Near-field triggering of microearthquakes along the Longitudinal Valley fault in eastern Taiwan

Dynamic earthquake triggering as a well-documented phenomenon can provide valuable information for studying the stress loading cycle from failure on faults.In this study,seismicity rate changes were investigated in the Longitudinal Valley fault(LVF)following the 2019 M_(L)5.2 Hualien earthquake,which occurred offshore in eastern Taiwan.After the matched filter technique was applied to continuous waveform data,twice as many microearthquakes were newly detected in the vicinity of the LVF compared with the number listed in the Taiwan Weather Bureau catalog.Seismicity rates in the northern segment of the LVF increased immediately following the Hualien mainshock,which indicated dynamic triggering during the passage of seismic waves.Statistical analysis suggested that following seismic events might be attributed to fault slipping or creeping.These findings show that the observation of earthquake triggering can provide valuable assistance in monitoring the stress perturbations of active faults.

Inversion of source process and related studies of the Taiwan Strait earthquake using genetic algorithm

pplying genetic algorithm to inversion of seismic moment tensor solution and using the data of P waveform from digital network and initial motion directions of P waves of Taiwan network stations, we studied the moment tensor solutions and focal parameters of the earthquake of M=7.3 on 16 September of 1994 in Taiwan Strait and other four quakes of ML5.8 in the near region (21°～26°N, 115°～120°E). Among the five earthquakes, the quake of M=7.3 on September 16, 1994 in Taiwan Strait is the strongest one in the southeastern coast area since Nan′ao earthquake of M=7.3 in 1918. The results show that moment tensor solution of M=7.3 earthquake is mainly doublecouple component, and is normal fault whose fault plane is near NW. The strike of the fault plane resembles that of the distributive bands of earthquakes before the main event and fracture pattern shown by aftershocks. The tension stress axis of focal mechanism is about horizontal, near in NE strike, the compressive stress axis is approximately vertical, near in NWW strike. It seems that this quake is controlled by the force of Philippine plate′s pressing Eurasian plate in NW direction. But from the viewpoint of P axis of near vertical and T axis of near horizontal, it is a normal fault of strong tensibility. There are relatively big difference between focal mechanism solution of this quake and those of the four other strong quakes. The complexity of source mechanism solution of these quakes represents the complexity of the process of the strait earthquake sequences.

The source rupture feature of the southern Taiwan Straits earthquake of September 16，1994 （Ms7．3） ＆ the analysis of earthquake circumstance in southeastern coast of China

According to the source mechanism of the main shock and the distribution feature of the aftershocks occurring in the southern Taiwan Straits on Sept. 16, 1994, in this paper the authors analysed the source rupture feature of the major earthquake, demonstrated that this seismic sequence possessed the charateristics of a large intraplate earthquake. And according to the seismotectonic background and the historical seismicity in the area, the authors clarified the active characteristics of the seismically active belts along northwestern direction and analysed preliminarily the earthquake circumstance in the southeastern coast of China.

A Study on the Early Warning Time of Strong Earthquakes in Taiwan

The paper collects the records by the Fujian Digital Seismic Network of 40 shallow earthquakes in Taiwan with M_S≥5.0 from 1999 to 2013,analyzes the seismic phase(Pn,Sn phase)characteristics and travel-time rules,determines travel-time models and develops a seismic phase travel-time equation based on the two-step fitting algorithm.With the deduction of processing time and network delay time,this method can provide an accurate estimation of early warning time of Taiwan earthquakes for the Fujian region,and has been officially employed in the earthquake early warning system of Fujian Province.

