Step-over of strike-slip faults and overpressure fluid favor occurrence of foreshocks:Insights from the 1975 Haicheng fore-main-aftershock sequence,China

This study analyzed and summarized in detail the spatial and temporal distributions of earthquakes,tidal responses,focal mechanisms,and stress field characteristics for the M 7.3 Haicheng earthquake sequence in February 1975.The foreshocks are related to the main fault and the conjugate faults surrounding the extension step-over in the middle.The initiation timing of the foreshock clusters and the original time of the mainshock were clearly modulated by the Earth's tidal force and coincided with the peak of dilational volumetric tidal strain.As a plausible and testable hypothesis,we proposed a fluid-driven foreshock model,by which all observed seismicity features can be more reasonably interpreted with respect to the results of existing models.Together with some other known examples,the widely existing step-over along strike-slip faults and associated conjugate faults,especially for extensional ones in the presence of deep fluids,favor the occurrence of short-term foreshocks.Although clustered seismicity with characteristics similar to those of the studied case is not a sufficient and necessary condition for large earthquakes to occur under similar tectonic conditions,it undoubtedly has a warning significance for the criticality of the main fault.Subsequent testing would require quantification of true/false positives/negatives.

A Review on Research of Foreshocks

This paper briefly introduces the current research progress in the field of foreshocks,both in China and abroad,and contrasts the common characteristics and the mechanisms of foreshocks of the sequence under the definition of different conditions of foreshocks. The main recognition methods of foreshocks are briefly reviewed,and their characteristics and existing problems are reviewed and discussed. Foreshocks are small earthquakes that occur before the mainshock and adjacent to the main source location. A foreshock sequence is constituted of a series of foreshock activities that occur before the mainshock. The proportion of earthquake cases includes direct foreshocks ranging from 10% - 40% at different defined conditions of foreshocks. Theoretically,cascade model or pre-slid model can explain foreshocks. Foreshocks are mainly concentrated in the range of 10km -75 km of the mainshock,but their time distribution form is very complicated,mostly prior to the mainshock from 1 or 2 days,the seismicity rate of part of foreshock sequences shows significant acceleration features,but many foreshock sequences often show the attenuation characteristics of mainshock-aftershock sequences. The most prominent feature of foreshocks is a focal consistent mechanism and low b-value of earthquakes of the earthquake sequence. Foreshocks seems to have a certain relationship with tectonic environment and the rupture form of the mainshock,in limited foreshocks earthquake cases,the dip-thrusting earthquakes seem to have relatively more foreshocks. The results of some of the earthquake cases show that the focal depth of foreshocks gradually moved downward with the mainshock approaching. So far,it is difficult to determine whether an earthquake or an earthquake sequence is a foreshock or foreshock sequence before the mainshock. The identification methods of foreshocks mainly include a statistical method of analogy,focal consistent mechanism and related derivative method,the fine detection of the earthquake nucleation process. From a few existing studies of earthquake cases,in spite of earthquake temporal clusters and focal consistent mechanism being the most significant features of foreshock sequence,there were not sufficient conditions for judging a foreshock sequence. Because the rupture rate of expansion and sliding displacement have a tendency to increase faster with time,the method of fine detection of the earthquake nucleation process is expected to play a more important role in the identification of foreshocks,but it needs more earthquake examples to be verified.

Characteristics of foreshock and its identi-fication

In the paper, we analyze 117 moderate-strong earthquakes occurred in Chinese mainland (Ms greater than or equal 5.5 in the east and Ms greater than or equal 6.0 in the west) since 1970, among them, 11 earthquakes (about 9%) have direct foreshocks and 63 earthquakes (about 51%) have generalized foreshocks. The predominant time interval between foreshock and main earthquake is no more than 30 days with a spatial distance less than 50 km and a magnitude difference over 1. From the digital seismic data in Liaoning Province, we know that direct foreshock had an obvious shear-stress background before the Ms=5.6 and Ms=5.1 Xiuyan earthquakes occurred on Nov. 29, 1999 and Jan.15, 2000.

