Normal Aftershock Research Articles

Aftershock deficiency of induced earthquake sequences during rapid mitigation efforts in Oklahoma

Induced seismicity provides a rare opportunity to study earthquake triggering and underlying stress perturbations. Triggering can be a direct result of induced stress changes or indirect due to elastic stress transfer from preceding events leading to aftershocks. Both of these processes are observable in areas with larger magnitude induced events, such as Oklahoma. We study aftershock sequences of M2.5 to M5.8 earthquakes and examine the impact of targeted injection rate reductions. In comparing aftershock productivity between California and Oklahoma, we find similar exponential scaling statistics between mainshock magnitude and average number of aftershocks. For events with M≥4.5 Oklahoma exhibits several mainshocks with total number of aftershocks significantly below the average scaling behavior. The sequences with deficient aftershock numbers also experienced rapid, strong mitigation and reduced injection rates, whereas two events with M4.8 and M5.0 with weak mitigation exhibit normal aftershock productivity. The timing of when aftershock activity is reduced correlates with drops in injection rates with a lag time of several days. Large mainshocks with significantly reduced aftershocks may explain decreasing seismicity rates while seismic moment release was still increasing in Oklahoma in 2016. We investigate the expected poroelastic stress perturbations due to injection rate changes within a layered axisymmetric model and find that stresses are lowered by 10s to 100s kPa within the injection-affected zone. For earthquakes induced by poroelastic stress-increase at several kilometers from wells, the rapid shut-in of wells may lead to elastic stress reductions sufficiently high to arrest unfolding aftershock sequences within days after mitigation starts.

On the deformation cycle of a strongly coupled plate interface: The triple earthquakes of 16 March 1963, 15 November 2006, and 13 January 2007 along the Kurile subduction zone

The great plate interface Kurile earthquake of 15 November 2006 (Mw = 8.3) was immediately followed by normal aftershocks in the outer rise of the central Kurile subduction zone. Two months later at a distance <50 km from this event the 13 January 2007 (Mw = 8.1) earthquake with a pure normal mechanism occurred in the outer rise of the central Kurile. The 15 November 2006 earthquake ruptured an area of 280 × 150 km2 with maximum slip of almost 7 m, while the 13 January 2007 earthquake ruptured an area of 240 × 40 km2 with a maximum slip of almost 14 m. These two significant events were preceded by a major reverse earthquake on 16 March 1963 (Mw = 7.7) that took place close to the location of the 13 January 2007 earthquake. These very different earthquakes demonstrate the different stages of the deformation cycle along strongly coupled plates at the central Kurile arc and present a rare case of stress regime reversal over a short time span. In order to understand the details of these three earthquakes, we inverted for the source rupture processes of the earthquakes and delineated the location and dimension of asperities. The role of gravity and topography anomalies and their correlation to the asperity distribution as well as the long‐term seismicity is studied in detail. Using trench‐parallel Bouguer anomaly, we have identified the differences in the asperities and the location of larger normal outer rise aftershocks as well as the location of 13 January 2007 earthquake. We also show that the long‐term seismicity of the central Kurile arc is confined to the intraslab and not the plate interface. The static stress transfer for these earthquakes showed that the stress on optimally oriented thrust faults is increased in the northeastern part of the rupture area of the 15 November 2006 earthquake. Absence of major and great earthquakes to the northeastern immediate vicinity of the rupture area of the 15 November 2006 earthquake supports the hypothesis that this area is mature for the next strong earthquake along the Kurile arc.

Stress triggering of normal aftershocks due to strike slip earthquakes in compressive regime

A few cases of occurrence of normal aftershocks after strike slip earthquakes in compressive regime have been reported in the literature. Occurrence of such aftershocks is intriguing as they occurred despite the apparent stabilizing influence of compressive plate tectonic stresses on the normal faults. To investigate the occurrence processes of such earthquakes, we calculate change in static stress on optimally oriented normal and reverse faults in the dilational and compressional step over zones, respectively, due to slip on a vertical strike slip fault under compressive regime. We find that change in static stress is much more pronounced on normal faults as compared to that on reverse faults, for all values of fault friction. Change in static stress on reverse fault is marginally positive only when the fault friction is low, whereas for normal faults it is positive for all values of fault friction, and is maximum for high fault friction. We suggest that strike slip faulting in compressive regime creates a localized tensile environment in the dilational step over zone, which causes normal faulting in that region. The aftershocks on such normal faults are considered to have occurred as an almost instantaneous response of stress transfer due to strike slip motion.

