculated by several institutes including USGS. This earthquake shows a reverse fault mechanism, which is in keeping with the tectonic structure of the area (compressive regime due to Eurasian and Pacific plate collision). The calculated value of the seismic moment is 2.4 1020 N.m. (USGS), 5.0 1020 N.m. (PPT).
A detailed study of the seismograms will be required to provide a precise estimate of the size of the rupture and source dislocation (static displacement).
Previous seismicity in the area
This area is characterized by regular seismicity. The largest earthquake previously recorded occurred on April 21, 1935 (M = 7.1), claiming 3000 lives. The chart below summarizes activity over the past 10 years.
| Date |
Location |
Magnitude |
Damage |
| 17/07/1998 |
South-west of Taiwan |
5.4 |
5 lives lost |
| 03/05/1998 |
Pacific Ocean, south-west of Taiwan |
7.3 |
|
| 05/09/1996 |
Pacific Ocean, south of Taiwan |
6.6 |
|
| 16/03/1996 |
Pacific Ocean, east of Taiwan |
6.4 |
|
| 23/06/1995 |
Center of Taiwan |
5.5 |
1 life lost |
| 23/02/1995 |
Center of Taiwan |
6.2 |
2 lives lost |
| 16/09/1994 |
South-west of Taiwan |
6.7 |
1 life lost |
| 05/06/1994 |
Center of Taiwan |
6.6 |
1 life lost |
| 24/05/1994 |
Pacific Ocean, east of Taiwan |
7.1 |
|
| 14/11/1986 |
South-east of Taiwan |
7.8 |
15 lives lost |
|
Athens
09/09/1999 3:00 pm report
On September 7, 1999 at 11:56 am UT (i.e. 1:56 pm in France), an earthquake of magnitude MS = 5.4 occurred near Athens (Greece) approximately 30 kilometers north-west of the Greek capital. This earthquake occurred at shallow depth and was felt in a large part of Greece. The number of casualties is estimated at close to 50 (AFP dispatch) and is not surprising considering the magnitude and shallow depth, as well as its proximity with Athens.
This earthquake triggered an EMSC alert, and its result was sent out one hour after the earthquake, based on all available data. Earthquake characteristics are as follows:
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ORIGIN TIME: 11:56:49 am |
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LATITUDE: 38.13 North |
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LONGITUDE: 23.58 East |
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DEPTH: 5 km |
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MAGNITUDE: MS = 5.4 (LDG), Mw = 5.9 (USGS) |
Its location was determined using 10 networks (270 arrival times) providing good coverage of the epicentral area (gap = 63°) and based on local Greek data as well as regional data from Italy, Switzerland, France, Germany, Spain and Israel, etc.
A precursor event occurred before the main shock (on 03/09/1999 at 5:26 am (UT), magnitude Ml = 4.1) and numerous aftershocks of magnitude above 4.0 occurred (over 10 as seen on the LDG network between 12:00 pm and 8:00 pm (UT)). To date, the strongest aftershock occurred on 07/09 at 8:44 pm (UT). Its characteristics are as follows:
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ORIGIN TIME: 8:44:55.2 pm |
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LATITUDE: 38.03 North |
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LONGITUDE: 23.64 East |
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DEPTH: 20 km |
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MAGNITUDE: Mb = 4.4 (LDG), MS = 4.2 (GSSC) |
Characteristics of the main shock source
The focal mechanism and the scalar seismic moment were calculated by several institutes including USGS. This earthquake shows a normal fault mechanism, which is in keeping with the tectonic structure of the area (extensive regime). The calculated value of the seismic moment is 7.8 1017 N.m.
A detailed study of the seismograms will be required to provide a precise estimate of the size of the rupture and source dislocation (static displacement).
Previous seismicity in the area
This area is marked by regular seismicity. Twelve earthquakes of magnitude above 5.0 have been recorded since 1974 within a 100 km radius of the epicenter. The largest earthquake occurred on 24/02/1981 (magnitude 6.8). It was located approximately 50 kilometers away from the September 7, 1999 earthquake.