Using the match-and-locate method to characterize foreshocks of the July 2019 M_(W)6.4 Ridgecrest,California earthquake

The July 2019 M_(W)6.4 Ridgecrest,California earthquake and its distinct foreshocks were well recorded by local and regional stations,providing a great opportunity to characterize its foreshocks and investigate the nucleation mechanisms of the mainshock.In this study,we utilized the match-and-locate(M&L)method to build a high-precision foreshock catalog for this M_(W)6.4 earthquake.Compared with the sequential location methods(matched-filter+cross-correlation-based hypoDD),our new catalog contains more events with higher location accuracy.The M_(W)6.4 mainshock was preceded by 40 foreshocks within~2 h(on July 4,2019 from 15:35:29 to 17:32:52,UTC).Their spatiotemporal distribution revealed a complex seismogenic structure consisting of multiple fault strands,which were connected as a throughgoing fault by later foreshocks and eventually accommodated the 2019 M_(W)6.4 mainshock.To better understand the nucleation mechanism,we determined the rupture dimension of the largest M_(L)4.0 foreshock by calculating its initial rupture and centroid points using the M&L method.By estimating Coulomb stress change we suggested that the majority of foreshocks following the M_(L)4.0 event and M_(W)6.4 mainshock occurred within regions of increasing Coulomb stress,indicating that they were triggered by stress transfer.The nucleation process before the M_(L)4.0 event remains unclear due to the insufficient sampling rate of waveforms and small magnitude of events.Thus,our study demonstrates that the M&L method has superior detection and location ability,showing potential for studies that require high-precision location(e.g.,earthquake nucleation).

A case study of large-amplitude ULF waves in the Martian foreshock

Foreshock ultralow frequency (ULF) waves constitute a significant physical phenomenon in the plasma environment of terrestrial planets. The occurrence of these waves, associated with backstreaming particles reflected and accelerated at the bow shock, implies specific conditions and properties of the shock and its foreshock. Using magnetic field and ion measurements from MAVEN, we report a clear event of ULF waves in the Martian foreshock. The interplanetary magnetic field connected to the Martian bow shock, forming a shock angle of ~51°. Indicating that this was a fast mode wave is the fact that ion density varied in phase with perturbations of the wave field. The peak frequency of the waves was about 0.040 Hz in the spacecraft frame, much lower than the local proton gyrofrequency (~0.088 Hz). The ULF waves had a propagation angle approximately 34° from ambient magnetic field and were accompanied by the whistler mode. The ULF waves displayed left-hand elliptical polarization with respect to the interplanetary magnetic field in the spacecraft frame. All these properties fit very well with foreshock waves excited by interactions between solar wind and backstreaming ions through right-hand beam instability.

Discrimination Between Foreshocks and Earthquake Swarm from the Change of First Motion Sign

Based on the data of first motion of 11 earthquake sequences with ML ≥ 3.0 recorded by the Telemetric Seismic Network of Shanxi since the 1980s, the first motion characteristics of each earthquake sequence were studied. It is known that earthquake sequences of different types have different consistency characteristics of focal mechanism. The decrease and increase of the first motion contradictory sign ratios could be taken as an index to judge whether there would still be a larger earthquake to come after the earthquake sequence.

Small-scale ion flux and magnetic field fluctuations in solar wind, foreshock and magnetosheath

We have continued investigation of waves in the regions of undisturbed solar wind, foreshock and magnetosheath. The analysis of ion flux and magnetic field variations with the time interval 1-240s was performed in the regions above. Very large variation in such a time interval can be considered the common feature of the foreshock and magnetosheath. The results of case and statistical studies showed that the level of relative variations of ion flux and magnetic field magnitude in foreshock is about 3 times larger than in undisturbed solar wind. Variations of these parameters in the magnetosheath topologically connected with the quasi-parallel bow shock are about two times larger than those behind the quasi-perpendicular. We also compared the results from Interball-1 data analysis with those from statistical analysis of cluster magnetic field measurements. The magnetic field variations obtained from the different satellite data coincide with each other very well not only in quality but also in quantity.

The Tongliao MS5.3 Earthquake and Its Foreshock Determination

Taking the 2013 Tongliao MS5. 3 earthquake as a research subject, on the basis of statistical analysis of earthquake sequence using the HypoDD location method and focal mechanism solutions,the paper analyzes and discusses the relationship between the ML4. 4 and MS5. 3 earthquakes. The results show that the Tongliao MS5. 3 earthquake occurred under the background of medium-small earthquakes long-term quiescence and short-term enhancement in the epicentral area. The results of accurate seismic location shows that the Tongliao MS5. 3 earthquake sequence is distributed in the NW direction,extending 10 km,and the ML≥3. 0 aftershocks are concentrated south of the mainshock. The distance between the MS5. 3 mainshock and the ML4. 4 foreshock is about 1. 8 km,with a focal depth of 7. 208 km and 7. 089 km,respectively,their focal location is very close,and may have occurred on the same fault plane. The results of focal mechanism shows that the Tongliao MS5. 3 earthquake is of the strike-slip type,the focal mechanism of aftershocks are disordered,and with time lapse,the type is changed from strike-slip to thrust and normal faulting. The bigger foreshocks had similar focal mechanism and were all normal fault types,which exhibits to some extent,an obvious crustal medium anisotropy in the epicentral area before macroscopic rupturing,as represented by alignment fractures,with stress action enhanced,this"consistency"of seismic precursor regime would gestate the mainshock. According to the characteristics of temporal-spatial distribution of earthquake sequence and similarity of focal mechanism,we judge that the Tongliao MS5. 3 earthquake sequence is a foreshock-mainshock-aftershock type.

