Changes In Seismic Activity Research Articles

Lidar monitoring of an aerosol precursor of seismic activity during the total solar eclipse of March 29, 2006

A decrease in the tectonic aerosol flux during a total solar eclipse was detected for the first time using a backscattering lidar. The experiment was performed in the tunnel of the Baksan Neutrino Observatory, Russian Academy of Sciences, on March 29, 2006 (11:17:30 UT). It was found that the decrease in the aerosol release is 2 h ahead of the decrease in the amplitude of seismic vibrations. The decrease in seismic activity is interpreted as a result of lithosphere bulging at the ridge of the eclipse-induced tidal wave, which arrives at the point of observation with a delay. The increased sensitivity of aerosol release to lithosphere deformation in comparison with a seismograph and the detected advance give grounds to consider the change in the lidar signal as a precursor of the change in seismic activity.

Preparatory process reflected in seismicity-pattern change preceding the M=7 earthquakes off Miyagi prefecture, Japan

Abstract Two M=7 interplate earthquakes recently occurred off Miyagi prefecture, northeastern Japan. These belong to a new sequence of earthquake series. The occurrence of this sequence was forewarned by the government, based primarily on a statistical approach utilizing historical records. Such a process, however, does not account for the present on-going status in the assessment. During the stage when stress approaches a critical level, there is a possibility of stress redistribution caused by a quasi-static slip, which will be reflected in a temporal change in seismic activity. Delineating a spatial map of the seismic-activity rate changes in the seismogenic zone off Miyagi prefecture, we found that there was an activation of microseismicity in the rupture zones several months prior to both M=7 earthquakes. This was interpreted as evidence of a preparatory process preceding each M=7 earthquake, during which a quasi-static slip progressed, driving stress redistribution and resulting in stress concentration on asperities of these earthquakes.

Change in seismic activity in the Tokai region related to weakening and strengthening of the interplate coupling

We investigate the relationship between changes in seismicity and crustal deformations in the Tokai region. We describe how seismicity in the subducted slab increased remarkably in the fall of 2000 and decreased in the fall of 2001, while in contrast, the crust seismicity decreased in the fall of 2000 and increased in the fall of 2001. We note that the trend of horizontal displacement at GPS stations changed coincidentally and we propose interpreting the increase and decrease in seismic activities and the changes in crustal deformations in a unified way based on changes in the state of the interplate coupling, i.e., the back-slip rate was reduced in the fall of 2000 and was partially restored in the fall of 2001. We explain why reduction of the back-slip rate increases seismogenic stress in the slab and decreases stress in the crust. We also describe the substantial positive dilatation observed in the region around Mt. Fuji in the fall of 2000 and suggest that the remarkable increase of low-frequency earthquakes beneath Mt. Fuji in October 2000 may have been caused by deceleration of the converging motion of the Izu micro-plate with the Eurasian plate. The decrease of the subduction velocity of the Izu micro-plate on the Suruga Trough in late 2000 would also have contributed to weakening of the interplate coupling beneath the Tokai region, since reduction of the relative velocity between overriding and subducting plates produces the same effect on the plate interface as a diminishing back-slip rate. However, subduction of the Izu micro-plate on the Suruga Trough was accelerated in early 2003, which may have caused increases in both slab and crust seismicities in that period.

Seismic activity changes progressing simultaneously with slow-slip in the Tokai area

Anomalous movements were detected simultaneously in both the seismic and the GPS observations in the Tokai area, the central part of the Japanese islands from the late 1990s to 2000. The anomalies are of great concern since the pending risk of a large megathrust earthquake in this area has been predicted for more than 20 years. The GPS data revealed that a slow-slip on the plate interface had commenced beneath Lake Hamana, the center of which is positioned around the edge of the assumed focal zone. On the other hand, the seismic data indicated that a delicate but clear quiescence appeared over a wide area that spreads into the main focal zone. Analyses of the seismicity changes in space and time confirmed that the contrast in the seismicity rate is distinct inside the focal zone. While the integrated seismicity indicated quiescence, some locations were distinguished as activated zones, possibly indicating the appearance of asperities. The rise of the seismicity rate in a quasi-stationary manner suggests an increase in the stress rate at that location. The following hypothesis is proposed based on the simultaneously detected evidences. The slow-slip progressing beneath Lake Hamana will induce a stress shift that invades the interior of the main locked zone, which will increase the contrast of the seismicity rate, possibly reflecting inhomogeneity in the locking strength. Even in this stage, the activated zones still maintain a locked state to prevent overall breakage. Investigations of the b-value changes and of tidal dependence in seismicity that reveal the stress-concentrated state also support the hypothesis. If this is the case, the observed change in seismicity would indicate the process of stress redistribution in the locking state, which represents the preparatory process toward final breakage. Tracking such seismicity changes would yield valid information for predictions of the next Tokai earthquake.

