The Statistical Analysis of Migration of Strong Earthquakes─Taking the North China Region as an Example

The migration of strong earthquakes is an important research topic because the migration phenomena reflect partly the seismic mechanism and involve the prediction of tendency of seismic activity. Research on migration of strong earthquakes has mostly focused on finding the phenomena. Some attempts on getting regularity were comparatively subjective. This paper suggests that there should be indices of migration in earthquake dataset and the indexes should have statistical meaning if there is regularity in the migration of strong earthquakes. In this study, three derivative attributes of migration, i.e., migration orientation, migration distance and migration time interval, were statistically analyzed. Results in the North China region show that the migration of strong earthquakes has statistical meaning. There is a dominant migration orientation (W by S to E by N), a dominant distance (≤100km and on the confines of 300～700km), and a dominant time interval (≤1a and on the confines of 3～4a). The results also show that the migration will differ slightly with different magnitude range or earthquake activity phase.

The Migration Characteristics of Strong Earthquakes on the North-South Seismic Belt and Its Relation with the South Asia Seismic Belt

Migration of strong earthquakes (M≥7.0) along the North-South Seismic Belt of China since 1500 AD shows three patterns: Approximately equal time and distance interval migration from N to S, varied patterns of migration from S to N and grouped strong earthquake activity in a certain period over the entire seismic belt. Analysis of strong earthquakes in the past hundred years shows that the seismicity on the North-South Seismic Belt is also associated with strong earthquake activities on the South Asia Seismic Belt which extends from Myanmar to Sumatra, Indonesia. Strong earthquakes on the former belt often lag several months or years behind the quakes occurring on the later belt. So, after the occurrence of the December 26, 2004 M_S8.7 great earthquake off the western coast of Sumatra, Indonesia, the possibility of occurrence of strong earthquakes on the North-South Seismic Belt of China cannot be ignored. The above-mentioned migration characteristics of strong earthquakes are related to the northeastward collision and subduction of the India Plate as well as the interaction between the Qinghai-Xizang (Tibet) Plateau and the stable and hard Ordos and Alashan Massifs at its northeastern margin.

The Datong earthquake and its anomalous tilt Field

In this paper, the similarity and the meanings of anomalous pattern of entropy for tilt tide amplitude factor, the distribution of entropy and dimension for tilt velocity, the similarity and relationships for long-period tilt deformation and earthquake migration are demonstrated. The deformation wave and its meanings have also been explained. The authors explored the relationships between distribution of similar pattern and epicenter. The possibility of prediction for next earthquake epicenter has also been explored based on the combining tilt field and earthquake activity.

Stress Transfer and the Impact of the India-Eurasia Collision and the Western Pacific Subduction on the Geodynamics of the Asian Continent

The interaction between the India-Eurasia collision and the Western Pacific subduction and their contribution to recent geodynamics of the Asian continent are discussed. We perform a comparative analysis of the data available from world literature and new data on the slow strain and earthquake migration from the India-Eurasia collision and the Western Pacific subduction zones. Based on the concepts of wave dynamics of the deformation processes, a localization scheme is constructed illustrating the migration of slow strain fronts in central and eastern Asia, and the wave geodynamic impact of collision and subduction on the Asian continent is shown.

Propagation of crust deformation anomalies related to the Menyuan M_(S) 6.9 earthquake

Decoding the variation laws of the deformation field before strong earthquakes has long been recognized as an essential issue in earthquake prediction research. In this paper, the temporal and spatial distribution characteristics of deformation anomalies in the northeastern margin of the Qinghai-Tibetan Plateau before and after the Menyuan M_(S)6.9 earthquake were studied by using the Fisher statistical test method. By analyzing the characteristics of these anomalies, we found that: 1) The deformation anomalies are mainly distributed in the marginal front area of the Qinghai-Tibetan Plateau, where short-term deformation anomalies are prone to occur due to a high gradient of gravity;2) The deformation anomalies along the northeastern margin of the Qinghai-Tibetan Plateau are characterized by spatial propagation, and the migration rate is about 2.4 km/d. The propagation pattern is counterclockwise, consistent with the migration direction of M_(S)≥ 6.0 earthquakes;3) The time and location of the Menyuan earthquake are related to the group migration of earthquakes with M_(S)≥ 6.0. Finally,based on the results of gravity field variation and the theory of crust stress wave, the law of deformation anomaly distribution was discussed. We suggest that both the deformation propagation along the northeastern margin of the Qinghai-Tibetan Plateau and the earthquake migration are possibly associated with the variation of the stress field caused by subsurface mass flow.