Izmit
17/08/1999 3:00 pm report
On August 17, 1999 at 00:01 am UT (i.e. 02:01 am in France), an earthquake of magnitude MS = 7.4 occurred in the immediate vicinity of Izmit (Turkey), in an area on the east bank of the Marmara Sea (refer to attached map). This earthquake occurred at shallow depth and was felt all over Turkey, in Greece and in Bulgaria. The number of casualties is estimated at close to 600 (AFP dispatch) and is not surprising considering the magnitude and the shallow depth as well as proximity with Izmit. The earthquake was located 150 km east of Istanbul and 600 km west of Ankara.
This earthquake triggered an EMSC alert that was sent out at 1:10 am (UT), i.e. just over one hour after the event. Renewed EMSC calculations produced a more precise location in the morning, based on all available data. Earthquake characteristics are as follows:
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ORIGIN TIME: 00:01:37.0 am |
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LATITUDE: 40.70 North |
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LONGITUDE: 29.98 East |
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DEPTH: 17 km |
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MAGNITUDE: Mw = 7.6 (GFZ Potsdam), MS = 7.4 (LDG) |
Its location was determined using 17 networks (216 arrival times) providing relatively good coverage of the epicentral area (gap = 57°) and based on local Greek and Russian data as well as regional data from Italy, Switzerland, France, Germany and Spain, etc.
The Edinburgh TREMORS station was triggered and provided a magnitude Mm = 7.6, which was fully compatible with the Mw value calculated by Potsdam.
The main shock is followed by numerous aftershocks (over 30 as seen on the LDG network between 00:00 am and 06:00 am (UT)), 10 of which had an Mb magnitude above 4.5. The strongest aftershock recorded occurred at 03:14 am (UT). Its characteristics are as follows:
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ORIGIN TIME: 03:14:00.0 am |
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LATITUDE: 40.60 North |
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LONGITUDE: 30.63 East |
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DEPTH: 10 km |
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MAGNITUDE: Mb = 5.2 (LDG), MS = 5.0 (GSSC) |
Characteristics of the main shock source
The focal mechanism and the scalar seismic moment were calculated by several institutes including GFZ Potsdam (as part of EMSC) and USGS. These mechanisms are perfectly consistent and indicate that the earthquake presented a near-pure right-slip fault. The calculated value of the seismic moment is 2.4 1020 N.m.
Generally an earthquake of this magnitude represents a fault rupture of approximately 100 km. A detailed study of the seismograms will be required to provide a precise estimate of the size of the rupture and source dislocation (static displacement).
Seismotectonic setting
Turkey's North Anatolian fault is on its northern border. This fault crosses the country from east to west from Karliova to Greece. As far as plate tectonics is concerned, this is a major structure as it is about 1200 km long and separates the European plate (north) from the Anatolian block (south).
Its activity is linked to the westward expulsion of the Anatolian block, caused by the collision between the Arabian and Eurasian plates. The North Anatolian fault extends westward, divides into separate segments and then disappears into the North Aegean trench.
The lateral expulsion of the Anatolian block triggers migration of the rupture along the segments of the North Anatolian fault, from east to west, or during high magnitude seismic episodes. The main earthquake and its aftershocks on 17/08/1999 were located on a segment of the western end of the North Anatolian fault.
This fault is a right-slip fault, its direction is confirmed by both GPS data analysis and morphological studies performed in situ and by the focal mechanisms of instrumental earthquakes. This fault has horizontal displacement rates of about 2cm/year.
This is an area of regular historical seismicity. Four other earthquakes of magnitude above 7.0 have been recorded since 1939. The last earthquake located in the immediate vicinity of the August 17 earthquake occurred in 1983 (magnitude 5.0).
| Area on the fault |
Year |
Magnitude |
| West |
1957 |
7.1 |
| West |
1967 |
7.2 |
| West |
1983 |
5 |
| East (Erzincan) |
1939 |
8 |
| East (Mudurnutal) |
1967 |
7.2 |
| East (Erzincan) |
1992 |
6.9 |
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