Relocation of the 2001 Earthquake Sequence in Aegion,Greece

The western part of the Corinth Gulf attracts attention because of its seismically active complex fault system and considerable seismic hazard. Close to the city of Aegion, damaged by the M _L 6.2 earthquake of 1995, a sequence of small earthquakes occurred from February to May 2001. The sequence, comprising 171 events of M _L 1.8 to 4.7, was recorded by a short-period network of the University of Patras, PATNET. As most stations have single component-recording, the S-wave arrival time readings were scarce. A sub-set of 139 events was recorded by at least 5 stations, and in this study we limit ourselves just to that sub-set. A preliminary location is performed by a standard linearized kinematic approach, with several starting depths and crustal models. Then the mainshock is re-located, and finally it is used as a master event to locate the remaining events. The mainshock relocation is performed by a systematic 3D grid search, and the trade-off between depth and origin time is eliminated by a special procedure, the so-called station difference (SD) method. In the SD method, instead of inverting arrival times directly, their intra-station differences are employed. The station corrections, determined from the master event, are also used. As a result, the sub-set is imaged as a relatively tight cluster, occupying space of about 5 by 5 km horizontally and 10 km vertically, with the mainshock inside (at a depth of 7 km). The results should be interpreted with caution, mainly as regards the absolute depth position of the cluster. A more accurate location would require a local network with both P and S readings.     

Using beamforming for the global network location

The aim of this study is the design and trial of a novel sparse network beamforming (NB) technique to improve earthquake location. For this purpose bulletin phase arrival time data were processed via the use of complex exponents form and then used in an optimization via grid-search to search for the maximum semblance in hypocenter space. The use of the robust semblance statistic, provides reliable location results. The NB location results for a set of test events were compared to standard iterative ISC location procedure “iscloc” and its prototype the Jeffreys maximum likelihood estimator. For this purpose a data-base of 139 reference ground-truth events was extracted from the catalog of the International Seismological Centre (ISC) (97 GT5 events, assumed earthquakes, and 42 GT0 nuclear tests). The tuned NB procedure has shown excellent location results for events with a “good” ISC location and demonstrated the large epicenter deviations have been decreased in “bad” cases. Further developments of the algorithm would include allowances for 3D earth structure and a priori site-specific information.     

Reliability of the automatic procedures for locating earthquakes in southwestern Alps and northern Apennines (Italy)

Reliable automatic procedure for locating earthquake in quasi-real time is strongly needed for seismic warning system, earthquake preparedness, and producing shaking maps. The reliability of an automatic location algorithm is influenced by several factors such as errors in picking seismic phases, network geometry, and velocity model uncertainties. The main purpose of this work is to investigate the performances of different automatic procedures to choose the most suitable one to be applied for the quasi-real-time earthquake locations in northwestern Italy. The reliability of two automatic-picking algorithms (one based on the Characteristic Function (CF) analysis, CF picker, and the other one based on the Akaike’s information criterion (AIC), AIC picker) and two location methods (“Hypoellipse” and “NonLinLoc” codes) is analysed by comparing the automatically determined hypocentral coordinates with reference ones. Reference locations are computed by the “Hypoellipse” code considering manually revised data and tested using quarry blasts. The comparison is made on a dataset composed by 575 seismic events for the period 2000–2007 as recorded by the Regional Seismic network of Northwestern Italy. For P phases, similar results, in terms of both amount of detected picks and magnitude of travel time differences with respect to manual picks, are obtained applying the AIC and the CF picker; on the contrary, for S phases, the AIC picker seems to provide a significant greater number of readings than the CF picker. Furthermore, the “NonLinLoc” software (applied to a 3D velocity model) is proved to be more reliable than the “Hypoellipse” code (applied to layered 1D velocity models), leading to more reliable automatic locations also when outliers (wrong picks) are present.     

