Increase In Seismic Activity Research Articles

Spatio-temporal evolution of induced seismicity at Açu reservoir, NE Brazil

We present the spatio-temporal evolution of seismicity recorded by eight three-component digital seismographs in operation continuously during a 3 yr period (1994 August to 1997 May) at Açu reservoir, NE Brazil. The Açu dam is a 34 m high earth-filled dam constructed in 1983 May on an area of Precambrian shield. Based on seismic monitoring between 1987 and 1989 using single-component analogue seismographs, previous workers concluded that the seismic activity was a case of reservoir-induced seismicity (RIS) associated with diffusion of pore fluid pressure beneath the reservoir. The digital data presented here reveal the seismic activity in remarkable detail with vertical and horizontal location errors ≈0.1 km. A total of 286 events were recorded by three or more stations and all occurred at a depth of <5 km. Using these data we demonstrate that the majority of the earthquake activity is clustered within several well-defined zones and that individual zones are active over discrete periods of time. Over the entire period of seismic monitoring between 1987 and 1997 there is no simple correlation between reservoir level and number of seismic events. Lateral migration of the locus of seismic activity in an unpredictable fashion is shown to be partly responsible for the poor correlation, as event detection is not uniform through time. We also show that the time delay between maximum water level and a subsequent increase in seismic activity varies systematically; longer time delays correspond to activation of an earthquake cluster with a greater average hypocentral depth. However, within any one cluster there is no correlation between time delay and depth. The 3-D distribution of seismic activity through time may only be explained in terms of triggering by the diffusion of pore fluid pressure if the rock properties (e.g. permeability, strength) are heterogeneous.

On possible causes of increase in seismic activity of mine fields in the Oktyabrsky and Taimyrsky mines of the Norilsk deposit in 2003. Part I: Seismic regime

A sharp increase in seismic activity was recorded within the mine fields of the Oktyabrsky and Taimyrsky Mines of the Norilsk deposit in 2003. This paper is an attempt to explain this event by a complex analysis of mine seismological and in situ geomechanical information accumulated over the period from 1993 to 2003. The first part of the paper is devoted to general data on the object under study as well as to description of some important regularities concerning seismic regime of the mine fields in the Oktyabrsky and Taimyrsky Mines.

Nature and origin of volcanic ash in the 2000 eruption of Usu volcano, southwestern Hokkaido, Japan

Usu volcano, one of the most active volcanoes in Japan, erupted at regular intervals of ca. 30 years during the 1900s. The 2000 eruption took place on March 31 following 23 years of dormancy. An increase in seismic activity and remarkable ground deformation preceded the eruption. Many small-scale craters were successively formed in two areas at the foot of the volcano and ash was ejected intermittently from them. Ten samples out of the 2000 ash collected around the volcano during half a year were analyzed. They were depleted in SiO2 but enriched in FeO compared with the essential ejecta of the five major historic eruptions and all were highly altered as a whole. XRD analysis revealed smectite as a major clay mineral in the ash and kaolin as a minor one. The clay mineral assemblage and the trend of change in chemical composition of the 2000 ash are quite similar to those of the 1977–1978 ash. No temporal changes in chemical composition and clay mineral assemblage of the 2000 ash were detected. The percentages of the LOI and the relative proportions of SiO2 in the ashes and the essential ejecta showed a linear correlation. These results indicate that the constituents of the 2000 ash were alteration products of the volcaniclastics of the historic eruptions in the subsurface.

Hydrological anomalies associated with crustal deformation before the 2000 eruption of Usu volcano, Japan

Abrupt and large rises in well water levels were observed three days before the 2000 eruption of Usu volcano, simultaneously with the increase of seismic activity. After the groundwater level in the DT1 well rose 4.07 m, the groundwater discharged one day before the eruption. The groundwater level rose 0.95 m in the DT2 well before the eruption. These water‐level rises are caused by crustal deformation from magma migration, and enable us to estimate 0.63−11.2 × 10−6 of compressional volumetric strain by applying the wells' responses to earth and ocean tides. These strain changes are explained by a 2.8–16.5 × 106 m3 spherical inflation source model located at the Usu volcano. A 5 × 106 m3 inflation source model is estimated by a horizontal displacement observed by GPS near DT2, which is consistent with the volumetric strain estimated by the water‐level rises in DT2.

