Performance of a Low-Cost Earthquake Early Warning System
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Performance of a Low-Cost Earthquake Early Warning System (P-Alert) and Shake Map Production during the 2018 M w 6.4 Hualien, Taiwan, Earthquake by Yih-Min Wu, Himanshu Mittal, Ting-Chung Huang, Benjamin M. Yang, Jyh-Cherng Jan, and Sean Kuanhsiang Chen ABSTRACT On 6 February 2018, an M w 6.4 earthquake struck near the city of Taiwan, the EP subducts eastward under the PSP (Tsai et al., of Hualien, in eastern Taiwan with a focal depth of 10.4 km. 1977; Wu et al., 2008). Tectonically, most of Taiwan is under The earthquake caused strong shaking and severe damage to northwest–southeast compression, with a measured conver- many buildings in Hualien. The maximum intensity during this gence rate of ∼8cm=yr (Yu et al., 1997). The damaging earth- earthquake reached VII (> 0:4g) in the epicentral region, which quakes happening in Taiwan can be divided into two is extreme in Taiwan and capable of causing damage in built categories, those associated with the plate boundary and the structures. About 17 people died and approximately 285 were others associated with active faults in western Taiwan (Wu injured. Taiwan was one of the first countries to implement an et al., 1999). earthquake early warning (EEW) system that is capable of issu- The Hualien earthquake is located in northeastern Tai- ing a warning prior to strong shaking. In addition to the official wan, where seismic activity is due to the subduction of the EEW run by the Central Weather Bureau (CWB), a low-cost PSP under the EP. In the neighborhood of the epicenter, there EEW system (P-alert) has been deployed by National Taiwan is the Milun fault (Fig. 1a). It caused an M 7.3 earthquake in University (NTU). The P-alert network is currently operational October 1951, although it has not been active for the past two and is capable of providing on-site EEW as well as a map of decades. A detailed study of in-land seismogenic features in expected ground shaking. In the present work, we demonstrate Taiwan by Shyu et al. (2016) suggests that the region is a the performance of the P-alert network during the 2018 curved reverse-fault system. The system consists of two reverse Hualien earthquake. The shake maps generated by the P-alert faults, the Milun fault and the other east–west-striking reverse network were available within 2 min and are in good agreement fault located offshore north of the Milun tableland (Fig. 1a). with the patterns of observed damage in the area. These shake The recent earthquake of 6 February 2018 may be associ- maps provide insights into rupture directivity that are crucial ated with the Milun fault. The peak ground acceleration (PGA) for earthquake engineering. During this earthquake, individual at some stations recorded by the P-alert network reached around P-alert stations acted as on-site EEWsystems and provided 2–8s 0:6g, giving rise to a maximum intensity of VII. This earthquake lead time in the blind zone around the epicenter. The coseismic was well located by the Central Weather Bureau (CWB) (offi- deformation (Cd) is estimated using the records of P-alert sta- cial agency in Taiwan). Based on the CWB rapid-reporting sys- 2 tions. The higher Cd values (Cd > ) in the epicentral region tem (Wu et al.,1997, 2000), the earthquake was located 18.3 km are very helpful for authorities for the purpose of responding to north-northeast of city of Hualien, with a focal depth 10.0 km. damage mitigation. The Broadband Array in Taiwan for Seismology (BATS) and the U.S. Geological Survey (USGS) reported focal mechanisms for this event (Fig. 1a), and both confirm oblique slip faulting INTRODUCTION with two nodal planes striking northeast–southwest and north- west–southeast. The notion that the Milun fault is responsible Located on the western circum-Pacific seismic belt, Taiwan is for this earthquake is based on the structure and location of the frequently struck by earthquakes. The collision between the Milun fault, which is mapped as a reverse fault with a strike-slip nearby Philippine Sea plate (PSP) and Eurasian plates (EP) component. The obtained focal mechanism seems to support contributes to the complex geological features of Taiwan. this finding. This earthquake caused severe damage to many To the east of Taiwan, the PSP subducts northward under buildings in the city of Hualien. About 17 people were reported the EP along the Ryukyu trench (Fig. 1a). To the southwest dead and 285 injured (Wikipedia, 2018). Maximum causalities doi: 10.1785/0220180170 Seismological Research Letters Volume 90, Number 1 January/February 2019 19 Downloaded from