Central Weather Bureau Seismic Network Research Articles

Construction and use of real-time seismic velocity change detection system to monitor the temporal and spatial changes of crustal stress and fault activity in Taiwan

Taiwan is a hotbed of seismic and orogeny activity and this is the basis of the research of Dr Hsin-Hua Huang, Earth Structures Laboratory, Institute of Earth Sciences, Academia Sinica, Taiwan. He has illuminated detailed crustal and slab structures that have furthered understanding of Taiwan orogeny, incorporated ambient noise data to resolve high-resolution imaging of the volcanic and fault structure in northern Taiwan and conducted research that led to the discovery of multi-layering magma beneath the Yellowstone supervolcano in the US. The ultimate goal of his diverse studies is to achieve an improved imaging of the structure of the Earth and establish better understanding of the orogenic tectonic process and assess and mitigate natural hazards, such as volcanoes and earthquakes. In their latest project, Huang and the team are using the seismic wave interferometry method, combined with Taiwan's existing real-time signal transmission Taiwan Broadband Seismic Network (BATS) and the Central Weather Bureau Seismic Network (CWB24), to develop an automated real-time seismicity change detection system. The idea is that by measuring changes in the transmission velocity of seismic waves in the crust, physical parameters such as the degree of rock compression or changes in porosity can be linked and discussed. The researchers have performed an extensive review of knowledge about the current stage of fault zone failure and healing processes,the advantages of seismic noise interferometry and the need for Taiwan fault zone monitoring and related seismic interferometry research. In order to understand the temporal changes in crustal structure and the behaviour of velocity changes across faults, a long-term and stable real-time seismic velocity change monitoring system is necessary.

Re-calculation of the attenuation functions for Local Magnitude from the upgraded Central Weather Bureau Seismic Network in Taiwan

Re-calculation of the attenuation functions for Local Magnitude from the upgraded Central Weather Bureau Seismic Network in Taiwan

2012 Seismicity Quiescence in Taiwan a Result of Site‐Effect Artifacts

ABSTRACT A significant seismic quiescence in Taiwan was reported by Wu and Chiao (2006) before the occurrence of the 1999 M w 7.6 Chi‐Chi earthquake. A similar kind of activity was also observed in the 2012 earthquake catalog reported by Central Weather Bureau (CWB). Seven months in the 2012 catalog seem to have lower monthly seismicity rates than the one standard deviation below the mean (673±82) for earthquakes with M L ≥2.0. Here, we checked the seismic network in Taiwan and found that some new seismic stations, including the Broadband Array in Taiwan for Seismology (BATS), were incorporated within the Taiwan Central Weather Bureau Seismic Network (CWBSN) since 2012. Most of these BATS stations are located on hard‐rock sites, which may affect magnitude estimation because of their site characteristics. To account for the impact of site effect, earthquake catalog data during the period of 1994 to 2014 was collected and station correction was calculated for each station. A strong correlation is found between station corrections determined in this study and geological settings of the region. Stations located on soil sites have high amplifications with negative station corrections. On the other hand, stations located on hard‐rock sites have low amplifications with positive station corrections. After applying station correction, the mean seismicity rate is found to be 716 events per month with a standard deviation of 76 events for earthquakes with M L ≥2.0. We conclude that the reason behind the apparent low seismicity in 2012 is due to the installation of new seismic stations on rock sites, which lead to underestimation of M L .

Tomography of the Chukou Fault Zone, Southwest Taiwan: Insights from Microearthquake Data

The vigorous collision between the Eurasian plate and Philippine Sea plate in Taiwan causes a series of imbricate fold and thrust belts to develop at the deformation front. The Chukou Fault (CKF), characterized by a thrust type fault, located in Chiayi County, southwest (SW) Taiwan, is a prominent boundary between the fold-thrust belts and the Western Coastal Plain. Most of the seismicity in SW Taiwan is associated with this fault and its neighboring fault systems. The seismotectonic structures in the CKF zone, especially in the east, are complex due to the interactions among fault systems with distinct slip motions. To gain better insights into the seismogenic characteristics in the CKF zone, we used 1661 microearthquakes recorded by a temporary dense broadband seismic network and the Central Weather Bureau Seismic Network (CWBSN) between 2003 and 2004 to investigate the physical properties of the crust in the CKF zone. A waveform cross-correlation technique was applied to 143086 pairs of waveform data to determine the relative differential travel time between the P- and S-waves. By combining both the absolute and relative differential travel time data, we were able to obtain a new 3-D crustal P-wave velocity structure and Vp/Vs ratios. This study suggests that by using both absolute and relative differential travel time data in tomographic inversion can obtain precise 3-D velocity images and also gain better correlation between seismic events and fault structures, which is crucial for understanding the seismogenic process in our study area.