The Coulomb Stress Change Associated with the Taiwan Straits M_S 7.3 Earthquake on September 16,1994 and the Risk Prediction of Its Surrounding Faults

Using the focal mechanism solutions and slip distribution model data of the Taiwan Straits MS7.3 earthquake on September 16, 1994, we calculate the static Coulomb stress changes stemming from the earthquake. Based on the distribution of aftershocks and stress field, as well as the location of historical earthquakes, we analyze the Coulomb stress change triggered by the Taiwan Straits MS7.3 earthquake. The result shows that the static Coulomb stress change obtained by forward modeling based on the slip distribution model is quite consistent with the location of aftershocks in the areas far away from the epicenter. Ninety percent of aftershocks occurred in the stress increased areas. The Coulomb stress change is not entirely consistent with the distribution of aftershocks near the epicenter. It is found that Coulomb stress change can better reflect the aftershock distribution far away from the epicenter, while such corresponding relationship becomes quite complex near the epicenter. Through the calculation of the Coulomb stress change, we find that the stress increases in the southwest part of the Min-Yue (Fujian-Guangdong) coastal fault zone, which enhances the seismic activity. Therefore, it is deemed that the sea area between Nanpeng Island and Dongshan Island, where the Min-Yue coastal fault zone intersects with the NW-trending Shanghang-Dongshan fault, has a high seismic risk.

The May 23 and 24, 1994 Taiwan earth quakes: comparison of source process between the two events

Inversion for the seismic fault rupture history is an important way to study the nature of the earthquake source. Inthis paper, we have selected two Taiwan earthquakes that occurred closely in time and located in the same region,inversed the distribution of the slip amplitudes, rakes, risetimes and the rupture times on the fault planes by usingGDSN broad-band and long-period records and the adaptive hybrid global search algorithm, and compared the twoevents. The slip rate of every subfault calculated provides information about the distribution of tectonic stress andfault strength. To the former event (Ms=6.0), the maximum slip amplitude 2.4 m and the minimum risetime 1.2 sare both located at the hypocentre. The latter earthquake (Ms=6.6) consisted of two subevents and the second source has 4 s delay. The maximum slip amplitUde 0.9 m located near hypocentre is corresponding to the minimumrisetime l.4 s, and the corresponding maximum slip rate 0.7 m.s~-1 is similar to the peak value of other large sliprate areas. We consider that the latter event has more complicated temporal-spatial distribution than the former.

Seismotectonics of the Taiwan Shoal Region in the Northeastern South China Sea:Insights from the Crustal Structure

A cluster of earthquakes occurred in the Taiwan Shoal region on the outer rise of the Manila Trench. Although most were of small to medium magnitudes, one strong earthquake occurred on September 16, 1994. Several previous studies have provided important information to progress our understanding of this single earthquake. However, little is currently known about the earthquake cluster, and it is necessary to investigate the deep crustal structure of the Taiwan Shoal region to understand the mechanisms involved in controlling and generating it. This study presents a two-dimensional seismic tomographic image of the crustal structure along the OBS2012 profile based on ocean-bottom seismograph(OBS) data, which exhibits a high-velocity anomaly flanked by low-velocity anomalies in the upper crust beneath the Taiwan Shoal. In this study, 765 earthquakes(Richter magnitude ML > 1.5) occurring between 1991 and 2015 were studied and analyses of earthquake epicenters, regional faults, and the crustal structure provides an improved understanding of the nature of active tectonics in this region. Results of analyses indicate firstly that the high-velocity area represents major asperities that correspond to the location of the earthquake cluster and where stress is concentrated. It is also depicted that the earthquake cluster was influenced by fault interactions. However, the September 1994 earthquake occurred independently of these seismic activities and was associated with reactivation of a preexisting fault. It is also determined that slab pull is resisted by the exposed precollision accretionary prism, and the resistive force is causing accumulation of inplane compressive-stress. This may trigger a future damaging earthquake in the Taiwan Shoal region.

