Long aftershock sequences in North China and Central US: implications for hazard assessment in mid-continents

Because seismic activity within mid-continents is usually much lower than that along plate boundary zones, even small earthquakes can cause widespread con- cerns, especially when these events occur in the source regions of previous large earthquakes. However, these small earthquakes may be just aftershocks that continue for decades or even longer. The recent seismicity in the Tangshan region in North China is likely aftershocks of the 1976 Great Tangshan earthquake. The current earthquake sequence in the New Madrid seismic zone in central United States, which includes a cluster of M- 7.0 events in 1811-1812 and a number of similar events in the past millennium, is believed to result from recent fault reactivation that releases pre-stored strain energy in the crust. If so, this earthquake sequence is similar to aftershocks in that the rates of energy release should decay with time and the sequence of earthquakes will eventually end. We use simple physical analysis and numerical simulations to show that the current sequence of large earthquakes in the New Madrid fault zone is likely ending or has ended. Recognizing that mid-continental earthquakes have long aftershock sequences and complex spatiotemporal occur- rences are critical to improve hazard assessments.

Earthquake Hazard Zonation and Seismotectonics of the Bandar Abbas Area, Zagros, Iran

The study area (Bandar Abbas area) is located in the Zagros fold-thrust belt as part of the Alpine-Himalayan orogenic belt as seismically active belt. This area is located between the Makran accretionary prism and Oman Mountains from east and the Zagros collision belt from west as transition zone. The Zagros fold-thrust belt from the viewpoint of seismicity, is very active and Iran’s major earthquake-prone area. The study area has main active faults and some high magnitude earthquakes occurred in current century. Because the Bandar Abbas area has high seismic activity, the main goal of this research is prepared to earthquake hazard zonation and identify hazardous seismic zones, based on Decision Support System method for define active seismotectonic in this area. The seismotectonic study has been done in 30 - 100 km radius, for Bandar Abbas area. In this research, we used Decision Support System method by in corporate and combine essential data such as seismic data from 1900-2015, Digital Elevation Model of the study area (DEM), surface geology, seismicity parameters, soil classification and location main faults. In this research the Decision Support System (DSS) base on GIS database is used for calculate seismicity parameters. Based on the relative risk of earthquake zonation map, the Bandar Abbas area is located from the north to the East and from the South to the East, in area with high seismic risk (with Orange color). Some small regions with very high relative seismic risk have been limited to these areas with high risk. Also from north to west and from south to west “the study area” is located mainly in the area with earthquake relative risk of in areas with moderate and low relative risk of earthquakes. In the far southwestern region of the study, the small area is located in an area with high and very high seismic relative risk and this case may be due to the activity of the Mountain Front Fault (MFF) and Zagros Fore deep Fault (ZFF). Finally, the study area has been affected by active faults and it causes high vulnerability of the study area in the face of a possible occurrence of earthquakes. Based on of Seismotectonic investigations, there are existed minor faults of the Zagros fault from East to West and in the middle part. This case has been caused some parts in the study area with low and moderate seismic risk to be considered in the face of possible earthquakes and seismic damages, as an area with high seismic risk.

Seismogenic mechanism of the Lancang and Gengma earthquakes

The typical earthquake disaster and its seismogenic mechanism in the meizoseismal regions of M7. 6 Lancangand M7. 2 Gengma earthquakes are introduced and analysed in this paper. Combining with the tectonic features in the earthquake regions, the difference of the tectonic activity between the two earthquakes is alsoshown in the paper: the strong strike offset feature of Muga fault for M7. 6 Lancang earthquake, while thedip-slip feature of Hanmuba fault for M7. 2 Gengma earthquake. The obvious difference in the displacementbetween Muga fault and Hanmuba fault probably related to the part adjustment and diversion for the principalcompressive stress axis in the focal area of M7. 2 Gengma earthquake. The M7. 6 Lancang earthquake triggered M7. 2 Gengma earthquake.