Study on Simulation of Foreshock Activity Properties before Strong Earthquake Using Heterogeneous Cellular Automata Models

Three different degrees of heterogeneous fault models are simulated by using 2-D random dynamic cellular automata models for analyzing macroscopic behaviors of seismic activity evolution influenced by heterogeneity of fault structures. The results show that the heterogeneities of fault structures can influence evolution properties of the foreshock activity and rupture process, such as the mediate heterogeneous and less heterogeneous structures, which show relatively higher ASR rates and more significant seismic gaps before main shocks. Besides, stress drop distribution ranges of the foreshock events when approaching a main shock show more homogenous (narrower) than that of the foreshock events far from a main shock. So the heterogeneity of fault structures plays an important role in strong earthquake preparation processes.

Some Foreshocks and Aftershocks Associated with the Moiyabana Earthquake of 2017 in Botswana

On the 3rd of April 2017, an earthquake of moment magnitude 6.5 occurred near Moiyabana in central Botswana. This paper is aimed at studying the spatial distribution of the foreshock and aftershock sequences associated with the Moiyabana earthquake. The foreshocks and aftershocks data used were from the Botswana Geoscience Institute (BGI) and the Seisan software was used to analyze the foreshock and aftershock events. The analyses revealed nine epicenter locations of foreshocks which are spread out across the country and most of them are located in the central and southern parts of Botswana, while the aftershocks are clustered around the mainshock. Although five of the nine foreshocks occurred far from the boundaries of major tectonic units, the other four occurred near key features such as the Zoetfontein, Lecha and Chobe faults. The spatial distribution of aftershocks indicates that the stress released by the mainshock, re-activated the planes of weakness in the vicinity of the mainshock and farther away from the mainshock. Hence, this affected the Zoetfontein fault, the boundary between Passarge basin and Magondi belt and the boundary between the Kaapvaal craton and Limpopo mobile belt. The aftershocks also show a northwest-southeast trend, which probably indicates the rupture plane;and mainly lie within the Limpopo mobile belt that is sandwiched between the Kaapvaal craton to the south and Zimbabwe craton to the north. Furthermore, the aftershocks concentration to the south reveals a close relation in demarcating the boundary of the Kaapvaal craton and the Limpopo mobile belt.

How to Identify Foreshocks in Seismic Sequences to Predict Strong Earthquakes

The time analysis of seismic events preceding several strong earthquakes occurred in recent decades throughout the world, has highlighted some foreshocks’ characteristics, which are helpful for their discrimination compared to other types of events. These features can be identified within the seismic sequence and used as strong events’ precursors. Through the energy release pattern analysis, which precedes any strong earthquakes, in this study we describe some graphical procedures suitable for distinguishing a foreshock from any other type of earthquake. We have broadly divided foreshocks into two classes, depending on their position within the energy release pattern, by describing some relationships between the foreshock’s magnitude and the following earthquake’s. The results obtained show how the energy release pattern of some major earthquakes has distinctive features and repeatability which it is possible to obtain information from in order to perform sufficiently reliable short-term forecasts.

Spectrum Deviation Method of Identification of Foreshocks or Generalized Foreshocks and Its Applications

Based on previous research work,we present a spectrum deviation method to recognize a foreshock or generalized foreshock in this paper. The criterion to determine whether an event is a foreshock is a wide spectrum for an ordinary event,however,a moderate earthquake with foreshock or generalized foreshock has the characteristics of a narrow frequency band,and it deviates to the low frequency. It may be explained by metastable extension in the rupture source or related area of the main shock or regional fragmentation damage and crack nucleation process. The calculation results of two foreshocks,the M_S4. 7 event which occurred before the Yushu M_S7. 1 earthquake on April 14,2010 and the M_S5. 3 event which occurred before the Yutian M_S7. 3 earthquake on February 12,2014,show that the spectra of foreshocks shift,and they are quite different from the nonforeshock seismic spectrum of equivalent size. Therefore,this result can verify the validity of the spectrum deviation method.