Seismicity and water level variations in the Lake Aswan area in Egypt 1982 1997

The data described here are obtained from the continuous record of earthquakeactivity and lake water-level variation in the Lake Aswan area in Egypt between 1982 and 1997. The seismicity is monitored by a local telemetered seismograph network. The hypocentral parameters of earthquakes have been determined using Hypo71. The earthquake foci are distributed in two seismic zones, shallow and deep in the crust. Shallow events have focal depths of less than 10 km. Deep events extend from 10 to 30 km. The temporal sequence of seismicity iscorrelated with both the water-level variation and the average daily change of the water level in the lake Aswan. The temporal variation of the seismicityindicates that there are only six sequences of increased seismic activity during 1982–1997. The correlation between the seismic activity and the daily variation of the lake water level is poorly observed except with the June 1987 events swarm, which was accompanied by the presence of an anomaly in rate of water level decreasing. It is concluded that the increase in seismic activity in the Aswan reservoir is demonstrating an example of rapid reservoir-triggered seismicity. The deeper seismic sequence in this area, which was associated with the November 14, 1981 mainshock (MD = 5.7), and the earlier seismicity (1981–82), has been correlated with a deeper high velocity anomaly (Awad and Mizoue, 1995-b).

A search for the volcanomagnetic signal at Deception volcano (South Shetland I., Antarctica)

After the increase in seismic activity detected during the 1991-1992 summer survey at Deception Island, the continuous measurement of total magnetic intensity was included among the different techniques used to monitor this active volcano. The Polish geomagnetic observatory Arctowski, located on King George Island, served as a reference station, and changes in the differences between the daily mean values at both stations were interpreted as indicators of volcanomagnetic effects at Deception. A magnetic station in continuous recording mode was also installed during the 1993-1994 and 1994-1995 surveys. During the latter, a second magnetometer was deployed on Deception Island, and a third one in the vicinity of the Spanish Antarctic Station on Livingston Island (at a distance of 35 km) and was used as a reference station. The results from the first survey suggest that a small magma injection, responsible for the seismic re-activation, could produce a volcanomagnetic effect, detected as a slight change in the difference between Deception and Arctowski. On the other hand, a long term variation starting at that moment seems to indicate a thermomagnetic effect. However the short register period of only two stations do not allow the sources to be modelled. The future deployment of a magnetic array during the austral summer surveys, throughout the volcano, and of a permanent geomagnetic observatory at Livingston I. is aimed at further observations of magnetic transients of volcanic origin at Deception Island.

