Determination of focal depth by two waveform-based methods:A case study for the 2008 Panzhihua earthquake

With the 2008 Ms6.1 Panzhihua earthquake as a case study, we demonstrate that the focal depth of the main shock can be well constrained with two approaches： （1） using the depth phase sPL and （2） using full waveform inversion of local and teleseismic data. We also show that focal depths can be well constrained using the depth phase sPL with single broadband seismic station. Our study indicates that the main shock is located at a depth of ii kin, much shallower than those from other studies, confirming that the earthquake occurs in upper crust. Aftershocks are located in the depth range of 11 16 kin, which is consistent with a ruptured near vertical fault whose width is about 10 km, as expected for an Ms6.1 earthquake.

Focal depth estimates of earthquakes in the Himalayan-Tibetan region from teleseismic waveform modeling

We estimate the focal depths and fault plane solutions of 46 moderate earthquakes in the Himalayan- Tibetan region by modeling the broadband waveforms of teleseismic P waves. The depths of 38 of these earth- quakes range between 0-40 km, with a peak at -5 km. One earthquake is located within the lower crust of the Indian shield. The remaining eight earthquakes occurred between depths of 80 -120 km and are all located in the Pamir-Hindu Kush and the Indo-Myanmar deep seismic zones. None of the earthquakes outside these deep seismic zones are located in the mantle. Global centroid moment tensor （CMT） solutions indicate that most earthquakes in northern Tibet and northern India had thrust-faulting mechanisms and that normal and strike-slip faulting earthquakes occurred primarily in central Tibet. These mechanisms are consistent with the predominantly NNW-SSE compression in the direction of current Himalayan-Tibetan continental collision.

Focal depth,magnitude,and frequency distribution of earthquakes along oceanic trenches

The occurrence of earthquakes in oceanic tren- ches can pose a tsunami threat to lives and properties in active seismic zones. Therefore, the knowledge of focal depth, magnitude, and time distribution of earthquakes along the trenches is needed to investigate the future occurrence of earthquakes in the zones. The oceanic trenches studied, were located from the seismicity map on： latitude ＋51 ° to＋53° and longitude -160° to 176° （Aleutian Trench）, latitude ＋40° to ＋53° and longitude ＋148° to＋165° （Japan Trench）, and latitude -75° to -64° and longitude -15° to ＋30° （Peru-Chile Trench）. The following features of seis- mic events were considered： magnitude distribution, focal depth distribution, and time distribution of earthquake. The results obtained in each trench revealed that the earthquakes increased with time in all the regions. This implies that the lithospheric layer is becoming more unstable. Thus, tectonic stress accumulation is increasing with time. The rate of increase in earthquakes at the Peru-Chile Trench is higher than that of the Japan Trench and the Aleutian Trench. This implies that the convergence of lithospheric plates is higher in the Peru-Chile Trench. Deep earthquakes were observed across all the trenches. The shallow earthquakes were more prominent than intermediate and deep earthquakes in all thetrenches. The seismic events in the trenches are mostly of magnitude range 3.0-4.9. This magnitude range may indi- cate the genesis of mild to moderate tsunamis in the trench zone in near future once sufficient slip would occur with displacement of water column.

Focal depths and mechanisms of Tohoku-Oki aftershocks from teleseismic P wave modeling

Aftershocks of the 2011 Tohoku-Oki great earthquake have a wide range of focal depths and fault plane mechanisms. We constrain the focal depths and focal mechanisms of 69 aftershocks with Mw 〉 5.4 by modeling the waveforms of teleseismic P and its trailing near-surface reflections pP and sP. We find that the ＂thrust events＂ are within 10 krn from the plate interface. The dip angles of these thrust events increase with depth from ~ 5~ to ~ 25~. The ＂non-thrust events＂ vary from 60 km above to 40 km below the plate interface. Normal and strike-slip events within the overriding plate point to redistribution of stress following the primary great earthquake; however, due to the spatially variable stress change in the Tohoku-Oki earthquake, an understanding of how the mainshock affected the stresses that led to the aftershocks requires accurate knowledge of the aftershock location.

