Search

Using seismic activity and the Meishan earthquake sequence that occurred from 1904 to 1906, a scenario for short-term probabilistic seismic hazards in the Chiayi region of Taiwan is assessed. The long-term earthquake occurrence rate in Taiwan was evaluated using a smoothing kernel. The highest seismicity rate was calculated around the Chiayi region. To consider earthquake interactions, the rate-and-state friction model was introduced to estimate the seismicity rate evolution due to the Coulomb stress change. As imparted by the 1904 Touliu earthquake, stress changes near the 1906 Meishan and Yangshuigang epicenters was higher than the magnitude of tidal triggering. With regard to the impact of the Meishan earthquake, the region close to the Yangshuigang earthquake epicenter had a +0.75 bar stress increase. The results indicated significant interaction between the three damage events. Considering the path and site effect using ground motion prediction equations, a probabilistic seismic hazard in the form of a hazard evolution and a hazard map was assessed. A significant elevation in hazards following the three earthquakes in the sequence was determined. The results illustrate a possible scenario for seismic hazards in the Chiayi region which may take place repeatly in the future. Such scenario provides essential information on earthquake preparation, devastation estimations, emergency sheltering, utility restoration, and structure reconstruction.

[1] The structure of the Taiwan mountain belt is thought to be that of an imbricate thrust and fold belt developed in a forward breaking sequence above a shallowly dipping basal detachment. In recent years, however, a growing amount of seismicity data from the internal part of the mountain belt indicates the existence of widespread fault activity in the middle and lower crust, suggesting that deeper levels of the crust must be involved in the deformation than predicted by the shallow detachment, imbricate thrust belt model. To address this issue, we present new geological mapping, together with earthquake focal mechanism and seismic energy release data from the central part of Taiwan. We concur with the interpretation that the foreland basin part of the Western Foothills comprises an imbricate thrust system that is developing as a forward breaking sequence that is structurally and kinematically linked to a basal detachment at between 7 and 10 km depth. To the east of the foreland basin, however, in the Hsuehshan and Central Ranges, our data show the presence of two fault systems. An earlier, inactive thrust system with a well-developed cleavage is cut by a system of steeply dipping active faults that penetrate to a depth of 25 to 30 km or more. In the Hsuehshan Range, the second fault system is best represented by a structural and kinematic model in which this part of the mountain belt forms a zone of transpression with a structural architecture similar to that of a crustal-scale positive flower structure. Eastward, in the Central Range, Mesozoic basement rocks are over thrusting strongly folded and cleaved deep water sediments of the first, now inactive, thrust system. The involvement of deep crustal levels and Mesozoic basement in the second fault system is suggestive of the reactivation of preexisting basin-bounding faults that were located on the Eurasian continental margin.

[1] The northeastern region of Taiwan is one of the few places on Earth that experienced a transition from collision to subduction and back-arc opening in a young orogenic belt. This provides us a rare opportunity to study the structural characteristics in such a transition. We attempt to analyze the seismicity and stress patterns in this transitional region using data obtained from a dense and high-quality seismic network. Our results show that from south to north, fault types change from thrust to strike-slip, then to normal faulting. The transitional domain between Ilan and Hualien is dominated by left-lateral strike-slip faulting with a NNW-SSE minimum principal stress axis, consistent with the opening of the Okinawa Trough. To the north, the Ilan Plain is formed by structural activities of a series of normal fault systems. To the south, the spatial distribution of an earthquake cluster north of Taiwan's Coastal Range appears to parallel the subducting interface between the Philippine Sea and Eurasian plates. This implies the northern extension of the Coastal Range has subducted. Consequently, the opening of the Okinawa Trough becomes an important tectonic driving force in northeastern Taiwan. The westernmost Ryukyu arc is pushed by the opening Okinawa Trough, and moves southward. The northern part of Taiwan's Central Range moves with the arc to bend southeastward, and the Ilan Plain forms in the space north of the range. The bending of the Central Range belt results in bedding plane slip with left-lateral strike-slip earthquakes in the transitional domain to accommodate the deformation.

The M(subscript w) 6.3, 21 February 2011 Christchurch, New Zealand, earthquake is regarded as an aftershock of the M 7.1, 4 September 2010 Darfield earthquake. However, it caused severe damage in the downtown Christchurch. Such a circumstance points out the importance of an aftershock sequence in seismic hazard evaluation and suggests the re-evaluation of a seismic hazard immediately after a large earthquake occurrence. For this purpose, we propose a probabilistic seismic hazard assessment (PSHA), which takes the disturbance of a short-term seismicity rate into account and can be easily applied in comparison with the classical PSHA. In our approach, the treatment of the background seismicity rate is the same as in the zoneless approach, which considers a bandwidth function as a smoothing Kernel in neighboring region of earthquakes. The rate-and-state friction model imparted by the Coulomb stress change of large earthquakes is used to calculate the fault-interaction-based disturbance in seismicity rate for PSHA. We apply this approach to evaluate the seismic hazard in Christchurch after the occurrence of the M 7.1, 4 September 2010 Darfield earthquake. Results show an increase of seismic hazards due to the stress increase in the region around the rupture plane, which extended to Christchurch. This provides a suitable basis for the application of a time-dependent PSHA using updating earthquake information.