Landslide Hazard Mapping During a Large Scale Earthquake

This paper reports a method to make hazard maps of sediment disasters resulting from an earthquake and following heavy rainfall for the entire region of Gunma prefecture, Japan. Firstly, we identified the slopes in the study area, which are susceptible to large-scale landslides and land failures during an earthquake with a magnitude of seven on the Richter scale. To analyze the sheer volume of the data, we employed a statistical method to evaluate the susceptibility, mainly considering geomorphologic conditions. Secondly, we extracted mudflow and slope failure susceptible areas and potential flooding zones resulting from a damming at a river triggered by the earthquake and heavy rainfall, and we identified the settlements which would be isolated by the road disruption caused by the sediment disasters. As the result, 359 settlements were classified as potential isolation areas. Combining the above-mentioned susceptibility maps, we obtained two types of sediment disaster hazard maps of the study area, depicting the potential hazards which would occur during the earthquake and the disasters which would be caused by heavy rainfall following the quake, respectively. These hazard maps and the disaster information would be useful for the regional disaster prevention planning and countermeasures in the future.

Seismic Observations and Seismic Hazard Studies in the Philippines

It has taken more than a hundred years for seismic observations in the Philippines to evolve to a modern observation system. The responsibility of seismic observations was likewise transferred from one agency to another during this same period of time. At present, the mandate of conducting seismic observations in the Philippines rests with the Philippine Institute of Volcanology and Seismology (PHIVOLCS). In 2000, through a grant aid from the Japan International Cooperation Agency (JICA), the Philippine seismic network was upgraded to a digital system. As a result, a new set of seismic monitoring equipments was installed in all of the 34 PHIVOLCS seismic stations all over the country. Digital waveforms are now available for high level seismic data processing, and data acquisition and processing are now automated. Included in the upgrade is the provision of strong motion accelerographs in all stations whose data can now be used for studying ground motion and intensity attenuation relations. The new setup is now producing high-resolution data that can now be used for conducting basic seismological researches. Earthquake locations have now improved allowing for the modeling and delineation of earthquake source regions necessary for earthquake hazard studies. Current seismic hazard studies in the Philippines involve the estimation of ground motion using both probabilistic and deterministic approaches, seismic microzonation studies of key cities using microtremor observations, paleoseismology and active faults mapping, and identification of liquefaction-prone, landslide-prone and tsunami-affected areas. The earthquake database is now being reviewed and completed with the addition of historical events and from data from regional databases. While studies of seismic hazards were primarily concentrated on a regional level, PHIVOLCS is now focusing on doing these seismic hazard studies on a microlevel. For Metro Manila, first generation hazard maps showing ground rupture, ground shaking and liquefaction hazards have recently been completed. Other large cities that are also at risk from large earthquakes are the next targets. The elements at risk such as population, lifelines, and vertical and horizontal structures for each of these urban centers are also being incorporated in the hazard maps for immediate use of planners, civil defense officials, policy-makers and engineers. The maps can also now be used to describe possible scenarios during times of strong events and how appropriate socio-economic and engineering responses could be designed. In addition, a rapid earthquake damage assessment system has been started which will attempt to produce immediate or rapid assessments including identification of elements at risk during times of strong earthquakes.

Preliminary Research on the Characteristics of the M_S8.0 Wenchuan Earthquake Hazard

The Ms8.0 Wenchuan earthquake on May 12, 2008 caused heavy casualties and economic loss. According to the field investigations, the characteristics can be described as follows： The meizoseismai region with an intensity of XI shows an obviously zonal distribution and suffered serious destruction from the earthquake, and the destruction perpendicular to the earthquake surface rupture decreased sharply. At the same time, the intensity X and IX regions perpendicular to the rupture are narrow and therefore their coverage area is small. The intensity on both sides of the rupture attenuates rapidly, but intensity VII and the VI regions are wide, the latter covering about 240,000 km^2. In intensity VI region, the damage area perpendicular to the rupture in the southern part is much larger than that of the northern part. Also, much new understanding about destruction types and destructive modes for all kinds of buildings, landforms and terrain is achieved in this paper.