Determination Method of Focal Depth of Local Earthquake Using the Travel-time Difference between Pn and sPn

In this paper,we derived the relationships between the travel time difference of sPn and Pn and the local earthquake focal depth.In these equations,the travel time difference of sPn and Pn is not related to the epicentral distance,but depends only on the regional crustal mode and the focal depth.According to the equations,we provided a simple and accurate method to determine local earthquake focal depth by using the travel time difference between phase sPn and Pn.This method has been used to determine the focal depths of two earthquake of MS6.1 and MS5.6 which occurred at the junction of Panzhihua and Huili,Sichuan on August 30 and 31,2008.The results were compared to those from other sources such as the China Earthquake Networks Center,and the comparison shows that the results are accurate and reliable.

Theoretical Error Analysis of the Accuracy of Focal Depth Determination of Near Earthquakes

Focal depth is one of the most difficult seismic parameters to determine accurately in seismology. The focal depths estimated by various methods are uncertain to a considerable degree, which affects the understanding of the source process. The influence of various factors on focal depth is non-linear. The influence of epicentral distance, arrival time residual and velocity model （crust model） on focal depth is analyzed based on travel time formula of near earthquakes in this paper. When wave propagation velocity is constant, the error of focal depth increases with the increase of epicentral distance or the distance to station and the travel time residual. When the travel time residual is constant, the error of focal depth increases with the increase of the epicentral distance and the velocity of seismic wave. The study also shows that the location error perhaps becomes bigger for shallower earthquakes when the velocity is known and the travel time residual is constant. The horizontal error caused by location accuracy increases with the increase of the epieentrai distance, the travel time residual and the velocity of seismic waves, thus the error of focal depth will increase with these factors. On the other hand, the errors of focal depth will lead to change of the origin time, therefore resultant outcomes will all change.

Focal Mechanism and Focal Depth of the May 22,2016 M S 4.6 Earthquake in Chaoyang,Liaoning

An earthquake with M_S4. 6 occurred at 17: 08 p. m.,May 22,2016 in Chaoyang County,Liaoning Province. We used the P-wave first motion method,TDMT method,and CAP method to determine the focal mechanisms and the PTD method and sP n-Pn method to determine the focal depth. The focal mechanism results of the three methods are consistent. The depth results of the CAP method,PTD method and sP n-Pn method are close. We used the double difference location method to relocate earthquakes in 2009-2016,and obtained the strikes and dip angles of the small earthquake distributions with the help of simulated annealing algorithm and gauss Newton algorithm fitting. According to the focal mechanism results,the depth results,the characteristics of small earthquake distributions and the structural characteristics of the source area,the seismogenic fault strike is NEE and the main pressure force direction is NNW. The earthquake focal mechanism is for a normal fault type with a little left-lateral strike slip motion.

The Focal Depth Analysis of the Inner Mongolia-Ningxia Border Area Earthquakes

We selected the 103 M_L≥2. 5 earthquake waveform data from the Inner Mongolia-Ningxia border during 2009-2015,which was recorded by the Inner Mongolia Digital Seismic Networks and used the simplex method, Hyposat method, double difference location method,and deterministic method (PTD) for the determination of focal depth,and compared the results. The CAP moment tensor inversion method is used to determine the focal depths of the Alxa Left Banner M5. 8 and M4. 2 earthquakes. The final results of the focal depth by the deterministic method (PTD) and the double difference location method fit well with the tectonic characteristics of the Inner Mongolia-Ningxia border area,but those of the simplex method and the Hyposat method do not. The average depth of the Inner Mongolia-Ningxia border seismic tectonic zone is 13. 32 ± 8 km.

