Seismicity pattern in north Sumatra-Great Nicobar region: In search of precursor for the 26 December 2004 earthquake

We analyse the seismicity pattern including b-value in the north Sumatra-Great Nicobar region from 1976 to 2004. The analysis suggests that there were a number of significant, intermediate and short-term precursors before the magnitude 7.6 earthquake of 2 November 2002. However, they were not found to be so prominent prior to the magnitude 9.0 earthquake of 26 December 2004 though downward migration of activity and a 50-day short-term quiescence was observed before the event. The various precursors identified include post-seismic and intermediate-term quiescence of 13 and 10 years respectively, between the 1976 (magnitude 6.3) and 2002 earthquakes with two years (1990–1991) of increase in background seismicity; renewed seismicity, downward migration of seismic activity and foreshocks in 2002, just before the mainshock. Spatial variation in b-value with time indicates precursory changes in the form of high b-value zone near the epicenter preceding the mainshocks of 2004 and 2002 and temporal rise in b-value in the epicentral area before the 2002 earthquake.     

国内外地震滑坡研究：现状、问题与展望

地震滑坡一直是地学界关注的研究热点之一,特别是2005年10月8日克什米尔M_s 7.8地震、2008年5月12日汶川M_s 8.0地震、2010年玉树M_s 7.1地震、2014年云南鲁甸M_s 6.5地震、2015年4月25日尼泊尔M_s 8.1地震以及2017年8月8日九寨沟M_s 7.0地震等一系列强震活动,都不同程度地触发了大量地震滑坡,并伴生了许多次生灾害,从而促进地震滑坡研究进入了新的阶段。本文通过总结国内外地震滑坡研究现状,归纳了地震滑坡的特征和分布规律、地震动参数与地震滑坡关系、边坡地质条件与地震滑坡关系、地震滑坡的动力响应特征、地震滑坡预测以及地震滑坡危险性与风险区划等6方面的研究现状,在此基础上进一步提出了未来地震滑坡应关注的主要研究方向和重点。     

Quiescence period prior to strong earthquakes in the Kuril-Kamchatka zone

The characteristics of the distributions of the time differences occurrence of the consecutive earthquakes in the Kuril-Kamchatka zone, including the Hokkaido Island area, have been investigated in various magnitude ranges. For the purpose of the analysis, we used the data from the regional and world earthquake catalogs for the last 20 years. As a result of this analysis, a new intermediate-term precursor effect has been found: a quiescence period observed prior to the strongest earthquakes. This precursor manifests itself in the form of the long-term (2–6 months) absence of events with M ≥ 5.5 within the territory. For the predictive purposes, it was proposed to replace the quiescence period by such a more stable parameter as the sum of the three longest intervals between the earthquakes with M ≥ 5.0 in a sliding time window. The prognostic informativeness of this parameter has been assessed.     

Process Analysis of In-situ Strain during the Ms8.0 Wenchuan Earthquake—Data from the Stress Monitoring Station at Shandan

Abstract: There were huge life and property losses during the Ms8.0 Wenchuan earthquake on May 12, 2008. Strain fluctuation curves were completely recorded at stress observatory stations in the Qinghai-Tibet plateau and its surroundings in the process of the earthquake. This paper introduces the geological background of the Wenchuan earthquake and the profile of in-situ stress monitoring stations. In particular, data of 174 earthquakes (Ms4.0-Ms8.5) were processed and analyzed with various methods, which were recorded at the Shandan station from August 2007 to December 2008. The results were compared with other seismic data, and further analyses were done for the recoded strain seismic waves, co-seismic strain stepovers, pre-earthquake strain valleys, Earth’s free oscillations before and after the earthquake and their physical implications. During the Wenchuan earthquake, the strainmeter recorded a huge extensional strain of 70 seconds, which shows that the Wenchuan earthquake is a rupture process predominated by thrusting. Significant precursory strain anomalies were detected 48 hours, 30 hours, 8 hours and 37 minutes before the earthquake. The anomalies are very high and their forms are very similar to that of the main shock. Similar anomalies can also be found in strain curves of other shocks greater than Ms7.0, indicating that such anomalies are prevalent before a great earthquake. In this paper, it is shown that medium aftershocks (Ms5.5-6.0) can also cause Earth’s free oscillations. Study of free oscillations is of great significance to understand the internal structure of the Earth and focal mechanisms of earthquakes and to recognize slow shocks, thus providing a scientific basis for the prevention and treatment of geological disasters and the prediction of future earthquakes.     