三维复杂速度模型的交切法地震定位

地震定位是地震监测与减灾研究重要基础.基于均匀或横向均匀介质模型,利用震源轨迹确定震源位置的交切法具有稳健和效率高的优点,但定位精度较低,特别是震源深度.为提高震源定位精度,我们提出适用于三维复杂速度模型的地震定位交切法.将地壳速度模型由均匀或横向均匀介质模型扩展为三维复杂速度模型;均匀或横向均匀介质模型对应的原假设为球面或双曲面的震源轨迹通过最小走时树射线追踪技术予以确定.确定震源位置的震源轨迹以到时差作为约束条件;将震源定位于震源轨迹交汇最密集的点处,即总的到时差残差(RDT)最小的点处.定位结果的不确定性可通过RDT值较小节点的空间分布予以定性表示.考察了准确速度模型、扰动速度模型、扰动观测到时及地震在台网外等4种情况下改进方法的地震定位效果,结果表明改进的交切法可用于三维复杂速度模型的地震定位;综合利用P波与S波的到时差信息,可明显改善震源位置约束;使用多条震源轨迹进行定位,有助于减少由随机因素导致的定位误差.     

Characteristics of isolated hybrid tremor (HBT) during a calm activity period at Aso Volcano

Isolated-type tremors having two events with different dominant frequencies are characteristic seismological phenomena observed during the fumarolic activity stage at Aso Volcano. These isolated tremors are called hybrid tremors (HBT) and comprise two parts: an initial part named the “HF-part” with a dominant frequency in the high-frequency region (approximately 10 Hz) and the following part named the “LF-part” with a dominant frequency in the low-frequency region (approximately 2 Hz). The LF-part is observed after the HF-part, and the HBT is accompanied by a long-period tremor (LPT). Hypocenters and source parameters are estimated using seismograms recorded at 64 stations around Nakadake crater. The amplitude distributions of all HF-parts have almost similar trends. Similarly, the amplitude distributions of all LF-parts have almost similar trends. However, the amplitude distributions of HF- and LF-parts are not similar. From these results, we proposed that the hypocenters and source parameters of HF- and LF-parts are not common, but each of them have common hypocenters and source parameters. The hypocenter region of HF-parts was estimated to be just beneath the fumarole region south of the 1st crater: the volume fluctuation is the major source factor. The hypocenter region of LF-parts is estimated to be at a depth of approximately 300 m beneath the first crater: the strike–slip component is the major source parameter. The hypocentral depth of LF-parts is located at the upper end of the crack estimated to be the source of the LPTs. The LPTs and HBTs are observed almost simultaneously. We consider that volcanic fluid is involved in the source mechanisms of both HBT and LPT.     

Two modes of dipole events in tropical Indian Ocean

By analyzing the distributions of subsurface temperature and the surface wind stress anomalies in the tropical Pacific and Indian Oceans during the Indian Ocean Dipole (IOD) events, two major modes of the IOD and their formation mechanisms are revealed. (1) The subsurface temperature anomaly (STA) in the tropical Indian Ocean during the IOD events can be described as a “<” -shaped and west-east-oriented dipole pattern; in the east side of the “<” pattern, a notable tongue-like STA extends westward along the equator in the tropical eastern Indian Ocean; while in the west side of the “<” pattern, the STA has opposite sign with two centers (the southern one is stronger than the northern one in intensity) being of rough symmetry about the equator in the tropical mid-western Indian Ocean. (2) The IOD events are composed of two modes, which have similar spatial pattern but different temporal variabilities due to the large scale air-sea interactions within two independent systems. The first mode of the IOD event originates from the air-sea interaction on a scale of the tropical Pacific-Indian Ocean and coexists with ENSO. The second mode originates from the air-sea interaction on a scale of the tropical Indian Ocean and is closely associated with changes in the position and intensity of the Mascarene high pressure. The strong IOD event occurs when the two modes are in phase, and the IOD event weakens or disappears when the two modes are out of phase. Besides, the IOD events are normally strong when either of the two modes is strong. (3) The IOD event is caused by the abnormal wind stress forcing over the tropical Indian Ocean, which results in vertical transports, leading to the upwelling and pileup of seawater. This is the main dynamic processes resulting in the STA. When the anomalous easterly exists over the equatorial Indian Ocean, the cold waters upwell in the tropical eastern Indian Ocean while the warm waters pileup in the tropical western Indian Ocean, hence the thermocline in the tropical Indian Ocean is shallowed in the east and deepened in the west. The off-equator component due to the Coriolis force in the equatorial area causes the upwelling of cold waters and the shallowing of the equatorial India Ocean thermocline. On the other hand, the anomalous anticyclonic circulations and their curl fields located on both sides of the equator, cause the pileup of warm waters in the central area of their curl fields and the deepening of the equatorial Indian Ocean thermocline off the equator. The above three factors lead to the occurrence of positive phase IOD events. When anomalous westerly dominates over the tropical Indian Ocean, the dynamic processes are reversed, and the negative-phase IOD event occurs. Supported by National Natural Science Foundation of China (Grant No. 40776013), National Basic Research Program of China (Grant No. 2006CB403601) and the Knowledge Innovation Project of Chinese Academy of Sciences (Grant No. KZCX-SW-222)     