The Present Status and Prospect of Earthquake Focal Depth Locating

Locating an earthquakes focal depth is always a key project in seismology. Precise focal depth is of critical importance for evaluating seismic hazards, deciphering dynamic mechanisms of earthquake generating,estimating aftershock evolutions and risk,as well as monitoring nuclear tests. However,how we determine an accurate focal depth is always a challenge in seismological studies. Aiming to solve these problems, we analyzed and summarized the present status and the future development of earthquake focal depth locating. In this paper we first reviewed the present status of focal depth locating in the world,and summarized the frequently-used relocating methods and ideas at present,and introduced two types of focal depth relocating ideas: arrival time relocating and waveform modeling methods. For these ideas,we systematically described the S-P and the Pn-Pg methods that belong to arrival time method,and polarization focal depth locating and amplitude focal depth locating that belongs to waveform modeling,and further analyzed the advantages and limitations of these methods. Since the depth phase methods are highly sensitive to focal depth,and are relatively free from the uncertainties of crustal models,we mainly reviewed the depth phases of s Pm P,s PL,s Pn,and s Sn,and quantitatively evaluated their availabilities and characteristics. Second,we also discussed the effects of crustal velocity models on the reliability of focal depth locating,and reviewed the advancements of seismic tomography techniques over recent years. Finally,based on the present status of the progress on the focal depth locating,and studies of seismic velocity structures,we proposed an idea of combining multiple datasets and relocating methods,jointly utilizing seismologic and geodetic techniques to relocate focal depth,which should be the major research field in investigating focal depth and source parameters in the near future.

Determination of Focal Depths of the MS5.8 Alxa Left Banner,Inner Mongolia Earthquake Sequence

Using the double-difference earthquake location algorithm,the deterministic method (PTD method) and the CAP seismic moment tensor inversion method,the paper selects the primary waveform data of 78 earthquakes recorded by the "China Earthquake Science Array Probe Project in the Northern Part of North South Seismic Belt ",the "China Earthquake Scientific Exploration Array Data Center"of Institute of Geophysics,China Earthquake Administration,and the Inner Mongolia Digital Seismic Network to calculate the focal depths of the mainshock and the seismic sequence of the M_S5.8 Alxa Left Banner earthquake in Inner Mongolia. The results show that the focal depth of the main shock is 20. 6 km,determined by the double-difference earthquake location method,18. 1 km by the PTD method,and 19. 2 km by the CAP method. The focal depth of the earthquake sequence calculated by the double-difference location method is larger. The deterministic method (PTD method) and double-difference location method are the methods that fit the tectonic characteristics of the seismic source area,and the CAP method is suitable for larger earthquakes.

Focal Depth of Earthquakes in the Qinghai-Xizang Plateau and Its Tectonic Implication

In the previous work of the present study,moment tensors of 11 major earthquakes in the Qinghai-Xizang Plateau and its surrounding region from 1966 to 1980 are estimated by generalized inversion technique.The seismic source time function and focal depth are immediately determined in the inversion.The results indicated that all earthquakes investigated here are shallow events within the upper crust.The purpose of this paper is to present a summary of the distribution of focal depths of earthquakes in and near the Qinghai-Xizang Plateau from the above result,combining the focal depths of 78 significant earthquakes from 1964 to 1986,which are relocated individually by other authors,as well as to discuss its tectonic implication.