Characteristics of three recent earthquake sequences in the Taupo Volcanic Zone, New Zealand

We have analysed three recent earthquake sequences in the northern part of the Taupo Volcanic Zone. A 1998 sequence at Haroharo with a largest event of M L 4.8, and a 2004 sequence near Lake Rotoehu (largest event M L 5.4), had normal b-values, and displayed an aftershock decay pattern, with most of the activity within the first few days. In contrast, a 2005 sequence a few tens of kilometres away at Matata (largest event M L 4.1), had very different characteristics, with a slow development and decay, no tendency for enhanced seismicity after the larger events, and a very high b-value. The focal mechanisms of the Rotoehu and Matata events are normal, and have stress patterns consistent with the geodetically observed extension of the Taupo Volcanic Zone in a northwest–southeast direction. The extensive recent volcanism in the Okataina Volcanic Centre does not seem to have affected the stress pattern in this area. The Rotoehu sequence showed a strong resemblance, particularly in the time distribution of events, to the well-known swarm activity in the Vogtland region on the German/Czech border, in which larger events were followed by a burst of seismicity, as in a normal aftershock sequence. Some of the arguments that have been advanced to explain the Vogtland swarm as seismicity induced by fluid injection apply to Rotoehu, but there is no direct evidence of fluid involvement. The Matata sequence appears to have a continuing trigger mechanism, either a slow injection of fluid, or a slow slip event, in an environment in which opening pore spaces prevent high overpressures developing. The Matata sequence occurred close to the area of the 1987 M L 6.3 Edgecumbe Earthquake, so exhibiting two extremes of seismic temporal pattern, namely mainshock–aftershock and a swarm with many events of similar magnitude, within a small area.

When and where the aftershock activity was depressed: Contrasting decay patterns of the proximate large earthquakes in southern California

Seismic quiescence has attracted attention as a possible precursor to a large earthquake. However, sensitive detection of quiescence requires accurate modeling of normal aftershock activity. We apply the epidemic‐type aftershock sequence (ETAS) model that is a natural extension of the modified Omori formula for aftershock decay, allowing further clusters (secondary aftershocks) within an aftershock sequence. The Hector Mine aftershock activity has been normal, relative to the decay predicted by the ETAS model during the 14 months of available data. In contrast, although the aftershock sequence of the 1992 Landers earthquake (M = 7.3), including the 1992 Big Bear earthquake (M = 6.4) and its aftershocks, fits very well to the ETAS up until about 6 months after the main shock, the activity showed clear lowering relative to the modeled rate (relative quiescence) and lasted nearly 7 years, leading up to the Hector Mine earthquake (M = 7.1) in 1999. Specifically, the relative quiescence occurred only in the shallow aftershock activity, down to depths of 5–6 km. The sequence of deeper events showed clear, normal aftershock activity well fitted to the ETAS throughout the whole period. We argue several physical explanations for these results. Among them, we strongly suspect aseismic slips within the Hector Mine rupture source that could inhibit the crustal relaxation process within “shadow zones” of the Coulomb's failure stress change. Furthermore, the aftershock activity of the 1992 Joshua Tree earthquake (M = 6.1) sharply lowered in the same day of the main shock, which can be explained by a similar scenario.

Increased probability of large earthquakes near aftershock regions with relative quiescence

It is shown that after a large earthquake of M6 class or over has taken place, another event of similar size or larger is relatively more likely (in the sense of rate per unit area) to occur in the near field than in the far field. In particular, we show that the aftershock activity of the first event provides useful information for assessing the probability of a following large event in the neighborhood (within a distance of a few degrees). Namely, if the aftershock activity from the first event becomes relatively quiet compared to the normal decay, the occurrence rate of larger events in the neighborhood is several times higher during the first decade after the main shock than would be the case for normal aftershock activity. For measuring such phenomena precisely, we need to model the normal aftershock activity. In fact, many aftershock sequences are more complex than the simple inverse power decay represented by the modified Omori formula. The epidemic type aftershock sequences (ETAS) model is a generalized version of the modified Omori formula, which fits well with various aftershock sequences, including nonvolcanic type swarms. Using this model, aftershock sequences are investigated for the 76 main shocks that occurred in and around Japan during the last three quarters of a century. The focus is placed on objective examination of whether or not, in an aftershock sequence, there exists a significant change point followed by relative quiescence, that is, a significant lowering of the seismicity from that predicted by the ETAS model. Relative quiescence can take place regardless of the seismicity level. The extent of such quiescence, if it exists, is seen from the diagrams of cumulative numbers versus the transformed occurrence times based on the predicted occurrence rate by the estimated ETAS model. It is thus demonstrated that relative quiescence can be a helpful factor in detecting anomalous aftershock activity. This can then be used to forecast whether or not a large event is more likely to follow shortly (within ∼6 years) in the neighborhood (within a distance of 3°) of the initial event.