The Spatial-temporal Distribution and Their Statistical Characteristics of Foreshocks in the Yunnan Region

We analyzed 223 earthquakes with M 1〉 5.0 that occurred in the Yunnan region during 1965 - 2014, among which 74 （ about 33. 2 % ） had foreshocks. There are great differences in foreshock populations in different tectonic blocks： the most abundant foreshocks occurred in the Lancang-Gengma and Tengchong-Baoshan blocks, which have the most abundant foreshocks in the Yunnan region. The predominant magnitude difference, time interval and spatial distance are 0. 5 -2. 9, within 10 days and within 20km, respectively. These characteristics can be used to forecast the mainshock after the identification of a foreshock.

Deformations at Earth’s dayside magnetopause during quasi-radial IMF conditions:Global kinetic simulations and Soft X-ray Imaging

The Solar wind Magnetosphere Ionosphere Link Explorer(SMILE)is a joint mission of the European Space Agency(ESA)and the Chinese Academy of Sciences(CAS).Primary goals are investigating the dynamic response of the Earth's magnetosphere to the solar wind(SW)impact via simultaneous in situ magnetosheath plasma and magnetic field measurements,X-Ray images of the magnetosheath and magnetic cusps,and UV images of global auroral distributions.Magnetopause deformations associated with magnetosheath high speed jets(HSJs)under a quasi-parallel interplanetary magnetic field condition are studied using a threedimensional(3-D)global hybrid simulation.Soft X-ray intensity calculated based on both physical quantities of solar wind proton and oxygen ions is compared.We obtain key findings concerning deformations at the magnetopause:(1)Magnetopause deformations are highly coherent with the magnetosheath HSJs generated at the quasi-parallel region of the bow shock,(2)X-ray intensities estimated using solar wind h+and self-consistentO7+ions are consistent with each other,(3)Visual spacecraft are employed to check the discrimination ability for capturing magnetopause deformations on Lunar and polar orbits,respectively.The SMILE spacecraft on the polar orbit could be expected to provide opportunities for capturing the global geometry of the magnetopause in the equatorial plane.A striking point is that SMILE has the potential to capture small-scale magnetopause deformations and magnetosheath transients,such as HSJs,at medium altitudes on its orbit.Simulation results also demonstrate that a lunar based imager(e.g.,Lunar Environment heliospheric X-ray Imager,LEXI)is expected to observe a localized brightening of the magnetosheath during HSJ events in the meridian plane.These preliminary results might contribute to the pre-studies for the SMILE and LEXI missions by providing qualitative and quantitative soft X-ray estimates of dayside kinetic processes.

2022年6月10日四川马尔康M_(S)6.0震群序列时空演化特征

2022年6月10日马尔康M_(S)6.0震群序列发生在松岗断裂与龙日坝断裂交会处,呈震群型活动特征。本文以四川地震台网目录为基础,对马尔康地震序列的参数特征开展研究,取该序列三次M_(S)5.0地震之后各一个小时,作为Ⅰ,Ⅱ,Ⅲ共计三个阶段,进行对比分析。由于较大地震后短时间内目录遗漏的余震较多,为增大研究所需的地震样本量,首先采用模板匹配法进行微小地震识别,以补充完备目录,并利用识别的地震目录及台网目录分别计算马尔康M_(S)6.0地震序列的b值、p值等参数。计算结果显示,相比于第Ⅱ和Ⅲ阶段,第Ⅰ阶段具有显著的低b值(0.59),随着时间的推移,序列b值逐渐上升,后两个阶段分别为0.84和0.86。第Ⅰ阶段低b值的结果反映了此阶段孕震区应力水平较高。另外,第Ⅰ阶段序列的p值为0.76,明显低于后两个阶段的1.81和1.64,反映出第Ⅰ阶段序列频次衰减速度较慢,应力释放不充分,而后两个阶段刚好相反,表明不同阶段序列的时间演化特征存在差异。综合分析认为,M_(S)5.8地震是M_(S)6.0地震的前震。序列西支与东支的参数计算结果呈现不一样的特征,可能与M_(S)5.8前震序列发生在西支有关。

Statistical study of magnetic holes in the upstream region of Mercury’s bow shock

