Developing a Next Generation of Earthquake Early Warning System. Contents

Transcription

1 Developing a Next Generation of Earthquake Early Warning System W. H. K. Lee, T. L. Teng, and Y. M. Wu Contents 1. Introdution Physial Basis and Limitations for Earthquake Early Warning A Next-Generation Earthquake Early Warning System Determining earthquake magnitude by the τ method Loating offshore earthquakes Future Perspetives...13 REFERENCES...14 Appendix 1. Intel Resaerh Sensor Network Operation...16 April 30, 2007

2 1. Introdution As inreasing urbanization is taking plae worldwide, earthquake hazards post strong threats to lives and properties for urban areas near major ative faults on land or subdution zones offshore. Earthquake early-warning systems an be a useful tool for reduing earthquake hazards, if ities are favorably loated with respet to earthquake soures and their itizens are properly trained to response to earthquake warning messages (Lee and Espinosa-Aranda, 2003). Lee, Shin, and Teng (1996) reported the design and implementation of two earthquake early warning systems in Taiwan. Subsequently, Wu and his assoiates greatly refined and improved the CWB s RTD system (Teng et al., 1997; Wu et al., 2007). It has been suessfully in operation for over a deade in providing rapid earthquake information release to the responsible government agenies, inluding the 1999 disastrous Chi-Chi earthquake (Wu et al., 2000). In late 2006, T. L Teng and W. H. K. Lee were asked by CWB to further develop the earthquake early warning systems. Sine then, we have done some exploratory work and in ollaboration with Y. M. Wu, we have begun to design a next generation of earthquake early warning system, speifially for CWB. The purpose of this report is to summarize the results of a re-examination of existing earthquake early warning systems worldwide from the tehnial point of views. During the past twenty years, advanes in omputer power and teleommuniation allow us to implement a more robust system. Better understandings of earthquake physis and ritial parameters useful for earthquake engineering designs also permit us to design a more effetive system. 2

3 2. Physial Basis and Limitations for Earthquake Early Warning The physial basis for earthquake early warning is simple: strong ground shaking is aused by shear (S) and the following surfae waves (whih travel at about half the speed of the primary (P) waves), and seismi waves travel muh slower than eletromagneti signals transmitted by telephone or radio. However, there are some physial limitations that we must onsider as shown in Figure 1. Figure 1. Travel time of P-waves and of S-waves versus distane for a typial earthquake. 3

4 Figure 1 is a plot of the travel time for P-wave and S-wave versus distane from an earthquake. We make the following assumptions of a typial destrutive earthquake: (1) foal depth at 20 km, (2) P-wave veloity at 8 km/se, and (3) S-wave veloity at 4.5 km/se. If an earthquake is loated 100 km away from a ity, the P-wave arrives at the ity in about 13 seonds, and the S-waves in about 22 seonds aording to Figure 1. If we deploy a dense seismi network in the earthquake soure area that is apable of loating and determining the size of the event in about 10 seonds, we will have about 3 seonds to issue a warning before the P-wave arrives, and about 12 seonds before the more destrutive S-waves and surfae waves arrive at the ity. Here, we have assumed that it takes very little time to send a signal from a seismi network to the ity by eletromagneti waves (e.g., telephone iruit) at nearly the speed of light (300,000 km/se). From Figure 1, it is lear that the above strategy may work for earthquakes loated about 60 km or more away from a ity. For earthquakes at shorter distanes (say, 20 to 60 km), we must redue the time for deteting the event and issuing a warning in about 5 to 10 seonds, respetively. This implies that we must deploy a very dense seismi network so that there are enough stations reeiving the seismi signals in 1 to a few seonds. However, this is not eonomial to deploy using existing seismi instrumentations. For earthquakes within 20 km of a ity, there is little one an do other than installing automati shut-off devies for ritial failities (gases, for example) that an be triggered by the onset of the P-wave. Normally an earthquake that is more than 100 km away from a ity does not pose a large threat to the ity, beause seismi waves would be attenuated by a fator of about 5 in general. There are exeptional ases due to unusual loal site onditions, suh as Mexio City, where large earthquakes are loated more than 200 km away and thus an provide about 2 minutes in warning. In the above disussion, we assume that we implement an earthquake early warning system using a traditional seismi network approah. Its limitation an be illustrated by Figure 2 for the expeted early warning time for the 1999 Chi-Chi earthquake. 4