Near-Surface Attenuation and Velocity Structures in Taiwan from Wellhead and Borehole Recordings Comparisons

By analyzing the data from 28 seismic borehole stations deployed by the Central Weather Bureau Seismic Network throughout Taiwan from 2007 to 2014, we estimated the near-surface velocity (Vp and Vs) and attenuation (Qp and Qs) structures from surface to depths of approximately 300 m. To ensure that the deeper recordings were on the ray path between the seismic source and upper receiver, only events with an incidence angle of less than 35° were selected. Local magnitudes of analyzed events were between 1.1 and 6.6. The subsurface Qp and Qs were well modeled in the 5 - 40 Hz frequency band using the spectral ratio of direct P- and S-waves, respectively, at each station, under frequency-independent Q and ω^2 source model assumptions. The estimated Vp in the Coastal Plain, the Western Foothills, the Longitudinal Valley, and the Yilan Plain were approximately 1000 - 2000 m s^(-1), which was lower than the Vp of 2500 - 4000 m s^(-1) in the Central Mountain Range. In addition, the Vs in the plain areas were lower than that in the Central Mountain Range. The low Vp and Vs and high Vp/Vs ratio in the Coastal Plain and the Western Foothills can be attributed to the unconsolidated soil and high pore-fluid content of subsurface sediments in the plain areas. In contrast to the velocity distribution, low Qp and Qs were observed in the Central Mountain Range. The low Qp and Qs with low Vp/Vs and low Qs/Qp ratios in the Central Mountain Range was consistent with the high thermal temperature observed in the field investigation. The obtained velocity and attenuation structures near surface could also provide important constraints in validation of the crustal structure of Taiwan.

The seismogenic deformation and Qp temporal variation before the M6.2 Mingjen earthquake, Taiwan

Detecting anomalies before an earthquake has become an important task in seismic hazard prevention. The purpose of this research is to use the intrinsic attenuative dispersion of the P-wave, Qp, to serve as an anomaly before an earthquake. On November 5, 2009, an earthquake (ML=6.2; depth=24km) occurred in Ming-Jen, Nantou county, Taiwan. The vertical-component of the first-cycle of P-wave data recorded from the Central Weather Bureau Seismic Network from 2009/01 to 2010/01 was calculated to obtain the Qp parameter. Meanwhile, this study also utilizes global positioning system stations to examine changes in surface displacements that occurred during the Mingjen earthquake. Analysis results indicate that 2-months before the mainshock occurred; the Qp began to decrease for all stations, which implies that the crustal area became more fractured owing to a high stress accumulation. Such a lead time of ∼2months is equal to the average lead time of Seismic Electric Signals (SES) activities being also compatible with a physical model suggested long ago for the SES generation. With stress accumulating a few days before the earthquake, an impeded region can be clearly identified by relatively-large difference values when the mainshock is going to occur. We conclude that the analysis of the Qp parameter and GPS index are very sensitive to changes in the stress state and may serve as a useful index in early warning of earthquake.

Incorporating Low-Cost Seismometers into the Central Weather Bureau Seismic Network for Earthquake Early Warning in Taiwan

