Quiescence Anomalies Research Articles

Statistical seismic quiescence evaluation in the Gulf of Aqaba source zone

The seismicity rate variations prior to the 1993 and 1995 Gulf of Aqaba mainshocks have been investigated using statistical analysis, focusing on the precursory phenomena of the seismic activity progression. A statistical estimation of the seismic quiescence was applied to detect a significant reduction in the rate of seismic activity, using the normal standard deviate Z-test. The catalogue data both from the Egyptian National Seismological Network (ENSN) and the International Seismological Center (ISC) which cover the interval from 1983 to 2003 was used in this analysis. For our computations, only events with completeness magnitude ≥3.5 in the source area of the two Gulf of Aqaba mainshocks were considered. We investigated seismicity rate variations as a function of time and space in terms of the Z value preceding the 1993 and 1995 Gulf of Aqaba mainshocks respectively, using a declustered catalogue. Mapping the spatial changes of the seismicity over the entire period of catalogue manifests clear seismic quiescence anomalies related to the epicenters of the 1993 and 1995 Gulf of Aqaba mainshocks. The long-term average and the corresponding cumulative number of events versus time plot for anomalous areas have been constructed as a way of determining the starting time of the seismic quiescence and its duration for both mainshocks. The plot of the 1993 mainshock shows a decrease in the seismicity rate with a maximum Z value level (Zmax) equal to 5.4 with an observed seismic quiescence starting two years before its occurrence while the 1995 mainshock plot gives a Zmax = 4.5 with 1.3 seismic quiescence duration before it. The estimated periods of quiescence are consistent with the duration of the second type seismic gap derived from the detailed investigations of the spatio-temporal variations of the seismicity pattern which precedes the 1993 and 1995 Gulf of Aqaba main earthquakes. Earthquakes of magnitudes smaller than those which have shown the second type of seismic gap reflect pattern similarities of the activity before both 1993 and 1995 earthquakes. Both mainshocks are preceded by anomalous patterns of activity, which include both swarm type activity and short-term seismic gap, respectively.

Space-time assessing of the earthquake potential in recent years in the Eastern Anatolia region of Turkey

In this study, a statistical analysis is carried out to reveal the current earthquake potential in the Eastern Anatolia region of Turkey. For this purpose, a space-time assessing based on the recent changes in Gutenberg-Richter b-value, seismic quiescence Z-value, cumulative moment, annual probability and recurrence time of earthquakes is achieved. Temporal variation of b-value indicates that average b-value decreased from 1.25±0.02 between 2002 and 2010 to 1.08±0.02 between 2011 and 2015. A clear quiescence in the seismic moment in recent years may be a clue foretelling the occurrence of an intense event. Annual probability of the earthquakes between 3.5 and 4.0 magnitude levels exhibits a value equal to and larger than 30. Recurrence time of the earthquakes has a value between 2.0 and 3.0 years for magnitude level of 6.0, and a value between 6.0 and 7.0 years for magnitude level of 7.0. A significant decrease in b-value and clear quiescence anomalies in Z-value at the beginning of 2015 are observed in the several same regions. These areas cover the Central Anatolia Fault Zone, Malatya and Ovacık faults, the southeastern part of the Eastern Anatolia Fault Zone, and the junction of the Eastern Anatolia and Dead Sea Fault Zones. In these anomaly regions, a decreasing trend in b-value may be an indicator of stress increases and an increasing trend in Z-value may show the quiescence areas before the next earthquake occurrences. As a remarkable fact, these areas in the Eastern Anatolia region may be interpreted as one of the most likely locations for the major earthquakes in the future.