The 1511 Eastern Alps earthquakes: a critical update and comparison of existing macroseismic datasets

Three earthquakes condition the seismic hazard estimates of the Eastern Alps: the 1348 “Villach”, the 1511 “Idrija”, and the 1976 Gemona events. Only the last one can be well documented, while doubts remain for location and size of the other two. New documents have been found about the 1511 quake that, together with a complete revision of the information already available, offer some new indications on the location and size of the event.     

单震相微地震事件识别与反演

为了对单震相微地震事件进行识别,同时将识别出来的微地震事件进行定位.根据单一震相任意两道到时差与微地震事件、检波器空间位置及震相速度关系的特征规律,研究了单震相微地震事件识别方法.首先分析到时差与以上各变量的内在变化规律,建立起到时差与各变量之间的定量计算关系,然后就相邻道到时差和检波器排列的首尾两道到时差,研究了具体...     

Using Ancillary Information to Improve Hypocenter Estimation: Bayesian Single Event Location (BSEL)

We have developed and tested an algorithm, Bayesian Single Event Location (BSEL), for estimating the location of a seismic event. The main driver for our research is the inadequate representation of ancillary information in the hypocenter estimation procedure. The added benefit is that we have also addressed instability issues often encountered with historical NLR solvers (e.g., non-convergence or seismically infeasible results). BSEL differs from established nonlinear regression techniques by using a Bayesian prior probability density function (prior PDF) to incorporate ancillary physical basis constraints about event location. P-wave arrival times from seismic events are used in the development. Depth, a focus of this paper, may be modeled with a prior PDF (potentially skewed) that captures physical basis bounds from surface wave observations. This PDF is constructed from a Rayleigh wave depth excitation eigenfunction that is based on the observed minimum period from a spectrogram analysis and estimated near-source elastic parameters. For example, if the surface wave is an Rg phase, it potentially provides a strong constraint for depth, which has important implications for remote monitoring of nuclear explosions. The proposed Bayesian algorithm is illustrated with events that demonstrate its congruity with established hypocenter estimation methods and its application potential. The BSEL method is applied to three events: 1) A shallow Mw 4 earthquake that occurred near Bardwell, KY on June 6, 2003, 2) the Mw 5.6 earthquake of July 26, 2005 that occurred near Dillon, MT, and 3) a deep Mw 5.7 earthquake that occurred off the coast of Japan on April 22, 1980. A strong Rg was observed from the Bardwell, KY earthquake that places very strong constraints on depth and origin time. No Rg was observed for the Dillon, MT earthquake, but we used the minimum observed period of a Rayleigh wave (7 seconds) to reduce the depth and origin time uncertainty. Because the Japan event was deep, there is no observed surface wave energy. We utilize the prior generated from the Dillon, MT event to show that even in the case when a prior is inappropriately applied, high quality data will overcome its influence and result in a reasonable hypocenter estimate.     

Development and Applications of Double-difference Seismic Tomography

Double-difference (DD) tomography is a generalization of DD location; it simultaneously solves for the three-dimensional velocity structure and seismic event locations. DD tomography uses a combination of absolute and more accurate differential arrival times and hierarchically determines the velocity structure from larger scale to smaller scale. This method is able to produce more accurate event locations and velocity structure near the source region than standard tomography, which uses only absolute arrival times. We conduct a stability and uncertainty analysis of DD tomography based on a synthetic data set. Currently three versions of the DD tomography algorithms exist: tomoDD, tomoFDD and tomoADD. TomoDD assumes a flat earth model and uses a pseudo-bending ray-tracing algorithm to find rays between events and stations while tomoFDD uses a finite-difference travel-time algorithm and the curvature of the Earth is considered. Both codes are based on a regularly distributed inversion grid, with the former for a local scale and the latter for a regional scale. In contrast, tomoADD adapts the inversion mesh to match with the data distribution based on tetrahedral and Voronoi diagrams. We discuss examples of applying DD tomography to characterize fault zone structure, image high-resolution structure of subduction zones, and determine the velocity structure of volcanoes.     