https://pubs.geoscienceworld.org/ssa/srl/article-pdf/90/1/19/4603292/srl-2018170.1.pdf by National Taiwan Univ - Lib Serials Dept user on 23 April 2019 (a) (b) 24.2°24.2° 550 Damaged building 900 600 750 850 Okinawa trough USGS BATS 800 Eurasian plate 750 Ryukyu Trench 2018.02.06 M 6.4 2018.02.06 Mw 6.4 w 700 24.1°24.1° 650 Philippine 650 Sea plate Manila Trench 700 600 550 600 450 550 500 Hualien city 500 30 (m/s) S Latitude (N) 450 350 Milun Fault V 450 400 300 24.0°24.0° 65 Milun Tableland 500 400 Pacific Ocean 350 300 250 5 km 600 5 km 200 Damaged building 550 23.9°23.9° 150 121.5° 121.6°121.6° 121.7°121.7° 121.8°121.8° 121.5°121.5° 121.6°121.6° 121.7°121.7° 121.8°121.8° Longitude (E) ▴ Figure 1. (a) Tectonics map of the Taiwan region and Hualien area. The red line indicates the Milun fault. (b) V S30 map of the Hualien area. Stars show the epicenter of the 6 February 2018 Mw 6.4 Hualien earthquake, and squares show the buildings damaged by the Hualien earthquake. were reported in a 12-story residential building that tilted due to in the plains (Mittal et al.,2013, 2016). During the Hualien collapse of some lower floors. Correlation of this earthquake earthquake, the P-alert system performed very well and recorded with the Milun fault is also supported by the damage pattern good-quality data. A detailed shake map was provided by the P- of buildings that are within a few hundred meters of this fault. alert network within 2 min of the occurrence of the Hualien Although five of the collapsed structures are in the vicinity of the earthquake. The areas with maximum values of PGA and peak Milun fault, soil liquefaction and resonance may also play a role ground velocity (PGV) show some correlation with the damaged because the city of Hualien is built on deep sediment (Fig. 1b). areas. For example, the damaged buildings are located in the area This earthquake occurred 2 yrs after the 6 February 2016 Mei- where PGV is maximum. The network also has the capability to nong earthquake that killed 117 people (Wu et al.,2016)in image rupture directivity, a key parameter in damage assessment; southern Taiwan. Wu et al. (2016) reported the performance stations in the direction of propagation of the rupture front will of the P-alert network-based earthquake early warning record larger acceleration or velocity values, whereas stations in (EEW) system during this earthquake. the other directions will observe smaller values. This phenome- Taiwan was one of the first countries to implement an non was well documented in previous studies (Hsieh et al.,2014; EEW system that is capable of issuing a warning prior to strong Wu, 2015). For regional EEW, signals are transferred continu- shaking. The CWB only issues regional warnings for cities more ously from field stations to a central recording station situated at than 50 km away from the epicentral region. For cities within NTU, as well as at Institute of Earth Science, Academia Sinica 50 km, an on-site EEWsystem would be helpful. In addition to (IESAS) for research purposes. In addition to regional EEW, the official EEW of CWB, a low-cost P-alert system deployed by individual P-alert stations act as on-site EEW systems and pro- National Taiwan University (NTU) is in operation and is vide warnings when predefined thresholds are exceeded (Wu capable of providing on-site EEW, as well as rapid shake maps. et al.,2011, 2013; Hsieh et al.,2015). In this article, we examine This network consists of 636 P-alert stations. This network was the data obtained from the P-alert network and its performance initiated by a research group at NTU and is based on low-cost during the Hualien earthquake. microelectromechanical system (MEMS) accelerometers (Wu et al., 2013; Wu, 2015). The NTU group is also helping other P-ALERT NETWORK countries build their own EEW networks using P-alert. Most recently, a network of 100 P-alert instruments was installed The P-alert seismic network is a dense array of 636 stations in one portion of the Himalayan belt of India (Kumar et al., in Taiwan (Fig. 2). As mentioned earlier, these sensors are 2014), where large earthquakes pose great danger to people living low-cost MEMS-based acceleration sensors introduced in 20 Seismological Research Letters Volume 90, Number 1 January/February 2019 Downloaded from https://pubs.geoscienceworld.org/ssa/srl/article-pdf/90/1/19/4603292/srl-2018170.1.pdf by National Taiwan Univ - Lib Serials Dept user on 23 April 2019 25.5° Each P-alert instrument provides three-component data Taipei city with a sampling frequency of 100 Hz. The system is designed Total : 699 stations 2 P-alert : 636 stations to have 16-bit resolution and a g clip level.