Time Distribution of Immediate Foreshocks Obtained by a Stacking Method

We apply a stacking method to investigate the time distribution of foreshock activity immediately before a mainshock. The foreshocks are searched for events with M> 3.0 within a distance of 50 km and two days from each mainshock with M> 5.0, in the JMA catalog from 1977 through 1997/9/30. About 33% of M> 5.0 earthquakes are preceded by foreshocks, and 50-70% in some areas. The relative location and time of three types of representative foreshocks, that is, the largest one, the nearest one to the mainshock in distance, and the nearest one in time, are stacked in reference to each mainshock. The statistical test for stacked time distribution of foreshocks within 30 km from and two days before mainshocks shows that the inverse power-law type of a probability density time function is a significantly better fit than the exponential one for all three types of representative foreshocks. Two explanations possibly interpret the results. One is that foreshocks occur as a result of a stress change in the region, and the other one is that a foreshock is the cause of a stress change in the region and it triggers a mainshock. The second explanation is compatible with the relationship between a mainshock and aftershocks, when an aftershock happens to become larger than the mainshock. However the values of exponent of the power law obtained for stacked foreshocks are significantly smaller than those for similarly stacked aftershocks. Therefore the foreshock–mainshock relation should not be explained as a normal aftershock activity. Probably an increase of stress during foreshock activity results in apparently smaller values of the exponent, if the second explanation is the case.

The Norris Lake Earthquake Swarm of June Through September, 1993; Preliminary Findings

Abstract During the summer of 1993, the residents in the Norris Lake community, Lithonia, Georgia, were bothered by an incessant swarm of earthquakes. The largest, a magnitude 2.7 on September 23, showed a normal aftershock decay and occurred after the main swarm. Over 10,000 earthquakes have been detected, of which perhaps 500 were felt. The earthquakes began June 8, 1993, with a 5-day swarm. The residents, accustomed to quarry explosions, suspected the quarries of irregular activities. To locate the source of the events, a visual recorder and a digital event recorder were placed in the epicentral area. Ten to 20 events were detected per day for the next three weeks. The swarm then escalated to a peak of over 100 per day by August 15, 1993. Activity following the peak died down to about 10 events per day. The magnitude 2.7 event of September 23 was followed by a normal aftershock sequence. The larger events were felt with intensity V within 2 km of their epicenter, and noticed (intensity II) to a distance of 15 km. Some incidents of cracked wallboard and foundations have been reported, but no significant damage has been documented. Preliminary locations, based on data from digital event recorders, suggest an average depth of 1.0 km. The hypocenters are in the Lithonia gneiss, a massive migmatite resistant to weathering and used locally as a building stone. The epicenters are 1 to 2 km south-southwest of the Norris Lake Community. The cause of the seismicity is not yet known. The earthquakes are characteristic of reservoir-induced earthquakes; however, Norris Lake is a small (96 acres), 2 to 5m deep recreational lake which has existed since the 1950s.

Considerations on the global seismic sequences: the second and the third largest aftershocks

Statistical considerations on the occurrence of the second largest aftershock of an earthquake sequence have been developed using global data. The time distribution of the second, M2, and the third, M3, largest aftershocks are studied here as well. The probability that these two aftershocks would occur in the first 24 hr after the main shock is more than 50 per cent. It was found that the second and the third largest aftershocks are distributed like the first largest aftershock. The difference, D2, is determined between the magnitudes of the main shock, M, and the second largest aftershock, M2, of an earthquake sequence. Using a complete data set, it was found that the difference, D2, has a mean of about 2.0. Two distinct cases of the M2 occurrence are observed. The first case (case 1) corresponds to normal aftershock sequence evolution, where M2 follows M1 (which is the largest aftershock of a sequence), while in the second one (case 2) M2 precedes M1. The values of the time distribution are totally different between case 1 and case 2. An attempt is made to interpret the temporal occurrence of the aftershocks, with respect to the surface (terrestrial) heat flow. The mean heat flow value for case 1 is qA= 1.61 ± 0.86, while for case 2 this is qB= 1.38 ± 0.82. It is shown that when the heat flow is high, the three largest aftershocks (M1, M2 and M3) temporally occur closer to the main shock.