The effect of focal depth error on moment tensor inversion

In the determination of focal mechanism and rupture process of earthquake sources by using moment tensor inversion technique, it is difficult to guarantee the focal depth used in calculating the Green′s functions (theoretical focal depth) is exactly equal to the real focal depth. The difference between the theoretical and real focal depths, i.e., the focal depth error, will affect the moment tensor inversion to some extent. Using synthetic seismograms , the effect of the focal depth error on moment tensor inversion for three basic types of faults is discussed systematically. For the normal and thrust fault, the focal depth error mainly affects the explosive ( EP ) component and the compensated linear vector dipole ( CLVD ) component. In the case that the theoretical focal depth is greater than the real focal depth, the focal depth error causes a false positive EP component and a false negative CLVD component for the normal fault. However, it produces a false negative EP component and a false positive CLVD component for the thrust fault. The absolute values of the false EP and CLVD components for both normal fault and thrust fault cases increase with increasing focal depth error. In the case that the theoretical focal depth is smaller than the real focal depth, the focal depth error causes a false negative EP component and a false positive CLVD component for the normal fault. However, it produces a false positive EP component and a false negative CLVD component for the thrust fault. Similarly, the absolute values of the false EP and CLVD components for both normal fault and thrust fault cases increase with increasing focal depth error. For a pure strike slip fault the focal depth error mainly affects the shape of source time function, unlike for the normal and thrust faults. The source time functions have artificially extended tails when either the theoretical focal depth is greater or smaller than the real focal depth. The numerical experiments show that the focal depth error less than 20 km has no significant effect on the overall focal mechanism of the earthquake. In addition, the effect of the focal depth error on the inversion result is slighter in case that the theoretical focal depth is greater than the real focal depth than in the case that the theoretical focal depth is smaller than the real focal depth.

Focal depth research of earthquakes in China's Mainland: Implication for tectonics

Focal depth data of earthquakes in China's Mainland are processed and analyzed in this paper, as well as the relationship between the focal depths and large-scale tectonic structures. As a basic parameter for earthquakes, focal depth is used to investigate deep environment of seis-mogenic regions, tectonic backgrounds for concentration and release of seismic energy, the inner crustal deformation and its mechanic features. Depth data of 31282 ML≥2.0 events with 1st class and 2nd class precision in China's Mainland from Jan. 1, 1970 to May 31, 2000 are used to get spatial features of earthquakes distributed with depth and to provide average depth for each grid area throughout China. Researches show that the average depth (D) for all the earthquakes used in this paper is (16±7) km, and (13±6) km and (18±8) km for the events in eastern China and western China, respectively. The area with the deepest focal depth is located in southwest Xinjiang region, near the western and southwestern ends of the Tarim

The focal depths of the 2008 Panzhihua earthquake sequence and the stress field in the source region

Focal depths of the 2008 Ms6.1 Panzhihua earthquake sequence and tectonic stress field in the source area are investigated.Source depths of 24 earthquakes in Panzhihua earthquake sequence with a magnitude M≥3.0 were determined using the seismic depth phase sPL;additionally,the focal depths of 232 earthquakes were measured by fitting the threecomponent waveforms of the P and S waves.The source depth of the main shock is^12 km.The majority of the aftershocks with magnitude M≥3.0 occurred in the brittle upper crust at the depths range of 12-18 km.Further,the Source mechanisms of the 232events around the Panzhihua earthquake source area were determined,and the results show that the earthquakes have predominantly strike-slip mechanisms in the Dianzhong Block,but display complexity of the focal mechanisms outside and near the boundary of the Dianzhong block.The 232 earthquake mechanisms from this study are combined with the solutions from the Global Centroid Moment Tensor (GCMT) catalog to derive 2D stress field.The inversion results show that the Dianzhong block is predominantly under a strike slip faulting regime and the direction of the maximum principal compressionσ1 is northwestsoutheast (NW-SE)-trending.The distribution is coincide with GPS velocity field.However,orientations of principal stress axes as well as the faulting types change outside and near the Dianzhong block.The results show that the tectonic stress field in the study area is predominantly controlled by the southeast (SE)-trending horizontal movement and clockwise rotation of the Dianzhong block as a result of the eastward movement of eastern Tibetan meeting the old and rigid South China block (SCB).The Panzhihua earthquake ruptured at^12 km depth where the tectonic stress regime is under the SE-direction horizontal compression and the NE-direction horizontal extension.