Spatiotemporal Variations of Seismic Quiescence prior to the 2011 M 9.0 Tohoku Earthquake Revealed by an Improved Region-Time-Length Algorithm

The region–time–length (RTL) algorithm, which takes into account the epicenter, time, and magnitude of earthquakes, is an effective technique in detecting seismic quiescence. In order to reduce the possible ambiguity due to the selection of model parameters in the RTL algorithm, we propose an improved technique of searching for the optimal model parameters. As a case study, we investigated the spatiotemporal variations of seismic quiescence prior to the M 9.0 Tohoku earthquake by applying the improved RTL algorithm to the declustered and complete earthquake catalog of the Japan Meteorological Agency (JMA). The temporal variation of the RTL parameter at the epicenter indicated that a seismic quiescence occurred between 2006 and 2008. The spatial distribution showed the consistent time window of the appearance of anomalous quiescence near the epicenter. The significance of the seismic quiescence anomaly was supported by the close investigations of model parameter effects and the stochastic test based on randomized earthquake catalogs.

Precursory seismic quiescence before theMw= 8.3 Tokachi-oki, Japan, earthquake on 26 September 2003 revealed by a re-examined earthquake catalog

[1] The 2003 Tokachi-oki earthquake (Mw = 8.3) occurred on 26 September 2003 off the pacific coast of Hokkaido, Japan. In the present study, an earthquake catalog is used that lists 2,000 earthquakes with M ≥ 3.3. All of the earthquake waveforms were recorded by the Institute of Seismology and Volcanology, Hokkaido University. In the present study, these waveforms are manually re-examined, and hypocenters and magnitudes are re-calculated. A detailed analysis of the re-determined earthquake catalog between 1994 and 2003 using a gridding technique (ZMAP) shows that the 2003 Tokachi-oki earthquake is preceded by two neighboring seismic quiescence anomalies that start around the beginning of 1999, and last about 5 years, until the main shock occurs. These quiescence anomalies are located around the asperity ruptured by the main shock, and the Z-values are +3.9 and +4.0 for a time window of Tw = 4 years, using a sample size of N = 100 earthquakes. The detected seismic quiescences can be interpreted as being caused by a decrease of 50% in the stressing rate based on Dieterich's theory. It is proposed that a quasi-static pre-slip occurs at the northeastern edge of the asperity ruptured by the main shock, and lasts for five years until the main shock occurs. By calculating the change in the Coulomb failure stress (ΔCFS), it is found that negative ΔCFS areas are consistent with the two quiescence anomalies, and a positive ΔCFS area corresponds to the hypocenter of the main shock, indicating that the quasi-static pre-slip model is a plausible one.

A long-term seismic quiescence started 23 years before the 2011 off the Pacific coast of Tohoku Earthquake (M = 9.0)

I find that a long-term seismic quiescence started 23.4 years before the 2011 off the Pacific coast of Tohoku Earthquake (M = 9.0). An earthquake catalog compiled by the Japan Meteorological Agency (JMA) is analyzed. The catalog includes 5770 earthquakes shallower than 60 km with M ≥ 4.5. A detailed analysis of the earthquake catalog between 1965 and 2010 using the gridding technique ZMAP shows that the 2011 Tohoku earthquake is preceded by a seismic quiescence anomaly that began in November 1987. The quiescence-anomaly area is located around the deeper edge of the asperity ruptured by the main shock, and the Z-value is +4.9 for a time window of Tw = 15 years, using a sample size of N = 150 earthquakes. It is suggested that a seismic quiescence which starts more than 20 years before the main shock is common to giant earthquakes (M ~ 9.0) in subduction zones.

Seismicity anomalies prior to 8 June 2008, <i>M<sub>w</sub></i>=6.4 earthquake in Western Greece

Abstract. The epicentral area of the Mw=6.4, 8 June 2008 main shock in northwestern Peloponesus, Western Greece, had been forecasted as a candidate for the occurrence of a strong earthquake by independent scientific investigations. This study concerns the seismicity of a large area surrounding the epicenter of the main shock using the seismological data from the monthly bulletins of the Institute of Geodynamics of the National Observatory of Athens. This data set is the most detailed earthquake catalog available for anomalous seismicity pattern investigations in Greece. The results indicate a decrease in seismicity rate seven years prior to the 8 June main shock which constituted a two and a half year long seismic quiescence surrounding the epicentral area. This quiescence anomaly was succeeded by a period of acceleration in seismic activity for five years approximately, until the occurrence of the main shock.