Some premonitory phenomena of the 1995 Hyogo-Ken Nanbu (Kobe) earthquake: seismicity, b-value and fractal dimension

This paper presents a detailed and complex analyses of the spatio-temporal evolution of seismic activity between 1976 and 1998, in a broad area surrounding the epicentre of the 1995 Hyogo-ken Nanbu (Kobe) earthquake as well as in the source area. Various precursory changes in seismicity, such as quiescence followed by increased seismic activity, b-value and fractal dimension (correlation dimension, D 2) anomalies appear approximately two years before the occurrence of the major event. An increased level of background seismicity is clearly observed after the occurrence of the Kobe earthquake, particularly in the northeast extension of the source area. All these ‘anomalous’ changes in seismic activity before and after the occurrence of the 1995 Hyogo-ken Nanbu earthquake appear in a relatively large region surrounding the epicentre of the main shock as well as in the source area.

Seismicity cycles in the Mt. Vesuvius area and their relation to solar flux and the variations of the Earth's magnetic field

A local seismic network has been continuously monitoring the earthquake activity around the volcano Mt. Vesuvius for about 25 years. A total number of 7800 events have been recorded. The earthquakes have not exceeded a magnitude of 3.4. This considerable volume of data makes it possible to carry out several analyses regarding temporal changes of seismic activity in the volcano region, which are of interest for geodynamic and risk considerations. From the analyses, results of general nature can also be obtained: The present study indicates that the variations of solar activity and of the Earth's magnetic field are in close relation to earthquake activity. The latter relationship between geomagnetic field and seismicity has already been suggested by an earlier study (Duma, 1996). However, mainly observational results will be presented in this paper, rather than explanations of these phenomena.

茨城県南西部鬼怒川側の地震活動に見られる周期的変化

茨城県南西部鬼怒川側の地震活動に見られる周期的変化

The stress field variation caused by faulting and the prediction for seismic risk

The stress field caused by faulting has an effect on the stability of the neighboring faults, and the study on the fault interaction has a close relation with the prediction of seismic risk. Stress field caused by the rectangle fault in the semi-infinite elastic medium is calculated on the basis of the elastic dislocation theory. The result shows that most of the successive large earthquakes, in the southwestern part of China and North China, occurred in the increasing area of shear stressS xy and the decreasing area of normal stressS yy. The increasing of earthquake occurrence probability has a function relation with the increasing of stress. Earthquake triggering is resulted from the increasing of shear stress and the decreasing of normal stress. An activation coefficientA t of the earthquake is defined to express the change of seismic activity. The concrete risk region can be obtained through space scanning ofA t value. Finally, the fault interaction in a large scope is discussed in this paper.

Statistical forecasting of repetitious dome failures during the waning eruption of Redoubt Volcano, Alaska, February–April 1990