A dense seismic network can increase Earthquake Early Warning (EEW) system capability to estimate earthquake information with higher accuracy. It is also critical for generating fast, robust earthquake alarms before strong-ground shaking hits the target area. However, building a dense seismic network via traditional seismometers is too expensive and may not be practical. Using low-cost Micro-Electro Mechanical System (MEMS) accelerometers is a potential solution to quickly deploy a large number of sensors around the monitored region. An EEW system constructed using a dense seismic network with 543 MEMS sensors in Taiwan is presented. The system also incorporates the official seismic network of Taiwan's Central Weather Bureau (CWB). The real-time data streams generated by the two networks are integrated using the Earthworm software. This paper illustrates the methods used by the integrated system for estimating earthquake information and evaluates the system performance. We applied the Earthworm picker for the seismograms recorded by the MEMS sensors (Chen et al. 2015) following new picking constraints to accurately detect P-wave arrivals and use a new regression equation for estimating earthquake magnitudes. An off-line test was implemented using 46 earthquakes with magnitudes ranging from M_L 4.5-6.5 to calibrate the system. The experimental results show that the integrated system has stable source parameter results and issues alarms much faster than the current system run by the CWB seismic network (CWBSN).

Quantifying the seismicity on Taiwan

SUMMARY We quantify the seismicity on the island of Taiwan using the frequency–magnitude statistics of earthquakes since 1900. A break in Gutenberg–Richter scaling for large earthquakes in global seismicity has been observed, this break is also observed in our Taiwan study. The seismic data from the Central Weather Bureau Seismic Network are in good agreement with the Gutenberg–Richter relation taking b ≈ 1 when M 8 earthquake in the study region about every 25 yr. However, our analysis shows a lower frequency of occurrence of large earthquakes so that the expected frequency of M > 8 earthquakes is about 200 yr. The level of seismicity for smaller earthquakes on Taiwan is about 12 times greater than in Southern California and the possibility of a M ≈ 9 earthquake north or south of Taiwan cannot be ruled out. In light of the Fukushima, Japan nuclear disaster, we also discuss the implications of our study for the three operating nuclear power plants on the coast of Taiwan.

Statistical evaluation of the effect of earthquake with other related factors on landslide susceptibility: using the watershed area of Shihmen reservoir in Taiwan as a case study

Landslide susceptibility evaluation is one of the most important issues in watershed management. After an earthquake, the landslide susceptibility decreases functionally with increases in the distance from the epicenter. Under the same rainfall intensity, landslides are more likely to occur in an area where earthquakes occur more frequently. However, the questions of how much an earthquake should be weighted and how to evaluate the effects of an earthquake still need to be studied. To understand how earthquakes affect rainfall-triggered landslides, the horizontal peak ground acceleration (PGA) data from the Central Weather Bureau Seismic Network is used as the earthquake factor and combined with other factors to determine the weight of earthquakes in landslide susceptibility using logistic regression. The results indicate that the ability of landslide prediction is better when considering the earthquake factor. This study also proved that although there are no co-seismic landslides (after earthquakes) in the study area, the earthquake factor is still required to increase the model accuracy. PGA has been described as a usable factor. In areas with frequent earthquakes and high geological activity, when using historical data to evaluate landslide susceptibility, the earthquake factor should be taken into consideration to prevent errors.

An Examination of Telemetry Delay in the Central Weather Bureau Seismic Network

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.

Bayesian Estimation of the Spatially Varying Completeness Magnitude of Earthquake Catalogs

Assessing the completeness magnitude Mc of earthquake catalogs is an essential prerequisite for any seismicity analysis. We employ a simple model to com- puteMc inspacebasedontheproximitytoseismicstationsinanetwork.Weshowthata relationship of the form M predd �� ad bc, with d the distance to the kth nearest seismic station, fits the observations well, k depending on the minimum number of stations being required to trigger an event declaration in a catalog. We then propose anewMc mappingapproach,theBayesianmagnitude ofcompleteness (BMC)method, based on a two-step procedure: (1) a spatial resolution optimization to minimize spatial heterogeneities and uncertainties in Mc estimates and (2) a Bayesian approach that merges prior information about Mc based on the proximity to seismic stations with locally observed values weighted by their respective uncertainties. Contrary to the cur- rentMc mappingprocedures,theradiusthatdefineswhichearthquakestoincludeinthe local magnitude distribution is chosen according to an objective criterion, and there are no gaps in the spatial estimation of Mc. The method solely requires the coordinates of seismic stations. Here, we investigate the Taiwan Central Weather Bureau (CWB) seismic network and earthquake catalog over the period 1994-2010.

The Preliminary Study of the 4 March 2010 Mw 6.3 Jiasian, Taiwan Earthquake Sequence

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.