The first attenuation relationships of peak ground acceleration (PGA) and peak ground velocity (PGV) for northern Vietnam are obtained in this study. Ground motion data are collected by a portable broadband seismic network in northern Vietnam as a part of cooperation between the Institute of Geophysics, Vietnamese Academy of Science and Technology, Vietnam and Institute of Earth Sciences, Academia Sinica, Taiwan. The database comprises a total of 330 amplitude records by 14 broadband stations from 53 shallow earthquakes, which were occurred in and around northern Vietnam in the period between 01/2006 and 12/2009. These earthquakes are of local magnitudes between 1.6 and 4.6, focal depths less than 30 km, and epicentral distances less than 500 km. The new attenuation relationships for PGA and PGV are: log 10 ( PGA ) = - 0.987 + 0.7521 M L - log 10 ( R ) - 0.00475 R , log 10 ( PGV ) = - 3.244 + 0.9008 M L - log 10 ( R ) - 0.00322 R , where PGA is in cm/s 2 , PGV is in cm/s, and R is the epicentral distance in kilometers. The site corrections are also derived in this study. These site corrections are very suitable with the station corrections for M L and imply the qualification of the resulting attenuation relationships.

[1] We collected the strong-motion accelerograms from thirteen crustal earthquakes in Taiwan recorded by the Taiwan Strong Motion Instrumentation Program (TSMIP) stations to find the empirical relationship between the areas of high initial P-wave displacement and the corresponding earthquake magnitudes. We found that the logarithm of the area has a linear relation to the corresponding earthquake magnitude in Mw for study earthquakes with magnitudes between 5.4 and 6.5. We propose that this relationship might be able to rapidly define earthquake magnitude without knowing the earthquake location in regions with sufficient seismic station coverage and might have practical application in earthquake early warning (EEW) and rapid reporting systems.

Modern technology allows real-time seismic monitoring facilities to evolve into earthquake early warning (EEW) systems, capable of reducing deaths, injuries, and economic losses, as well as of speeding up rescue response and damage recovery. The objective of an EEW system is to estimate in a fast and reliable way the earthquake’s damage potential, before the strong shaking hits a given target. The necessary framework for EEW implementation is provided by the observed relationships between different parameters measured on the signal onsets and the final earthquake size. The implication of these observations on the physics of fracture processes has given rise to a significant debate in the seismological community. Currently, EEW systems are implemented or under testing in many countries of the world, and different methodologies and procedures have been studied and developed. The leading experience of countries like Japan or Mexico shows that, with a proper education of population and end-users, and with the design of real-time systems for the reduction of vulnerability/exposure, EEW can be an effective approach to the mitigation of the seismic risk at short time-scales.

We performed inversions of earthquake focal mechanisms in central Taiwan to investigate the heterogeneity of the stress field and fault strength, and temporal variations of stress parameters, friction and pore pressure associated with the 1999 Chi-Chi earthquake. We divided the focal mechanism data into two groups: before and after the Chi-Chi earthquake, and analysed them separately. With the assumption of a uniform stress field, the friction coefficient is mostly within a range of 0.2–0.4 in central Taiwan, which is lower than the commonly quoted laboratory result, 0.6–0.85. The low friction coefficient is also inferred by the rotation of principal stress axes after the Chi-Chi earthquake. By contrast, if we assume that the friction is constant and failures occur on optimally oriented planes, we find that the resulting stress orientations must be spatially variable. However, a large dispersion of stress orientations is not seen in borehole breakouts and fault slip data, implying a constant friction model might be ruled out. Our analysis suggests that either the distribution of the coefficient of friction or pore pressure changed during the 1999 Chi-Chi earthquake. We infer that the pore pressure probably rose in the Chi-Chi rupture area and northern Longitudinal Valley and dropped in the areas south of the coseismic rupture area after the main shock.

[1] We collected the strong-motion accelerograms with peak ground acceleration (PGA) larger than 100 Gal (1 Gal = 1.0 cm/s 2 ) from large crustal earthquakes in Taiwan recorded by the Taiwan Strong Motion Instrumentation Program (TSMIP) stations to find an empirical relationship between the area of high PGA and the corresponding earthquake magnitude. We found that the logarithms of the areas inside the PGA contours have a linear relation to the corresponding earthquake magnitudes. We propose that this relationship might be able to rapidly define the earthquake magnitude while providing sufficient seismic station coverage and might have practical application in earthquake early warning (EEW) and rapid reporting systems. The proposed magnitude estimation method is directly related to the level of strong surface shaking and is inherently suitable for the purpose of the EEW and rapid reporting systems.