Studies of Earthquake Hazard Using Microseismicity Data in Modern Times

This paper selects some representative regions to obtain their G-R relation curves according to their seismicity characteristics,by using ML≥2.0 microseismicity data(1970~1993)in North China.The annual occurrence rate of events of each magnitude can be inferred from the G-R relation.At the same tune,the actual annual occurrence rate of earthquakes of higher magnitudes can be calculated from historical earthquakes(1300-1993)recorded in the same region.It seems that both results are almost the same.Therefore,the rate of events of higher magnitudes can be obtained by using microseismicity data when the proper region is selected.However,two points should be noticed:(1)The method can only give the annual occurrence rate in a seismicity system and estimate the whole situation of the system.(2)When there is a very large earthquake in and near the period in which the microseismicity data are applied,the actual occurrence rate of the system,including this larger earthquake,cannot be obtained by this method.

Detailed sedimentary structure of the Mianning segment of the Anninghe fault zone revealed by H/V spectral ratio

The Anninghe fault is a major left-lateral strike-slip fault in southwest China and a seismic gap with a potential earthquake larger than MW 7.0 lies in the Mianning-Xichang segment according to recent observations.The shallow structure of this region can offer a glimpse into the geometry of the fault,which plays an important role in earthquake hazard mitigation.To further investigate the sedimentary structure of the Anninghe fault zone,two dense linear arrays with a station spacing of around 80 m were deployed across the fault.In this study,the H/V spectral ratio(HVSR),together with its peak frequency at each station site,was obtained by applying the Nakamura method.Our findings demonstrate that the peak frequency behaves in high correlation with lithology and is controlled by topography.HVSR in foothills or regions with magmatic intrusion shows a single peak at about 2–3 Hz.In locations with abundant Quaternary sedimentation,such as Anninghe valleys and fracture zones,another low-frequency peak around 0.4 Hz can be noticed in HVSR.By using the empirical relationship,the thickness of the sedimentary layer around the fault fracture zone is estimated to be 300–600 m.Furthermore,the sedimentary interface shows a downward dip to the east,possibly influenced by the east-west extrusion stress.Considering the resonance effect,buildings with 6–9 stories in the valley area of the Anninghe require additional attention in earthquake hazard prevention.

Relationships between moment magnitude and fault parameters:theoretical and semi-empirical relationships

Fault parameters are important in earthquake hazard analysis.In this paper,theoretical relationships between moment magnitude and fault parameters including subsurface rupture length,downdip rupture width,rupture area,and average slip over the fault surface are deduced based on seismological theory.These theoretical relationships are further simplified by applying similarity conditions and an unique form is established.Then,combining the simplified theoretical relationships between moment magnitude and fault parameters with seismic source data selected in this study,a practical semi-empirical relationship is established.The seismic source data selected is also to used to derive empirical relationships between moment magnitude and fault parameters by the ordinary least square regression method.Comparisons between semi-empirical relationships and empirical relationships show that the former depict distribution trends of data better than the latter.It is also observed that downdip rupture widths of strike slip faults are saturated when moment magnitude is more than 7.0,but downdip rupture widths of dip slip faults are not saturated in the molnent magnitude rangcs of this study.

The spatiotemporal variation of the b-value and its tectonic implications in North China

In this study, we adopt an improved Bayesian approach based on free-knot B-spline bases to study the spatial and temporal distribution of the b-value. Synthetic tests show that the improved Bayesian approach has a superior performance compared to the Bayesian approach as well as the widely used maximum likelihood estimation （MLE） method in fitting the real variation of b-values. We then apply the improved Bayesian approach to North China and find that the b-value has a clear relevance to seismicity. Temporal changes of b-values are also investigated in two specific areas of North China. We interpret sharp decreases in the b-values as useful messages in earthquake hazard analysis.

