Yu. F. Kopnichev

On the Nature of an Unusual Wave Train Observed in the Northern Tien Shan Region

We describe an unusual wave group that propagates in the Northern Tien Shan region along the border between mountain ranges and the Chu and Ili depressions to the north. This wave group, which we call S L, stands out against the short-period coda on the recordings of local earthquakes and quarry explosions (at distances up to 100 km). The time t m needed for the S L group to reach maximum amplitude varies between 98 and 131 sec, and at the same time, the t m value does not depend on epicentral distance. At distances up to 30 km, the S L group has a very narrow spectrum ( T = 1.5–1.8 sec), but when the epicentral distance increases, the predominant period also increases up to approximately 2.3 sec (at Δ ∼ 100 km). Characteristics of the wave group are significantly different for the Zaili fault zone than for the regions to the west of 76° E and to the east of 79° E. In the Zaili fault region, the S L group is characterized by great variability of velocity, amplitude, and polarization parameters even for nearby paths. The S L group maintains an amplitude within 0.7 of its maximum for 4–36 sec depending on the path. The polarization of the S L group is in the vertical plane in the Zaili fault-zone region and in the horizontal plane in other regions. Particle motion is elliptical or almost circular. We suggest that the S L group consists of shear waves propagating in a subvertical wave guide and reflected from a thin upper mantle layer.

On the correlation between seismicity characteristics and S-wave attenuation in the ring structures that appear before large earthquakes

We discuss seismicity characteristics in the source zones of two great earthquakes: the December 26, 2004 Sumatra (Mw = 9.0) and the November 14, 2001 Kunlun (Mw = 7.8) events. Ring structures of low magnitude seismicity have been forming prior to these earthquakes for several decades. We studied the short period shear-wave attenuation field in the area of these ring structures. The method we used is based on the analysis of the rate of attenuation for the early Sn and Lg codas to detect attenuation inhomogeneities in the uppermost mantle. We show that the ring structures have comparatively high attenuation of shear waves compared with the crustal volumes inside the rings. The fact that there is no recent volcanism in the area of the seismicity rings shows that this effect is due to a high content of free fluids in the uppermost mantle. Proceeding by analogy with our results, we identified a zone in northern Tien Shan that is anomalous for these parameters; the zone may be related to the precursory process of a large earthquake. We discuss the geodynamic mechanisms that may be responsible for fluid concentration in the seismicity rings.

Space-time variations of the shear wave attenuation field in the upper mantle of seismic and low seismicity areas

This paper deals with characteristics of the short period S-wave attenuation field in the rupture zones of 37 large and great earthquakes with Ms = 7.0–8.6, as well as in low seismicity areas. We estimate the effective quality factor from Sn and Lg coda envelopes in two time intervals (Q1 and Q2). The quantity Q1 is a measure of shear wave attenuation in the uppermost mantle, at depths of down to approximately 200–250 km, while Q2 is relevant to deeper horizons of the upper mantle. We studied variations in the attenuation field in the rupture zone of the 1950 Assam earthquake. We examined the parameters Q1, Q2, and Q1/Q2 as functions of the time ΔT elapsed after a large earthquake. It is shown that the parameter Q2 in rupture zones is practically independent of ΔT, while the quantities Q1 and Q1/Q2 increase until ΔT ∼ 20–25 years, especially rapidly for normal, normal-oblique, and strike-slip earthquake mechanisms. This analysis provides evidence that, as ΔT increases, so does the quality factor in the upper mantle for shear waves. It is supposed that this is related to the rise of mantle fluids to the crust. Geodynamic mechanisms are discussed that can support a comparatively rapid “drying” of the upper mantle beneath earthquake rupture zones.

Characteristics of ring seismicity in different depth ranges before large and great earthquakes in the Sumatra region

Characteristics of the seismicity in depth ranges 0–33 and 34–70 km before ten large and great (Mw = 7.0−9.0) earthquakes of 2000–2008 in the Sumatra region are studied, as are those in the seismic gap zones where no large earthquakes have occurred since at least 1935. Ring seismicity structures are revealed in both depth ranges. It is shown that the epicenters of the main seismic events lie, as a rule, close to regions of overlap or in close proximity to “shallow” and “deep” rings. Correlation dependences of ring sizes and threshold earthquakes magnitudes on energy of the main seismic event in the ring seismicity regions are obtained. Identification of ring structures in the seismic gap zones (in the regions of Central and South Sumatra) suggests active processes of large earthquake preparation proceed in the region. The probable magnitudes of imminent seismic events are estimated from the data on the seismicity ring sizes.

Heterogeneities in the field of short period seismic wave attenuation in the lithosphere of central Tien Shan

Records of deep-focus Hindu Kush earthquakes in the depth ranges 70–110 and 190–230 km made by 45 digital and analogue seismic stations were analyzed to study the attenuation field of short period seismic waves in the lithosphere of central Tien Shan. The dynamic characteristics studied include the ratio of peak amplitudes in S and P waves (S/P) and the ratio of the S-wave maximum to the coda level in the range t = 400 ± 5 s, where t is the lapse time (S/c400) for 1.25 Hz. Comparatively high values of S/P are shown to prevail in most of the area, corresponding to lower S-wave attenuation. Upon this background is a band of high and intermediate attenuation in the west of the area extending along the Talas-Fergana fault in the south and afterwards turning north-northeast. The rupture areas of the two largest (M ≥ 7.0) earthquakes which have occurred in Tien Shan during the last 25 years are confined to this band. Abnormally high values of S/c400 were obtained for stations situated in the rupture zone of the August 19, 1992, magnitude 7.3 Suusamyr earthquake and around it. For two of the stations we found considerable time variations in the coda envelope before the earthquake. The effective Q was derived from compressional and shear wave data for the entire area, as well as for the band of high attenuation. Comparison with previous data shows that the attenuation field in the area has changed appreciably during 20–25 years, which can only be due to a rearrangement of the fluid field in the crust and uppermost mantle. It is hypothesized that a large earthquake is very likely to occur in the northern part of the attenuating band.