Search for reliable precursors: A case study of the seismic quiescence of the 2000 western Tottori prefecture earthquake

How to search for reliable precursors is one of the key problems of the study on earthquake forecast. In this paper a method is presented to judge whether or not a precursor is reliable. Before a potential anomaly can be nominated as a reliable precursor, it should pass or be proved by the following tests or analyses: whether or not it is an artificial anomaly, whether or not it correlates with an investigated event, whether or not it is a random anomaly. As a case study, I have investigated the reliability of seismic quiescence for an M = 7.3 earthquake, which occurred in the western region of Tottori prefecture, Japan, on 6 October 2000. The earthquake data (1975–2000) of the Japan Meteorological Agency (JMA) were used in this study. After analyzing the completeness of the catalog and removing aftershocks, a statistical method, which is called the RTL (region‐time‐length) algorithm and takes into account the information of magnitude, occurrence time and location of earthquakes, was applied to the above preprocessed earthquake data. A clear seismic quiescence anomaly was detected before the 2000 western Tottori prefecture earthquake. Close investigation indicated that the above anomaly is not due to artifacts of the selections of model parameters. After quantifying the anomaly and making statistical analysis over the whole investigated spatiotemporal domain, reasonable correlation between the above anomaly and the main shock in 2000 was recognized. The stochastic test using synthetic randomized catalogs showed the above anomaly is unlikely due to chance. Thus the above seismic quiescence anomaly before the 2000 main shock should be a significant and reliable precursor and the research method revealed in this study can enhance the reliability of precursors.

A Method of Evaluating Reliability of Earthquake Precursors

A method is presented to evaluate the reliability of earthquake precursors. The method that can improve the significance and reliability of earthquake precursors includes: extracting precursory information from observation data, distinguishing various artificial effects, analyzing the correlation in time and space domains between the detected precursor and the investigated earthquake, and making stochastic test. The above evaluation method can be also applied to other non-earthquake precursors. As a case study, I investigated the seismicity changes before the M=7.3 earthquake in the western region of Tottori prefecture, Japan on October 6, 2000, using the RTL (Region-Time-Length) algorithm. The earthquake data (1975~2000) of the Japan Meteorological Agency (JMA) were chosen in this study. A clear seismic quiescence anomaly was detected before the 2000 western Tottori prefecture earthquake. The close investigation using the above evaluation method indicated that the above quiescence anomaly before the mainshock is significant and reliable.

Seismic quiescence precursors to two M7 earthquakes on Sakhalin Island, measured by two methods

Two large earthquakes occurred during the last decade on Sakhalin Island, the M w 7.6 Neftegorskoe earthquake of 27 May 1995 and the M w 6.8 Uglegorskoe earthquake of 4 August 2000, in the north and south of the island, respectively. Only about five seismograph stations record earthquakes along the 1000 km, mostly strike-slip plate boundary that transects the island from north to south. In spite of that, it was possible to investigate seismicity patterns of the last two to three decades quantitatively. We found that in, and surrounding, their source volumes, both of these main shocks were preceded by periods of pronounced seismic quiescence, which lasted 2.5 ± 0.5 years. The distances to which the production of earthquakes was reduced reached several hundred kilometers. The probability that these periods of anomalously low seismicity occurred by chance is estimated to be about 1% to 2%. These conclusions were reached independently by the application of two methods, which are based on different approaches. The RTL-algorithm measures the level of seismic activity in moving time windows by counting the number of earthquakes, weighted by their size, and inversely weighted by their distance, in time and space from the point of observation. The Z-mapping approach measures the difference of the seismicity rate, within moving time windows, to the background rate by the standard deviate Z. This generates an array of comparisons that cover all of the available time and space, and that can be searched for all anomalous departures from the normal seismicity rate. The RTL-analysis was based on the original catalog with K-classes measuring the earthquake sizes; the Z-mapping was based on the catalog with Ktransformed into magnitudes. The RTL-analysis started with data from 1980, the Z-mapping technique used the data from 1974 on. In both methods, cylindrical volumes, centered at the respective epicenters, were sampled. The Z-mapping technique additionally investigated the seismicity in about 1000 volumes centered at the nodes of a randomly placed regular grid with node spacing of 20 km. The fact that the two methods yield almost identical results strongly suggests that the observed precursory quiescence anomalies are robust and real. If the seismicity on Sakhalin Island is monitored at a completeness-level an order of magnitude below the present one, then it may be possible to detect future episodes of quiescence in real time.