Magnetic holes are magnetic depression structures that exist widely in many plasma environments.The magnetic holes with durations of>1 s in the solar wind at Mercury’s orbit have drawn much attention,but the properties of the magnetic holes with shorter durations are still unclear.Here,we investigate the magnetic holes with durations of 0.1-100 s in the upstream region of Mercury’s bow shock based on observations by the MESSENGER(MErcury Surface,Space ENvironment,GEochemistry,and Ranging)spacecraft.They can be divided into two groups according to the distribution of their duration:small-duration magnetic holes(SDMHs,<0.6 s)and large-duration magnetic holes(LDMHs,>0.6 s).The duration of each group approximately obeys a log-normal distribution with a median of~0.25 s and 3 s,respectively.Approximately 1.7%(32.6%)of the SDMHs(LDMHs)reduce the magnetic field strength by more than 50%.For both groups,some structures have a linear or quasi-linear polarization,whereas others have an elliptical polarization.The magnetic hole events in both groups tend to have a higher rate of occurrence when the interplanetary magnetic field strength is weaker.Their occurrence rates are also affected by Mercury’s foreshock,which can increase(decrease)the occurrence rate of the SDMHs(LDMHs).This finding suggests that Mercury’s foreshock might be one source of the SDMHs and that the foreshock can destroy some LDMHs.These observations suggest that a new group of magnetic holes with durations of<0.6 s exist in the upstream region of Mercury’s bow shock.

魏晋·说说地震活动性——漫步地震五千年(7)

谈及中国的地震活动性研究,必然涉及一位杰出的女科学家——梅世蓉。她没有院士的头衔,却是公认的、无人可替代的中国地震预报的领军者。本文以梅世蓉先生在地震活动性上的研究成果为代表,选择了3个方面加以介绍。南北地震带的提出促进了深浅构造研究的深入,龙日坝断裂带的发现具有重要的大地构造意义。分析了地震空区和震中迁移现象,给出了南北地震带与南亚地震带间的地震呼应震例,介绍了力学机制。直接前震和前兆震群的正确判断在地震预测上十分重要,简介了直接前震的特征,给出了中国大陆震群的分布。

The April 14th, 2010 Yushu earthquake, a devastating earthquake with foreshocks

In the early morning of April 14th, 2010, a strong earth- quake (Ms7.1, http://www.csndmc.ac.cn; Mw6.9, http://neic. usgs.gov) occurred in Yushu district, Qinghai Province, China. More than 2000 lives were lost and over 100000 injured,

维西-乔后断裂带及其周边地区震群和前震序列活动特征

使用云南地震台网2009—2021年的地震目录和波形数据,采用双差定位方法、基于P波初动极性和S/P振幅比的HASH方法和最大似然法等,获取了维西—乔后断裂带及附近地区M_(L)≥4.0震群和前震序列精确的震源位置、震源机制解、视应力和b值等地震活动参数,对比分析了维西—乔后断裂带及其周边震群及前震序列的活动特征。结果显示:除前震序列频次呈现出反大森定律外,前震序列与普通震群在空间分布、震源机制一致性、视应力和b值等单一参数方面并无明显的差异;但前震序列同时具备震中优势分布方向与区域主要构造展布方向一致、震源机制一致性较好、低b值等特征,建议综合使用时空演化特征、震群参数识别前震序列。

Propagation properties of foreshock cavitons: Cluster observations

Foreshock cavitons are transient phenomena observed in the terrestrial foreshock region.They are characterized by a simultaneous depression of magnetic field magnitude and plasma density,which are bounded with enhancements of these two parameters and surrounded by ultralow frequency(ULF)waves.Previous studies focused on the interplanetary magnetic field(IMF)conditions,solar wind(SW)conditions,and the growth of the foreshock waves related to the generation of foreshock cavitons.Previously,a multipoint spacecraft analysis method using Cluster data was applied to analyze only two foreshock cavitons,and this method did not consider uncertainties.In this study,multipoint spacecraft analysis methods,including the timing method,the minimum directional derivative(MDD)method,and the spatiotemporal difference(STD)method are applied to determine the velocity in both spacecraft and solar wind frames.The propagation properties show good agreement with previous results from simulations and observations that most cavitons move sunward in the solar wind frame,with the velocities larger than the Alfvén speed.The propagation properties of foreshock cavitons support the formation mechanism of cavitons in previous simulations,which suggested that cavitons are formed due to the nonlinear evolution of compressive ULF waves.We find that there is clear decreasing trend between the size of cavitons and their velocity in the solar wind frame.In addition,the timing method considering errors has been applied to study the evolution properties by comparing the velocities with errors of the leading and trailing edges,and we identify three stable cavitons and one contracting caviton,which has not been studied before.Most cavitons should remain stable when they travel toward the Earth’s bow shock.The relationship between the size of foreshock cavitons and their distance from the bow shock is also discussed.