A statistical procedure for the analysis of seismotectonically induced hydrochemical signals: A case study from the Eastern Carpathians, Romania

Changes in the stress field of an aquifer system induced by seismotectonic activity may change the mixing ratio of groundwaters with different compositions in a well, leading to hydrochemical signals which in principle could be related to discrete earthquake events. Due to the complexity of the interactions and the multitude of involved factors the identification of such relationships is a difficult task. In this study we present an empiric statistical approach suitable to analyse if there is an interdependency between changes in the chemical composition of monitoring wells and the regional seismotectonic activity of a considered area. To allow a rigorous comparison with hydrochemistry the regional earthquake time series was aggregated into an univariate time series. This was realized by expressing each earthquake in form of a parameter “e”, taking into consideration both energetic (magnitude of a seismic event) and spatial parameters (position of epi/hypocentrum relative to the monitoring site). The earthquake and the hydrochemical time-series were synchronised aggregating the e-parameters into “earthquake activity” functions E, which takes into account the time of sampling relative to the earthquakes which occurred in the considered area. For the definition of the aggregation functions a variety of different “e” parameters were considered. The set of earthquake functions E was grouped by means of factor analysis to select a limited number of significant and representative earthquake functions E to be used further on in the relation analysis with the multivariate hydrochemical data set. From the hydrochemical data a restricted number of hydrochemical factors were extracted. Factor scores allow to represent and analyse the variation of the hydrochemical factors as a function of time. Finally, regression analysis was used to detect those hydrochemical factors which significantly correlate with the aggregated earthquake functions.This methodological approach was tested with a hydrochemical data set collected from a deep well monitored for two years in the seismically active Vrancea region, Romania. Three of the hydrochemical factors were found to correlate significantly with the considered earthquake activities. A screening with different time combinations revealed that correlations are strongest when the cumulative seismicity over several weeks was considered. The case study also showed that the character of the interdependency depends sometimes on the geometrical distribution of the earthquake foci. By using aggregated earthquake information it was possible to detect interrelationships which couldn't have been identified by analysing only relations between single geochemical signals and single earthquake events. Further on, the approach allows to determine the influence of different seismotectonic patterns on the hydrochemical composition of the sampled well. The method is suitable to be used as a decision instrument in assessing if a monitoring site is suitable or not to be included in a monitoring net within a complex earthquake prediction strategy.     

Seismic reversal pattern for the 1999 Chi-Chi, Taiwan, MW 7.6 earthquake

We in this study have calculated the standard normal deviate Z-value to investigate the variations in seismicity patterns in the Taiwan region before and after the Chi-Chi earthquake. We have found that the areas with relatively high seismicity in the eastern Taiwan became abnormally quiet before the Chi-Chi earthquake while the area in the central Taiwan with relatively low seismicity showed unusually active. Such a spatially changing pattern in seismicity strikingly demonstrates the phenomenon of “seismic reversal,” and we here thus present a complete, representative cycle of “seismic reversal” embedding in the changes of seismicity patterns before and after the Chi-Chi earthquake.     

Application of the region–time–length algorithm to study of earthquake precursors in the Thailand–Laos–Myanmar borders

In order to examine the precursory seismic quiescence of upcoming hazardous earthquakes, the seismicity data available in the vicinity of the Thailand–Laos–Myanmar borders was analyzed using the Region–Time–Length (RTL) algorithm based statistical technique. The utilized earthquake data were obtained from the International Seismological Centre. Thereafter, the homogeneity and completeness of the catalogue were improved. After performing iterative tests with different values of the \(r_{0}\) and \(t_{0}\) parameters, those of \(r_{0}\) = 120 km and \(t_{0}\) = 2 yr yielded reasonable estimates of the anomalous RTL scores, in both temporal variation and spatial distribution, of a few years prior to five out of eight strong-to-major recognized earthquakes. Statistical evaluation of both the correlation coefficient and stochastic process for the RTL were checked and revealed that the RTL score obtained here excluded artificial or random phenomena. Therefore, the prospective earthquake sources mentioned here should be recognized and effective mitigation plans should be provided.     