Seismicity Pattern Changes Prior to Large Earthquakes An Approach of the RTL Algorithm

A statistical method, which is called the Region-Time-Length (RTL) algorithm and takes into account information such as magnitude, occurrence time and place of earthquakes, was applied to earthquake data to investigate seismicity pattern changes prior to large earthquakes. Based on the RTL algorithm and some newly developed parameters such as the Qparameter (average of the RTL values over some time window) and S-parameter (an index of seismic activation), I quantified both the temporal and spatial characteristics of seismicity pattern changes in various tectonic regions. The results indicated that seismic quiescence anomalies generally started a few years before the occurrence of the earthquakes and lasted from 1 to 2.5 years. The duration of the subsequent stage of seismic activation generally lasted several months. The linear dimension of the quiescence zone reached a few hundred kilometers (several times the rupture dimension of the mainshock), while the activation zone was generally in order of several tens of kilometers (comparable to the rupture dimension). An earthquake is most likely to occur once the relevant source region has passed through the quiescence and activation stages. Close investigation of possible artifacts due to the selection of model parameters and the changes of seismological networks are important in identifying real seismicity changes from man-made ones. Further stochastic testing using random earthquake catalogs was also done and it supports that the anomalies revealed in my works are significant. Besides studying on seismicity changes before large earthquakes, I also performed the first test of the above statistical method for investigating seismicity changes of earthquake swarms. It indicated that an increased RTL parameter would be a new potentially useful index for the risk alarm of earthquake swarms.

Phases of earthquake's preparation and by chance test of seismic quiescence anomaly

A recently proposed predictive parameter of region–time–length (RTL) was tested using the seismological catalogues of Kamchatka (Russia) and Japan Meteorological Agency (JMA). We introduced the RTL algorithm and summarized the main results of the seismicity variations before the strong earthquakes in Kamchatka. We also investigated the seismicity pattern changes before the disastrous 1995 Kobe earthquake with M=7.2. It was found that a significant seismic quiescence had taken place one year before this event and it had been followed by seven months foreshock activation period. The detailed investigation indicated that the number of clusters of weak earthquakes increased comparatively to the background events during the stages of foreshock activation in the epicenter area of the Kobe earthquake. The by chance test based on randomized earthquake catalogues indicated that the quiescence anomaly in 1994 is a significant one. The results strengthen the physical basis of intermediate-term prediction for large earthquakes.

The examples of earthquake preparation in Kamchatka and Japan

A recently proposed predictive parameter RTL was tested using seismological catalogues of Kamchatka and Japan. We analysed three earthquakes in Kamchatka with M>7 and disastrous Hyogo-ken Nanbu (Kobe) earthquake in Japan with M=7.2. It was found that a seismic quiescence had taken place during a period of three years before these earthquakes and been followed by foreshock activation. The dimensions of the quiescence areas reached the first hundreds of kilometers, whereas the epicenters of strong events were located on the edge of quiescence anomalies. The areas of foreshock activation were about 50–100 km, and strong events occurred in the region of maximum anomalies. The number of clusters increased comparatively to the background events during the stages of foreshock activation in epicenter areas of strong earthquakes. The results strengthen the physical basis of earthquake prediction.

