Seismo-Watch

Special Earthquake Report No: 99-022

Regional Location:

Preliminary Magnitude:

Moment Magnitude:

Greenwich Mean Date:
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Latitude:
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Analysis Quality (A-D):

TAIWAN

7.7 Ms (NEIC)

Mw 7.7 (HRV), Mw 7.6 (NEIC)

99/09/20
18:16:18
23.69N
121.31E
20.7 km
A

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An earthquake registering a preliminary magnitude of Ms 7.6 (NEIC), Mw 7.7 (Harvard), Mw 7.6 (NEIC) -was detected 17:47:19 UTC, September 20, on the island of Taiwan. The epicenter of the main shock is located about 90 miles southwest of Taipei, about 30 miles southeast from Taichung and 30 miles northeast of Chiaya in the remote mountainous area of Nantou Province near the center of the island. (See epicenter maps)
 
The fault plane solutions from Harvard Geophysical Laboratory (HRV solution) and the USGS National Earthquake Information Center (NEIC solution) both indicated thrust faulting along a north-northeast trending, east dipping fault plane, but their moment magnitude estimates differ by a tenth of a magnitude.
 
The Jiji earthquake occurred along the Shundong Fault and geologists from Taiwan's National Central University have discovered stunning surface fault ruptures. Preliminary reports indicate average vertical displacements are on the order of 2 -3 m (6-10 ft) and there is supposedly one location with an 8 m (25 ft) displacement. The length of the surface fault ruptures have not be determined as of yet.
 
There are reports of widespread damage and casualties, including collapsed buildings, landslides, liquefaction failures, power outages and road closures. Television footage showed several collapsed buildings and stunning rescues. Preliminary reports indicate more than 2,000 people have been killed over 5,000 were injured and more than 100,00 have been left homeless. Many people are still missing and believed trapped in the rubble and rescue attempts are ongoing. Several areas of the islands are inaccessible at this time and there has been little word from these regions. (Photo Courtesy of TUES-CNN)
 
Television footage showed damaged and collapsed buildings as far away as Taipei, the nation's capital and some 90 miles from the epicenter. Much of the rescue operations are focusing on the 78-room Sungshan Hotel which had the bottom stories collapse, trapping occupants in side. Many structures are tilted and lean precariously against other adjacent buildings. One 12-story building had much of the interior cave in leaving the exterior as a skeleton of rebar, broken concrete and shattered windows. In the photo above, also notice the leaning structure behind the collapsed one in the foreground.
 
Hardest hit appears to be Taichung and nearby Nantou, located near the epicentral region, where at least 1,200 people have been killed. Reports indicate the area has seen a burst of development in recent years, often with shoddy construction. Yet in all the devastation and as an effected region, these building appear better built than those in Western Turkey where a M7.4 earthquake on August 17 killed over 15,000 people and injured some 47,000 others.
 
Some fires have broken out and there are reports of out of control blazes in the epicentral area. Many utility lines were severed make fire response difficult.
 
Transportation routes throughout the country have been compromised. Landslides have taken out portions while other were blocked by debris strung across the roads. Other areas have heaved and buckled pavement all of which is hampering aid and rescue workers from reaching many portions of the island.
 
Many said the quake began slowly, then steadily intensified to where it was difficult to stand. Many people walking outside were shaken from their feet and tossed to the ground. Severe shaking lasted up to 20-30 seconds and perhaps longer on soft sediments. Some reported a deafening roar that came with the shaking.
 
The U.S. Geological Survey's National Earthquake Information Center (NEIC) said the quake prompted tsunami warnings for Taiwan, Japan, the Philippines, Yap, Guam, and Palau. There has been no news of any waves at this time.

There has been a robust aftershock sequence which has included at least five strong aftershock registering about M6 within the first few hours of the main shock. Here is a list of the largest events so far:

99/09/20 17:47:19 23.78N 121.09E 33.0 7.6Ms A TAIWAN
99/09/20 17:57:16 23.80N 121.34E 33.0
6.0Mb B TAIWAN
99/09/20 18:03:44 23.65N 121.36E 33.0
5.9Mb B TAIWAN
99/09/20 18:11:53 23.75N 121.19E 33.0
6.1Mb B TAIWAN
99/09/20 18:16:18 23.69N 121.31E 33.0
6.1Mb B TAIWAN
99/09/20 21:46:44 23.61N 121.12E 33.0
6.4Ms B TAIWAN
99/09/20 21:54:49 23.64N 120.94E 33.0
5.4Mb C TAIWAN
99/09/21 17:38:39 23.90N 121.29E 33.0
5.1Mb A TAIWAN
99/09/21 22:17:05 24.18N 121.05E 33.0
5.3Mb B TAIWAN
99/09/22 00:14:39 23.71N 121.11E 33.0
6.3Ms A TAIWAN
99/09/22 00:49:42 23.56N 121.18E 33.0
6.0Mb B TAIWAN
99/09/23 12:44:34 23.76N 121.26E 33.0
5.0Mb C TAIWAN
99/09/25 08:43:31 23.56N 121.27E 33.0
5.1Mb B TAIWAN
99/09/25 23:52:51 23.78N 121.12E 33.0
6.5Mw B TAIWAN

Taiwan receives thousands of earthquakes each year with most centered off the coast. A M7.8 quake struck on November 14, 1986, in about the same region, killing 15 and injuring 44 others. The M7.4 quake in 1935 killed more than 3,250 people. Here is a list of M7+ and M6.5+ earthquakes in the Taiwan region since 1900 (List).

Taiwan is located at the junction of the Manila and Ryukyu Trench in the Western Philippine Sea where the Philippine plate is being forced under the Eurasia plate. The Philippine plate is moving in a northwest direction which causes a significant strike-slip component along the northern portion of the Manila Trench and essentially creates a "transpressional" effect which has popped up the island of Taiwan microplate relative to its larger tectonic neighbors.

Several large thrust faults extend across the length of the island of Taiwan. Faults on the western side of the island tend to dip toward the east and seem to be an extension of the Manila Trench. Faults along the eastern seaboard tend to dip toward the west and back under the island. There are several elevated Holocene shorelines along the eastern coast of Taiwan which are believed to be due to the tectonic uplift along the eastern fault system.

The Jiji earthquake appears to have occurred along the complex western fault zone which is associated with the Manila Trench.


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Update: 7:00 p.m., September 25, 1999.

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Subtract 8 hours from Greenwich Mean Time to obtain PST or 4 hours for EDT
Location Quality: A (good), B (fair), C (poor), D (bad)
Magnitude: Ml (local or Richter magnitude), Lg (mblg), Md (duration), Mb (body wave), Ms (surface wave), Mw (moment)

Standard Sources Include:
U.S. Geological Survey, National Earthquake Information Center, Golden, CO (NEIC)
Harvard Geophysical Observatory, Harvard University, Cambridge, MA (HRV)
U.S. Geological Survey, Northern California Seismic Network (NCSN)
University of California, Berkeley, Seismological Laboratory (UCBSL)
Southern California Seismic Network (USGS & Caltech), Pasadena, California (SCSN)
California Institute of Technology, Pasadena, California (Caltech)
various wire reports and/or personal communications

All data are preliminary and subject to change.
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