Precursors Of Large Earthquakes Research Articles

Are Foreshocks Fore‐Shocks?

AbstractForeshocks are spatially clustered seismic events preceding large earthquakes. Since the dawn of seismology, their occurrence has been identified as a possible mechanism leading to further crustal destabilization, hence, to major failures. However, several cases occurred without any previous anomalous seismic activity, so that the hypothesis of foreshocks as reliable seismic precursors fails to pass statistical tests. Here, we perform an all‐round statistical comparative analysis of seismicity in Southern California to assess whether any differences can be identified between swarms and foreshocks. Our results suggest that extremely variable seismic patterns can forerun mainshocks, even though they tend to be preceded by clusters with more numerous events spread over larger areas than swarms and with a wider range of magnitudes. We provide a physical explanation of such dissimilarity and conclude, despite it, that foreshocks can hardly be reliable short‐term precursors of large earthquakes in California.

High S-wave attenuation anomalies and ringlike seismogenic structures in the lithosphere beneath Altai: Possible precursors of large earthquakes

This paper addresses inhomogeneities in the short-period S-wave attenuation field in the lithosphere beneath Altai. A technique based on the analysis of the amplitude ratios of Sn and Pn waves is used. High S-wave attenuation areas are identified in the West Altai, which are related to the source zones of recent large earthquakes, viz., the 1990 Zaisan earthquake and the 2003 Chuya earthquake. Associated with the Chuya earthquake, a large ringlike seismogenic structure had been formed since 1976. It is also found that ringlike seismogenic structures are confined to high S-wave attenuation areas unrelated to large historical earthquakes. It is supposed that processes paving the way for strong earthquakes are taking place in these areas. The magnitudes of probable earthquakes are estimated using the earlier derived correlation dependences of the sizes of ringlike seismogenic structures and the threshold values of magnitudes on the energy of principal earthquakes with prevailing focal mechanisms taken into consideration. The sources of some earthquakes are likely to occur near to the planned gas pipeline route from Western Siberia to China, which should be taken into account. The relationship of anomalies in the S-wave attenuation field and the ringlike seismogenic structures to a high content of deep-seated fluids in the lithosphere is discussed.

Fractal analysis and yule statistics for seismic prediction based on 2009 L’Aquila earthquake in Italy

Earthquakes represent a five-dimensional set of events: time, three space dimensions, and magnitude. To study the clustering between the space dimensions and the magnitude, a comparative analysis of epicenter and magnitude distributions was performed between the earthquake time series in Italy and around the L’Aquila region. The b value analysis reveals a slightly stronger seismic activity in the L’Aquila region compared with the Italian seismicity. Analysis of epicenter distribution shows that earthquakes in the L’Aquila region tend to fill the plane more than the overall distribution. Fractal dimension of hypocenter distribution for the L’Aquila region is approximately twice the b value, fulfilling the relation D = 2b. To investigate the relation between time and magnitude, an original interpretation was given for the variation of the bY value based on the Yule statistics. This approach reveals the relationship between earthquakes happened to propagate the existing faults and events creating new faults. The application of bY value analysis to the L’Aquila region highlights that clustering of earthquakes may be precursors of large earthquakes, and shows a reliable approach to predict violent earthquakes.

Erratum : Studies on deep groundwater changes for detecting the precursors of anticipated large earthquakes off Miyagi Prefecture, Japan: Observation results from June 2004 to December 2007

宮城県沖大地震に関連した前兆を検出するため，深層地下水変動観測システムを構築した．本論文で取り扱った2008年と2009年の観測では，2008年5月8日の茨城県沖地震（MJ7.0; MJは気象庁マグニチュード），同年6月14日の岩手・宮城内陸地震（MJ7.2）， 7月19日の福島県沖地震（MJ6.9），および7月24日の岩手県北部沿岸地震（MJ6.8）に関連した水位・水温変動が捉えられた．潮汐に対する観測システムの応答を調べ，3本の孔井の中の2本について，水位換算体積歪を2.27×10－9/mm，および0.5×10－9/mmと見積もった．水位換算体積歪と理論的な静的体積歪変化から求められた地下水変動の期待値と観測値がよく一致し，地下水変動の要因が静的歪変化であると示唆された．我々の観測システムが地震直後の地下水変動を検知できる下限は，MJ＝2.4logr＋1.0 （rは震源距離震で，単位はkm）で近似できる．このような式は静的歪変化の距離減衰だけでなく，地震動の距離減衰からも導かれる．

