The ML 6.7 (MW 7.1) Taiwan Earthquake of December 26, 2006
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Learning from Earthquakes The ML 6.7 (MW 7.1) Taiwan Earthquake of December 26, 2006 Yuan-Tao Weng, Chu-Chieh Jay Lin, Wen-Yu Jean, Tao-Ming Chang, Vivek Walia, Jiunn- Shyang Chiou, Yeong-Kae Yeh, Tsung-Chih Chiou, Fan-Ru Lin National Center for Research on Earthquake Engineering (NCREE) Ching-Yun Kao, Ming-Wey Huang, Siao-Syun Ke, Wen-Ray Su, Hao-Jan Hsing, Tzu-Hsiu Wu, Bing-Ru Wu National Science and Technology Center for Disaster Reduction (NCDR) George C. Yao Dept. Architecture, National Cheng-Kung University This report presents the observations of several reconnaissance teams that assessed the earthquake damage. The information mainly came from NCREE, NCDR, Pingtung University of Science and Technology and National Cheng-Kung University. Overview story was collapsed. Sev- sia, Singapore, and Thai- eral fire outbreaks were land. The reconnaissance On December 26, 2006 at reported right after the team formed by NCREE 8:26 pm local time, a ML earthquakes. There were and NCDR visited the sites 6.7 (MW 7.1) earthquake totally 2 casualties, 45 peo- of damaged buildings with rocked southern coast of ple injured, 3 apartment the following observation. Taiwan, about 22.8 km buildings collapsed as well southwest from Hengchun as 134 schools damaged as Tectonics/Strong (90 km from Kaohsiung). reported by Pingtung Ground Motion About 8 minutes later (8:34 County government. Be- Taiwan island is in the in- pm local time), the first af- sides, the earthquakes also teraction zone of the Eura- tershock with M 6.4 (M damaged several undersea L W sia and Philippine Sea 6.9) hit the region again fiber-optic cables used to plates. The 3 tectonic units followed by the second af- route internet and telephone of southern Taiwan as iden- tershock with M 5.2 at services, disrupted the L tified are, the Eurasia 8:40 pm. Most of the build- businesses in Taiwan, Hong Kong, Japan, China, South (South China Sea), the Tai- ings survived without any wan Island (Kenting penin- damages during these Korea, Philippines, Malay- earthquakes. However, a few street-front commercial /residential buildings, ho- tels and elementary schools have experienced moderate to severe damages. Espe- cially a 4-story furniture store with possible soft first TWK1 Fig. 1 The earthquake focal mechanisms of the main shock and its biggest aftershock are shown. The other aftershocks are shown as stars. The red stars are shallower than 30 km, the blue stars are those deeper than 30 km. The small dots indicate the seismicity for the past 16 years. The red, green, yellow dots are shallower than 10, 20, 30 km respectively. The blue and black dots are deeper than 30 and 50 km. sula) and its surrounding utes later represent two ma- the Philippine Sea plate. accretion wedges, and the jor type focal mechanisms The rest of the aftershocks Philippine Sea plate. The (Fig.1). The main shock is a can be grouped as two parts Philippine Sea plate is mov- normal faulting event according to their focal ing north-westward and is which means the upper part depths. The deeper after- forcing the Eurasia plate to of subducting Eurasia plate shocks (> 30 km) are submerge eastward. The is undergoing the tensional roughly perpendicular to loosely marine sediments force caused by its over- the Manila Trench and be- have been pushed westward burden of thickening accre- come deeper north- to form the accretion tion wedges. Therefore the eastward. The shallow af- wedges and finally emerges strike of the rupture plane is tershocks (< 25 km) are as the Taiwan Island. The parallel to the Manila parallel to the trench. The Manila trench is the bound- Trench. The biggest after- preliminary results suggest ary between these two shock is a strike-slip fault- the main shock released the oblique colliding plates. ing event which means the tensional strains in the sub- accretion wedges undergo ducting Eurasia plate and The main shock and its the north-western compres- immediately trigger to re- biggest aftershock 8 min- sion force being pushed by lease the accumulated com- 80 80 TWK1 : UD TWK1 : UD 80 60 60 KAUP : UD 60 40 40 20 20 40 0 0 20 -20 -20 0 -40 -40 Acceleration, [gal] Acceleration, Acceleration, [gal] Acceleration, -60 -60 [gal]Acceleration, -20 -80 -80 -40 0 20 40 60 80 100 0 20406080100 