The 2005 Ilan earthquake doublet and seismic crisis in northeastern Taiwan: evidence for dyke intrusion associated with on-land propagation of the Okinawa Trough

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.

Improved seismic tomography offshore northeastern Taiwan: implications for subduction and collision processes between Taiwan and the southernmost Ryukyu

SUMMARY To improve the resolution in imaging the 3-D VP and VP/VS structures in the Taiwan region, especially offshore eastern Taiwan, we combine the arrival times from eleven ocean bottom seismometers (OBSs) and from seismic stations of the Japan Meteorological Agency (JMA) with those from the permanent stations of Taiwan Central Weather Bureau Seismic Network (CWBSN) and Taiwan Strong Motion Instrumentation Program (TSMIP). By doing so, we have obtained a new 3-D tomographic model for Taiwan and its surrounding regions with a better resolution, especially in the area offshore northeastern Taiwan. We also used this new tomography model to relocate the hypocentres of the earthquakes in northeastern Taiwan and determined the focal mechanisms of relatively large events. Our tomography results indicate that in the region northeast of Taiwan, the subducting oceanic Philippine Sea Plate beneath the Eurasian continental lithosphere is characterized by a high VP layer, surrounded by lower VP areas. This P-wave velocity characteristics of the subducting slab provides a better constraint on the geometry of the subduction interface, especially in its shallower portion. In the hangingwall (Eurasian) block above the subduction interface, a vertically elongated high VP/VS body appears to originate from the interface at depths between 100 and 140 km. We suggest that it represents the partially melted materials that are related to the magmatic activity of the Ryukyu volcanic arc.

Intrinsic and attenuative dispersion characteristics of direct P-waves in and near the source area of the 1999 MW7.6 Chi-Chi, Taiwan, earth- quake before and after the mainshock

Based on the measurement of the arrival time of maxima magnitude from band-pass filtering signals which were determined using a new Morlet wavelet multiple-filter method, we develop a method for measuring intrinsic and attenuative dispersion of the first cycle direct P-wave. We determine relative group delays of spectral components of direct P-waves for 984 ray paths from SML and ALS stations of the Taiwan Central Weather Bureau Seismic Network (CWBSN). Using continuous relaxation model, we deduce a new transfer function that relates intrinsic dispersion to attenuation. Based on the genetic algorithm (GA), we put forward a new inversion procedure for determining which is defined the flat part of quality factor Q(ω) spectrum, τ1 and τ2 parameters. The results indicate that (1) The distribution of Qm values versus epicentral distance and depth show that Qm values linearly increase with increasing of epicentral distance and depth, and Qm values is clearly independent of earthquakes magnitude; (2) In the different depth ranges, Qm residual show no correlation with variations in epicentral distance. Some significant changes of Qm residual with time is likely caused by pre-seismic stress accumulation, and associated with fluid-filled higher density fractures rock volume in the source area of 1999 Chi-Chi Taiwan earthquake. We confirm that Qm residual with time anomaly appears about 2.5 years before the Chi-Chi earthquake; (3) A comparison of Qm residual for different depth range between SML and ALS stations show that the level of stress has vertical and lateral difference; (4) The area near observation station with both anomalously increasing and decreasing averaged Qm residual is likely an unstable environment for future strong earthquake occurrence. This study demonstrates the capability of direct P-waves dispersion for monitoring attenuation characteristics and its state changes of anelastic medium of the Earth at short propagation distance using seismograms recorded from very small events.

Relocation of the 2006 Pingtung Earthquake sequence and seismotectonics in Southern Taiwan

We combine the arrival time data from eleven ocean bottom seismometers (OBS) and from permanent stations of the Central Weather Bureau Seismic Network (CWBSN) and the Taiwan Strong Motion Instrumentation Program (TSMIP) to improve the three-dimensional Vp and Vp/Vs structures in the Taiwan region, relocate hypocenters of earthquakes in southern Taiwan including the 2006 Pingtung Earthquake sequence, and determine the focal mechanisms of some of the larger events. Our tomography result suggests that in the region south of Taiwan the Eurasian continental lithosphere is being subducted beneath the oceanic Philippine Sea plate, leading to a region of low P-wave speed with seismicity on top of a relatively high P-wave speed zone. The hypocentral distribution of the Pingtung Earthquake sequence indicates that the first event in the initial two largest shocks was a normal-faulting quake caused by the bending of the subducting lithosphere. The second event, perhaps triggered by the first one, was a strike-slip earthquake.

