M=6.1 earthquake strikes offshore Taiwan

By David Jacobson, Temblor

Check your hazard rank

This image shows Taiwan’s Coastal Range, which has been thrust up by the Longitudinal Valley Fault. The Longitudinal Valley Fault is the most seismically active fault in eastern Taiwan, and 30% of the country’s earthquakes occur on or near it. (Photo by: Johann Champenois, University Paris-Est / GTMC, Marne-la-vallée, France)

 

Over the weekend, a M=6.1 earthquake struck off the coast of eastern Taiwan near the city of Hualien. This quake occurred at a shallow depth (5-12 km depending on the source), and registered moderate shaking near the epicenter. Fortunately, due to the earthquake’s location, there was no reported damage or injuries. This is also likely due to the fact that eastern Taiwan is not densely-populated, and the country as a whole is one of the most earthquake-prepared countries on earth.

This Temblor map shows the location of this weekend’s M=6.1 earthquake off the coast of Taiwan. This quake struck at the junction of the Longitudinal Valley Fault and the Ryukyu Trench.

 

Even though this quake did not have an impact on the country, what makes it interesting is its location. This earthquake struck at the junction of the Longitudinal Valley Fault and the Nansei-Shoto (Ryukyu) Trench, where the Philippine Sea plate subducts beneath the Eurasian plate. Because of relative plate motions at this location, the dominant movement in this earthquake was compression, with a small strike-slip component.

The Longitudinal Valley Fault is the most seismically active fault in Eastern Taiwan, slips at approximately 20-30 mm/yr, dips 54° to the east, and marks the collision boundary between the Philippine Sea and Eurasian plates. Additionally, up to 30% of the earthquakes in Taiwan every year take place on or near this fault. The Longitudinal Valley Fault is also unique because the southern portion creeps at a rate of 5-28 mm/yr, while the northern portion is locked. This means that in the south, there is very slow continuous movement, while in the north, slip only occurs in earthquakes. While not a black and white rule, creeping faults tend to not rupture in large earthquakes. The creeping aspect of the Longitudinal Valley Fault makes it similar to California’s Hayward Fault, which runs through Oakland and the rest of the San Francisco East Bay.

This figure from Huang et al., 2010 shows the generalized tectonic setting of Taiwan, as well as a zoomed in map of the Longitudinal Valley Fault. Yesterday’s M=6.1 earthquake has been added to these maps with a yellow star. What is evident from these maps is that yesterday’s event occurred very close to three historic offshore earthquakes. In the left map, the red area marks the surface trace of the 1951 earthquakes.

 

Because of the high strain rates off the eastern coast of Taiwan, the area is susceptible to large earthquakes. Between 1811 and 1951 three large earthquake struck off the eastern coast. By using the Global Earthquake Activity Rate (GEAR) model, which is available in Temblor, we can determine if yesterday’s earthquake or the ones between 1811 and 1951 should be considered surprising. This model uses global strain rates and seismicity since 1977 to determine what the likely earthquake magnitude in your lifetime is for any location on earth. From the image below, you can see that that magnitude is M=7.75+. Therefore, yesterday’s earthquake, and the historic offshore quakes, should not be considered surprising and instead provide an opportunity to learn about one of the most seismically active places on earth.

This Temblor map shows the location of the weekend’s M=6.1 earthquake off the coast of Taiwan. This model also shows the Global Earthquake Activity Rate (GEAR) model for Taiwan. This model uses global strain rates the the last 40 years of seismicity to forecast the likely earthquake magnitude in your lifetime anywhere on earth. This figure shows that the weekend’s M=6.1 earthquake should not be considered surprising as a much larger earthquake is possible.

 

References

USGS

EMSC

Central Weather Bureau

Huang, W.-J., K. M. Johnson, J. Fukuda, and S.-B. Yu (2010), Insights into active tectonics of eastern Taiwan from analyses of geodetic and geologic data, J. Geophys. Res., 115, B03413, doi:10.1029/2008JB006208

Taiwan Earthquake Research Center Report – Click here