5 Expeted Early Warning Time of the Earthquake of Sep. 20, 1999 (Mw7.6) 25 Latitude (N) se 20 se 0 se 30 se 22 Warning time VSN no warning area Longitude (E) Figure 2. Expeted EWS early warning times (indiated by irles) in Taiwan with respet to the ourrene of an event similar to the Chi-Chi earthquake of 20 September Triangles give the loation of elementary shools, whih an be regarded as the population density of Taiwan. 5

6 However, Nakamura and his olleagues had been suessful in applying a singlestation approah (Nakamura, 1988; Saita and Nakamura, 2003), where seismi signals are proessed loally and an earthquake warning is issued when ground motion exeeds the trigger threshold. More reently, Wu and Kanamori (2005a) experimented an onsite early warning method for the Taiwan early warning system that make use of theτ, from the initial 3 se of P waves. We will disuss this method in more details in the next setion. Unlike the Mexio City, major ities in Taiwan are loated too lose (< ~100 km) to potential damaging earthquakes that greatly limited the pratial use of an earthquake early warning system for the publi. Nevertheless, a next generation earthquake early warning system must be designed within the physial limitations, i.e, 1) For large (magnitude 7 or greater) earthquakes at a distane of greater than 120 km, some pratial use of earthquake early warning an be utilized if the system an reliably determine earthquake hypoenter and magnitude within 30 seonds after several stations reeived seismi signals near the soure regions. 2) For earthquakes at a distane from 60 to 120 km, some pratial use of earthquake early warning an be utilized if the system an reliably determine earthquake hypoenter and magnitude within several seonds after the several stations reeived seismi signals near the soure regions. 3) For earthquakes at a distane from 20 to 60 km, a single-station approah to earthquake early warning is neessary. 4) Within a distane of 20 km of an earthquake, an earthquake early warning system is not pratial, but some automati shut-down devies an be installed to redue fire hazards or equipment damages. 6

7 3. A Next-Generation Earthquake Early Warning System The simplest and surest way to develop a next-generation earthquake early warning system is to refine and improve the existing CWB s RTD system, so that its potentials for earthquake early warning are fully realized. In the following sub-setions, we will explore various methods and tehniques that are promising. 3.1 Determining earthquake magnitude by the τ method One of the major problems for robust earthquake early warning is to determine the earthquake magnitude rapidly and reliably. Kanamori (2005) extended the method of Nakamura (1988) and Allen and Kanamori (2003) to determine a period parameter, τ, from the initial 3 se of P waves. τ is defined as: where τ = 2 π / r (1) τ = & r u() tdt / u() tdt 0 0 τ (2) ut () is ground-motion displaement; τ 0 is the duration of reord used, usually 3 se, and an be omputed from the inoming data sequentially. τ represents the size of an earthquake. Wu and Kanamori (2005a, 2005b), and Wu et al. (2007a; 2007b) applied this method to earthquake early warning in southern California, Taiwan, and Japan by determining a ground-motion period parameter τ and a high-pass filtered displaement amplitude parameter Pd from the initial 3 se of the P waveforms. At a given site, the magnitude of an event from τ and the peak ground-motion veloity (PGV) from Pd were estimated. The inoming 3-omponent signals are reursively onverted to ground aeleration, veloity and displaement. The displaements are reursively filtered with a one-way Butterworth high-pass filter with a utoff frequeny of Hz, and a P-wave trigger is 7

8 onstantly monitored. The relationships between τ and magnitude (M), and Pd and peak ground veloity (PGV) for southern California, Taiwan, and Japan were established. Figure 3 shows that a good linear trend was found between τ and M w from the Japan K-NET reords. Figure 4 shows the Pd versus PGV plot for southern California, Taiwan, and Japan. These relationships an be used to detet the ourrene of a major earthquake and provide onsite warning in the area around the station where onset of strong ground motion is expeted within seonds after the arrival of the P-wave. When the station density is high, the methods an be applied to multi-station data to inrease the robustness of onsite early warning and to supplement the regional warning approah. In an ideal situation, suh warnings would be available within 10 se of the origin time of a large earthquake whose subsequent ground motion may last for tens of seonds. Although the τ method works well for earthquakes with magnitude up to about 7.5, physis of earthquake indiate that for great (M >= 8), and mega (M >= 9) earthquakes that more than 3-seond of P-waves will be required to determine their magnitudes reliably. The 26 Deember 2004 Sumatra mega-earthquake showed that the traditional near realtime magnitude determinations were inadquate and many authors had proposed alternative solutions. Bormann and Saul (2007) provided an up-to-date review of this problem. 8