Spatial-temporal characterization of the San Andreas Fault by fault-zone trapped waves at seismic experiment site,Parkfield,California

In this article,we review our previous research for spatial and temporal characterizations of the San Andreas Fault(SAF)at Parkfield,using the fault-zone trapped wave(FZTW)since the middle 1980s.Parkfield,California has been taken as a scientific seismic experimental site in the USA since the 1970s,and the SAF is the target fault to investigate earthquake physics and forecasting.More than ten types of field experiments(including seismic,geophysical,geochemical,geodetic and so on)have been carried out at this experimental site since then.In the fall of 2003,a pair of scientific wells were drilled at the San Andreas Fault Observatory at Depth(SAFOD)site;the main-hole(MH)passed a~200-m-wide low-velocity zone(LVZ)with highly fractured rocks of the SAF at a depth of~3.2 km below the wellhead on the ground level(Hickman et al.,2005;Zoback,2007;Lockner et al.,2011).Borehole seismographs were installed in the SAFOD MH in 2004,which were located within the LVZ of the fault at~3-km depth to probe the internal structure and physical properties of the SAF.On September 282004,a M6 earthquake occurred~15 km southeast of the town of Parkfield.The data recorded in the field experiments before and after the 2004 M6 earthquake provided a unique opportunity to monitor the co-mainshock damage and post-seismic heal of the SAF associated with this strong earthquake.This retrospective review of the results from a sequence of our previous experiments at the Parkfield SAF,California,will be valuable for other researchers who are carrying out seismic experiments at the active faults to develop the community seismic wave velocity models,the fault models and the earthquake forecasting models in global seismogenic regions.

The formation of the Wulipo landslide and the resulting debris flow in Dujiangyan City, China

The Wulipo landslide, triggered by heavy rainfall on July 10, 2013, transformed into debris flow,resulted in the destruction of 12 houses, 44 deaths, and 117 missing. Our systematic investigation has led to the following results and to a new understanding about the formation and evolution process of this hazard. The fundamental factors of the formation of the landslide are a high-steep free surface at the front of the slide mass and the sandstone-mudstone mixed stratum structure of the slope. The inducing factor of the landslide is hydrostatic and hydrodynamic pressure change caused by heavy continuous rainfall. The geological mechanical model of the landslide can be summarized as "instability-translational slide-tension fracture-collapse" and the formation mechanism as "translational landslide induced by heavy rainfall". The total volume of the landslide is 124.6×104 m3, and 16.3% of the sliding mass was dropped down from the cliff and transformed into debris flow during the sliding process, which enlarged 46.7% of the original sliding deposit area. The final accumulation area is found to be 9.2×104 m2. The hazard is a typical example of a disaster chain involving landslide and its induced debris flow. The concealment and disaster chain effect is the main reason for the heavy damage. In future risk assessment, it is suggested to enhance the research onpotential landslide identification for weakly intercalated slopes. By considering the influence of the behaviors of landslide-induced debris flow, the disaster area could be determined more reasonably.

Source rupture process of the 2015 Gorkha, Nepal Mw7.9 earthquake and its tectonic implications

On 25 April, 2015, an Mw7.9 earthquake occurred in Nepal, which caused great economic loss and casualties. However, almost no surface ruptures were observed. Therefore, in order to interpret the phenomenon, we study the rupture process of the earthquake to seek answers. Inversion of teleseismic body-wave data is applied to estimate the rupture process of the 2015 Nepal earthquake. To obtain stable solutions, smoothing and non-negative constraints are introduced. 48 teleseismic stations with good coverage are chosen. Finite fault model is established with length and width of 195 km and 150 km, and we set the initial seismic source parameters referring to CMT solutions. Inversion results indicate that the focal mechanism of this earthquake is a thrust fault type, and the strike, dip and rake angle are in accordance with CMT results. The seismic moment is 0.9195 ×10^（21）Nm（Mw7.9）, and source duration is about 70s. The rupture nucleated near the hypocenter and then propagated along the dip direction to the southeast, and the maximum slip amounts to 5.2 m. Uncertainties on the amount of slip retrieved by different inversion methods still exist, the overall characteristics are inconsistent. The lack of shallow slip during the 2015 Gorkha earthquake implies future seismic hazard and this region should be paid more attention to.