B71 1996 年口永良部島火山の地震活動の活発化について

B71 1996 年口永良部島火山の地震活動の活発化について

The crustal dynamics and the tectonic trends in the Bengal Basin

Seismological data of the events that took place in the Bengal Basin during 1918 to 1989 have revealed an increased frequency of earthquakes in the last 30 years. The increase in seismic activity is an indication of fresh tectonic activity or propagation of fractures from the adjacent seismic zones. The tectonic trend TT3 as determined from the tectonic flux and from the crustal model is in good coincidence with the NE-SW trending linear zone of gravity high, the zone of Moho upwarping and the location of earthquake events having fault-plane solutions of prominent strike-slip component. It is further observed that most of the earthquakes that occurred in the Bengal Basin in the 20th century follow the tectonic trend TT3. The NE-SW trending tectonic trend TT3 is inferred as one of the most remarkable features in the Bengal Basin. The extension of this trend is well marked by the “Halflong-Disang Thrust” in the NE and by the “Swatch of No-Ground” in the SW. The tectonic flux has also revealed some other striking tectonic trends distributed over broad regions and is not confined to definite geologic or physiographic provinces but instead is transverse to major structural elements of the region, thus forming conjugate sets of active zones. The focal mechanism solutions of 12 earthquake events reveal the nature of faulting which is predominantly strike-slip. The strike-slip fault solutions for most of the events are indicative of a changing pattern from convergence and subduction to strike-slip displacement in the Bengal Basin. The focal mechanism of two events having solutions of strike-slip with a normal fault component located on the tectonic trend TT3, the nature of Moho upwarping and the crustal configuration lend support to a process of crustal extension prevailing in the Bengal Basin. The crustal segment to the east of TT3 is relatively more mobile than that of the crustal segment to the west. The general trend of compression ( P-axis) is N57 °W in the Bengal Basin, while it is N40 °E in the Bay of Bengal. The oblique convergence of the crustal segments situated in the Bengal Basin and in the Bay of Bengal has been inferred.

Moderate-magnitude seismicity remotely triggered in the Taiwan region by large earthquakes around the Philippine Sea plate

Abstract Seismicity in the Taiwan region was studied before and after the 12 largest seismic events with body-wave magnitudes exceeding 6.5, occurring around the Philippine Sea plate in the 1973 to 1994 period. The total local seismicity rate involving events with magnitudes as small as 2.0 was not affected by remote large events. However, the number of local earthquakes with magnitude ML ≧ 4.5 in the 15 days following a large event exceeded the number before it in 9 out of 12 cases studied. When earthquakes with ML ≧ 4.0 were considered, then 10 cases showed an increase in seismic activity after the big event. This suggests a 75 to 83% probability of remote triggering.

Development of an early-warning system for monitoring remote volcanoes

Many andesitic volcanoes are quiescent for long time periods: usually (but not always) an increase in seismic activity and in deformation precedes an eruption by a few months or a few days. A UNESCO panel has put forward the concept of an early warning system for monitoring dormant volcanoes in remote regions. Simple seismic or deformation measuring devices can in principle be built for monitoring remote volcanoes. These instruments are composed of two units: 1) a processor that measures the baseline «activity» of the volcano and decides when the activity increases above a certain threshold; 2) a transmitter for long distance communication. For slow parameters like tilt or extensometry, the signal can be transmitted every few minutes or hours. For seismology, signals include a large quantity of data and therefore they are usually not transmitted. The processing unit is not easy to design because a single seismic station can record noises that are very similar to «volcanic events». Average noise level on a given time interval, event detection counters and high amplitude ground motion counters are a simple (but not exhaustive) way to summarize seismic activity. The transmission of data from the field to a monitoring center is feasible by present and future satellite telemetry. We present our attempt to develop an early warning system for remote volcano monitoring with data transmission by satellite.

Spatiotemporal Patterns in the Energy Release of Great Earthquakes

For the past 80 years, the energy released in great strike-slip and thrust earthquakes has occurred in alternating cycles of 20 to 30 years. This pattern suggests that a global transfer mechanism from poloidal to toroidal components of tectonic plate motions is operating on time scales of several decades. The increase in seismic activity in California in recent years may be related to an acceleration of global strike-slip moment release, as regions of shear deformation mature after being reached by stresses that have propagated away from regions of great subduction decoupling earthquakes in the 1960s.

Seismotectonics of the eastern Swiss Alps and evidence for precipitation-induced variations of seismic activity

One of the most striking features revealed by a regional study of seismic activity in the eastern part of the Swiss Alps, conducted with a dense network of seismic stations between 1986 and 1988, is the restricted, around 13 km vertical extension of the seismogenic zone (defined by approximately 300 located earthquakes) compared with the earthquake distribution down to the Moho, both north and south of the Alpine body. Studies of focal mechanisms, station diagrams and very precise relative locations of clustered earthquakes all reveal a consistent picture of the stress field in this region: σ Hmax and σ Hmin are oriented northwest-southeast and northeast-southwest, respectively. All three types of focal mechanisms are present. Because of an increase in seismic activity following an exceptionally long period of intense precipitation, the possibility was investigated whether under some circumstances, as in the well known cases of reservoir-induced seismicity, very intense precipitation coupled with seasonal snow melt could trigger shallow seismic activity. Some weak indications supporting this hypothesis were found.

