Coupled large earthquakes in the Baikal rift system: Response to bifurcations in nonlinear resonance hysteresis

The current lithospheric geodynamics and tectonophysics in the Baikal rift are discussed in terms of a nonlinear oscillator with dissipation.The nonlinear oscillator model is applicable to the area because stress change shows up as quasi-periodic inharmonic oscillations at rifting attractor structures （RAS）.The model is consistent with the space-time patterns of regional seismicity in which coupled large earthquakes,proximal in time but distant in space,may be a response to bifurcations in nonlinear resonance hysteresis in a system of three oscillators corresponding to the rifting attractors.The space-time distribution of coupled MLH ＞ 5.5 events has been stable for the period of instrumental seismicity,with the largest events occurring in pairs,one shortly after another,on two ends of the rift system and with couples of smaller events in the central part of the rift.The event couples appear as peaks of earthquake ‘migration＇ rate with an approximately decadal periodicity.Thus the energy accumulated at RAS is released in coupled large events by the mechanism of nonlinear oscillators with dissipation.The new knowledge,with special focus on space-time rifting attractors and bifurcations in a system of nonlinear resonance hysteresis,may be of theoretical and practical value for earthquake prediction issues.Extrapolation of the results into the nearest future indicates the probability of such a bifurcation in the region,i.e.,there is growing risk of a pending M ≈ 7 coupled event to happen within a few years.

On the generation and decay of the long-period coda energy of large earthquakes

Seismic coda wave is the tail portion of the earthquake record after main arrivals.Studies on the coda usually focus on high-frequency data within several hours after regional events and attribute them to the scattering effect of the heterogeneities inside the earth.Here,we use records of seven large earthquakes at globally distributed seismic stations to examine the decay of long-period(100 s to 300 s)coda in the time window of 10,000 s to 140,000 s after the origin time and fit it with a statistical model.The geometric spreading effect in the estimated initial energy and a locationindependent equivalent attenuation coefficient indicate that the long-period coda energy is less affected by the heterogeneity-induced scattering effect than that of shorterperiod coda.The coda energy can reach the earth’s inner core and can be explained by a 1D earth model,making it more effective for constraining the global attenuation model.It also has the potential to determine the magnitudes of large earthquakes and to explore the interior of planetary bodies.

Quantitative Tectonic Markers of Large Coastal Earthquakes in Fujian, Guangdong, Taiwan and Hainan——with a Discussion of Qualitative Indicators

This paper expounds the quantitative tectonic indicators and some qualitative indicators of large earthquakes in the coast areas of Fujian, Guangdong, Taiwan and Hainan. The main quantitative indicators include uplift amplitude of the Moho, Quaternary and Late Holocene coasts. The paper also gives a brief account of the research method on quantitative indicators of surface uplifted zones. Taiwan is a famous neotectonic zone and an area of large earthquakes in the world. There is only one large-earthquake area in each of Fujian, Guangdong and Hainan Provinces. Along the coast large earthquake areas there are certainly many remains of crustal activity. Among these remains, coast activity, taking the sea level as the accurate marker horizon, can determine not only the amplitude of coastal elevation and subsidence in a certain period, but also the cycle and rate of positive or negative movements.

Aftershock of the Wenchuan Earthquake in Sichuan Triggers Large Earthquake in Minxian County, Gansu

Because of the confining compression of the Pacific, Eurasian and Indian plates, the Chinese mainland is frequently stricken with earthquakes, especially in the Qinghai Tibet Plateau and surrounding areas and along the NS-trending tectonic zone from Yinchuan to Lanzhou, Chengdu, and finally to the Kunming tectonic belt （Fig. 1）. Historical records show that there are 14 earthquakes of 〉Ms 8 occurring in the two regions, eight of which occurred along the latter tectonic belt. The 2008 Wenchuan earthquake located on both the NS tectonic zone and the Longmenshan fault zone along the eastern margin of the Qinghai-Tibet Plateau.