[1] We use 1661 strong-motion accelerograms with peak ground acceleration (PGA) larger than 80 Gal (1 Gal = 1.0 cm/s2) from 77 earthquakes recorded by the Taiwan Strong Motion Instrumentation Program (TSMIP) stations to derive a strong-motion attenuation relationship. This relationship can be used to dynamically define a “Mpga magnitude” for earthquakes using earthquake locations determined by earthquake early warning process. The Mpga magnitude using this strong-motion attenuation relationship corresponds well with Mw given a sufficient number of PGA readings. MEMS (Micro Electro Mechanical Systems) acceleration sensor could be widely used for ground motion monitoring purposes. Thus, we propose that once a large earthquake has begun, that we might be able to use strong, near-field (tens of kilometers) PGA values to quickly estimate the earthquake's magnitude, which would improve earthquake early warning.

SUMMARY Northern Taiwan underwent mountain building in the early stage of the Taiwan orogeny but is currently subjected to post-collisional crustal extension. It may be related to gravitational collapse or to the rifting of the Okinawa Trough, which lies offshore northeastern Taiwan. The Ilan Plain, northeastern Taiwan, which is bounded by the normal fault systems and filled up with thick Pliocene–Pleistocene sedimentary sequences, formed under such an extension environment. Over there on 2005 March 5 two earthquakes with about the same magnitude (M L = 5.9) occurred within 68 s and produced intense aftershocks activity according to the records of Central Weather Bureau Seismic Network of Taiwan. We relocated the earthquake sequence by the three-dimension earthquake location algorithm with the newly published 3D Vp and Vp/Vs velocity model, and determined the first-polarity focal mechanisms of the earthquake doublet. One major cluster of aftershocks which trends E–W and dips steeply to the south can be identified and picked up as a potential fault plane. The focal mechanisms of the two main shocks are both classified as normal type by first-polarity but strike-slip by centroid moment tensor inversion; however two methods both yield consistent E–W strike. Static coseismic deformation was additionally determined from Global Positioning System (GPS) daily solutions at a set of continuous GPS stations and from strong-motion seismographs. These data show NW–SE extension at high angle to the fault plane, which cannot be explained from a simple strike-slip double-couple mechanism. On the other hand, the small vertical displacements and steep fault plane cannot be explained from a simple normal event as well. We present from elastic dislocation modelling that the geodetic data are best explained by significant component of tensile source with centimetre-scale of opening on a 15-km-long fault extending from 1 to 13 km depth. We therefore interpret the crisis as the result of dyke intrusion at the very tip of the Okinawa Trough, which is reasonably driven by backarc spreading.

We use preseismic, coseismic, and postseismic GPS data of the 1999 Chi-Chi earthquake to infer spatio-temporal variation of fault slip and frictional behavior on the Chelungpu fault. The geodetic data shows that coseismic slip during the Chi-Chi earthquake occurred within a patch that was locked in the period preceding the earthquake, and that afterslip occurred dominantly downdip from the ruptured area. To first-order, the observed pattern and the temporal evolution of afterslip is consistent with models of the seismic cycle based on rate-and-state friction. Comparison with the distribution of temperature on the fault derived from thermo- kinematic modeling shows that aseismic slip becomes dominant where temperature is estimated to exceed 200 at depth. This inference is consistent with the temperature induced transition from velocity-weakening to velocity-strengthening friction that is observed in laboratory experiments on quartzo-feldspathic rocks. The time evolution of afterslip is consistent with afterslip being governed by velocity-strengthening frictional sliding. The dependency of friction, l, on the sliding velocity, V, is estimated to be ol=o ln V ¼ 8 � 10 � 3 : We report an azimuthal difference of about 10-20 between preseismic and postseismic GPS velocities, which we interpret to reflect the very low shear stress on the creeping portion of the decollement beneath the Central Range, of the order of 1-3 MPa, implying a very low friction of about 0.01. This study highlights the importance of temperature and pore pressure in determining fault frictional sliding.

We have implemented and tested an algorithm for onsite earthquake early warning (EEW) in California using the infrastructure of the Southern California Seismic Network (SCSN). The algorithm relies on two parameters derived from the initial 3 sec of P waveform data at a single seismic sensor: period parameter τ_c and high-pass filtered displacement amplitude P_d. Previous studies have determined empirical relationships between c and the moment magnitude M_w of an earthquake, and between P_d and the peak ground velocity (PGV) at the site of observation. In 2007, seven local earthquakes in southern California with 4.0≤M_L≤4.7 have triggered the calculation of M_w and PGV by the EEW algorithm. While the mean values of estimated parameters were in the expected range, the scatter was large, in particular for the smallest events. During the same time period the EEW algorithm produced a large number of false triggers due to low trigger thresholds. To improve the real-time performance of the onsite approach, we have developed a new trigger criterion that is based on combinations of observed τ _c and P_d values. This new criterion removes 97% of previous false triggers and leads to a significant reduction of the scatter in magnitude estimates for small earthquakes.