9 events, τ = M w ± R = τ (se) M w.5 Figure 3. τ estimates from 17 events using the nearest 6 stations of the K-NET, small open irles show single-reord results and large irles show event-average values. Solid line shows the least squares fit and the two dashed lines show the range of one standard deviation. 9

10 100 Taiwan 507 reords Southern California 199 reords Japan 45 reords PGV (m/se) Linear regression over 751 reords log(pgv)=0.915 log(pd) SDV = Pd(m) Figure 4. Relationship between peak initial displaement amplitude (Pd) measurements and peak ground veloity (PGV) for the reords with epientral distanes less than 30 km from the epienter in Southern California (red solid irles), Taiwan (blue diamonds) and Japan (blak sloid triangles). Solid line shows the least squares fit and the two dashed lines show the range of one standard deviation. 10

11 3.2 Loating offshore earthquakes Although the traditional method of loating earthquake hypoenter works well for earthquakes that ourred within a seismi network, it often fails for offshore earthquakes. Sine many potential damaging earthquakes ourred offshore of Taiwan, the next generation earthquake early warning system must address this problem. Most ommonly used algorithms for loating earthquakes on omputers are based on an inverse formulation (Geiger, 1912). Numerous software implementations have been made using the Geiger method, whih applies the Gauss-Newton nonlinear optimization tehnique to find the origin time and hypoenter by iterative linearized steps starting from a trial solution. Reent advanes in earthquake loation methods are mostly onentrated on obtaining the best relative loations for a group of earthquakes using high-quality data (inluding waveforms) reorded by a dense seismi network (e.g., Rihards et al., 2006). However, Geiger-like loation programs do not work well for poorly onstrained earthquakes (e.g., offshore events), beause the available arrival times may not be suffiient to solve the inverse problem, and the hosen trial solution may lead to a loal minimum. A physial problem involving observations is muh easier solved by a forward or diret formulation, if the large amounts of omputations required an be managed. Reently, omputers beame fast enough that the forward approah has been explored (e.g., Sambridge and Kennett, 2001; Oye and Roth, 2003). Instead of a brute grid searh, we use the downhill simplex algorithm to searh the neighborhood of thousands of grid points that oarsely over the solution spae. The downhill simplex algorithm was hosen for its robustness (Press et al., 1986). Computation is intensive, but a robust solution an be found in about 100 seonds on a Pentium 4 PC for a typial ase, along with a 3-D residuals map for visualizing the solution s unertainty. This diret approah has three major advantages over the inverse formulation: (1) a global minimum in the solution spae an be obtained, (2) the omputation is simple and straight forward, and (3) it an be adapted to perform either a L1-norm or L2-norm minimization. 11

12 Lee, Dodge, and Baker are now developing a general software pakage (JLOC written in Java) with 3-D visualization (using MATLAB), speifially for loating poorly onstrained earthquakes, suh as offshore earthquakes (Lee and Baker, 2006). Unfortunately, the present single-proessor PC is not yet fast enough for near-realtime proessing. However, reently development of multi-ore proessors will make it possible for suh appliation in the near future. 12

13 4. Future Perspetives Some radially different design of an earthquake early warning system is now possible using Sensor Network developed by Intel Researh, whih is working with the aademi ommunity and industry ollaborators in atively exploring the potential of wireless sensor networks. Their researh is already demonstrating the potential of this new tehnology to enhane publi safety, redue the ost of doing business, and bring a host of other benefits to business and soiety ( A summary of Intel Researh Sensor Network Operation is given in Appendix 1. Another exiting omputer development is the reent introdution of multi-ore proessors by AMD, IBM, and Intel. A traditional omputer has just one CPU and is able to run multi-tasking by swapping eah task in-and-out of its CPU after exeuting a tiny fration of a seond. This is rather ineffiient and often reates a rash in the omputer. In designing the CWB RTD system, we were areful in limiting the number of tasks to be exeuted. To expand the omputer power, omputer lusters have been introdued in the past deade by linking hundreds or thousands of inexpensive omputers together. But the software for assigning different tasks to different members of the luster is rather diffiult and often not effiient. With thousands of omputers running, there is also the problem of individual omputer failures. However, with multi-ore proessors, multiple programs an be exeuted onurrently in different ores, greatly improving the overall omputer power and reliability. Unfortunately, with suh a new omputer arhiteture, how we implement the software for an earthquake early warning system will have to be re-onsidered. We will be making some exploratary programming of software by purhasing a quad-ore omputer this fall. 13