基于波形相似特征的微地震事件初至拾取及全局校正

微地震事件初至拾取是井下微地震监测数据处理的关键步骤之一.初至误差的存在会使微地震震源定位结果产生较大偏差,进而影响后续的压裂裂缝解释.通常初至拾取过程对所有的微地震事件选择相同的特征函数并采用一致的拾取参数进行统一处理,然而当事件的能量、震源机制、传播路径以及背景噪声等存在明显差异时,所得初至拾取结果差别显著.为了提高微地震事件初至拾取标准一致性,本文提出基于波形相似特征的初至拾取及全局校正方法.该方法首先利用互相关函数对每个事件内的各道记录进行时差校正,得到初始初至信息并形成叠加道,再对所有事件的叠加道进行全局互相关得到事件间初至相对校正量,最终初至结果可以通过各个事件的初始初至信息与其相对校正量相加得到.方法将所有微地震事件初至结果作为一个整体处理,从而能够克服常规方法初至拾取标准一致性差的缺陷.实际资料处理结果表明,相比于常规方法,该方法可以有效提高事件初至拾取和定位结果的一致性.     

Inversion of travel times obtained during active seismic refraction experiments CELEBRATION 2000, ALP 2002 and SUDETES 2003

A series of kinematic inversions based on robust non-linear optimization approach were performed using travel time data from a series of seismic refraction experiments: CELEBRATION 2000, ALP 2002 and SUDETES 2003. These experiments were performed in Central Europe from 2000 to 2003. Data from 8 profiles (CEL09, CEL10, Alp01, S01, S02, S03, S04 and S05) were processed in this study. The goal of this work was to find seismic velocity models yielding travel times consistent with observed data. Optimum 2D inhomogeneous isotropic P-wave velocity models were computed. We have developed and used a specialized two-step inverse procedure. In the first “parametric” step, the velocity model contains interfaces whose shapes are defined by a number of parameters. The velocity along each interface is supposed to be constant but may be different along the upper and lower side of the interface. Linear vertical interpolation is used for points in between interfaces. All parameters are searched for using robust non-linear optimization (Differential Evolution algorithm). Rays are continuously traced by the bending technique. In the second “tomographic” step, small-scale velocity perturbations are introduced in a dense grid covering the currently obtained velocity model. Rays are fixed in this step. Final velocity models yield travel time residuals comparable to typical picking errors (RMS ∼ 0.1 s). As a result, depth-velocity cross-sections of P waves along all processed profiles are obtained. The depth range of the models is 35–50 km, the velocity varies in the range 3.5–8.2 km/s. Lowest velocities are detected in near-surface depth sections crossing sedimentary formations. The middle crust is generally more homogeneous and has typical P wave velocity around 6 km/s. Surprisingly the lower crust is less homogeneous and the computed velocity is in the range 6.5–7.5 km/s. The MOHO is detected in the depth ≈30–45 km.     

Back‐projection stacking of P‐ and S‐waves to determine location and focal mechanism of microseismic events recorded by a surface array

We present an automatic method of processing microseismic data acquired at the surface by a star‐like array. The back‐projection approach allows successive determination of the hypocenter position of each event and of its focal mechanisms. One‐component vertical geophone groups and three‐component accelerometers are employed to monitor both P‐ and S‐waves. Hypocenter coordinates are determined in a grid by back‐projection stacking of the short‐time‐average‐to‐long‐time‐average ratio of absolute amplitudes at vertical components and polarization norm derived from horizontal components of the P‐ and S‐waves, respectively. To make the location process more efficient, calculation is started with a coarse grid and zoomed to the optimum hypocenter using an oct‐tree algorithm. The focal mechanism is then determined by stacking the vertical component seismograms corrected for the theoretical P‐wave polarity of the focal mechanism. The mechanism is resolved in the coordinate space of strike, dip, and rake angles. The method is tested on 34 selected events of a dataset of hydraulic fracture monitoring of a shale gas play in North America. It was found that, by including S‐waves, the vertical accuracy of locations improved by a factor of two and is equal to approximately the horizontal location error. A twofold enhancement of horizontal location accuracy is achieved if a denser array of geophone groups is used instead of the sparse array of three‐component seismometers. The determined focal mechanisms are similar to those obtained by other methods applied to the same dataset.     