Mapping Seismic Quiescence in California

The decrease of microearthquake activity is a controversial possible precursor for mainshock prediction. The calculation of seismic quiescence is a convenient tool to quantify this decrease. However, the actual size of an anomaly significantly depends on the calculation parameters. The necessary averaging over space and time acts as 3D lowpass filter, the sliding statistics windows introduce highpass behavior, and the magnitude threshold shall ensure catalog completeness but also biases the data selection. To argue on any claimed anomaly, the only conclusive solution would be an exhaustive search over all parameters, over an area significantly larger than just the epicenter region, and against a test of the null hypothesis. QMAP is a new mapping approach to process full catalogs, that is, some twenty years of data for areas of one million km 2 , with one fixed set of parameters. It utilizes fractil statistics to avoid errors otherwise induced by clusters of aftershock series. QMAP features two different scaling modes, one to display absolute decrease and a second for weighted anomalies by erosion filtering. Applied to California, one can recognize quiescence anomalies of regional scale prior to the Coalinga and Loma Prieta earthquakes. But there are more and greater anomalies distributed all along the San Andreas fault system. Without further criteria to understand these patterns, the additional anomalies degrade seismic quiescence to an observation with high false alarm rate for earthquake prediction.

Seismic quiescence before the M 7, 1988, Spitak earthquake, Armenia

A detailed analysis of the 35yr of seismicity between 1962 and 1997 using a gridding technique shows that the M7, Spitak earthquake of 1988 December 7 was preceded by a quiescence anomaly that started at approximately 1984±0.5, and lasted about 5±0.5yr, up to the main shock. This quiescence anomaly had a radius of about 20±3km, estimated from circular areas with 75 per cent rate decrease, centred at the point of maximum significance of the anomaly. The quiescence was clearly present in the aftershock volume during the 5yr before the 1988 main shock, but its statistically strongest expression was located 30km NW of the epicentre. This anomaly fulfills the association rules between precursory quiescence anomalies and main shocks, even for a tight definition, and is therefore proposed as a case of precursory quiescence. The largest value of the standard deviate Z, found by random selection of samples by gridding, was Z=14 for a time window of Tw=3yr, using a sample size of N=300 events. This makes this anomaly the strongest observed so far, and it is the first documented in an environment of continental collision. There are no false alarms exceeding in significance the precursor. The Armenian earthquake catalogue used for this study had 4600 earthquakes with M≥Mmin=2.2 in the area bounded by 39.5° to 42°N/42.5° to 47°E. From the point of view of homogeneous reporting this is the best catalogue we have analysed so far. The limits of the data used and the density of the grid are dictated by the data, and have no influence on the results. The choice of free parameters does not influence the results significantly within the following limits: 100≤N≤500, 2≤Tw≤7, 2.2≤Mmin≤2.8.

Two current seismic quiescences within 40 km of Tokyo

Two highly significant seismic quiescence anomalies that started in 1993.0 exist currently within about 40 km of Tokyo. They are located in central Saitama province at 35.99°N/139.56°E and in Tokyo Bay, south of Funabashi, at 35.59°N/l39.94°E. The radii of the anomalous volumes are approximately 15 km, centred at about 25 and 20 km depth, respectively. The seismicity rate during the quiescence was less than 10 per cent background rate in both cases. The significance of these quiescences, measured by the standard deviate Z-test, comparing the rate of the last three years to the backgound rate, is higher than for any other rate changes present in the catalogue (Zmax=7.5 and 7.2 respectively). However, episodes of highly significant quiescences, which could have been interpreted as precursors but were not followed by main shocks, also exist in the data set. A third seismic quiescence was found to be in progress since 1993.3 ± 0.5 in NW Saitama province at 36.18°N/139.22°E. It has a radius of 14 km and is less significant than the other two anomalies (Zmax=6.8). In a polygon that covers the areas of all three quiescences defined above, the seismicity rate since 1994 is only 30 per cent of the previous background rate (i.e. 100 earthquakes that would normally have occurred did not). This analysis is based on the earthquake catalogue of the National Institute for Earth Science and Disaster Prevention for the years 1980–1996.2 and Mmin=1.0, as well as for the years 1986.2–1996.2 with Mmin=0.1. The anomalies are equally clear in the clustered and in the full catalogues. To avoid contamination by explosions, only nighttime data (19:00 to 7:00 hours local time) were used. The frequency-magnitude distributions in the quiescent volumes at central Saitama. NW Saitama and Funabashi have b-values of 1.3, 1.5 and 1.1, respectively. Based on the hypothesis that crustal volumes with high b-values may not be capable of M6 + earthquakes, these three quiescent volumes may be ruled out as source volumes for main shocks of these magnitudes. Using lilt seismic quiescence hypothesis alone, it is a reasonable interpretation to suggest that M=6.5 ± 0.5 earthquakes are expected to terminate each of the three quiescences within 1 year ± 1 year from now, with a 50 per cent probability each. The interpretation that all three quiet volumes could rupture together in a large earthquake seems less likely. However, based on the information that the quiescences are located in volumes with b-values above the average b=0.8, it can be argued that the three current quiescences are false alarms. The fact that these two lines of evidence allow opposing interpretations in these cases shows that neither hypothesis is sufficiently tested. Thus, the quiescence hypothesis is not advanced enough for issuing earthquake predictions. Nevertheless, it is important that scientific tests of prediction scenarios are formulated in real time, so we can learn how to modify our hypotheses to make them useful for prediction in the future.