地震の前兆現象発生のメカニズム

The process of the occurrence of an earthquake is discussed from the mechanical point of view, and it is emphasized that the structural heterogeneity in and around the earthquake source region is one of the most important factors which cause the occurrence of precursory phenomena. Earthquake precursors are classified into the following two types. The one is the precursors which are caused by the increase in stress. In general, long-term precursors are those of this type. The gradual increase of earthquake swarm activity before the 1983 Japan Sea earthquake, the gradual increase in seismic activity in the surrounding region of the 1989 Loma Prieta earthquake before the main shock and the appearence of the seismic gaps of the second kind are discussed as examples of this type. The other is the precursors which are caused by a slowly progressive rupture process just before the sudden main fracture. Short-term precursors are mainly those of tis type. Foreshocks prior to the 1934 and 1966 Parkfield earthquakes are discussed as the typical examples of this type.

Possible tectonomagnetic effect observed from mid‐1989 to mid‐1990, in Long Valley Caldera, California

Precise measurements of local magnetic fields have been obtained with a differentially connected array of three proton magnetometers in the Long Valley caldera region since 1984. Two magnetometers are located inside the caldera with a third reference magnetometer located 26km southeast of the caldera. After correction for secular variation, it is apparent that an anomalous 2 nT decrease in the magnetic field occurred from mid‐1989 to mid‐1990 at the magnetometer located closest to the center of the resurgent dome inside the caldera. During this period a significant increase in geodetic strain rate of 8.5 ppm/a was observed on the two‐color geodimeter network within the caldera from October, 1989, to mid‐1990 and a dramatic increase in seismic activity occurred from December, 1989 to July, 1990. A simple dilatational point‐source model with pressure increasing by 52 Mpa from October 1989 to August 1990 at a depth of about 7km beneath the center of the resurgent dome can be fit to the strain data. If this same model is used to calculate piezomagnetic fields in the caldera, the results obtained agree with the observed local magnetic field data provided the Curie point isotherm is at a depth of ≤5km. Taken together, these magnetic, seismic and geodetic data suggest that an episode of active magmatic intrusion occurred from late 1989 to mid‐1990 at a depth of about 7–8km beneath the resurgent dome within the Long Valley caldera. Other indications of this intrusion should be evident in measurements of leveling, local gravity, and seismic imaging data.

A low magnitude seismic sequence near Isernia (Molise, Central Italy) in January 1986

Following the increase in seismic activity which occurred near Isernia (Molise, Central Italy) in January 1986, a digital seismic network of four stations with three-component, short-period seismometers, was installed in the area by the Osservatorio Vesuviano. The temporary network had an average station spacing of about 8–10 km and, in combination with permanent local seismic stations, allowed the accurate determination of earthquake locations during an operating period of about one month. Among the 1517 detected earthquakes, 170 events were selected with standard errors on epicentre and depth not greater than respectively 0.5 and 1.5 km. The most frequent focal depths ranged between 4 and 8 km, while the epicentres distribution covered a small area NE of Isernia of about 10 km2. A main rupture zone could not be clearly identified from the spatial distribution of the earthquakes, suggesting a rupture mechanism involving heterogeneous materials. The activity was characterized by low energy levels, the largest earthquake, on January 18, 1986, havingMD=4.0. The time sequence of events and pattern of seismic energy release revealed a strong temporal clustering of events, similar to the behaviour commonly associated with seismic swarms.