The waning phase of the 1989–1990 eruption of Redoubt Volcano in the Cook Inlet region of south-central Alaska comprised a quasi-regular pattern of repetitious dome growth and destruction that lasted from February 15 to late April 1990. The dome failures produced ash plumes hazardous to airline traffic. In response to this hazard, the Alaska Volcano Observatory sought to forecast these ash-producing events using two approaches. One approach built on early successes in issuing warnings before major eruptions on December 14, 1989 and January 2, 1990. These warnings were based largely on changes in seismic activity related to the occurrence of precursory swarms of long-period seismic events. The search for precursory swarms of long-period seismicity was continued through the waning phase of the eruption and led to warnings before tephra eruptions on March 23 and April 6. The observed regularity of dome failures after February 15 suggested that a statistical forecasting method based on a constant-rate failure model might also be successful. The first statistical forecast was issued on March 16 after seven events had occurred, at an average interval of 4.5 days. At this time, the interval between dome failures abruptly lengthened. Accordingly, the forecast was unsuccessful and further forecasting was suspended until the regularity of subsequent failures could be confirmed. Statistical forecasting resumed on April 12, after four dome failure episodes separated by an average of 7.8 days. One dome failure (April 15) was successfully forecast using a 70% confidence window, and a second event (April 21) was narrowly missed before the end of the activity. The cessation of dome failures after April 21 resulted in a concluding false alarm. Although forecasting success during the eruption was limited, retrospective analysis shows that early and consistent application of the statistical method using a constant-rate failure model and a 90% confidence window could have yielded five successful forecasts and two false alarms; no events would have been missed. On closer examination, the intervals between successive dome failures are not uniform but tend to increase with time. This increase attests to the continuous, slowly decreasing supply of magma to the surface vent during the waning phase of the eruption. The domes formed in a precarious position in a breach in the summit crater rim where they were susceptible to gravitational collapse. The instability of the February 15–April 21 domes relative to the earlier domes is attributed to reaming the lip of the vent by a laterally directed explosion during the major dome-destroying eruption of February 15, a process which would leave a less secure foundation for subsequent domes.

Precursory swarms of long-period events at Redoubt Volcano (1989–1990), Alaska: Their origin and use as a forecasting tool

During the eruption of Redoubt Volcano from December 1989 through April 1990, the Alaska Volcano Observatory issued advance warnings of several tephra eruptions based on changes in seismic activity related to the occurrence of precursory swarms of long-period (LP) seismic events (dominant period of about 0.5 s). The initial eruption on December 14 occurred after 23 years of quiescence and was heralded by a 23-hour swarm of LP events that ended abruptly with the eruption. After a series of vent-clearing explosions over the next few days, dome growth began on December 21. Another swarm, with LP events similar to those of the first, began on the 26th and ended in a major tephra eruption on January 2. Eruptions continued over the next two weeks and then ceased until February 15, when a large eruption initiated a long phase of repetitive dome-building and dome-destroying episodes that continued into April. Warnings were issued before the major events on December 14 and January 2, but as the eruptive sequence continued after January 2, the energy of the swarms decreased and forecasting became more difficult. A significant but less intense swarm preceded the February 15 eruption, which was not forecast. This eruption destroyed the only seismograph on the volcanic edifice and stymied forecasting until March 4, when the first of three new stations was installed within 3 km of the active vent. From March 4 to the end of the sequence on April 21, there were eight eruptions, six of which were preceded by detectable swarms of LP events. Although weak, these swarms provided the basis for warnings issued before the eruptions on March 23 and April 6. The initial swarm on December 13 had the following features: (1) short duration (23 hours); (2) a rapidly accelerating rate of seismic energy release over the first 18 hours of the swarm, followed by a decline of activity during the 5 hours preceding the eruption; (3) a magnitude range from −0.4 to 1.6; (4) nearly identical LP signatures with a dominant period near 0.5 s; (5) dilatational first motions everywhere; and (6) a stationary source location at a depth of 1.4 km beneath the crater. This occurrence of long-period events suggests a model involving the interaction of magma with groundwater in which magmatic gases, steam and water drive a fixed conduit at a stationary point throughout the swarm. The initiation of that sequence of events is analogous to the failure of a pressure-relief valve connecting a lower, supercharged magma-dominated reservoir to a shallow hydrothermal system. A three-dimensional model of a vibrating fluid-filled crack recently developed by Chouet is found to be compatible with the seismic data and yields the following parameters for the LP source: crack length, 280–380 m; crack width, 140–190 m; crack thickness, 0.05–0.20 m; crack stiffness, 100–200; sound speed of fluid, 0.8–1.3 km/s; compressional-wave speed of rock, 5.1 km/s; density ratio of fluid to rock, ≈0.4; and ratio of bulk modulus of fluid to rigidity of rock, 0.03–0.07. The fluid-filled crack is excited intermittently by an impulsive pressure drop that varies in magnitude within the range of 0.4 to 40 bar. Such disturbance appears to be consistent with a triggering mechanism associated with choked flow conditions in the crack.