Seismic quiescence before the landers (M = 7.5) and big bear (M = 6.5) 1992 earthquakes

The Landers earthquake of 28 June 1992 was preceded by a seismicity rate decrease of 75% in a volume approximately 11 by 23 by 15 km, located adjacent to and north of the epicenter. This anomaly started, for all magnitude bands, in January 1988, lasting 4.5 yr up to the mainshock. A smaller volume (7 by 14 by 15 km), to the south of the epicenter, showed a seismicity rate decrease of 75% starting in November 1989. The Big Bear earthquake of the same day was preceded by a 100% decrease of the seismicity rate, within a volume of approximately 13 by 22 by 15 km around the hypocenter. This volume contained 192 earthquakes of M ≧ 1.6 during the background period of 9 yr, but none during the 1.6 yr from December 1990 to 27 June 1992. The standard deviate z values characterizing these rate changes are very high, 6.3 and 12.3, respectively, and the anomalies are unique, not surpassed in significance by any other rate decrease in portion of the southern California earthquake catalog examined (117°06′W to 115°40′W and 33°45′N to 35°10′N). No seismicity rate change was found in the vicinity of the M = 6.1 Joshua Tree earthquake of 23 April 1992. Based on magnitude signatures and frequency-magnitude analyses, we conclude that the two quiescence anomalies could not have been due to a shift or a compression of the magnitude scale, and we interpret them as precursor anomalies. This analysis was carried out using the declustered earthquake catalog for the Landers area with corrections of 0.2 and 0.1 (August 1985 and May 1990) for two suspected magnitude shifts. With a new visualization technique we can investigate the stability, and changes, of the seismicity rate as a function of time and space. By mapping z values for rate changes at every time interval, and in volumes centered on a dense grid of points (latitude, longitude), this computer code furnishes an almost continuous sequence of smooth contour maps of the degree of rate changes. Any anomalies, artificial or real, may thus be defined accurately.

Precursory seismic quiescence

Seventeen cases of precursory seismic quiescence to mainshocks with magnitudes fromM L=4.7 toM S=8.0 are summarized. The amount of rate decrease ranges from 45% to 90%. The significance of these changes varies between 90% and 99.99%. The assumption that the background rate is approximately constant is fulfilled in most crustal volumes studied. All quiescence anomalies seem to have abrupt beginnings, and the rate during the anomalous period is fairly constant. The duration of the precursors ranges from 15 to 75 months, and it is not clear what factors determine that time. At least three successful predictions have been based on seismic quiescence. These cases have shown that mainshocks can be predicted based on quiescence, but they have also shown that the interpretation of the data in real time is difficult and nonunique. If a false alarm is defined as a period of quiescence with a significance level larger than a precursory quiescence in the same tectonic area, then we estimate, based on searches in four areas, that the false alarm rate may be on the order of 50%. Failure to predict may be expected in perhaps 50% of mainshocks, even in carefully monitored areas. Quiescence cannot be used as a precursor in tectonic environments with low seismic activity. Most characteristics of the phenomenon are still poorly defined, but data exist which probably permit at least a doubling of the presently available data on case histories.