Quake swarm in California

A series of four earthquakes shook an area centered 24 km north of Bishop, Calif., on July 20–21, 1986. The earthquake swarm prompted the director of the U.S. Geological Survey (USGS) to send a letter to the California Office of Emergency Services, advising state officials of increased risk that additional earthquakes of the same or greater magnitude could soon follow. The earthquakes near Bishop had no apparent connection to two quakes in other parts of the state earlier in the month. As Eos went to press, the seismic activity near Bishop was tapering off, “as it does in a normal aftershock sequence,” according to David P. Hill, a geophysicist with USGS in Menlo Park, Calif.

The McNaughton Lake earthquake of May 14, 1978

abstractAt 22h37m02s UTC on May 14, 1978, a magnitude (ML) 4.8 earthquake occurred in the Canadian Rockies near McNaughton Lake, the large reservoir behind the Mica Dam in eastern British Columbia. The data suggest it was not a reservoir-induced earthquake.The earthquake was within a seismic array monitoring the reservoir and consequently, the epicenter is well constrained in a northwest-southeast direction. The preferred epicenter is 52.65°N, 118.89°W, slightly east of the Rocky Mountain Trench, with a shallow focal depth of about 10 km. There were no foreshocks but a normal aftershock sequence. The focal mechanism, the first for the eastern Canadian Cordillera, indicates predominantly right-lateral strike-slip faulting along the strike of the mountains with a significant thrust component. The moment is 4 ± 2 × 1023 dyne-cm with a low stress drop (<10 bars). A well-developed Lg phase was recorded to the south of the earthquake. The isoseismals are elongated in a north-south direction and the rate with which intensity attenuates in a southerly direction is comparable to that for eastern North America.

1968年十勝沖地震に伴う岩手県北部の微小地震活動

A temporary observation of microearthquakes was carried out at Karumai, Iwate Prefecture (40°19′N, 141°32′E) after the occurrence of the Tokachi-oki Earthquake of May 16, 1968. It was made for about 100 days since 22 o'clock on May 18, about 60 hours later than the occurrence of main shock. Total number of observed earthquakes is as many as 52, 000. Most of them were, of course, the aftershocks in usual sense and their hypocenters were located within the aftershock area, of which the areal size is approximately accordant with the Utsu-Seki's law.Besides these normal aftershocks, however, an activity of microearthquakes with the S-P times less than 5 seconds at the station was clearly observed. These earthquakes, which are evidently located outside of the aftershock area, are the main concern of the present paper. The following natures of these events are found:a) The time distribution of these events is similar to the normal aftershock activity, and the number of shocks decreases in accordance with the hyperbolic law.b) The activity for these events is far lower than that of the aftershocks.c) The region of activity is estimated to lie on the northern part of Morioka-Shirakawa tectonic line, which is about 100km distant from the western boundary of the aftershock area.A similar phenomenon was reported by KURIHARA and HIKAWA (1968) to occur at the Tokachi-dake Volcano in Hokkaido, although the volcano is far from the aftershock area. A small activity in the northern part of Iwate Prefecture was found at the time of the great Sanriku Earthquake, 1933. This region may be near the place of the present activity. At the time of the Nankaido-oki Earthquake of 1946, activities in the middle part of Kyushu and in Kyoto were also reported. It should be noted that these activities apparently accompanied the great earthquakes and that the excited regions were on the tectonic lines.From these facts, it is concluded that the effect of a big earthquake is not limited within the so-called aftershock area but the occurrence of big shock sometimes causes the activity in the region far distant from it. It may be reasonable to consider that the microearthquake activity in the present case was resulted by the relative deformation of two crustal blocks of Kitakami mountaineous land and western area, Morioka-Shirakawa line being the boundary of the two blocks.