Slow Strain Waves: Models and Observations, a Review

The theoretical discovery of slow strain (tectonic) waves, the so-called strain waves in the Earth, served as a motivation to develop physical backgrounds of the mathematical theory of propagation of these waves and to search for methods of their experimental detection. For fifty years, scientists from different countries in different regions of the Earth, using direct and indirect methods, discovered the migration of crustal deformation and revealed its wave nature, and, therefore, proved the reality of the existence of strain waves of the Earth. This overview briefly describes the history of the development of the concept of strain waves on the Earth, the observation methods and properties of strain waves, and the main types of geological structures generating these waves. The most prominent results of the theoretical, laboratory, and in-situ observations of slow strain migration, including slow earthquakes and periodic Episodic Tremor and Slow (ETS) slip effects, are presented. In the near future, studies of slow strain waves may lead to a fundamental revision of the current concepts about the physics of the seismic process.

A Preliminary Analysis of Seismotectonics for the Indonesia M8.7 Earthquake on December 26, 2004

The Indonesian region is one of the most seismically active zones on the earth. On December 26, 2004, an M_S 8.7 earthquake (as measured by the China Seismograph Network, or M_w = 9.3 as measured by USGS) struck the west coast of northern Sumatra, Indonesia. By its magnitude it is classified as the world’s fourth largest earthquake since 1900 and the largest one since the 1964 Alaska earthquake. The spatial distribution of the relocation of larger aftershocks (M>4.5) following the main shock suggests a length and width of the rupture of about 1200km and 200km, respectively. The shock triggered massive tsunamis that affected several countries throughout South and Southeast Asia. It is a shallow interplate event of thrust type in the trench. Its epicenter is located at the northwestern end of the Indonesia-Melanesia plate boundary tectonic zone. In 2004, eight shocks of M≥7.0 occurred in this area, showing a migration from east to west. It implies that these shocks represent a correlated and consistent dynamic process along this subduction zone. These interplate events are associated with convergence of several plates and their fast motion in this region, which result in strong and complex structures and deformation. The India-Australia plate is underthrusting toward the Sunda continental block or Burma plate at a low angle, producing a great locked area on the shallow portion of the subduction zone where enormous strain is accumulated. Interseismic uplift recorded by coral growth and horizontal velocities measured by GPS show the geometry of the locked portion of the Sumatra subduction zone. The vertical and horizontal data reasonably match with a model in which the plate interface is fully locked over a significant width. This locked fault zone extends to a horizontal distance of 132km from the trench, which corresponds to a depth of 50km. The sudden ruptures and large-scale slip of this locked area as a release of stress occurred, are the direct cause of the M8.7 earthquake near Indonesia in 2004.

The seismogenic structures and migration characteristics of the 2021 Yangbi M6.4 Earthquake sequence in Yunnan,China

We constructed a more complete earthquake catalog in the 2021 Yangbi M6.4 focal area by re-scanning the continuous waveforms integrated with deep learning and template matching techniques,to explore the seismogenic structures of the Yangbi mainshock and its nucleation process.The new catalog has three times the number of earthquakes than the CENC catalog,and the magnitude completeness has dropped from 1.1 to 0.5.The distribution of earthquakes indicates a broom-shaped structure consisting of several oblique secondary faults and a strike-slip main fault which strikes to 315°with 80°dipping to NE.The earthquakes extend along the fault strike about 27 km in width and 2-13 km at depth and have noticeable variations on seismicity in the mainshock’s north and south.Compared with the north,the south has denser and higher magnitude aftershocks and also has a seismic gap probably weakened by the fluid at the depth range of about 5-6 km.The foreshocks were mainly active in the 8-kilometer-long fault zone south of the mainshock,which show a steady drop in b-values over time and a migration pattern toward the epicenter of two steep jumps,stagnation,and then acceleration which finally triggered the mainshock.While in the north,seldom foreshock occurred,and the aftershocks were delayed triggered 3 hours after the mainshock,and sparsely scattered shallow at depth and small in magnitude.To summarize,the northern part of the Yangbi seismogenic fault is thought to be relatively locked,whereas the southern part has a weakening zone and promotes pre-slip.The nucleation mechanism of the mainshock and its onset at the junction of the locked and pre-slip zones may be a combination of pre-slip and cascade triggering.