Strain Event in 1985-1987 in the Tokai Region, Central Japan

Electro-optical distance measurements (EDM) for the baseline networks of Sangane and Awagatake in the Tokai region began in 1978. The rangings of two baselines, Sangane-Zaoh and Awagatake-Magami, have been repeated four times a year on average. The directions of the baselines are parallel to that of the convergence of the Philippine Sea plate. In order to minimize the EDM error, distance measurements are carried out at sunset. The results of distance measurements for the 13 years are summarized as follows: (1) The accuracy of EDM around sunset is generally better than 5×10-7. (2) A NW-SE contraction and a NE-SW extension are common to both networks. A shear strain at a rate of 1 to 3×10-7/year is consistent with the geodetic surveys carried out in the last 100 years. (3) Both baselines, 80 km apart from each other, indicated a similar strain event, suggesting the accelerated contraction amounting to 1 to 2×10-6 in 1985-1987. The contraction is concordant with the change in geodetic tilt near Zaoh and the change in seismic activity in the Ise Bay area. The result obtained from observations might be evidence of the remarkable strain event that occurred in the Tokai region in 1985-1987.

The Baltic Shield earthquakes

The Baltic Shield area is a low-seismicity area with few seismic events exceeding M L = 4. Since 1979 dense regional seismic networks have been in operation and have allowed accurate location, including focal depths and fault plane solutions, of more than 200 earthquakes in the range M L = 0.6–4.5. The location of the southern Sweden seismicity shows that a long part of the Protogine zone, the border between the younger western and older eastern crustal rocks, marks a change in seismic activity. The focal depths of the earthquakes indicate three seismically different crustal layers: upper crust (0–13 km in northern Sweden, 0–18 km in southern Sweden), middle crust down to 35 km, and the seismically quiet lower crust. The Gutenberg b-value is smaller for the middle crust than for the upper crust. The fault plane solutions show that strike-slip is dominant. Along the Tornquist Line (the southwestern border of the Baltic Shield area) normal faulting occurs. In northern Sweden reverse faulting is more common than normal faulting. The stresses released by the earthquakes show a remarkable consistency considering the small sizes of the earthquakes. The data are consistent with a N60W direction of the regional principal compression, supporting the view that the Baltic Shield seismicity is related to plate tectonic processes rather than to uplift. The spatial distribution of the Baltic Shield seismicity is consistent with a model in which the earthquakes are considered as sudden breakdowns of asperities of normally, stably sliding faults. Aseismic sliding is estimated to be more than 20,000 times more extensive than seismic sliding. The rate at which southern Sweden presently deforms horizontally is estimated to be ⩾ 1 mm/yr.

Recent seismic activity in the Tokai (Japan) region where a large earthquake is expected in the near future

A large shallow earthquake is expected to occur in the Tokai region of central Honshu, Japan in the near future, and various intensive observations are being carried out in the region with the goal of predicting its occurrence. So far there have been no reports of any changes which could be regarded as precursors of the expected large earthquake in this area, but the following noteworthy changes have been observed in the seismic activity over the past decade. 1. (1)There has been a spate of large earthquakes of M 6.5–7 in a wide area surrounding the focal region of the expected Tokai earthquake. 2. (2) There has been a significant decline in seismic activity in the area along the Suruga Trough and the Nankai Trough, which includes the focal region of the anticipated earthquake. 3. (3) The hypocenters of earthquakes that have occurred in the Tokai district have migrated downward from shallow depths. Since these changes resemble the changes that have appeared before other large shallow earthquakes along the trench in the past, it is conceivable that they are long-term precursors of a large earthquake. Future changes in seismic activity in this area demand close attention.