[1] With the implementation of a real-time strong-motion network by the Central Weather Bureau (CWB), an earthquake early warning (EEW) system has been developed in Taiwan. In order to shorten the earthquake response time, a virtual sub-network method based on the regional early warning approach was utilized at first stage. Since 2001, this EEW system has responded to a total of 225 events with magnitude greater than 4.5 occurred inland or off the coast of Taiwan. The system is capable of issuing an earthquake report within 20 sec of its occurrence with good magnitude estimations for events up to magnitude 6.5. Currently, a P-wave method is adopted by the CWB system. Base on the results from 596 M > 4.0 earthquakes recorded by the real-time strong-motion network, we found that peak displacement amplitudes from initial P waves (P d ) can be used for the identification of M > 6.0 events. Characteristic periods τ c and τ max p of the initial P waves can be used for magnitude determination with an uncertainty less than 0.4. We expect to achieve a 10-second response time by the EEW system in Taiwan in the near future.

[1] We determined the τc and τpmax parameters from the K-NET strong motion records of 16 earthquakes in Japan with moment magnitude (Mw) ranging from 6.0 to 8.3. A 0.075 Hz high-pass Butterworth filter was applied for determination of τc based on our previous studies. It was found that different pole selections of the Butterworth filter lead to different uncertainty in magnitude determination. Our results show that using two poles in the filters results in the best magnitude estimates, i.e., minimized the standard deviation in magnitude determination in comparison to Mw using τc. The τpmax parameters (Allen and Kanamori, 2003) were also determined with the same dataset using the Wurman et al. (2007) procedure. It was found that τpmax values obtained from this dataset, and using the Wurman procedure, had a larger uncertainty. However, when a 0.075 Hz high-pass Butterworth filter with five poles was added, the uncertainty in τpmax-derived magnitude estimates decreased minimizing the standard deviation in magnitude determination using τpmax. This difference in the behavior of τc and τpmax can be used to further reduce the uncertainty in rapid magnitude determination for earthquake early warning. When the magnitude estimations from τc and τpmax of each event are averaged to provide a new magnitude estimate, the standard deviation in magnitude estimates is reduced further to 0.27 magnitude units.

In this study, a large collection of 41,141 S-P times from the untapped records of the Taiwan Strong Motion Instrumentation Program (TSMIP) network is combined with the P and S wave arrival times from the Taiwan Central Weather Bureau Seismic Network (CWBSN) to image the V_p and V_p /V_s structures beneath Taiwan. The records from the 680 TSMIP stations throughout Taiwan in the past 15 years enhance the path coverage and the resolution in the tomography inversions tremendously. Our result for the V_p structure largely confirms previous studies but brings better constraint on the V_p /V_s structure. The colliding Luzon volcanic arc is characterized by a belt of high V_p and high V_p /V_s with high seismicity that includes the offshore islands of Lutao and Lanyu and the Coastal Ranges in eastern Taiwan, at the depth between about 13 and 25 km. This high V_p /V_s belt can be traced to the subduction zone in the region between Hualian and Ilan in the deeper portion. The shallow portions of the southwestern coastal plain and the Pingtung region are also characterized by a belt of high V_p /V_s with lower seismicity. Most of the events occurred at the base of the high V_p /V_s zones. We suggest that material strength in those regions may be too low to accumulate stress, which may indicate water-saturated young sediments. Finally, the Central Range region is characterized by a low V_p /V_s belt.

We report here the recent progress and future development of real-time seismic monitoring in Taiwan, especially on earthquake early warning system (EWS) developed at the Central Weather Bureau (CWB), using the telemetered signals from strong-motion instruments. Utilizing a quick magnitude determination based on the first 10 sec of signals from a virtual and sub-network which is configured automatically, we were able to reduce the earthquake rapid reporting time to about 30 sec or less. This represents a significant step towards a more realistic earthquake early warning capability. This early warning system has been in operation at CWB since 2002. Comprehensive earthquake reports have been issued mostly in less than 30 sec, with an average of about 22 sec from the origin time. At 3 km/sec for a typical crustal shear-wave velocity, the present operation is not useful if an earthquake occurs less than about 70 km from a city, but the lead time will increase to more than 10 sec for cities at distances greater than 100 km 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. In order to offer earlier warning within an epicentral distance of less than 70 km, a τc and Pd method was experimented. We used the Taiwan Strong-Motion Instrumentation Program (TSMIP) accelerograms recorded within epicentral distances of less than 30 km from Mw > 5.0 crustal earthquakes. This method uses the first 3-sec length records from the onset of P-waves and could issue an on-site early warning within 10 sec after the event origin time, reducing the radius of the blind zone from 70 to 25 km from the epicenter.

SUMMARY We explore a practical approach to earthquake early warning in southern California by determining a ground-motion period parameter τc and a high-pass filtered displacement amplitude parameter Pd from the initial 3 s of the P waveforms recorded at the Southern California Seismic Network stations for earthquakes with M > 4.0. At a given site, we estimate the magnitude of an event from τc and the peak ground-motion velocity (PGV ) from Pd. The incoming threecomponent 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 cut-off frequency of 0.075 Hz, and a P-wave trigger is constantly monitored. When a trigger occurs, τc and Pd are computed. We found the relationship between τc and magnitude (M) for southern California, and between Pd and PGV for both southern California and Taiwan. These two relationships can be used to detect the occurrence of a major earthquake and provide onsite warning in the area around the station where onset of 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 multistation data to increase the robustness of onsite early warning and to add the regional warning approach. In an ideal situation, such warnings would be available within 10 s of the origin time of a large earthquake whose subsequent ground motion may last for tens of seconds.