Heterogeneities of the field of short-period shear wave attenuation in the lithosphere of Central Asia and their relationship with seismicity

The characteristics of the short-period shear wave attenuation field in the lithosphere of the Turanian Plate, West Tien Shan, Pamir, and Hindu Kush have been studied. The method based on analysis of the logarithm of the ratio between maximal amplitudes of Sn and Pn waves (Sn/Pn parameter) has been applied. More than 400 records of earthquakes, obtained at distances of ∼400–1000 km from the AAK digital station, have been processed. It has been found that relatively weak attenuation is observed in the regions of the West Tien Shan and Pamir. The largest area of strong attenuation is located in the region of the Afghan-Tadjik Depression adjacent to Hindu Kush. A wide band of low Sn/Pn parameter values, stretched northeastwards, has been distinguished. Along with the analogous band of strong attenuation, distinguished before in the regions of Central Tien Shan and Dzungaria, it is the continuation of the largest Chaman Fault, which stretches 850 km along the boundary of the Indian Plate. Source zones of strong earthquakes with M ≥ 7.0 that occurred in the first half of 20th century correspond to relatively weak attenuation. Areas of high attenuation, where strong seismic events have not occurred for the last 110 years, are outlined. Analogously to other seismoactive regions, it is supposed that these areas are related to preparation of strong earthquakes.

Annular seismicity structures and the March 11, 2011, earthquake ( M w = 9.0) in Northeast Japan

The characteristics of seismicity prior to the series of eight very strong earthquakes (Mw = 7.0–9.0) in Northeast Japan are discussed. Ring seismicity structures that appeared prior to all eight events in two depth ranges of 0–33 and 34–70 km are identified. The epicenters of the main shocks were located near areas of crossing or touching of shallow and deep rings. It was shown that the sizes of shallow rings and threshold magnitudes corresponding to seismicity rings grow with the energy of the main shocks. It was noted that the prognosis with respect to the place and magnitude of the catastrophic earthquake on March 11, 2011, had been made before it based on the data obtained prior to July 1, 2009. Use of the new data obtained prior to March 10, 2011, enabled us to specify this prognosis significantly. We obtained correlation dependences of threshold magnitudes on the energy of the main shocks (with a high correlation coefficients). It was shown that the duration of the period for seismicity rings to emerge in the considered region nearly did not depend on magnitude. The nature of annular structures and the possibility of application of their parameters for prognosis of strong earthquakes were discussed.

Ring seismicity in different depth ranges before large and great earthquakes in subduction zones

Characteristics of seismicity before eight strong and very strong shallow earthquakes in the subduction regions (in Sumatra, New Britannia, Mexico, Hokkaido, and Peru) are studied. The data on earthquakes are considered in two depth ranges: 0–33 and 34–70 km. It was found that ring structures of seismicity were formed in both ranges before large earthquakes. The epicenters of the main events were located near the regions of crossing or maximum closeness of shallow and deep rings. An interpretation of the revealed effects is offered related to the migration of deep fluids.

Heterogeneities in the absorption field of short-period S waves in the lithosphere of Tien Shan and Dzungaria with their relation to seismicity

1119 The mapping for the absorption field of transversal waves in the lithosphere of Tien Shan and Dzungaria is carried out. The method based on analysis of the ratio between maximal values of amplitudes for Sn and Pn waves is used. Earthquake records obtained at dis tances of ~300–1300 km by the MKAR station were processed. It was found that S wave absorption in the lithosphere is significantly higher in the northwestern part of the area than in the southeastern one. It is shown that the source zones for strong earthquakes with М ≥ 7.0 that occurred in 1978–1992 are charac terized by a higher absorption. Areas of higher absorp tion, where there were no strong earthquakes for the last 130 years, are found. It is supposed that these areas are related to preparation of strong earthquakes.

Ring structures of seismicity in central Tien Shan and Dzungaria: Possible precursory processes of large earthquakes

It is shown that episodes of comparative seismic quiescence that lasted about 20–25 years in the areas of study alternated with intervals of sharply increased seismicity as series of large (M ≥ 6.9) earthquakes occurred during two to three decades. Since no M ≥ 6.6 earthquake has occurred in the area for as long as 21 years after the 1992 Susamyr event, middle-term prediction would require identification of zones of imminent large earthquakes. More reliable identification of such zones rests on data relating to inhomogeneities in the field of S-wave attenuation in the lithosphere, as well as on the characteristics of ring structures of seismicity. Such structures are formed as zones of seismic quiescence that are bounded by M ≳ Mth earthquake epicenters, where Mth is the threshold magnitude value. Correlative relationships were previously derived, lgL(Mw) and Mth(Mw), for events with different focal mechanisms (L is the length of the longer axis of a seismicity ring and Mw is the magnitude of the associated large earthquake). These relationships were used to estimate the Mw of large events that can occur in these ring structures. The greatest earthquake with Mw ≳ 7.5 is probably about to occur in southern Tien Shan, east of the 1949 Khait earthquake rupture. A smaller event (Mw ∼ 7.0) can occur in the Kyrgyz Range area. Still smaller earthquakes probably have their precursory areas north and east of Lake Issyk-Kul, as well as in Dzungaria.