Earthquake location in transversely isotropic media with a tilted symmetry axis

The conventional intersection method for earthquake location in isotropic media is developed in the case of transversely isotropic media with a tilted symmetry axis (TTI media). The hypocenter is determined using its loci, which are calculated through a minimum travel time tree algorithm for ray tracing in TTI media. There are no restrictions on the structural complexity of the model or on the anisotropy strength of the medium. The location method is validated by its application to determine the hypocenter and origin time of an event in a complex TTI structure, in accordance with four hypotheses or study cases: (a) accurate model and arrival times, (b) perturbed model with randomly variable elastic parameter, (c) noisy arrival time data, and (d) incomplete set of observations from the seismic stations. Furthermore, several numerical tests demonstrate that the orientation of the symmetry axis has a significant effect on the hypocenter location when the seismic anisotropy is not very weak. Moreover, if the hypocentral determination is based on an isotropic reference model while the real medium is anisotropic, the resultant location errors can be considerable even though the anisotropy strength does not exceed 6.10%.     

Relocation of hypocenters from DOMERAPI and BMKG networks: a preliminary result from DOMERAPI project

Merapi volcano located in central Java, Indonesia, is one of the most active stratovolcanoes in the world. Many Earth scientists have conducted studies on this volcano using various methods. The geological features around Merapi are very attractive to be investigated because they have been formed by a complex tectonic process and volcanic activities since tens of millions of years ago. The southern mountain range, Kendeng basin and Opak active fault located around the study area resulted from these processes. DOMERAPI project was conducted to understand deep magma sources of the Merapi volcano comprehensively. The DOMERAPI network was running from October 2013 to mid-April 2015 by deploying 46 broad-band seismometers around the volcano. Several steps, i.e., earthquake event identification, arrival time picking of P and S waves, hypocenter determination and hypocenter relocation, were carried out in this study. We used Geiger’s method (Geiger 1912) for hypocenter determination and double-difference method for hypocenter relocation. The relocation result will be used to carry out seismic tomographic imaging of structures beneath the Merapi volcano and its surroundings. For the hypocenter determination, the DOMERAPI data were processed simultaneously with those from the Agency for Meteorology, Climatology and Geophysics (BMKG) seismic network in order to minimize the azimuthal gap. We found that the majority of earthquakes occurred outside the DOMERAPI network. There are 464 and 399 earthquakes obtained before and after hypocenter relocation, respectively. The hypocenter relocation result successfully detects some tectonic features, such as a nearly vertical cluster of events indicating a subduction-related backthrust to the south of central Java and a cluster of events to the east of Opak fault suggesting that the fault has an eastward dip.     

Seismic tomography of the crust and upper mantle in the Bohai Bay Basin and its adjacent regions

In the Bohai Bay Basin and its adjacent regions(112°―124°E,34°―42°N),there exists abundant gas-petroleum while modern inter-plate seismic activity is robust.Although the tectonic structure of this region is very complicated,plenty of geological,geophysical and geochemical data and results are obtained through previous researches.On the basis of absorbing previous results,especially various kinds of geological and geophysical results,we collect and process the arrival time of P-wave phases of local events and tele-seismic events recorded by the station within this region from 1978 to 2004,build the responding initial model,and image the velocity structure of the crust and upper mantle of this region via tomography.The perturbation images of various depths and velocity profiles imply that the velocity structure of the crust and upper mantle in the Bohai Bay Basin and its adjacent regions is mainly influenced by the surface tectonic units,and is characterized by "Stripped along east-west,and zoned along south-north";some large-scaled faults penetrate Moho and lithosphere,and provide the channels for the basic lava or hot mass upwelling from the mantle.     