Gravity changes before large earthquakes in China: 1998–2005

Although it is well known that coseismic gravity changes take place during an earthquake, previous research has not yielded convincing evidence demonstrating that significant gravity changes occur before large earthquakes. Furthermore, even if we suspect that gravity changes occur before large earthquakes, we have yet to demonstrate how to consistently observe these changes for useful earthquake forecast that would bring benefits to society. We analyzed ground gravity survey data obtained in 1998, 2000, 2002, and 2005 at stations of the Crustal Movement Observation Network of China (CMONOC) and examined gravity changes before the occurrence of nine large (Ms⩾6.8) earthquakes that ruptured within or near mainland China and Taiwan from November 2001 to August 2008. Results from this analysis show that significant gravity changes occurred across a large region before each of these nine large earthquakes, and these changes were detected by repeated ground gravity surveys through CMONOC. Although these gravity changes were significant, more research is needed to investigate whether these gravity changes could be viewed as precursors of large earthquakes. Limitations and uncertainties in the data include sparseness of the gravity monitoring network, long time intervals between consecutive gravity surveys, inevitable measurement errors, hydrological effects on gravity, and effects of vertical crustal movements on gravity. Based on these observations, we make several recommendations about possible future directions in earthquake-related research using gravity monitoring data.

The Precursory Significance of Tectonic Stress Field Transformation Before the Wenchuan MW7.9 Earthquake and the Chi‐Chi MW7.6 Earthquake

AbstractThe 2008 Wenchuan earthquake and the 1999 Chi‐chi earthquake are both intraplate thrust fault earthquakes occurred under the tectonic setting of thrust‐nappe. Through comparative analysis on CMT solutions, focal mechanism solutions of small and moderate earthquakes and the tectonic stress field derived from focal mechanisms near the mainshcok region, we found that the thrust type focal mechanism of the Chi‐Chi mainshock coincides with the tectonic stress field from the CMT solutions of earthquakes occurred in Taiwan central and western inland; but for the small and moderate earthquakes randomly occurred near mainshock region, the strike slip type focal mechanism of regional stress field is not consistent with the tectonic stress field before the Chi‐Chi mainshock; the Chi‐Chi mainshcok occurred a few months after the local stress field became consistent with the tectonic stress field. Similarly, the compressive stress axis of the thrust fault type focal mechanism of Wenchuan mainshock coincides with the compressive stress axis of tectonic stress field determined by the CMT solutions of earthquake occurred in the eastern part of Qinghai‐Tibet Plateau, the local stress field from the focal mechanisms of small and moderate earthquake near Wenchuan mainshock is consistent with the tectonic stress field. When the local stress field is consistent with the thrust fault type stress field, the Wenchuan mainshock occurred immediately. With the stress accumulation near the source region, there are similar local stress field transformation phenomena before the thrust fault mainshock. When the local stress state transformed to the stress state of mainshock, the stress level is up to very high near the source region. It may be a precursor of large earthquake and must arouse further concerns.

Reverse tracing of precursors and earthquake precursors in Taiwan

In this paper, we investigate the precursors of large earthquakes in the eastern region of Taiwan by means of the reverse tracing of precursors. We discuss the parameters which are suitable for the seismic chains and intermediate-term patterns in this region and obtain the threshold of the patterns. Applying the linear discriminate method to the intermediate-term patterns of seismic chains, we present an approach for exploring the precursors of large earthquakes. The results show that this method can reduce the false alarm rate for large earthquakes in this region, and the reverse tracing of precursors can be applied to the eastern region of Taiwan.

Increased correlation range of seismicity before large events manifested by earthquake chains

“Earthquake chains” are clusters of moderate-size earthquakes which extend over large distances and are formed by statistically rare pairs of events that are close in space and time (“neighbors”). Earthquake chains are supposed to be precursors of large earthquakes with lead times of a few months. Here we substantiate this hypothesis by mass testing it using a random earthquake catalog. Also, we study stability under variation of parameters and some properties of the chains. We found two invariant parameters: they characterize the spatial and energy scales of earthquake correlation. Both parameters of the chains show good correlation with the magnitudes of the earthquakes they precede. Earthquake chains are known as the first stage of the earthquake prediction algorithm reverse tracing of precursors (RTP) now tested in forward prediction. A discussion of the complete RTP algorithm is outside the scope of this paper, but the results presented here are important to substantiate the RTP approach.

Evidence of electro-seismic phenomena

Abstract Ground-based observations of electromagnetic precursors to large earthquakes have been reported in Japan, Iran, Chile, Greece and the Soviet Union. Ionospheric disturbances measured by Soviet satellites have been claimed to be seismic precursors. In addition, observations of very low frequency anomalies measured by GEOS satellites have been reported by French researchers. To confirm reports of electromagnetic precursors to earthquakes, a series of low-frequency, broadband, radio-frequency receivers were deployed along the San Andreas fault system. This system of monitors has recorded electromagnetic anomalies occurring just before earthquakes between 1983 and 1986. These anomalies are of two types and are presumably caused by separate effects, both of which may be related to stress-induced changes in the crust just before fault rupture. The first of these types of anomalies appears to be an interruption in the propagation of radio waves (radio depressions), lasting for many hours, before an earthquake. The second type is similar to those in previously published reports and consists of short-duration broadband radio emissions. Given the results reported by other researchers and the results from our own experiments, we believe that electromagnetic data may be useful as a tool for predicting earthquakes.