Paleoseismology of the Palu–Lake Hazar segment of the East Anatolian Fault Zone, Turkey

The East Anatolian Fault Zone (EAFZ) is among the most important active continental transform fault zones in the world as testified by major historical and minor instrumental seismicity. The first paleoseismological exploratory trenching study on the EAFZ was done on the Palu–Lake Hazar segment (PLHS), which is one of the six segments forming the fault zone, in order to determine its past activity and to assess its earthquake hazard.The results of trenching indicate that the latest surface rupturing earthquakes on this segment may be the Ms=7.1+ 1874 and Ms=6.7 1875 events, and there were other destructive earthquakes prior to these events. The recurrence interval for a surface rupturing large (M>7) earthquake is estimated as minimum 100±35 and maximum 360 years. Estimates for the maximum possible paleoearthquake magnitude are (Mw) 7.1–7.7 for the Palu–Lake Hazar segment based on empirical magnitude fault rupture relations.An alluvial fan dated 14,475–15,255 cal years BP as well as another similar age fan with an abandoned stream channel on it are offset in a left-lateral sense 175 and 160.5 m, respectively, indicating an average slip rate of 11 mm/year. Because 127 years have elapsed since the last surface rupturing event, this slip rate suggests that 1.4 m of left-lateral strain has accumulated along the segment, ignoring possible creep effects, folding and other inelastic deformation. A 2.5 Ma age for the start of left-lateral movement on the segment, and in turn the EAFZ, is consistent with a slip rate of 11 mm/year and a previously reported 27 km total left-lateral offset. The cumulative 5–6 mm/year vertical slip rate near Lake Hazar suggests a possible age of 148–178 ka for the lake. Our trenching results indicate also that a significant fraction of the slip across the EAFZ zone is likely to be accommodated seismically. The present seismic quiescence compared with the past activity (paleoseismic and historic) indicate that the EAFZ may be “locked” and accumulating elastic strain energy but could move in the near future.     

Temporal behavior of the background seismicity rate in central Japan, 1998 to mid-2003

We studied the temporal behavior of the background shallow seismicity rate in 700 circular areas across inland Japan. To search for and test the significance of the possible rate changes in background seismicity, we developed an efficient computational method that applies the space–time ETAS model proposed by Ogata in 1998 to the areas. Also, we conducted Monte Carlo tests using a simulated catalog to validate the model we applied. Our first finding was that the activation anomalies were found so frequently that the constant background seismicity hypothesis may not be appropriate and/or the triggered event model with constraints on the parameters may not adequately describe the observed seismicity. However, quiescence occasionally occurs merely by chance. Another outcome of our study was that we could automatically find several anomalous background seismicity rate changes associated with the occurrence of large earthquakes. Very significant seismic activation was found before the M6.1 Mt. Iwate earthquake of 1998. Also, possible seismic quiescence was found in an area 150 km southwest of the focal region of the M7.3 Western Tottori earthquake of 2000. The seismicity rate in the area recovered after the mainshock.     

Earthquake generation in Cyprus revealed by the evolving stress field

Strong earthquake occurrence (M ≥ 6.0) onshore and offshore the Cyprus Island constitutes significant seismic hazard because they occur close to populated areas. Seismicity is weak south of the Island along the Cyprean Arc and strong events are aligned along the Paphos transform fault and Larnaka thrust fault zone that were already known and the Lemessos thrust fault zone that defined in the present study. By combining the past history of strong (M ≥ 6.0) events and the long-term tectonic loading on these major fault zones, the evolution of the stress field from 1896 until the present is derived. Although uncertainties exist in the location, magnitude and fault geometries of the early earthquakes included in our stress evolutionary model, the resulting stress field provides an explanation of later earthquake triggering. It was evidenced that the locations of all the strong events were preceded by a static stress change that encouraged failure. The current state of the evolved stress field may provide evidence for the future seismic hazard. Areas of positive static stress changes were identified in the southwestern offshore area that can be considered as possible sites of future seismic activity.     