0 20406080100 150 200 TWK1 : NS TWK1 : NS 50 KAUP : NS 150 100 100 50 0 50 0 0 -50 -50 -50 Acceleration, [gal] Acceleration, Acceleration, [gal] -100Acceleration, -100 [gal] Acceleration, -150 -150 -100 0 20406080100 0 20 40 60 80 100 0 20406080100 200 150 50 TWK1 : EW TWK1 : EW KAUP : EW 150 100 100 50 0 50 0 0 -50 -50 -50 Acceleration, [gal] Acceleration, Acceleration, [gal] Acceleration, -100 -100 Acceleration, [gal] -150 -150 -100 0 20406080100 0 20 40 60 80 100 0 20406080100 Time, [Sec.] Time, [Sec.] Time, [Sec.] Fig. 2 The ground accelerograms recorded at RTD stations TWK1, KAU and KAUP in ML 6.7 main shock 80 80 80 KAU : UD 60 KAUP : UD KAU : UD 60 40 60 20 40 0 40 20 -20 -40 20 0 -60 -80 0 -20 -100 Acceleration, [gal] -120Acceleration, Acceleration, [gal] Acceleration, -40 Acceleration, [gal] Acceleration, -20 -140 -60 -160 -40 0 20 40 60 80 100 020406080100 0 20406080100 150 100 KAUP : NS 100 KAU : NS KAU : NS 100 50 50 50 0 0 0 -50 -50 -50 -100 Acceleration, [gal] Acceleration, [gal] -100Acceleration, Acceleration, [gal] -100 -150 -150 0 20406080100 0 20 40 60 80 100 020406080100 100 150 150 KAU : EW KAU : EW KAUP : EW 100 100 50 50 50 0 0 0 -50 -50 -50 Acceleration, [gal] Acceleration, Acceleration, [gal] Acceleration, Acceleration, [gal] -100 -100 -100 -150 -150 0 20406080100 0 20 40 60 80 100 020406080100 Time, [Sec.] Time, [Sec.] Time, [Sec.] Fig. 3 The ground accelerograms recorded at RTD stations TWK1, KAU and KAUP in ML 6.4 aftershock Table 1 PGA records of main shock (ML6.7) Epicentral PGA Sa(T=0.3) Sa(T=1.0) Station ID Dist.(km) (cm/sec2) (cm/sec2) (cm/sec2) TWK1 26.206 159.6 316.1 244.8 KAU 78.805 64.5 171.4 60.3 KAUP 77.794 61.3 154.9 67.6 SSD 94.727 73.5 117.2 24.3 SPT 86.962 64.9 176.8 91.5 WCH 242.819 60.7 104.3 12.9 SCK 147.193 61.5 129.0 26.0 SGL 92.458 53.3 135.4 95.8 SCL 146.878 47.9 104.4 24.3 SCZ 53.831 48.9 88.1 28.6 TAW 61.989 47.4 132.3 67.9 CHN3 132.453 46.9 102.1 26.2 TAI2 126.441 40.0 80.9 20.1 TCUP 261.677 32.7 90.8 30.6 PNG 211.931 35.9 46.6 7.2 TWK1 26.206 159.6 316.1 244.8 KAU 78.805 64.5 171.4 60.3 Table 2 PGA records of aftershock (ML6.4) Epicentral PGA Sa(T=0.3) Sa(T=1.0) Station ID Dist.(km) (cm/sec2) (cm/sec2) (cm/sec2) TWK1 59.526 179.6 277.8 272.9 KAU 27.289 123.5 375.4 134.5 KAUP 26.683 156.4 347.1 167.2 SCK 92.914 118.3 270.9 109.1 SPT 29.588 90.7 275.1 144.3 SGL 35.085 81.5 204.0 157.9 CHN3 75.323 74.9 187.9 44.6 SSD 39.180 74.6 173.4 35.8 SCL 90.951 70.6 163.9 62.5 TAI2 71.410 64.1 134.8 48.8 SCZ 11.620 56.8 151.5 54.2 TAW 39.867 57.2 147.1 66.9 WCH 185.662 57.4 LAY 114.711 59.2 pressive strain inside the http://www.cwb. gov.tw). The spectral acceleration is accretional wedges. How- Figure 4 shows the re- a little higher than that of ever it needs more intensive sponse spectra of three the design earthquake for studies to fully understand RTD stations, which ex- the TWK1 station, which is the seismological meanings. perienced the largest located at Hengchun and ground excitation (159.88 near the damaged area. A The strong ground motion gal at Kenting), in the similar result of comparison were well recorded by Cen- ML6.7 main shock. The de- of the largest aftershock tral Weather Bureau (CWB) sign spectrum of current (biggest PGA is 156.42 gal TSMIP array and are shown Taiwan building code at Kaohsiung Harbor) of in Figures 2 & 3 as well as (TBC ’05) for Hengchun, magnitude ML6.4 is also Tables 1 & 2 (Further in- the most damaged site, is shown in Figure 5. formation can be found on also shown for comparison. After Shock: ML6.4 0.6 UD Design Code'05(S1) 0.5 Station: TWK1 0.4 Station: KAU 0.3 Station: KAUP 0.2 0.1 Kaohsiung Spectral Acceleration, [g] Acceleration, Spectral 0.0 0.01 0.10 1.00 10.00 0.6 NS 0.5 0.4 0.3 0.2 0.1 Spectral Acceleration, [g] Acceleration, Spectral 0.0 0.01 0.10 1.00 10.00 0.6 EW 0.5 0.4 Pingtung 0.3 0.2 0.1 Spectral Acceleration,0.0 [g] 0.01 0.10 1.00 10.00 Structural Period, [Sec.] Fig. 4 The comparisons of Fig.6 The map for estimated damaged house from response spectra (5% TELES damping) at selected RTD stations in main shock (ML6.7) Main Shock: ML6.7 0.3 UD Design Code'05(S1) Station: TWK1 Kaohsiung 0.2 Station: KAU Station: KAUP 0.1 Spectral Acceleration, [g] Acceleration, Spectral 0.0 0.01 0.10 1.00 10.00 0.6 NS 0.5 0.4 0.3 0.2 Pingtung 0.1 Spectral Acceleration, [g] Acceleration, Spectral 0.0 0.01 0.10 1.00 10.00 0.6 EW 0.5 0.4 0.3 0.2 Fig.