General overview of geophysical studies at Cerro Prieto

Geophysical investigations by the CFE in the Mexicali Valley near the Cerro Prieto volcano began nearly 20 years ago. Initially, gravity and seismic refraction methods were used for structural information related to faults and basement configuration. Geophysical emphasis shifted to d.c. resistivity surveys in the early 1970's after it was determined that a large area of low resistivity coincided with the thermal manifestations south of the volcano and the known high temperature zone at depth. Supplemented by ground magnetic and gravity measurements, the resistivity data are being interpreted to yield a detailed picture of the structure concealed by valley fill and to identify promising areas for future exploration. Under the DOE/CFE agreement, the LBL has been conducting various types of electrical, passive seismic and precision gravity surveys over the field. This work is less concerned with geothermal exploration than the CFE geophysical program. It involves applying exploration methods to reservoir-related questions; namely, can the reservoir area be delineated and can changes in reservoir characteristics be detected and monitored by means of surface measurements? As the second question involves a long time scale, repeated measurements and correlations to production data, only baseline information is presently being obtained. Dipole-dipole resistivity, magnetotelluric (MT) and self-potential (SP) surveys have been completed and results will be discussed. The large dipolar SP anomaly found over the field has encouraged further use of the method in the Mexicali Valley. Careful interpretation of dipole-dipole and Schlumberger data set yields a subsurface resistivity model which provides a surprising amount of useful detail. Permanent current electrodes have been emplaced and repeat measurements will be made at later times to determine whether changes in resistivity occurred within the reservoir area. Reference magnetic MT technique, developed recently at U.C. Berkeley, was applied at Cerro Prieto. Earlier tests have shown that in the presence of electro-magnetic noise, such as cultural noise, this technique yields more accurate subsurface information than conventional MT. Regional seismicity has been studied for many years. Recently, the CICESE has maintained a four station network in the Northern Mexicali Valley. The LBL passive seismic results show epicenters within the reservoir boundary associated with right-lateral strike-slip movement. V P/V S data from these events indicate a Poisson's ratio of 0.4 for the upper 2–3 km of the field. Although limited, P-wave velocity and attenuation data imply a local low near the western edge of the field superimposed upon a slight high for the entire production region. Beginning in the fall of 1978, continuous monitoring by the LBL and CICESE will provide more complete data on field characteristics. The first phase of precision gravity surveys at 60 monuments is nearly complete. Repeated gravity measurements coupled with repeated first-order leveling by the CFE will be needed to distinguish subsidence or tectonic induced gravity changes from the small changes (80–100 μgal) that might be associated with mass depletion due to fluid withdrawal. A five-station seismic array is being installed in shallow holes so that the long-term changes in seismic activity and relationship to subsidence and geothermal fluid production can be studied.

Earthquakes related to hydraulic mining and natural seismic activity in western New York State

We have monitored the seismic activity of western New York since 1970 using a combination of up to eight permanent seismograph stations and several portable stations. Our investigation centered on the Attica-Dale area, the site of several damaging earthquakes in this century. Although the background level of seismicity was found to be extremely low, less than one event per month in the first 12 months of our study, this near quiescence was broken in 1971 by a sharp increase in seismicity at our site near Dale following the initiation of fluid injection under high pressure (120 bars tophole) at a hydraulic mining operation nearby. This facility, which mines salt from the Vernon Formation of Silurian age, is centered near the Clarendon-Linden Fault, a major north-south trending system of high-angle thrust faults that extends for over 100 km from Lake Ontario to Allegheny County, New York. Although the seismic events were small (none were recorded at our station 30 km to the northwest), as many as 80 occurred per day, and many were felt locally. The marked increase in seismic activity after attaining high pressures, the closeness of these events to the bottom of the injection well, and the near cessation of activity within 48 hours of the shutdown of injection strongly suggest that this activity was caused by the triggering of tectonic strain on or near the Clarendon-Linden Fault by the high fluid pressures of the mining operation. The minimum pressure (41–48 bars) at which seismic activity occurred is consistent with predictions made by applying the Hubbert and Rubey theory of effective stress to hydrofracturing stress measurements from Alma, New York. Since 1972, five other wells at Dale have been hydrofractured at pressures of about 110 bars, but none had abrupt changes in seismic activity associated with them during the high-pressure phases of pumping. The well used in 1971 (0.43 km deep) is the closest (about 50 m) of the six to the Clarendon-Linden Fault. It was hydrofractured near the base of the salt layer, whereas the others were hydrofractured well within the salt layer. Thus fluids under high pressure appear to have reached the Clarendon-Linden Fault in 1971 but not in the other five cases. A focal mechanism involving thrust faulting on a plane nearly parallel to the Clarendon-Linden Fault was obtained from events that continue to occur but at a much reduced level in the brine field in 1974 and 1975. An earthquake of magnitude 2.7, which occurred about 7 km to the west of the brine field in 1973, is apparently unrelated to the injection operation and is of natural origin. This and other nearby natural events appear to be associated with the western and southwestern branches of the Clarendon-Linden Fault. Hydraulic diffusivity values calculated from the space-time relationship of earthquakes triggered by fluid injection at Denver and Rangely, Colorado, Matsushiro, Japan, and Dale are about 104 and 105 cm2/s and are similar to those obtained from precursory anomalies of earthquakes. The similarity of the diffusivity values suggests that the precursory changes do involve the movement of water. In each of these four places the wells used for fluid injection bottomed into or very close to major fault zones.