Additional evidence on some relationship between Seismic Electric Signals (SES) and earthquake focal mechanism

Investigating the period 1983–1994 for western Greece, a possible correlation between the selectivity characteristics of the SES (seismic electric signals of the VAN method) and earthquake parameters has been reported by Uyeda et al. [Uyeda, S., Al-Damegh, K.S., Dologlou, E., Nagao, T., 1999. Some relationship between VAN seismic electric signals (SES) and earthquake parameters, Tectonophysics, 304, 41–55.]. They found that the earthquake source mechanism changed from largely strike-slip type to thrust type at the end of 1987, and this coincided with a shift in the SES sensitive site from Pirgos (PIR) to Ioannina (IOA) VAN station. Here, we report the results for the period January 1, 2002–July 25, 2004, during which the SES sensitive site of PIR became again active, after a 10-year period of “quiescence”. This activation was followed by strike slip earthquakes (on August 14, 2003 and March 17, 2004 with magnitude 6.4 and 6.5, respectively) in the Hellenic arc, which provides additional evidence on the correlation reported by Uyeda et al. The SES activities recorded at PIR have been discriminated from “artificial” noise by employing the natural time-domain analysis introduced recently.     

Evaluation of a seismic quiescence pattern in southeastern sicily

Southeastern Sicily experienced a very peculiar seismic activity in historic times, with a long series of ruinous earthquakes. A last large event, with magnitude probably in excess of 7.5, occurred on Jan., 11, 1693, totally destroying the city of Catania and killing 60,000 people. Only a few moderate events were reported since then, and a seismic gap issue has been proposed on this basis. A close scrutiny of the available data further shows that all significant seismic activity ceased after year 1850, suggesting one of the largest quiescence patterns ever encountered. This is examined together with the complex tectonic setting of the region, characterized by a wrenching mechanism with most significant seismicity located in its northern graben structure. An attempt to ascertain the imminence and the size of a future earthquake through commonly accepted empirical relations based on size and duration of the quiescence pattern did not provide any feasible result. A precision levelling survey which we recently completed yielded a relative subsidence of ~ 3 mm/yr, consistent with an aseismic slip on the northern graben structure at a rate of ~ 15 mm/yr. Comparing these results with sedimentological and tidal data suggests that the area is undergoing an accelerated deformation process; this issue is further supported by Rikitake's ultimate strain statistics. If the imminence of a damaging (M = 5.4) event is strongly favoured by Weibull statistics applied to the time series of occurrence of large events, the accumulated strain does not appear sufficient for a large earthquake (M 7.0). Within the limits of reliability of present semi-empirical approaches we conclude that the available evidence is consistent with the occurrence of a moderate-to-large (M 6.0) event in the near future. Several questions regarding the application of simple models to real (and complex) tectonic settings remain nevertheless unanswered.     

A Systematic Test on Precursory Seismic Quiescence in Armenia

A systematic test on seismic quiescence occurring before largeearthquakes is conducted. For a fixed geographical location, the degree ofclustering in space and time is analysed and the results are testedagainst randomized earthquake catalogs.A gridding technique allows to investigate the entirespatial volume covered by a certain earthquake catalog. The result is a significance K(x,t) for seismic quiescence as a function ofspace and time. A point (x,t) is considered as quiet, if K(x,t) exceeds a threshold value K⁽⁹⁹⁾ such that the nullhypothesis is rejected with a probability of p 99%. Becauseearthquake clusters, like aftershocks and swarm events, generateerroneous quiescence, declustered catalogs are also investigated andthe influence of the clusters is discussed.Applying this method to an earthquake catalog from Armenia,several cases of seismic quiescence before mainshocks are obtained.These quiescence periods occur in the originaldata as well as in the declustered data. Using alarm conditions, itis found that quiescence periods and mainshocks are correlated`better-than-chance'. Thus, the results support the claim thatseismic quiescence makes a contribution to the improvement of seismichazard assessment.     