The middle-long term prediction of the February 3,1996 Lijiang earthquake(M_S=7) by the "criterion of activity in quiescence

Earthquake activities in history are characterized by active and quiet periods. In the quiet period, the place where earthquake M_≥6 occurred means more elastic energy store and speedy energy accumulation there. When an active period of big earthquake activity appeared in wide region, in the place where earthquake (M_≥6) occurred in the past quiet period, the big earthquake with magnitude of 7 or more often occur there. We call the above-mentioned judgement for predicting big earthquake the 'criterion of activity in quiescence'. The criterion is relatively effective for predicting location of big earthquake. In general, error of predicting epicenter is no more than 100 km. According to the criterion, we made successfully a middle-term prediction on the 1996 Lijiang earthquake in Yunnan Province, the error of predicted location is about 50 km. Besides, the 1994 Taiwan strait earthquake (M_s=7.3), the 1995 Yunnan-Myanmar boundary earthquake (M_s=7.2) and the Mani earthquake (M_s=7.9) in north Tibet are accordant with the retrospective predictions by the 'criterion of activity in quiescence'. The windows of 'activity in quiescence' identified statistically by us are 1940-1945, 1958-1961 and 1979-1986. Using the 'criterion of activity in quiescence' to predict big earthquake in the mainland of China,the earthquake defined by 'activity in quiescence' has magnitude of 6 or more; For the Himalayas seismic belt, the Pacific seismic belt and the north-west boundary seismic belt of Xinjiang, the earthquake defined by 'activity in quiescence' has magnitude of 7, which is corresponding to earthquake with magnitude of much more than 7 in future. For the regions where there are not tectonically and historically a possibility of occurring big earthquake (M_s=7), the criterion of activity in quiescence is not effective.

Relation between Gravity Field Feature and Tectonics and Earthquakes in Taiwan and Its Adjacent Seas

Short wave gravity anomaly is correlated to sea floor topography in the gravity field of ×Taiwan and its adjacent seas. Gravity values of 20010-5ms-2 at Yushang and -×16010-5ms-2 at Liuqiu sea trench are respectively the maximum and minimum gravity values in this area. Bouguer gravity anomaly reflects not only Moho interface undulation, but also fault distribution. The inflexion of gradient belt of Bouguer gravity anomaly is a spot liable to earthquakes . Middle-long wave geoid is the best data to invert crustal thickness. We calculate crustal thickness by using geoid data, and the maximum value is 38km; the minimum value is 12km in Taiwan and its adjacent seas.

Effect of Seismicity in the Taiwan Straits on the Southeast Coastal Area of the Chinese Mainland

Based on the available and supplementary survey data,it analyzes the effect of seismicity in Taiwan and the Taiwan Straits on the southeastern coastal area of the Chinese mainland and discusses its roles in seismic hazard prevention and textual research of historical earthquakes. The results show that the frequency of strong earthquake in Taiwan Region is high,with a time interval ranging from several to dozens of years,but the maximum influence intensity of seismicity from there to the coastal areas of the Chinese mainland is only VI degree; while the maximum influence intensity of the seismicity along the littoral fault zone located on the west of the straits reaches VIII ～ IX degree because of the shorter distance to the Chinese mainland,though the frequency of strong earthquakes is lower than that of the Taiwan Region. Strategies for protecting against seismic hazards in the southeastern coastal area of China are proposed. Besides focusing on the effect of strong earthquakes of the littoral fault zone,attention also has to be paid to the low-cycle fatigue failure of engineering structures induced by the earthquakes in Taiwan and the stir effect on society induced by earthquake phobia. It is concluded that it would be more accurate and proper to take the May 19,1517 earthquake recorded in the Chinese mainland area as the influence of a strong earthquake in the Taiwan Region.

Discussion of the Prospective Danger of Earthquake Damage to the Xiamen Jinmen District

It has been long discussed already about the characteristics of the recent earthquake of the Xiamen Jinmen district. However, attention should be paid to that a large shock unexpected before such as the 1999 Chi Chi earthquake in Taiwan west plain would take place in this area and its neighborhood, affecting the stability of the construction during the long duration after completion and usage of the future Xiaman Jinmen Bridge. The probability of such case should be considered from studying the earthquake mechanism of the surrounding regions: The Taiwan Strait basin, Taiwan region (Taiwan island) and Fujian coastal region respectively. In this paper the features of earthquakes which occurred and will occur in any one of these three districts and then their influences on the construction of the future Xiamen Jinmen Bridge are described and discussed.

