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.

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.

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.

2023年塔吉克斯坦M_(W)7.2地震InSAR同震形变场与滑动分布

利用Sentinel-1A卫星SAR影像数据,对2023年塔吉克斯坦M_(W)7.2地震开展同震形变提取,基于弹性位错模型进行断层反演,并以本文反演得到的右旋节面解为接收面,计算不同深度的静态库仑应力。同震形变结果显示,升轨LOS向最大形变量达15 cm,降轨LOS向最大形变量达16 cm。断层反演结果表明,此次地震最优发震断层走向为131.1°、倾角为85.7°,同震主滑移区分布在深度10~30 km范围内,以右旋走滑为主,最大滑移位置位于地下约20 km深度处,滑移量为3.49 m,未破裂至地表,矩震级为M_(W)7.16。库仑应力结果显示,该区域库仑应力符合帕米尔高原已有的应力场及地质学研究结果,随着深度增加,其影响范围以发震断层为中心向外扩张,且自5 km深度往下,应力加载区逐渐侵蚀应力卸载区,并开始以加载区为主,在约10 km深度处开始发生余震活动,与本次发震断层相邻的2条断层未来短时间内地震风险性较小。

Application of High-Resolution Remote Sensing Technology in Quantitative Study on Coseismic Surface Rupture Zones: An Example of the 2008 M_w7.2 Yutian Earthquake

Objective Nowadays, high-resolution remote sensing technology has brought new changes to surveys of earthquakes, and the quantitative study of seismic faults based on this technology has become a trend in the world(Barzegari et al., 2017). An Mw 7.2 earthquake occurred in Yutian of Xinjiang on the western end of the Altyn Tagh fault on March 21 st, 2008. It is difficult to access this depopulated zone because of the high altitude and only 1–2 months of snowmelt. This study utilized high-resolution

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.

2011年3月24日缅甸7.2级地震对云南地区的影响研究

利用2011年1—6月云南地区的连续波形资料,采用背景噪声和波形互相关方法分别反演该地区的速度结构以及2011年3月24日缅甸7.2级地震前后60d的速度变化图像。同时,根据云南地区中小地震计算缅甸地震前后应变能释放响应比空间分布,并利用缅甸地震的震源参数,计算了缅甸地震对云南地区主要断裂产生的库仑破裂应力影响。结果显示:(1)禄劝至华坪一带、永定至泸水区域和通海至建水地区震后波速增加,同时该地区地震活动增强,相应断裂上库仑破裂应力增加,说明缅甸地震对这些区域具有加速构造活动的正影响;(2)小江断裂带以东马龙至宣威地区和南汀河断裂带以南临沧至景洪地区震后波速降低,地震活动减弱,断裂上库仑破裂应力降低,说明缅甸地震对该区域具有减缓构造活动的负影响。

由苏门答腊Mw9.0级大震预测中国台湾南端2006年7.2级大震

历史上苏门答腊大震与台湾大震有呼应性,故2004年苏门答腊发生巨震后,我们在2005年曾预测台湾南端及附近海域可能会发生7级地震,结果2006年12月26日在台湾南端恒春附近海域发生了7.2级地震。本文是预测性回顾。

1976年松潘、平武7.2级地震前后甘肃一侧震情监视的回顾

在重新全面深入收集整理历史档案和资料的基础上，回顾性研究总结了1976年8月16—23日松潘、平武两次7．2级地震的监测预报和防震抗震工作。研究表明，对于该次地震甘肃省和四川省地震工作者从中期预测开始，到短期、临震阶段，从地震预测到强震观测，组织开展了诸多有成效的工作。使松潘地震这个称之为重要的成功预报的实例，在我国地震工作发展史的记载中有更加充实、完整和科学的记载。并根据修订过的中国地震台网中心国家科学数据共享中心数据库给出的地震系列参数，对该次地震命名提出商榷，认为这次地震应当称之为“松潘-文县地震”，或“平武-文县地震”或“松潘-文县-平武地震”更为妥当。

2006年12月26日台湾恒春7.2级地震的预测讨论

回顾了台湾2006年恒春7.2级地震的长期和中长期预测,并对临震预测的有关问题进行了讨论。

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.

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.

1976年松潘、平武2次M_(S)7.2地震前相关气象因子的变化

1976年8月16日和23日,四川省北部松潘、平武相继发生MS 7.2地震,震中均位于岷山NS向构造带的虎牙断裂。关于这2次地震的大量研究表明,震前存在流体、形变、宏观等长中短临异常,为后续气象三要素研究提供了重要参考。同时,气压、湿度和温度等气相因子受干扰较少,记录时间、可回溯周期长,且保留较为完整,计算并分析其震前变化,寻找可能存在的异常,为地震预报研究工作提供了新的思路。基于此,以松潘和平武两地气象站1960-1975年记录的气温、气压和湿度数据的日均值为背景场,计算数据在1976-1978年与背景场距平及标准差,研究气象因子在2次地震前的异常变化。

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.

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.

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.

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.