Oil and Gas Developments in Southeastern States in 1983

Total drilling activity in Mississippi, Alabama, Florida, and Georgia, including both exploratory and development wells, decreased 8.9% from 855 in 1982 to 779 in 1983. However, exploratory drilling in the southeastern states in 1983 increased 7.2%. Exploratory tests totaled 357 versus 333 tests in 1982, and 52 were successfully completed for a 14.6% success rate. This compares to 46 successful completions and a 13.8% success rate in 1982. The shallow Eocene Wilcox trend in southwest Mississippi accounted for 39.8% of exploratory tests in the southeastern states during 1983. This active trend had 11 new-field discoveries compared to only 6 new-field discoveries in 1982. The Paleozoic trend of northeast Mississippi and northwest Alabama had 17 new-field discoveries compared with 16 the previous year. This trend represented 18.4% of the exploratory drilling activity in the southeast. The Jurassic trend had 4 new-field discoveries for a 50.0% decline relative to 1982 when 8 new-field discoveries were made. The Jurassic accounted for 17.4% of the total exploratory effort in the southeast in 1983. The Upper Cretaceous, Lower Cretaceous, and Miocene trends accounted for 12.9%, 8.1%, and 3.4%, respectively, of the total explor tory tests. Six new-field discoveries were reported in the Upper Cretaceous, 2 in the Lower Cretaceous, and 3 in the Miocene. Seismic activity was down 18.1% as crew-week totals dropped from 1,306 in 1982 to 1,070 in 1983. Oil and gas production also showed a decline. Oil production decreased 12.1%, and gas production decreased 7.9%. In 1984, a modest increase in exploratory drilling can be expected. Most of the activity will be concentrated in the Wilcox, Paleozoic, and Jurassic trends. These 3 trends represented approximately 75% of the exploratory activity in the southeastern states in 1983, and a similar level of activity in these trends is expected to occur in 1984. Natural gas exploration below 15,000 ft will continue to be slow because of continuing poor economic conditions for deep gas. Seismic activity should show a modest improvement in 1984 as companies attempt to improve prospect inventories. The increase in seismic activity will be accompanied by an increase in acreage availability as 3- and 5-yr term lease blocks taken during the boom years of 1979-81 expire. Even with increased exploratory activity, 1984 oil and gas production in the southeastern states will show a decline relative to 1983 unless a number of significant discoveries are made. The independents and smaller companies will continue to conduct the bulk of the exploratory drilling in the southeastern states.

Strength changes due to reservoir‐induced pore pressure and stresses and application to Lake Oroville

The association of a significant increase in seismic activity with the filling of some large water reservoirs is well documented. One possible such case is the earthquake sequence at Oroville, California, in 1975. It has been suggested that this activity may be triggered on faults close to failure by the weight of the water, the increase in pore pressure, or both. Two‐dimensional half‐space models with surface loading are used to study the change in strength produced by these mechanisms for thrust, normal, and strike slip faulting. For the case of the water weight alone the strength increased across fault planes dipping as in the Oroville sequence. It is concluded that the water load alone is an unlikely mechanism. The inclusion of pore pressure effects in these models, however, is shown to be very encouraging. Using the Biot linearized quasi‐static elasticity theory for fluid‐infiltrated porous materials, two types of loaded half‐space models were studied. The first was homogeneous with respect to permeability and found to produce broad zones of weakening. The second type included a fault zone of different permeability. This model intensifies the magnitude and broadens the zone of strength drop if the fault zone is highly permeable in relation to the rest of the half space. For fault dips roughly corresponding to the Oroville situation it was found that although weakening occurs for ambient stresses favoring either thrust or normal faulting, the magnitude and breadth of strength drop were significantly greater for normal faulting: Similar effects were observed for strike slip faulting. It was also found that all types of models developed significant zones of weakening very rapidly, permitting reasonable values of permeability to be estimated. The case of the impervious reservoir bottom was examined, with significantly differing results. Just as in the permeable case, zones of weakening were initially developed. However, in the impermeable case these dissipated with time, eventually reaching the same strength distribution as that of the water weight alone. Thus with sufficiently slow impoundment the impervious reservoir may not weaken at all, whereas the permeable reservoir will continue to weaken in time. The results also show that rapid unloading may cause instantaneous crustal weakening owing to excess pore pressure. Although the models are very simplified, it is interesting to apply them to the case of Lake Oroville. Using reasonable parameters, strength drops for normal faulting of 8–14 bars were calculated, which compared favorably to the fluid injection experiments at Rangely and Matsushiro. The observed lag time between initial impoundment and seismicity yielded a permeability of 0.2 mdarcy for the homogeneous model and 20 mdarcy for the greater strength drops produced by the heterogeneous model, both of which are reasonable rock values. If the period of rapid filling is assumed to have triggered the activity, then the permeability was found to be 3.6 mdarcy for the homogeneous model and 360 mdarcy for the heterogeneous model, the latter of which is appropriate for fractured rock. It is concluded that the pore pressure trigger mechanism is very promising and that the methods of this paper may be extended to three dimensions to study reservoirs in more detail. The mechanism is limited, however, by the lack of knowledge of, first, the ambient pore pressure in situ and, second, the in situ permeability, coupling parameters, reservoir bottom permeability, and local tectonic stress. Perhaps the most important implications of the results is the presence of water to depths of 10 km or more in the crust. Thus at least in some places the crust may be weak, and tectonic stresses low. This also suggests an important potential for geothermal energy at great depth in the crust.