武汉某超高层建筑对台湾花莲6.9级地震的结构响应分析

为研究2022年台湾花莲6.9级地震对武汉市某超高层建筑结构的影响,初步分析了该建筑结构4个加速度测点记录的各向PGA放大效应、傅立叶谱及反应谱,分别利用互功率谱法和基于协方差的随机子空间法分别识别结构在频域和时域上的一阶模态。结果表明:结构的峰值加速度随楼层的增高而有显著的放大效应,最大放大倍数达7.322倍;各向傅立叶谱峰值均集中在0.275 Hz和1.171 Hz;加速度响应反应谱远小于该地区Ⅵ度设防地震设计谱;两种方法识别的结构一阶模态频率基本一致,均接近数值分析值。台湾花莲6.9级地震没有对该高层建筑结构造成损伤。

2024年4月3日台湾省花莲县海域7.3级地震的快速产出参数

2024年4月3日07时58分(北京时间)在台湾省花莲县海域发生7.3级地震。中国地震局地球物理研究所在震后启动快速响应,组织相关领域研究人员对此次地震的震源模型、地震辐射能量等进行了估计,同时基于震源模型进行了震动图模拟、同震形变场模拟。结果表明,此次地震发生在菲律宾海板块和欧亚板块的边界,以逆冲机制为主,呈现双侧破裂,能量集中在前约35 s内释放;极震区震动烈度可能达Ⅹ度以上,可能的受灾范围近35 000 km^(2);此次地震引起了显著的同震位移,最大水平向位移达到0.52 m、垂直向位移达到1.18 m。

台湾地区花莲县海域7.3级地震及其触发火山活动的可能性分析

北京时间2024年4月3日07时58分在台湾地区花莲县海域(23.81°N,121.74°E)发生7.3级地震,地震灾害影响较大,引起国内外广泛关注。本文对此次地震的地质构造背景及其灾害影响,以及受地震作用导致山体崩塌的龟山岛火山活动等方面进行了介绍,重点针对此次地震是否会触发龟山岛火山活动的可能性进行了综合分析。通过地震及高光谱气体数据分析可知,龟山岛火山尚未出现明显的异常响应;随后通过引入地震对潜在火山触发的无维指数来定量判断触发的可能性,经测算触发指数TRIGI值为19.8(>10),短期内地震触发龟山岛火山喷发的可能性较低,从定性定量上分析龟山岛火山当前仍处于休眠状态。未来将继续收集完善火山喷发前的宏微观前兆异常资料,跟踪研判火山活动状态,得出更精准可靠的触发可能性结论。

GPS约束下的2022年台湾地区M_(W)6.9地震破裂滑动分布

利用9个GPS站水平和垂直向形变数据反演2022年台湾地区M_(W)6.9地震破裂滑动分布。结果表明,此次地震破裂至地表,滑移以走滑为主兼逆冲分量,主要沿NNE向延伸,有2个破裂集中区,共释放地震矩约5.73×10^(19) Nm。基于GPS观测和位错理论模型对此次地震的地表形变进行分析,认为同震形变整体上符合台湾岛东海岸区域构造运动特征,菲律宾海板块俯冲欧亚板块是中央山脉断裂活动的主要动力来源。

Seismicity gap and seismic quiescence before 1999 Jiji(Chi-Chi) M_W7.6 earthquake

The September 21, 1999, Jiji （Chi-Chi） Mw7.6 earthquake is the strongest event occurred since 1900 in Taiwan of China. It is located in the middle segment of the western seismic zone of Taiwan. Based on several versions of China earthquake catalogue this study found that a seismic gap of M≥5 earthquakes appeared, in and around the epicenter region, 24 years before and lasted up to the mainshock occurrence. This study also noticed that there existed a lager seismically quiet region of M≥4 earthquakes, which lasted for about 2.5 years before the mainshock occurrence, The spatial variation pattern of regional seismicity before the mainshock seems to match with its coseismic source rupture process. The mentioned seismicity gap and seismic quiescence might be an indication of the preparation process of the Jiji strong earthquake.