Seismicity changes associated with reservoir loading

Changes in seismic activity have been related to the filling of large reservoirs in over thirty cases. These changes range from variations in the level of micro-earthquake activity detectable only with instruments of high sensitivity to destructive earthquakes with magnitudes greater than 6. On the other hand, the filling of many other large reservoirs has not been accompanied by increased seismicity. A number of factors may contribute to the generation or absence of post-impounding seismicity. Increased vertical stress due to the load of the reservoir and decreased effective stress due to increased pore pressure can modify the stress regime in the reservoir region. Whether or not these stress changes are sufficient to generate earthquake activity will depend on a complex interaction of the induced stress with the state of pre-existing stress near the reservoir, and on the geologic and hydrologic conditions at the site. The combined effect of increased vertical load and increased pore pressure will have the greatest tendency to increase activity in regions where the maximum compressive stress is vertical (normal faulting). In regions where the minimum compressive stress is vertical (thrust faulting) increased stress due to a vertical load should have a minimum effect. For all of the larger reservoir-induced earthquakes the stress system determined from fault plane solutions is in agreement with the pre-existing stress field in the region of the reservoir. These earthquakes are all of strike-slip or normal type, there being no reported cases of large induced earthquakes with thrusting mechanisms. The potential for major changes in seismicity may be highest in regions of moderate strain accumulation (low to moderate natural seismicity). In areas of high strain accumulation and high levels of natural seismicity, the stress changes induced by the reservoir will be small compared to natural variations. In aseismic areas, with low strain accumulation, the reservoir-induced stresses may be insufficient to raise the stress level to a state of failure.

Evidence for earthquake triggering stress

Earthquake time series are constructed by making counts per unit time of earthquakes with magnitudes greater than a chosen threshold. The time series from widely separated regions show strong correlations with one another, and it is suggested that tectonic plates are subjected to a fluctuating stress field that results in these changes in seismic activity.

Principal regularities in the distribution of major earthquakes relative to solar and lunar tides and other cosmic forces

Magnitudes of strong earthquakes in the period 1903–1956 fall into two ranges: an upper of magnitudes 8.4–8.9 and a lower of magnitude 7.9–8.3. Earthquakes of the upper range are distinctly related to (a) rotational parameters of the Earth (the so-called V numbers) and (b) lunar declination at culmination. Earthquakes of lower magnitudes do not reveal this relationship. Annual and diurnal trends in the seismic activity of the Earth's crust and the upper parts of the mantle are considered. Changes of seismic activity are compared with those of tide-generating forces, these changes reaching some 100%. The storing of seismic activity, its intensification and dimunition within different structural levels in the Earth's crust and mantle at different times, and the approximate coincidence of seismic events in various tectonic levels of continental and oceanic border regions are discussed. Characteristics of the changes of seismic activity are related to the fixed position of the Earth (i.e., the Greenwich Meridian relative to the Sun). Regularities in the distribution of seismic activity are related to the axes of the Earth's deformation. The association of the strongest earthquakes with the position of maxima of geoid amplitudes in circumsolar space is described.