New observational information on the precursory accelerating and decelerating strain energy release

Recent reliable data are used to study the behavior of seismic activity before 46 strong shallow earthquakes (M ≥ 6.0), which correspond to five complete samples of mainshocks. These samples include 6 mainshocks (M = 6.0–7.1) that occurred in western Mediterranean since 1980, 17 mainshocks (M = 6.0–7.2) which occurred in the Aegean (Greece and surrounding area) since 1980, 5 mainshocks (M = 6.4–7.5) that occurred in Anatolia since 1980, 12 mainshocks (M = 6.0–7.3) that occurred in California since 1980 and 6 mainshocks (M = 7.0–8.3) that occurred in Japan since 1990. In all 46 cases, a similar precursory seismicity pattern is observed. Specifically, it is observed that accelerating Benioff strain (square root of seismic energy) release caused by preshocks occurs in a broad circular region (critical region), with a radius about eight times larger than the fault length of the mainshock, in agreement with results obtained by various research groups during the last two decades. However, in a much smaller circular region (seismogenic region), with a radius about four times the fault length, the corresponding preshock strain decelerates with the time to the mainshock. The time variation of the strain follows in both cases a power law but the exponent power is smaller than unit (m ¯ = 0.3) in the case of the accelerating preshock strain and larger than unit (m ¯ = 3.0) in the case of the decelerating preshock strain. Predictive properties of this “Decelerating In–Accelerating Out Strain” model are expressed by empirical relations. The possibility of using this model for intermediate-term earthquake prediction is discussed and the relative model uncertainties are estimated.     

Seismic potential of Southern Italy

To improve estimates of the long-term average seismic potential of the slowly straining South Central Mediterranean plate boundary zone, we integrate constraints on tectonic style and deformation rates from geodetic and geologic data with the traditional constraints from seismicity catalogs. We express seismic potential (long-term average earthquake recurrence rates as a function of magnitude) in the form of truncated Gutenberg–Richter distributions for seven seismotectonic source zones. Seismic coupling seems to be large or even complete in most zones. An exception is the southern Tyrrhenian thrust zone, where most of the African–European convergence is accommodated. Here aseismic deformation is estimated to range from at least 25% along the western part to almost 100% aseismic slip around the Aeolian Islands. Even so, seismic potential of this zone has previously been significantly underestimated, due to the low levels of recorded past seismicity. By contrast, the series of 19 M6–7 earthquakes that hit Calabria in the 18th and 19th century released tectonic strain rates accumulated over time spans up to several times the catalog duration, and seismic potential is revised downward. The southern Tyrrhenian thrust zone and the extensional Calabrian faults, as well as the northeastern Sicilian transtensional zone between them (which includes the Messina Straits, where a destructive M7 event occurred in 1908), all have a similar seismic potential with minimum recurrence times of M ≥ 6.5 of 150–220 years. This potential is lower than that of the Southern Apennines (M ≥ 6.5 recurring every 60 to 140 years), but higher than that of southeastern Sicily (minimum M ≥ 6.5 recurrence times of 400 years). The high seismicity levels recorded in southeastern Sicily indicate some clustering and are most compatible with a tectonic scenario where the Ionian deforms internally, and motions at the Calabrian Trench are small. The estimated seismic potential for the Calabrian Trench and Central and Western Sicily are the lowest (minimum M ≥ 6.5 recurrence times of 550–800 years). Most zones are probably capable of generating earthquakes up to magnitudes 7–7.5, with the exception of Central and Western Sicily where maximum events sizes most likely do not exceed 7.     

Usage of an early warning and information system Web-site for real-time seismicity in Iceland