14 REFERENCES Allen, R. M., and Kanamori, H., (2003). The potential for earthquake early warning in Southern California, Siene, 300, Bormann, P., and J. Saul, (2007). Earthquake magnitude. In: Enylopedia of Complexity and System Siene, Springer, in press. Geiger, L. C., (1912). Probability method for the determination of earthquake epienters from the arrival time only, Bull. St. Louis Univ., 8, Kanamori, H., (2005). Real-time seismology and earthquake damage mitigation, Annual Review of Earth and Planetary Sienes, 33, Lee, W. H. K., and Espinosa-Aranda, J. M., (2003). Earthquake early warning systems: Current status and perspetives, in "Early Warning Systems for Natural Disaster Redution", edited by J. Zshau and A. N. Kuppers, p , Springer, Berlin. Lee, W. H. K., and L. M. Baker, (2006). Development of a diret searh software pakage for loating poorly onstrained earthquakes (Abstrat). Seism. Res. Lett., 77, Lee, W. H. K., T. C. Shin, and T. L. Teng, (1996). Design and implementation of earthquake early warning systems in Taiwan. Pro. 11th World Conf. Earthq. Eng., Paper No Nakamura, Y., (1988). On the urgent earthquake detetion and alarm system (UrEDAS), Pro. 9 th world Conf. Earthq. Eng., 7, Oye, V., and M. Roth, (2003). Automated seismi event loation for hydroarbon reservoirs, Computers & Geosi., 29, Press, W. H., B. P. Flannery, S. A. Teukolsky, and W. T. Vetterling, (1986). Numerial Reipes: The Art of Sientifi Computing, Cambridge University Press, Cambridge. Rihards, P. G., F. Waldhauser, D. Shaff, and W. Y. Kim, (2006). The appliability of modern methods of earthquake loation, in: Advanes on Studies of Heterogeneities in the Earth's Lithosphere: The Keiiti Aki Volume II, edited by Y. Ben-Zion and W. H. K. Lee, p , Pageoph Topial Volumes, Birkhauser Verlag, Basel. Saita, J., and Nakamura, Y., (2003). UrEDAS: the early warning warning system for mitigation of disasters aused by earthquakes and tsunamis. In Early Warning Systems 14

15 for Natural Disaster Redution, edited by J. Zshau and A. N. Kuppers, p , Springer, Berlin. Sambridge, M., and B. Kennett (2001). Seismi event loation: nonlinear inversion by using a neighbourhood algorithm, Pure Appl. Geophys., 158, Teng, T. L., Y. M. Wu, T. C. Shin, Y. B. Tsai, and W. H. K. Lee, (1997). One minute after: strong-motion map, effetive epienter, and effetive magnitude, Bull. Seism. So. Am., 87, Wu, Y. M., W. H. K. Lee, C. C. Chen, T. C. Shin, T. L. Teng, and Y. B. Tsai, (2000). Performane of the Taiwan Rapid Earthquake Information Release System (RTD) during the 1999 Chi-Chi (Taiwan) earthquake, Seismo. Res. Lett., 71, Wu, Y. M., and H. Kanamori, (2005a). Experiment on an onsite early warning method for the Taiwan early warning system, Bull. Seism. So. Am., 95, Wu, Y. M., and H. Kanamori, (2005b). Rapid assessment of damaging potential of earthquakes in Taiwan from the beginning of P Waves, Bull. Seism. So. Am., 95, Wu, Y. M., N. C. Hsiao, W. H. K. Lee, T. L. Teng, and T. C. Shin, (2007a). State of the art and progresses of early warning system in Taiwan. In: Seismi Early Warning, edit by Paolo Gasparini, Gaetano Manfredi, and Johen Zshau, Springer in press. Wu, Y. M., H. Kanamori, R. Allen, and E. Hauksson, (2007b). Determination of earthquake early warning parameters, τ and P d, for southern California, Geophys. J. Int. in press. 15

16 Appendix 1. Intel Resaerh Sensor Network Operation 16

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