SUMMARY A moderate earthquake of Mw= 6.8 occurred on 2003 December 10. It ruptured the Chihshang Fault in eastern Taiwan which is the most active segment of the Longitudinal fault as a plate suture fault between the Luzon arc of the Philippine Sea plate and the Eurasian plate. The largest coseismic displacements were 13 cm (horizontal) and 26 cm (vertical). We analyse 40 strong motion and 91 GPS data to model the fault geometry and coseismic dislocations. The most realistic shape of the Chihshang fault surface is listric in type. The dipping angle of the seismic zone is steep (about 60°–70°) at depths shallower than 10 km and then gradually decreases to 40°–50° at depths of 20–30 km. Thus the polygonal elements in Poly3D are well suited for modelling complex surfaces with curving boundaries. Using the strong motion data, the displacement reaches 1.2 m dip-slip on the Chihshang Fault and decreases to 0.1 m near surface. The slip averages 0.34 m, releasing a scalar moment of 1.6E26 dyne-cm. For GPS data, our model reveals that the maximal dislocation is 1.8 m dip-slip. The dislocations decrease to 0.1 m near the surface. The average slip is 0.48 m, giving a scalar moment of 2.2E26 dyne-cm. Regarding post-seismic deformation, a displacements of 0.5 m were observed near the Chihshang Fault, indicating the strain had not been totally released, as a probable result of near-surface locking of the fault zone.

[1] A Mw 6.1 earthquake occurred on April 1st, 2006 near Taitung, eastern Taiwan. It produced significant coseismic ground displacements and a large number of aftershocks in the ensuing month. This event provides an opportunity to diagnose the seismogenic structure in the southern Longitudinal Valley (LV) of eastern Taiwan, long viewed as one of the collision sutures between the Philippine Sea and the Eurasian plates. With precisely relocated main- and aftershock hypocenters, focal mechanisms for M ≥ 3.8 events, and coseismic ground displacements from strong motion records, we determine a main shock dislocation model. Our results indicate that the main shock occurred on a high angle fault (azimuth 198°, dip 77°). The model comprises a fault with two segments; the main shock and a large number of aftershocks are associated with the northern segment that exhibited predominantly left-lateral strike-slip motion, in agreement with P-wave first motions and waveform (USGS) solutions. The southern segment exhibits a slightly larger thrust component, in agreement with CMT solutions. Tectonically, this event highlights a NNE-trending fault on the west side of the LV, which is predominantly strike-slip. The aftershocks clustered to the east of the main shock, which exhibit mainly thrust mechanisms, indicate that shortening is still acting on the sedimentary materials deposited between the Coastal and Central ranges prior to collision. As a result, the southern LV is undergoing slip partitioning along different faults, which has never been specified before.

[1] Prompted by recent success in the research and application of earthquake early warning systems, we conducted an investigation into the relationship between the magnitudes of earthquakes and the properties of the first three seconds of the P waves at a single station. Using waveforms at station NACB of the Broadband Array in Taiwan for Seismology from 46 earthquakes within 100-km epicentral distance, we found a linear correlation between the magnitudes and the logarithmic characteristic periods of the initial P waves for earthquakes up to magnitude 6.5. We also detected a deterministic relationship between the magnitudes and the peak amplitudes of the initial P waves for earthquakes of magnitude M < 6.5. While further study needs to be done on the behavior of the initial P-wave peak amplitudes from larger earthquakes, we propose here that the characteristic periods and peak amplitudes of the first three seconds of the P waves be used jointly in onsite (single-station) earthquake early warning operations.

[1] The termination of the Ryukyu trench against Eurasia and the oblique subduction of the Philippine Sea plate create a subduction-collision complex offshore Taiwan, which has not previously been elucidated in detail. We combine traveltime data from the seismic networks in Taiwan and Japan to better illuminate how the subducting Ryukyu slab deforms in this subduction-collision zone. More than 5000 events recorded by both networks were relocated with the double-difference method using an optimal regional one-dimensional velocity model. The offshore seismicity indicates that the double seismic zone, with a gap of 15–20 km, exists in the subducting slab in the depth range of 40–80 km. Focal mechanisms suggest that the double seismic zone is caused by east-west compression resulting from oblique convergence. The improved hypocentral locations for the first time reveal folding of the slab into a horizontal curvature larger in magnitude than and opposite in sign to that of the Ryukyu trench in the depth range 50–100 km. The anomalous curvature, together with the focal mechanisms, suggests that the slab folds against the Eurasian lithosphere and that this deformation cannot be fully elastic. We model this deformation mode as the developing instability of a viscoelastic Maxwell layer embedded in a viscous medium. The characteristic wavelength of the instability, i.e., ∼250 km, is consistent with folding of a slab whose viscosity is 100 times higher than that of the surrounding mantle for an along-strike elastic membrane strain as small as 0.01, or more than 3 orders of magnitude higher if 5% elastic strain is allowed.