Iceland has been subjected to destructive earthquakes and volcanic eruptions throughout history. Such events are often preceded by changes in earthquake activity over varying timescales. Although most seismicity is confined to micro-earthquakes, large earthquakes have occurred within populated regions. Following the most recent hazardous earthquakes in 2000, the Icelandic Meteorological Office (IMO) developed an early warning and information system (EWIS) Web-site for viewing near-real-time seismicity in Iceland. Here we assess Web-site usage data in relation to earthquake activity, as recorded by the South Iceland Lowland (SIL) seismic network. Between March 2005 and May 2006 the SIL seismic network recorded 12,583 earthquakes. During this period, the EWIS Web-site logged a daily median of 91 visits. The largest onshore event (M _L 4.2) struck 20 km from Reykjavík on 06 March 2006 and was followed by an immediate, upsurge in usage resulting in a total of 1,173 unique visits to the Web-site. The greatest cluster of large (≥M _L 3) events occurred 300 km offshore from Reykjavík in May 2005. Within this swarm, 9 earthquakes ≥M _L 3 were detected on 11 May 2005, resulting in the release of a media bulletin by IMO. During the swarm, and following the media bulletin, the EWIS Web-site logged 1,234 unique visits gradually throughout the day. In summary, the data reveal a spatial and temporal relationship between Web-site usage and earthquake activity. The EWIS Web-site is accessed immediately after the occurrence of a local earthquake, whereas distant, unfelt earthquakes generate gradual interest prompted by media bulletins and, possibly, other contributing factors. We conclude that the Internet is a useful tool for displaying seismic information in near-real-time, which has the capacity to help increase public awareness of natural hazards.     

Possibility of a major earthquake in the tokai district, Japan and its pre-estimated seismotectonic effects

Crustal deformations, tsunamis and seismic intensity are pre-estimated for a large hypothetical earthquake, which it is feared may occur in the Tokai district along the Nankai trough. The long-term seismic quiescence since 1854, as well as the high rate of the present crustal movements in the district, form the principal evidence for the risk of the approaching catastrophe. The location and the mode of faulting in this earthquake are hypothesized in reference to the source mechanisms of the recent and historical earthquakes there. The fault parameters thus assumed are as follows: dip direction: N30°W; dip angle: 25°; fault dimension : 100 km × 70 km; dislocation: 4 m (reverse dip-slip: 3.8 m; right-lateral strike-slip: 1.3 m). The following are the principal conclusions: (a) the eastern part of the epicentral region including the Point Omaezaki will rise up about 100 cm, whereas the western part covering Ise and Mikawa bays will subside about 10–30 cm; (b) the coast extending from Omaezaki to the Shima peninsula will receive tsunami waves as high as 3 m in maximum, which may be locally amplified by the factor 2 or more on the rias coast along the Shima peninsula; (c) the Tokai coastal region with thick alluvium layers may suffer seismic damages as severe as those experienced in the 1854 Ansei I earthquake.     

Prediction of strong earthquake within the southwestern shelf of Sakhalin Island and its realization during the August 2, 2007, Nevelsk earthquake

Materials of the long- and short-term predictions of the destructive earthquake with the magnitude M _LH = 6.6 ± 0.6 within the southwestern shelf of Sakhalin Island are described. The long-term prediction was issued in December 2005 and was affirmed by the Russian Council of Experts on Earthquake Forecasting and Seismic Hazard Assessment in August 2006. The August 17(18), 2006, Gornozavodsk earthquake with a magnitude of M _w = 5.6 was the beginning of the realization of this prediction. Six days after its occurrence, the short-term prediction of a much more serious seismic event in the alarm region was prepared. One year later, the prediction of the August 2, 2007, Nevelsk earthquake with a magnitude of M _w = 6.2 (M _LH = 6.2) proved to be correct.     

Statistical relations between source parameters for low and intermediate magnitudes (Friuli, 1976)

A data set of three-component short-period digital seismograms recorded in Friuli after the strong earthquake of 6th May 1976, allowed the local magnitude M_L and the seismic moment M₀ to be estimated in the range 0 < M_L < 2. The data set including the same parameters for the higher-magnitude Friuli events (M_L 5) shows two different slopes for the relation Log M₀ = CM_L + d for the two different ranges of M_L. One finds C ~ 1.0 (for 0 < M_L < 2) and C ~1.5 (for5 M_L 6.2), respectively.This implies that apparent stress release increases at low magnitudes, while it appears to be comparatively independent of the magnitude and to have an average value of about 100 bar for higher-magnitude earthquakes. Conversely, the fault dimensions do not appear to be magnitude-dependent for M_L < 2; for higher-magnitude events the linear fault dimensions range from about 1 km at M_L ~ 5 to about 12 km for the strong earthquake of 6th May 1976 (M_L = 6.2).