The influences of large earthquake signals on the recovery of surface waves from ambient noise cross-correlation functions

Ambient noise tomography(ANT)has been widely used to image crust and upmost mantle structures.ANT assumes that sources of ambient noise are diffuse and evenly distributed in space and the energy of different modes is equipartitioned.At present,the sources of the primary and the secondary microseisms are well studied,but there are only a few on the studies of long-period ambient noise sources.In this study,we study the effects of large earthquake signals on the recovery of surface waves from seismic ambient noise data recorded by seismic stations from the US permanent networks and Global Seismographic Network(GSN).Our results show that large earthquake signals play an important role on the recovery of long-period surface waves from ambient noise cross-correlation functions.Our results are consistent with previous studies that suggest the contribution of earthquake signals to the recovery of surface waves from cross-correlations of ambient noise is dominant at periods larger than 20–40 s.

Tectonic Stress Analysis of Future Large Earthquake Zones along the Bayan Har Block Boundary,Tibet Plateau

The Bayan Har block is mainly bounded by the east Kunlun fault zone to the north, Garze-Yushu -Xianshuihe fault zone to the south and Longmenshan fault zone to the east （Fig. 1）. In the past 20 years, large earthquakes have occurred frequently along this block＇s boundaries, which has received much attention among geoscientists. Whether large earthquakes will happen （and where） along this block＇s boundary faults in the future are two key problems that need to be addressed. This study calculates the accumulated tectonic stress and superposition of the coulomb stress caused by fault slip of 16 large earthquakes since 1904, and evaluates the possible locations of future earthquakes that may occur around this block.

Regularities of Seismicity of Western and Central Uzbekistan (Southwestern Part of Western Tien-Shan Region)

The paper discusses the spatial and temporal distribution of seismic activity in the southwestern part of the Western Tien Shan. The study identifies four groups of strong earthquakes that occurred in Western and Central Uzbekistan. The regularity of redistribution of the accumulated tectonic energy in the form of shuttle migration of seismicity with the recurrence rate of a group of strong earthquakes has been revealed. The synchronicity of periods of seismic activation of the Western Tien Shan (regional structure) and the Asian part of the Mediterranean-Asian seismic belt (structure of a large-regional scale) was found. At the same time, deviations from synchronicity are noted in the form of delays in the periods of seismic activation between the considered structures. The seismicity study revealed a deviation from the regularity of grouping of strong earthquakes in the seismic regime of the region. This was observed in the Gazli earthquakes. The strongest triple Gazli earthquakes of 1976 and 1984 with М = 7.0 - 7.3 were preceded by natural and technogenic triggers. Natural triggers included active fracturing (large fractures up to 100 km in length) in the north of Tamdybulak area, the absence any of tangible earthquakes in the epicentral area of Gazli for more than 40 years, and the appearance of small mud volcanoes—griffins before a strong earthquake. The technogenic triggers included a 40-year gas pumping at the Gazli fields and two underground nuclear explosions in 1966 (#273) and 1968 (#142) produced near the epicenter of the tectonic Gazli earthquakes of 1976 and 1984.

A complete introduction to the SCJ proposal and its commentary on the development of seismically resilient cities

In modern society, the population, wealth and social functions are increasingly concentrated in a few large cities. Such concentration brings about efficiency while at the same time it increases the vulnerability of the society. To address this issue, the Science Council of Japan（SCJ） published a proposal entitled ＂Shape Cities and Societies Safer against Severe Earthquakes＂. It makes a variety of suggestions for enhancing the seismic resilience of large cities, including：（1） comprehensive thinking based on the latest scientific knowledge and rich imagination;（2） selection of sites suitable for residence and social activities;（3） introducing the urban seismic coefficient;（4） promoting the enhancement of the seismic performance of buildings and civil structures;（5） easing the concentration of population and functions;（6） building communities that enable shelter and escape;（7） resilient technology of information and communication system and its effective utilization;（8） preparation and implementation of emergency response after earthquakes;（9） development and application of new structural seismic technology;（10） learning from domestic and foreign earthquake disasters and launching of international cooperation and knowledge sharing and（11） taking actions from a multidisciplinary perspective. The proposal is introduced in this paper.