[1] Pd is the peak amplitude of displacement in the first three seconds after the arrival of the P wave. We investigated the attenuation of Pd with the hypocentral distance R in southern California as a function of magnitude M, and obtained the following relationship: log (Pd )= � 3.463 + 0.729 � M � 1.374 � log (R) ± 0.305. Given an earthquake location determined by the P-wave arrival times at stations close to the epicenter, this relationship can be used to define a so-called ‘‘Pd magnitude’’ of earthquakes. Our result shows that for earthquakes in southern California the Pd magnitudes agree with the catalog magnitudes with a standard deviation of 0.18 for events less than magnitude 6.5. Therefore, Pd is a robust measurement for estimating the magnitudes of earthquakes and has practical application in earthquake early warning systems. Citation: Wu, Y.-M., and L. Zhao (2006), Magnitude estimation using the first three seconds P-wave amplitude in earthquake early warning, Geophys. Res. Lett., 33, L16312, doi:10.1029/2006GL026871.

[1] The 2003 Chengkung earthquake (Mw 6.8) provided diagnostic evidence for a source model showing the deformation process of the seismogenic Chihshang fault in eastern Taiwan. The aftershocks show a fault-bend at a depth of 18 km. Coseismic ground displacements recorded by strong-motion records allow us to deduce instant rupturing of this event. Our resulting model shows a fault length of ∼33 km and dip-slip dominant rupture on fault-plane deeper than 18 km. Estimated coseismic displacements constrain two fault planes: one at 5–18 km depth dipping 60°E and 18–36 km depth dipping 45°E. The uppermost fault-plane of the Chihshang Fault (0–5 km) did not break immediately after the main shock; however, it may have a major role in after-slip and even interseismic ground deformation. The Taiyuan basin developed in the hanging wall is a geomorphic feature consistent with and adequately explained by coseismic ground displacements.

As increasing urbanization is taking place worldwide, earthquake hazards pose serious threats to lives and property in urban areas. For seismic hazard mitigation, a practical earthquake forecast method appears to be far from realization, because of the extreme complexity involved in earthquake processes (e.g., Kanamori et al., 1997). Another approach to mitigate seismic hazards is the development of early warning systems (ews) (Nakamura, 1984, 1988; Heaton, 1985; United States National Research Council, 1991; Teng et al., 1997; United States Geological Survey, 1998; Wu et al., 1998; Wu and Teng, 2002; Allen and Kanamori, 2003). An ews provides a few seconds to tens of seconds of warning time for impending ground motions, allowing for mitigation measures in the short term. Early warning systems that estimate the severity of ground shaking and its onset time are in operation in Japan (Nakamura, 1984, 1988, 1989), Mexico (Espinosa-Aranda et al., 1995), and Taiwan (Teng et al., 1997; Wu et al., 1998, 1999; Wu and Teng, 2002). This timely information can be used to minimize property damage and the loss of lives in urban areas. It can also be used for real-time loss estimation to aid emergency response and recovery (Wu et al., 2002).

An efficient earthquake auto-location algorithm has been developed by the Central Weather Bureau (CWB), Taiwan. The CWB can now routinely obtain earthquake information in one minute after the occurrence of an earthquake using this new algorithm. In order to take full advantage of this capability, four automatic information broadcast media, namely, e-mail, World Wide Web, FTP server and pager system, have been configured to receive and transmit automatically the earthquake information from the CWB seismic monitoring system. This new automatic earthquake information broadcast system will enable the CWB to disseminate information about felt earthquakes even more quickly and widely than its current practice through the fax and paper reports.

Two major earthquakes of ML greater than 6.0 occurred in Taiwan in the first half of 2013. The vibrant shaking brought landslides, falling rocks and casualties. This paper presents a seismic network developed by National Taiwan University (NTU) with 401 Micro-Electro Mechanical System (MEMS) accelerators. The network recorded high quality strong motion signals from the two events and produced delicate shaking maps within one minute after the earthquake occurrence. The high shaking regions of the intensity map produced by the NTU system suggest damage and casualty locations. Equipped with a dense array of MEMS accelerometers, the NTU system is able to accommodate 10% signals loss from part of the seismic stations and maintain its normal functions for producing shaking maps. The system also has the potential to identify the rupture direction which is one of the key indices used to estimate possible damage. The low cost MEMS accelerator array shows its potential in real-time earthquake shaking map generation and damage avoidance.

