Strong Motion Networks in Taiwan

There are several island-wide strong motion networks in Taiwan and the earliest network can be traced back to the 1970s (Shin et al., 2003). The first one, named Strong-Motion Accelerographs (SMA) network, was deployed by Institute of Earth Sciences (IES), Academia Sinica. This network began in 1974 and consisted of 72 stations by 1983. In the early 1990s, IES decided to close the SMA network and, as a consequence, several new island-wide strong motion networks were deployed by different institutions in Taiwan. The most important goal of these networks is to routinely estimate the source parameters; however, these strong motion networks with dense seismometers can also be used to provide high-quality ground motion data for the investigation of the far distant earthquakes.

In this study, the seismic ground motion data from the National Center for Research on Earthquake Engineering (NCREE) network and the Broadband Array in Taiwan for Seismology (BAS) network are majorly used for ultra-sensitive equipment. The seismometer distribution of the NCREE and BATS networks is shown in Figure 3.2.

3.1.1 National Center for Research on Earthquake Engineering (NCREE) Network Seismic activities are frequent in Taiwan; on average, more than 4000 earthquakes, including over 200 sensible earthquakes, happen each year. Moreover, several disastrous earthquakes have occurred over the past hundred years. Although the energy accumulation for these disastrous earthquakes may take time, the government needs a real-time decision support system (DSS) for managing emergency response during such seismic events. To support this, NCREE developed the Taiwan Earthquake Loss Estimation System (TELES) and the corresponding strong motion network.

The NCREE network incorporated with the TELES entered operation in early 2012 and became a prototype system for the government’s decision-making process in Taiwan. Now, this network has implemented 33 permanent stations and the distribution of the permanent stations has covered the entire Taiwan area.

Except for the main purpose of supporting the DSS, the NCREE network is also designed to provide hazard mitigation, refine building codes, and improve seismic-resistant designs. Hence, all permanent stations are equipped with state-of-the-art broadband seismometers and up to 24-bits digital recorders. The ground motion data are continuously recorded on hard disks with a very high

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sampling rate (100 samples per second). All the permanent stations are connected to the data center through the internet for real-time communication. The central platform is provided by Güralp System and the peripheral programs are developed by NCREE to provide various functionalities. After a seismic event, the network can immediately obtain the ground motion data and estimate earthquake information within 30 to 60 minutes. Currently, the NCREE network has produced plentiful ground motion data for the research on earthquake (Chang, 2008).

3.1.2 Broadband Array in Taiwan for Seismology (BATS) Network

In mid-1992, IES, along with several domestic and foreign institutions, began to prepare a prospectus for establishing a broadband seismic network in the Taiwan region. The BATS network is initially designed with 15 permanent stations located on Taiwan and the surrounding islands, approximately covering an area of 350 km by 400 km. The network entered test operation in late 1994 and has produced high-quality data in sufficient quantity since early 1996. Currently, the number of permanent broadband stations is 28, including 2 located in the South China Sea with satellite transmission (Kao et al., 2001; Kao and Jian, 2001).

The BATS network is designed to cover a variety of research demands. All permanent stations are equipped with state-of-the-art broadband seismometers and 24/26-bits digital recorders. The ground motion data with high sampling rates (higher than 80 samples per second) were recorded in trigger mode before and, after 2002, the network was gradually upgraded to have the ability to continuously record the data on both hard disks and magnetic tapes (but the sampling rate reduces to 20 samples per second). Besides, all permanent stations are capable of internet connection for retrieving the real-time data. BATS data acquisition in the data center is operated using the Antelope software platform made by Kinemetrics since 2007. After performing the preliminary data quality control, the ground motion data are distributed to the public through the web-based data service at the official BATS website. Now, the IES Data Management Center (DMC-IES) has taken care of the routine network operation, data processing, quality control, and data release, exchange, contribution, etc. (Institute of Earth Sciences, 1996).

3.1.3 Other Strong Motion Networks and Seismometers in High-tech Fabs

There are two different island-wide strong motion networks in Taiwan other than the NCREE and BATS networks. Moreover, to avoid the impact of earthquakes, some high-tech fabs in Taiwan have their own earthquake monitoring systems.

Taiwan Strong Motion Instrumentation Program (TSMIP)

Instead of the SMA network, a new strong motion network, named Taiwan Strong Motion

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Instrumentation Program (TSMIP), was widely deployed by the Central Weather Bureau (CWB) in 1992. The TSMIP includes more than 600 stations at the beginning and increases to 700 stations in 2007. However, most of the TSMIP stations are placed on the plain area, with special emphasis in nine metropolitan areas, and only limited stations are located at mountain area. Two types of digital instruments were deployed throughout Taiwan; one is a tri-axial accelerograph for recording free-field ground shakings and the other is a multi-channel accelerograph array system for monitoring the shakings caused by earthquakes in buildings and other structures. Now, the TSMIP connects over 800 strong motion stations (including the borehole stations) that consist of 110 real-time stations and around 60 building arrays (Kuo et al., 1995; Liu et al., 1999).