Contemporary kinematic models and moment deficit of Chinese mainland

There are lots of achievements about kinematics of Chinese mainland and its vicinity determined from historic earthquake data, Quaternary fault rates and geodesy observations, which provide basic data for analysis of seismic hazard in study areas. Based on impreciseness in using energy of 47 earthquakes with magnitude greater than 7.0 in previous study, we firstly collected source parameters, surface ruptures and displacements of major earthquakes carefully, and divided them into small segmentations with these data gathered. Secondly, we determined contemporary deformation field from latest earthquake mechanisms, Quaternary fault slip rates and geodesy observations. Finally, we evaluated moment deficit of study areas from historic earthquake data and predicted deformation field, and removed 10 percent of aseismic deformation. Combining with previous results, we analyzed the seismic hazard of study areas. The results show that there are some areas with large moment deficit in Chinese mainland. There are also large moment deficit areas, including regions around the Ordos Block, southeastern coast of China and the Bakal rift zone. Previous studies show that there may be some potential earthquakes in the near future in Darts of areas mentioned above.

Seismological Research in Indonesia1

Indonesia is situated at the juncture of three major tectonic plates: the Eurasian, the Pacific and the Indo-Australian plates. As a result, many parts of the country are classified as being at a high risk of earthquake and tsunami disasters. In Indonesia, the Bureau of Meteorology and Geophysics (BMG) is the agency responsible for the monitoring of tectonic earthquakes and tsunamis induced by them. For this purpose, BMG operates seismic networks throughout Indonesia. In order to serve better, BMG will have to improve its monitoring system. Many agencies and universities in Indonesia have done research in the field of seismology, including seismotectonics, earthquake hazards, and so on. Joint research has been done with agencies/universities from abroad. Considering the fact that some earthquake prone areas in Indonesia suffer from frequent disasters, a study on earthquake prediction in some areas of interest would contribute to achieving some long-term goals of the Bureau.

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.

The Analysis of the Van-Ercis Earthquake,October 23,2011 Turkey,for the Transportation Systems in the Region

This paper investigates the effect of the Van-Ercis, Turkey, (Mw: 7.2)earthquake occurring on 23rd of October, 2011 on the transportation networks in the region. The basic incentivefor this research is to conceptualise the reliability and performance of the networks after the earthquake through the operational and topological analysis of the system. The demand and composition of the traffic along with the behaviour of the pedestrians were taken into account to evaluate the performance of the networks. In addition, the general structure of the cities and towns, as far as planning is concerned, is also paid attention and regarded as one of the main elements for the appraisal. The outcomes obtained are thought very important to be guidance for the expected Istanbul earthquake in the near future.

Experimental analysis for the dynamic initiation mechanism of debris flows

Debris flow is one of the major secondary mountain hazards following the earthquake. This study explores the dynamic initiation mechanism of debris flows based on the strength reduction of soils through static and dynamic triaxial tests. A series of static and dynamic triaxial tests were conducted on samples in the lab. The samples were prepared according to different grain size distribution, degree of saturation and earthquake magnitudes. The relations of dynamic shear strength, degree of saturation, and number of cycles are summarized through analyzing experimental results. The findings show that the gravelly soil with a wide and continuous gradation has a critical degree of saturation of approximately 87%, above which debris flows will be triggered by rainfall, while the debris flow will be triggered at a critical degree of saturation of about 73% under the effect of rainfall and earthquake(M>6.5). Debris flow initiation is developed in the humidification process, and the earthquake provides energy for triggering debris flows. Debris flows are more likely to be triggered at the relatively low saturation under dynamic loading than under static loading. The resistance of debris flow triggering relies more on internal frication angle than soil cohesion under the effect of rainfall and earthquake. The conclusions provide an experimental analysis method for dynamic initiation mechanism of debris flows.

Review and Prospects of the Global Earthquake Model Project ( GEM)

This paper reviewed the main target,functions,tool( Open Quake software) and research achievements of the Global Earthquake Model( GEM) Foundation,and made a simple prospect on the development and application of projects of GEM in the future. Learning from GEM and Open Quake is helpful to improve the seismic hazard model of China and enhance the scientificity of the seismic hazard assessment for metropolitans and major engineering facilities near major seismogenic structures.