利用sPL震相测定2022年山西古交M_(L)4.1地震震源深度

选取2022年2月20日山西古交M_(L)4.1地震事件波形,筛选出震中距在10~70 km之间的5个地震台站,基于sPL震相特征对波形数据进行处理,在震中距23 km的GUJ台观测到sPL震相。使用山西2015一维地壳速度模型,通过SEIS-CAP软件计算该地震的震源机制解,并利用F-K方法计算理论地震图,与观测波形拟合对比,确定该地震震源深度约4 km。sPL震相方法、CAP方法测定的震源深度和正式编目结果基本一致,表明利用sPL震相测定山西古交M_(L)4.1地震震源深度是可靠的。

2022年平山4.3级地震震源机制及发震机理

基于震中附近地震台站波形资料,采用CAP方法得到平山4.3级地震的震源机制解和矩心深度,同时利用sPL震相对震源深度进行了精确测定。结果显示:平山4.3级地震震源机制双力偶解节面Ⅰ走向311°、倾角43°、滑动角33°;节面Ⅱ走向196°、倾角68°、滑动角128°;P轴方位角259°、倾角15°,表现为NEE-SWW向的挤压应力状态,与华北地区构造应力场方向基本相同。结合区域应力状态和地质构造活动,推测其发震断层为一条兼有走滑性质的逆断型隐伏断裂。

利用初至震相定位法测定2013年辽宁灯塔M_(S)5.1地震震源深度

以2009年至2022年辽宁测震台网的观测资料为基础,拟合辽宁地区小震Pg波走时曲线,得到上地壳P波速度为6.11 km/s,并构建该地区双层地壳P波速度模型。提取正式观测报告中的20个清晰Pg和9个Pn震相到时,基于初至震相定位法测定2013年辽宁灯塔M S5.1地震的震源深度为10.8 km,初至P震相最小走时残差为0.31 s;使用考虑误差的定位算法测定震源深度为10.27 km,定位残差为0.35 s,与使用其他速度模型计算得到的震源深度基本一致,可信度较高。分析认为,灯塔M S5.1地震震源深度在11 km左右。以该深度作为主要输入参数计算其在周围产生的位移场和应变场,显示出典型的走滑地震特征。

耦合震级、震源深度和滑动角不确定性效应的地震海啸危险性估计

选取中国东南沿海地区6个近岸场点,以马尼拉海沟俯冲带和琉球海沟俯冲带作为潜在地震海啸源区,采用广义极值地震活动性模型和广义帕累托地震活动性模型分析震级不确定性特征,通过统计震源深度的优势分布和拟合滑动角分布函数,耦合震级、震源深度和滑动角不确定性效应,得到两个俯冲带对6个场点未来30 a、50 a、100 a海啸波高超过0.4 m的地震海啸危险性估计结果。结果表明:百年内舟山近海和宁德近海特定场点遭受地震海啸袭击风险较低,随着时间的推移,从位于厦门近海至香港近海、海口近海和高雄近海的特定场点,地震海啸危险性递增。

2023年甘肃积石山M_(S)6.2地震:一次逆冲为主的浅源强震

基于区域数字地震台网波形数据,在三维空间内利用波形拟合反演方法获得2023年12月18日甘肃积石山M_(S)6.2地震最佳震源机制解,结果表明:节面Ⅰ走向305°,倾角56°,滑动角61°;节面Ⅱ为走向169°,倾角43°,滑动角125°;矩震级M_(W)6.02。最佳拟合的空间精细位置为102.377°E,35.968°N,深度9 km。结果显示,地震破裂沿NW方向扩展。初步推断此次地震的发震断层面为节面Ⅱ,发震断层是拉脊山北缘断裂(积石山东缘断裂),是一次逆冲为主,兼少量走滑分量的浅源强震事件。

2023年甘肃积石山6.2级地震震源位置测定

基于地震P波到时,利用模拟退火法对积石山6.2级地震震源位置进行了测定,同时采用样本集合统计分析方法对测定结果的可靠性进行了定量评价。结果显示:积石山6.2级地震震源位置为(35.750°N,102.833°E),深度为12.0 km,在95%置信水平下,震源位置参数解的置信区间为(35.748°N~35.753°N,102.830°E~102.836°E),深度为6.8~16.9 km。