Taiwan Rapid Earthquake Information Release System (TREIRS)

The demand for real-time seismological observation has long been recognized and the significant advances have successfully been made during the 1990s. The idea of an island-wide EEW system using the existing telemetry in Taiwan was first proposed in the early 1990s. The Taiwan Rapid Earthquake Information Release System (TREIRS) based on a simple hardware/software design was first introduced in 1989 and subsequently improved and refined from 1996 to 1999. This system consists of 61 telemetered strong motion accelerographs in Taiwan. The seismic ground motion data are continuously sampled with 50 samples per second and telemetered to the CWB headquarters through 4800-baud leased telephone lines. Therefore, TREIRS is capable to routinely determine the location and magnitude of earthquakes in the Taiwan region within one minute of occurrence and the earthquake information is immediately dispatched to emergency response agencies in various ways (Wu et al., 1997; Wu et al., 2000).

Seismometers in High-tech Fabs

In addition to the above-mentioned island-wide strong motion networks, some high-tech fabs in Taiwan have also implemented their own seismometers, and since the Great East Japan earthquake (March 11, 2011), many systems were set to continuous recording mode rather than trigger mode.

Thus, far distant earthquakes, such as the Indian Ocean earthquake (April 11, 2012), the Samar (Philippine) earthquake (August 31, 2012), and the Lushan (China) earthquake (April 20, 2013), have been sensed and recorded. The specifications of seismometers are listed as Table 3.2.

3.1.4 Comparison between Different Networks

Before the following analysis, a comparison between the BATS and NCREE networks needs to be done to ensure the correspondence across all the stations. The first example is the comparison between the MASB station (from the BATS network) and the B810 station (from the NCREE network)

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during the Samar (Philippine) earthquake. Both these two stations are located in Pingtung County, Taiwan. The MASB station is in Wanluan Township and the B810 station is in Neipu Township, but they are just about 4 kilometers apart. According to the WGS (World Geodetic System) 84 standard, the longitude, latitude, and elevation of the MASB station are 22.6109, 120.6326, and 106, respectively, and the ones of the B810 station are 22.641, 120.6278, and 104, respectively. Figure 3.3 shows the seismic waveforms collected individually from these two stations. Obviously, during this far distant earthquake, both the two stations are sensitive and can correctly record the seismic waveforms, including the primary waves (from 50 to 200 seconds) and the secondary waves (after 200 seconds). The comparison also shows the two stations share the same phases. The only difference is that the amplitudes of the MASB station (represented by the gray line) are slightly larger than the B810 station (represented by the black line).

Another example also comes from the MASB station and the B810 station, but the ground motion data were collected during the Lushan (China) earthquake. Figure 3.4 shows the comparison between the recorded seismic waveforms. Clearly, they are much more consistent than the previous example. These two stations have almost the same waveforms before 500 seconds, and the amplitudes of the MASB station are slightly larger than the B810 station after 500 seconds; however, the difference is small and the phases are near the same. Overall, these two stations are in a very good agreement on both the amplitudes and phases.

The third example is the comparison between the YHNB station (from the BATS network) and the B400 station (from the NCREE network) during the Lushan (China) earthquake, as shown in Figure 3.5. Both these two stations are located in Fusing District, Taoyuan City, Taiwan, and they are just 1 kilometer apart. The longitude, latitude, and elevation of the YHNB station are 24.6695, 121.3757, and 777, respectively, and the ones of the B400 station are 24.675, 121.3668, and 741, respectively. Because the two stations are very close to each other, the recorded waveforms are nearly identical throughout the earthquake duration. This further confirms the correspondence between the BATS and NCREE networks.

The last example is the seismic waveforms collected individually from the RLNB station (from the BATS network) and the B550 station (from the NCREE network) during the Indian Ocean earthquake. Both these two stations are located near Central Taiwan Science Park, which is the main target in this study. The longitude, latitude, and elevation of the RLNB station are 23.8911, 120.3594, and 63, respectively, and the ones of the B810 station are 24.1231, 120.7548, and 155, respectively.

The distance between the two stations is about 48 kilometers and Figure 3.6 shows the seismic

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waveforms collected during the far distant earthquake. Again, the comparison displays no significant difference across these two networks except that the B550 station has a saturation level of 0.3 cm/sec.

In fact, to collect the seismic waveforms of the micro-earthquakes, the NCREE network equips with the seismometers that focus on the amplitudes below 0.3 cm/sec. This limited measurement range enlarges the sensitivity and ensures the higher quality of the ground motion data. To further examine the correspondence between the BATS and NCREE networks, the FFT is applied to the seismic waveforms showed in Figure 3.6. The window is limited from 0 to 915 seconds because of the saturation happened after this point. The Fourier spectrum is shown in Figure 3.7 and the black and gray lines represent the ones from the B550 station and the RLNB station, respectively. The result displays that the major frequency collected from both these two stations is between 0.05 Hz and 0.1 Hz which means a major period up to 20 seconds. Again, they exhibit a good agreement, even in the frequency content.