After the occurrence of a large earthquake, rapid reporting is useful and necessary for resource dispatch management and quick damage assessment. In practice, a ground-motion prediction or a shaking-intensity estimation can be realized once the location and magnitude of an earthquake become available. Rapid availability of event information can also fulfill inquiries and alleviate the misgivings of the public and media. Using the traditional travel-time method, rapid earthquake location is a simple task once a few P -wave arrivals are known. In contrast, a quick and accurate estimate of earthquake magnitude is a nontrivial problem, especially for an extremely large regional event. For a teleseismic-determined moment magnitude, it may take tens of minutes for the propagation of teleseismic waves. For a method using first-arrival P waveforms of broadband seismograms for moment magnitude determinations (Tsuboi et al. , 1995, 1999), the reporting time for moderate-sized earthquakes still takes several minutes. In addition, common high-gain broadband stations will suffer a severe amplitude-clipping problem from regional large earthquakes. The M w 9.0 (Global Centroid Moment Tensor [CMT]) Tohoku-Chiho Taiheiyo-Oki (Tohoku) earthquake that occurred on 11 March 2011 was unprecedented in size since Japan started modern instrumental recordings 130 years ago. The megathrust Tohoku earthquake ruptured the Pacific–North American (Okhotsk) plate boundary off the Pacific coast of northeastern Japan. The work presented here employed the empirical method of Wu and Teng (2004) for quick M w determinations of crustal earthquakes in Japan using time integration over the strong-shaking duration for absolute values of acceleration records. In the study of Wu and Teng (2004), the maximum magnitude was for the 1999 M w 7.6 (Global CMT) Chi-Chi earthquake in Taiwan. In this work, the largest event was the 2011 M w 9.0 Tohoku earthquake in Japan. We show that the method of Wu and …

Timing is a key factor for an earthquake monitoring system and may affect the determination of earthquake location and related studies. In this work, we examined the telemetry delay within the Central Weather Bureau Seismic Network (CWBSN) from 1991 to 2011 and found that the timing systems, at most of the stations in the CWBSN, could contain an approximate 0.2 sec telemetry delay. Based on our results, the telemetry delay was found to cause a 0.2 sec shift in earthquake origin times during the earthquake location process. However, the delay may not cause a significant difference in earthquake location results.

The purpose of the present study was to document the furcation entrance dimensions (FEDs) of the maxillary and mandibular 1st and 2nd molars and relate them to the choice of periodontal therapy. Study samples consisted of 89 maxillary molars (49 1st and 40 2nd molars) and 93 mandibular molars (50 1st and 43 2nd molars). All the FEDs of the molars were examined and measured under a stereomicroscope at 2.5 x equipped with a Bioscan OPTIMAS Image Analyzer (BOIA). The results may be summarized as follows. (1) The mean FEDs in the buccal, distal and mesial furcations of maxillary 1st and 2nd molars were 0.74 mm, 0.99 mm and 1.04 mm in the 1st molars, and 0.63 mm, 0.67 mm, 0.90 mm in the 2nd molars, respectively. In the buccal and lingual furcations of mandibular 1st and 2nd molars, they measured 0.88 mm and 0.81 mm, and 0.73 mm and 0.71 mm, respectively. (2) The %s of FEDs of 0.56 mm or less (the tip width of a Cavitron tip being 0.56 mm) in the buccal, distal and mesial furcations of maxillary 1st and 2nd molars, accounted for 32%, 8% and 6% of 1st molars, and 40%, 40% and 18% of 2nd molars. In the buccal and lingual areas of mandibular 1st and 2nd molars, they accounted for 16% and 26%, and 35% and 33% of the furcations, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

On 4 March 2010, an inland M(subscript w) 6.3 earthquake occurred near the town of Jiasian in Kaohsiung County, Taiwan causing large ground shaking and extensive damage. In this study, we integrate the records from the Central Weather Bureau Seismic Network (CWBSN) and Taiwan Strong Motion Instrumentation Program (TSMIP) to obtain the relocated earthquake sequence and its first-motion focal mechanisms. This dataset offers us precise and reliable results which suggest a focal depth of 23 km and a possible fault plane of strike 313°, dip 41°, and rake 42° for the Jiasian earthquake. This fault plane significantly differs from the N-S striking Chaochou Fault (CCF) as well as the principal trend of Taiwan orogenic belt, and should be an undiscovered fault in southern Taiwan. The relocated Jiasian earthquake sequence initiating from the 23-km-deep mainshock and terminating at around 10 km in depth also indicates it is a blind fault. Peak ground acceleration (PGA) and peak ground velocity (PGV) recorded by the TSMIP stations reveal a distinct NW-SE-shape pattern from the epicenter area toward the Chiayi region, likely due to the directivity and site effects. Such phenomena should be considered for future regional hazard assessments.

A list of 9631 three-component PGA and PGV recordings from 60 large and shallow events in Taiwan from our article, “Near Real-Time Mapping of Peak Ground Acceleration and Peak Ground Velocity following a Strong Earthquake,” is presented as a computer readable file on the attached CD-ROM. …

To improve location identification, we employed layered mode joint hypocenters determination (JHD) and double-difference earthquake location algorithm (hypoDD), three-dimensions velocity structure location (3DLOC), and three-dimensions velocity structure with stations corrections location (3DCOR) for the Rueyli, Taiwan, earthquake sequence. We found that the 3DCOR can greatly improve the location identification precision of the Rueyli earthquake sequence. The Rueyli mainshock was relocated to 23.510˚N, 120.660˚E at the depth of 5.6 km. The focal depth is significantly deeper than the original CWBSN value of 2.8 km. Using this new location, a first-motion fault-plane solution shows that this earthquake is a thrust event with strike N53˚E, dip 42˚E, and rake 121˚. Based on the hypocentral distribution pattern ascertained after relocation and focal mechanism, the Rueyli earthquake was probably caused by faulting in the Tachienshan fault.