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.

Continued seismic activity at the Koyna reservoir site, India

Following the impounding of the Shivaji Sager Lake in 1962, tremors became prevalent in the Koyna region, considered previously to be aseismic. During ensuing years the tremor frequency appears to have been dependent on the rate of increase of water level, maximum water level reached, and the period for which high levels were retained. This culminated in a burst of seismic activity from September 1967 to January 1968 following the record water levels in the reservoir and included the earthquake of September 13, 1967 with magnitude 5.5 and the damaging December 10, 1967 earthquake of magnitude 6.0. During the next five years water levels were kept low and no significant earthquakes occurred subsequent to the October 29, 1968 earthquake of magnitude 5. The reservoir was filled to maximum capacity during September 1973 and this was followed by a conspicuous increase in seismic activity which included an earthquake of magnitude 5.1 on October 17, 1973. However, seismic activity during 1973 was much less severe than that of 1967. This relative decrease in seismicity may indicate that (a) the “threshhold level” for relatively large magnitude earthquakes had increased; (b) a major portion of the accumulated strains had been released; and/or (c) the importance of the longer period of high loading in 1967. Similar observations have been made at other seismically active reservoir sites.

Weight-induced stresses and the recent seismicity at Lake Oroville, California

Lake Oroville is a large artificial lake created by the construction of a 235-m-high earth dam on the Feather River, California, near the city of Oroville. Its storage capacity is about 4.4 × 10^9 m^3, and its maximum depth is about 200 m. There was no significant increase in seismic activity in the lake region following impoundment of the dam late in 1967 until the occurrence of many small seismic events which began in June 1975. This activity lead to a M = 5.7 main shock on August 1, 1975 with an epicenter about 11 km SSW of the Oroville dam. The main shock produced significant damage in the city of Oroville which lies about 7 km NNW of the epicenter. With several cases of reservoir-induced activity already documented, it is natural to inquire whether the Oroville seismicity was due to the presence of the reservoir. As part of such a study, the stresses induced in the neighboring lithosphere by the weight of Lake Oroville are determined. On the basis of present geological data, it is unlikely that these stresses were responsible for the main shock of August 1, 1975. The weight-induced shear stress across the fault plane in the hypocentral region has a component of about 0.04 bar parallel to the reported fault movement but in opposition to this movement. The greatest weight-induced shear stress is about 3.4 bars and this occurs under the deepest portion of the lake. The greatest vertical deflection at the surface due to the weight of Lake Oroville is calculated to be about 5.5 cm.