A Comprehensive Relocation of Earthquakes in Taiwan from 1991 to 2005

We have carried out a comprehensive relocation of a total of 267,210 earthquakes in Taiwan that occurred during the past 15 yr. We based our relocation process on the earthquake catalog of the Taiwan Central Weather Bureau Seismic Network (CWBSN) and made improvements in three aspects. First, we incorporated a large dataset of the S-P times from 680 Taiwan Strong-Motion Instrumentation Pro- gram (TSMIP) stations distributed throughout the island of Taiwan to improve the coverage of earthquakes on the island. Secondly, we added 18 Japan Meteorological Agency (JMA) stations in the southern Ryukyu Island chain to enhance the station coverage for eastern offshore events, especially around the subduction zone northeast of Taiwan. Thirdly, we adopted 3D VP and VP=VS models in predicting the travel times of P and S waves. The effectiveness of these improvements in earthquake re- location can be seen in three aspects: (1) the reduction in the residuals of P-wave arrival times and S-P times, (2) a better understanding of the attenuation relationship between the peak-ground acceleration and epicentral distance, and (3) the geologically meaningful patterns of station corrections to P-wave arrival times and S-P times. Online Material: Catalog of relocated earthquakes in Taiwan from January 1991 to

Study on Temporal Variations of Shear‐Wave Splitting in the Chiayi Area, Aftershock Zone of 1999 Chichi Earthquake, Taiwan

AbstractBased on the data from three seismic stations CHY, CHN2 and CHN8 of Taiwan Central Weather Bureau Seismic Network (CWBSN), the systematic analysis method (SAM) of shear‐wave splitting has been used to do the near‐source stress‐forecast research of temporal preseismic variations of polarization directions and time delays for nearly 22 months before 1999 ML 6.4 and ML 6.0 in the Chiayi area‐aftershock zone of 1999 Chichi earthquake with MW7.6 on September 20, Taiwan. The result shows that the normal polarization directions of station CHY, CHN2 and CHN8 are approximately EW, in good agreement with the direction of maximum compressional stress, suggesting that the anisotropy in this area is controlled by local tectonic stress field. The results strongly indicate that in the stress critical stage, the polarization 90°‐flips occurred frequently, and that there were abrupt decreases in time delays shortly before the Chichi mainshock and two Chiayi earthquakes. These can be considered as two precursors of stress‐forecast. We think that the temporal variations of shear‐wave splitting are of great importance, since they provide information about source stress processes and critical warning information before large earthquakes.

Seismic tomography of Taiwan: Improved constraints from a dense network of strong motion stations

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 Vp and Vp/Vs 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 Vp structure largely confirms previous studies but brings better constraint on the Vp/Vs structure. The colliding Luzon volcanic arc is characterized by a belt of high Vp and highVp/Vs 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 highVp/Vs 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 Vp/Vs with lower seismicity. Most of the events occurred at the base of the high Vp/Vs 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 Vp/Vs belt.

Aftershocks of the 1999 Chi-Chi, Taiwan, Earthquake: The First Hour

Within the hour immediately after the 1999 Chi-Chi mainshock, only 40 aftershocks were located by the Taiwan Central Weather Bureau Seismic Network (cwbsn) because of the power outage in more than half of the island and the limited dynamic range of the cwbsn high-gain instruments. Here, by analyzing 20 near- field, on-scale records from the Taiwan Strong-Motion Instrumentation Program (tsmip), we determined a catalog of 296 aftershocks with M L ≥3.4 within the first hour after the Chi-Chi mainshock. Focal mechanisms were also determined for 24 of these aftershocks. The frequency-magnitude relation obtained from the 296 aftershocks indicates that the catalog is complete above magnitude M L 4.3. Most of the aftershocks occurred in a small-slip region of the mainshock immediately to the north of the mainshock epicenter. Spatially, the aftershocks appear to migrate downward from the mainshock hypocenter. Later in the hour, the aftershocks began to concentrate in the fringe area of the main rupture. During the first hour, the b -value as defined by events in the 4.3 M L