Active tectonics in Taiwan:insights from a 3-D viscous finite element model

Taiwan is a young orogenic belt with complex spatial distributions of deformation and earthquakes. We have constructed a three-dimensional finite element model to explore how the interplays between lithospheric struc- ture and plate boundary processes control the distribution of stress and strain rates in the Taiwan region. The model assumes a liberalized power-law rheology and incorporates main lithospheric structures; the model domain is loaded by the present-day crustal velocity applied at its bound- aries. The model successfully reproduces the main features of the GPS-measured strain rate patterns and the earth- quake-indicated stress states in the Taiwan region. The best fitting model requires the viscosity of the lower crust to be two orders of magnitude lower than that of the upper crust and lithospheric mantle. The calculated deviatoric stress is high in regions of thrust faulting and low in regions of extensional and strike-slip faulting, consistent with the spatial pattern of seismic intensity in Taiwan.

Calculation of landslide occurrence probability in Taiwan region under different ground motion conditions

In this study,Bayesian probability method and machine learning model are used to study the real occurrence probability of earthquake-induced landslide risk in Taiwan region.The analyses were based on the 1999 Taiwan Chi-Chi Earthquake,the largest earthquake in the history in this Region in a hundred years,thus can provide better control on the prediction accuracy of the model.This seismic event has detailed and complete seismic landslide inventories identified by polygons,including 9272 seismic landslide records.Taking into account the real earthquake landslide occurrence area,the difference in landslide area and the non-sliding/sliding sample ratios and other factors,a total of 13,656,000 model training samples were selected.We also considered other seismic landslide influencing factors,including elevation,slope,aspect,topographic wetness index,lithology,distance to fault,peak ground acceleration and rainfall.Bayesian probability method and machine learning model were combined to establish the multi-factor influence of earthquake landslide occurrence model.The model is then applied to the whole Taiwan region using different ground motion peak accelerations(from 0.1 g to 1.0 g with 0.1 g intervals)as a triggering factor to complete the real probability of earthquake landslide map in Taiwan under different peak ground accelerations,and the functional relationship between different Peak Ground Acceleration and their predicted area is obtained.

Some thoughts on seismotectonics of major earthquake occurrence zones in China

A major earthquake occurrence zone means a place where M ≥6 events have occurred since the Holocene and similar shocks may happen again in the future. The dynamic context of the major earthquake occurrence zones in China is primarily associated with the NNE-directed push of the India plate, next with the westward subduction of the Pacific plate. The Chinese mainland is a grand mosaic structure of many crust blocks bounded by faults and sutures. When it is suffered from boundary stresses, deformation takes place along these faults or sutures while the block interiors remain relatively stable or intact. Since the Quaternary, for example, left slip on the Xianshuihe-Xiaojiang fault zone in southwestern China has produced a number of fault-depression basins in extensional areas during periods Q1 and Q2. In the Q3, the change of stress orientation and enhancement of tectonic movement made faults of varied trends link each other, and continued to be active till present day, producing active fanlt zones in this region. Usually major earthquakes occur at some special locations on these active fault zones. During these events, in the epicenter areas experience intensive deformation character- ized by large-amplitude rise and fall of neighboring sections, generation of horst-graben systems and dammed rivers. The studies on palaeoearthquakes suggest that major shocks of close magnitudes often repeated for several times at a same place. By comparison of the Chi-Chi, Taiwan event in 1999 and Yuza, Yunnan event in 1955, including contours of accelerations and intensities, destruction of buildings, and in contrast to the Xigeda formation in southwestern China, a sandwich model is established to account for the mechanism of deformation caused by major earthquakes. This model consists of three layers, i.e. the two walls of a fault and the ruptured zone intercalated between them. This ruptured zone is just the loci where stress is built up and released, and serves as a channel for seismic waves.