Having the ability of near real-time damage assessment would benefit earthquake emergency response operations in Taiwan greatly. Thus, we established an empirical method of assessing the near real-time damage using the rapid reporting system in Taiwan. Relationships between peak ground velocity and damage rates (fatality rate, total and partial household collapsing rates) during the 1999 Chi-Chi earthquake were determined in this study. The distribution of the peak ground velocity can be mapped within minutes of post-initiation of a strong earthquake by the rapid reporting system of the Taiwan Central Weather Bureau (Wu et at 2001). By correlating peak ground velocity with damage rates gathered by the rapid reporting system, a near real-time damage assessment can be issued, in addition to the epicenter, magnitude and intensity.

The 1999 Chi-Chi earthquake in Taiwan represented the island’s largest inland seismic event of the last century. This earthquake severely struck the western-middle part of the island. Abundant high-resolution digital seismic data were recorded by two seismic networks operated by the Central Weather Bureau (CWB). In this paper, we combined data from two CWB seismic networks to analyze the characteristics of the Chi-Chi earthquake sequence. The mainshock was relocated to 23.853°N, 120.816°E at the depth of 8 km. The focal mechanism was of a thrust type with strike 5°, dip 34° and rake 65° based on the new location and the first motion polarities. The distribution of the aftershocks revealed a special distribution pattern which marked out an aseismic dipping plane with the aftershocks mostly occurring on its upper side. It is highly suggested that there is a close relationship between this aseismic dipping plane and the Chelungpu fault.

The purpose of this paper is to document the climate characteristics ofthe Global Forecast System(GFS),which is an atmospheric general circulationmodel developed at the Central Weather Bureau(CWB),Taiwan.This paper documents the winter(December-February)and summer(June-August)climate characteristics of simulated hydrological processes andatmospheric circulation from a 2-year control simulation conducted withthe GFS using an annually repeating prescribed sea surface temperatureclimatology.In most regards,the climate characteristics of hydrological processesand atmospheric circulation are reproduced reasonably well by the GFSwhen compared to observations and analyses of the atmosphere.As for theclimate characteristics of hydrological processes,the major features of observedprecipitation,such as the Intertropical Convergence Zone(ITCZ),the Asian monsoon regimes,and the extratropical storm tracks,are wellcaptured in the GFS simulation.Similarly,other climate features of observedprecipitation,namely the regions of low precipitation rates over thesubtropical subsidence zones and polar areas,are also well defined by theGFS.The simulated precipitation pattern,however,exhibits some obviousdiscrepancies from the observed in the tropics.Excessive precipitation issimulated by the GFS over some tropical regions where there are complextopographic variations among oceans and lands.Otherwise,the GFS precipitationin the remaining tropical regions is generally underestimated.Inparticular,the underestimate of model precipitation over the tropical easternPacific results in a local ITCZ that is less organized in spatial structurethan the observed.This model precipitation deficiency is linked to underestimatesof precipitable water content and water vapor convergence overthe tropical eastern Pacific in the GFS simulation.Regarding the climate characteristics of the zonal mean state,the zonalmean climatologies of temperature and zonal wind are adequately simulatedby the GFS when compared to analyses.The major difference betweenthe simulated and analyzed zonal mean temperatures is a systematiccold bias in the model troposphere.This cold bias is generally within 4°K ofthe analyses for most of the tropospheric domain bounded by 40°S and40°N.The modal cold bias becomes significant at the polar tropopause,wherethe simulated zonal mean temperature can be from 8°K to 18°K colderthan the analyzed.Also noteworthy is the spatial relationship between thezonal mean temperature bias and zonal mean zonal wind bias.This is foundto be consistent with the spatial relationship between the real temperatureand the real zonal wind fields known as the thermal wind relationship.Thisfinding suggests that interactions between the thermal and dynamic fieldsin the GFS simulation must be to a great extent consistent with analyseswith regard to the thermal wind relationship.Regarding the climate characteristics of atmospheric circulation,theprimary circulation features associated with the summer monsoon systemand winter teleconnection pattern are well represented in the GFS simulationwhen compared with analyses.Nevertheless,in winter,major differencesbetween the analyzed and simulated circulation fields include theunderestimate of the East Asia subtropical jet features and the overestimateof the North America subtropical jet features in the GFS simulation.In the summer simulation,the major circulation bias is that the zonalwavenumber-2 component of the Northern-Hemisphere stationary eddy issimulated with larger amplitude than analyses.This circulation bias is accompaniedby excessive precipitation biases over the subtropical centralNorth Pacific west of the date line and the Central America/Caribbean Searegion.