Research

Our research has mainly focused on earthquake early warning and observational seismology in theTaiwan region.

One of our major efforts has been the establishment of an earthquake early warning system in Taiwan to enable speedy response to damaging earthquakes. Following a large earthquake in Taiwan, utilizing a quick magnitude determination based on the first 10 sec of signals from a virtual and sub-network of seismic stations configured automatically, this system reduces the earthquake reporting time to 30 sec or less. This represents a significant step forward in earthquake early warning capability. This early warning system has been in operation at CWB since 2002 (Wu and Teng, BSSA, 2002; Wu et al., Springer Book chapter, 2007). Comprehensive earthquake reports have been issued mostly in less than 30 sec, with an average of about 22 sec from earthquake origin times. At 3 km/sec for a typical crustal shear-wave speed, the present performance is not useful if an earthquake occurs within 70 km from a city, but the lead time can be more than 10 sec for cities 100 km away from the earthquake source. In the latter case, a lead time of several seconds will allow pre-programmed emergency response to take place prior to the arrival of strong ground shaking. 

Recently, we also explored a practical approach to earthquake early warning (Wu and Kanamori, Sensors, 2008) in southern California (Wu and Zhao, GRL, 2006; Wu et al., GJI, 2007), Taiwan (Wu and Kanamori, BSSA, 2005a, 2005b; Wu et al., GRL, 2006a; Hsiao et al., 2008), and Japan (Wu and Kanamori, EPS, 2008; Shieh et al., 2008) by determining a ground-motion period parameter tc and a high-pass filtered displacement amplitude parameter Pd from the initial 3 sec of the P waveforms.  At a given site, we estimate the magnitude of an event from  and the peak ground-motion velocity (PGV) from Pd. The incoming 3-component signals are recursively converted to ground acceleration, velocity and displacement.  The displacements are recursively filtered with a one-way Butterworth high-pass filter with a cutoff frequency of 0.075 Hz, and a P-wave trigger is constantly monitored.  When a trigger occurs,  and Pd are computed. We found the relationships between  and magnitude (M) for southern California, Taiwan, and Japan, and betweenPd and PGV for both southern California and Taiwan. Those relationships can be used to detect the occurrence of a major earthquake and provide onsite warning in the area around the station where strong ground motion is expected within seconds after the arrival of the P-wave.  When the station density is high, the methods can be applied to multi-station data to increase the robustness of onsite early warning and to complement the regional warning approach.  In an ideal situation, such warnings would be available within 10 sec of the origin time of a large earthquake whose subsequent ground motion may last for tens of seconds.  Pd method may offer early warning within 1 second after P arrival for site near epicenter (Wu and Kanamori, EPS, 2008). Currently, our method is being tested in Taiwan, Southern California, Beijing, Pacific Tsunami Warning Center and Italy.

For the research on Taiwan regional observational seismology, I have focused on seismotectonics, seismicity, velocity structures, earthquake relocation and earthquake sequence analysis, major achievements include:

1.                     Determination of the Taiwan regional 3D Vp and Vp/Vs structures using dense arrays from real-time seismic network and strong motion network (Wu et al., JGR, 2007). Earthquakes of the Taiwan region from 1991 to 2005 in the Central Weather Bureau catalog were relocated using this new 3D velocity models (Wu et al., BSSA, 2008a). A total of 1,635 earthquake focal mechanisms for M ≥ 4 in the Taiwan region from 1991 to 2005 were also determined using the Genetic Algorithm (Wu et al., BSSA, 2008b). These datasets offer a new viewpoint to study the seismotectonics of the Taiwan region. Further, we combined the data from ocean bottom seismometers and Japan Meteorological Agency to improve velocity structures and quality of earthquake location for offshore region. To offer a new viewpoint of the subduction zones (Wu et al., Tectonophysics, 2008a; Wu et al., GJI submitted, 2008) and arc-continental collision zone.  

2.                     Within the hour immediately after the 1999 Chi-Chi earthquake, only 40 aftershocks were located by the CWB seismic network. However, by analyzing 20 near-field, on-scale strong motion records, we determined a catalog of 296 aftershocks with M_L ³ 3.4 within the first hour after the Chi-Chi earthquake (Chang et al, BSSA, 2007). This offers a good opportunity to study the earthquake physics and aftershock decay problems.

3.                     A detailed post-earthquake examination of the seismicity in the source region revealed that the 1999 Chi-Chi Mw7.6 earthquake was preceded by a noticeable decrease in regional seismicity rate (Wu and Chiao, BSSA, 2006). Wu and Chen (Tectonophysics, 2007) also reported a cycle of the seismic reversal embedded in the changes of seismicity before and after the Chi-Chi earthquake. The 2003 Mw6.8 Chengkung earthquake was also preceded by a noticeable decrease in regional seismicity rate, with low b values in the source region (Wu et al., Tectonophysics, 2008b).

4.                     We proposed a new concept for M_L determination by which different regions could have the same magnitude scale (Wu et al., BSSA, 2005).

5.                     We proposed an approach for recovering the coseismic deformation from strong motion record (Wu and Wu, Journal of Seismology, 2007) and applied it to the 2003 Chengkung Mw 6.8 earthquake (Wu et al., GRL, 2006b) and the 2006 Taitung Mw6.1 earthquake (Wu et al., GRL, 2006c). Our study on the source ruptures of these two events reveals the existence of the Central range fault and costal range fault. These are very important results to the tectonics of the eastern Taiwan.

6.         We relocated the 1998 Rueyli Mw5.7 earthquake sequence (Wu et al., TAO, 2003), the 2003 Chengkung Mw 6.8 earthquake sequence (Kuochen et al., J. Asian Earth Science, 2007), the 2006 Pingtung Mw 7.0 earthquake sequence (Wu et al., Tectonophysics, 2008a) and the shallow earthquakes of the eastern Taiwan (Kuochen et al., TAO, 2004) region, and studied their seismotectonics.