Simulating time-histories and pseudo-spectral accelerations from the 1992 Cairo earthquake at the proposed El-Fayoum New City Site,Egypt

El-Fayoum New City represents one of the new urban settlements that are recently erected all over Egypt. Because seismic recordings are not available, I used the stochastic method to simulate the largest damaging earthquake from the closest seismic source to the proposed area of the city. To verify the method and its computed results in Egypt, a study termed “method verification” was performed. I found that the October 12, 1992, earthquake (M _b = 5.8) that occurred southwest of Cairo in the vicinity of the Dahshour region, at the coordinates 29.77°N, 31.07°E, is a significant earthquake to the city. The parameters of the path from the hypocenter of the event to the city were taken into consideration. To determine the site parameters, a shallow seismic refraction survey was carried out in the studied area. Accordingly, I simulated time-histories and pseudo-spectral accelerations from the October 12, 1992, earthquake at the location of seismic profiles. Finally, it is demonstrated that the site is characterized by high ground motion amplification factors, producing a high ground motion acceleration value.     

邻近算法在微地震震源位置和一维速度模型同时反演中的应用

微地震事件的空间分布可以用来监测水力压裂过程中裂缝的发育情况.因此,震源定位是微震监测中重要的环节.震源定位依赖准确的速度模型,而震源位置和速度模型的耦合易导致线性迭代的同时反演方法陷入局部极小值.邻近算法作为一种非线性全局优化算法,能够最大程度地避免陷入局部最优解.本文将邻近算法应用于单井监测的微震定位和一维速度模型...     

Suspended sediment transport in the German Wadden Sea—seasonal variations and extreme events

The German Wadden Sea (southern North Sea) sediments are composed of both cohesive and non-cohesive deposits. The spatial distribution patterns are mainly driven by wind-induced waves and tidal currents. Transport intensity and duration depend on the hydrodynamic conditions, which vary over time. In this paper, the transport of suspended sediment was investigated on seasonal, tidal and hourly time scales in the back-barrier system of Spiekeroog Island. Long- and short-term data of fair weather periods and two storm events were investigated based on stationary and mobile measurements of currents and waves by Acoustic Doppler Current Profiler (ADCP), in situ particle size and suspended sediment concentration (SSC) measurements by laser in situ scattering and transmissometry (LISST) as well as wind records. The ADCP backscatter intensities were calibrated by means of LISST volume concentration data in order to quantify longer term SSCs and fluxes in the back-barrier system. Values up to 120 mg l⁻¹ were recorded, but concentrations more commonly were below 60 mg l⁻¹. The long-term results confirm former observations of a balanced budget during low-energy (fair weather) conditions in the study area. In general, SSCs were higher during spring tides than during neap tides. The data also clearly show the remobilisation of sediment by tidal current entrainment. The records include two severe storm events, “Britta” (1st November 2006) and “Kyrill” (18th January 2007). The data reveal very complex temporal flow and transport patterns. During both storm events, the export of material was mainly controlled by the interaction of wind, waves and tidal phase. The typical ebb-dominance occurring during fair-weather conditions was temporarily neutralised and even reversed to a flood-dominated situation. During “Kyrill”, the wind and high-waves setup in conjunction with the tidal phase was even able to compress the duration of two successive ebb cycles by over 70%. Although SSCs increased during both storms and higher turbulence lifted particle clouds upwards, an export of suspended matter towards the North Sea was only observed under the conditions taking place during “Britta”. Such fluxes, however, are currently still difficult to quantify because the backscatter intensity during high energy events includes a substantial amount of noise produced by the high turbulence, especially near the water surface.     

Some statistic characteristics of “Underground Hot vortex” in China during 1980–1993 (I)

Using ground temperature data from meteorological stations as well as earthquake, ground tilt and precipitation data, the spatial-temporal distribution of “Underground Hot Vortex” (UHV) in China was analyzed in detail. The results show that concerning an “Underground Hot Vortex” cell, its life-span is 3–8 seasons, 1.5 years on average; the mean horizontal scale is 600 km and its characteristic velocity is about 400 km/a; UHV is likely to appear in some areas where the crustal movement is intense and the absolute value of vertical deformation rate is relatively high; its activity could hardly be detected in the area where the crust is stable and the vertical deformation is weak; most of “Underground Hot Vortex Groups” originate from the edge of Indian Plate, then migrate eastwards with a leaping-frog style. 5–10 years are needed for their arrival in the eastern border of China. Their horizontal migrating velocity is 200–500 km/a which is nearly equal to the characteristic velocity of a single UHV. Project sponsored by the National Climbing Project and Key Project of the Chinese Academy of Sciences.