OPINION - Seismological Research Letters

OPINIONMechanical PollutionSome things are "in" and others are "out." Fashion plays a rolein most ofour choices, from clothes to topics in seismology.A certain amount ofcollective irrationality is good; it holds ustogether and simplifies our decisions. But culture must keepup with the rapidly changing environment and we need topoke at it relentlessly with radical thought. The emphasis ison "radical" because thought can also be wasted on rationalizations.Even in science, rationalizations built on wrongassumptions can be a cover for outdated cultural values or,worse, for narrow private agendas. Aglaring example ofa research topic thatseems to be unjustifiably "out" isanthropogenic seismicity, earthquakestriggered by human activities.The concept that humans cancause earthquakes is more than a centuryold, yet it is still astonishing. Reservoir-inducedearthquakes were notnecessarily the first anthropogenicearthquakes, but they were widely recognizedand studied in the mid-20thcentury because they could be explained by straightforwardand physically plausible models. Perturbations of subsurfaceporomechanical conditions from the load of water in largereservoirs are clearly substantial, even at seismogenic depths,and were calculated to be ofsufficient amplitude to cause failureon faults already loaded by tectonic stress. The importantrole of pore pressure and interstitial fluid flow became obviouswhen earthquakes were triggered by deep fluid injectionnear Denver, Colorado in the early 1960's. The propagationof seismicity as far as 5 km away from the well was interpretedas the effect of an expanding fluid pressure front.Injection was halted to reduce earthquake hazard, yet seismicitycontinued, producing the largest earthquake a year later.Once recognized, anthropogenic earthquakes have beenassociated with many engineering activities, and the minimumperturbation considered significant has become progressivelysmaller. But the interest in anthropogenicseismicity was short-lived, probably because the seismologicalcommunity concluded, erroneously, that the hazard fromanthropogenic earthquakes was negligible and, correctly, thatrecognition ofthis hazard would engender strong resistance.[T]he seismological communityconcluded, erroneously, thatthe hazard from anthropogenicearthquakes was negligible and,correctly, that recognition of thishazard would engender strongresistance.In spite of the current interest in earthquake-earthquaketriggering, anthropogenic earthquakes remain largely off ourprofession's radar screen. While old but persistent misconceptionsreveal why we have not been interested in anthropogenicearthquakes, new results make a strong case that weshould, particularly in stable continental regions (SCR). SCRare characterized by relatively low seismicity, but they covermuch ofthe Earth's land surface and have experienced a numberofnasty earthquake disasters in the last decade.Hazard is the first concern. Anthropogenic earthquakesare common, but fortunately potential sources of anthropogenicearthquakes are not only relativelyeasy to identify but can also becontrolled. Another reason to studyanthropogenic earthquakes is that theymay offer useful engineering applications.In spite of their bad reputation,earthquakes can be helpful in illuminatingfaults and folds and in monitoringstress change and fluid flow.Abundant, low-magnitude anthropogenicseismicity is being studied inhydrocarbon and hydrothermal fieldsas a way to monitor physical properties. Finally, anthropogenicseismicity offers unique laboratory conditions to studyseismogenesis in general. We have only begun to take advantageof research opportunities offered by the ability to monitorstress, pore pressure, and fluid flow as earthquakes areturned on and offOne ofthe main reasons that anthropogenic earthquakesare out of fashion is that they are relatively unimportant inactive areas where natural seismicity is high and where mostof seismology happens. In SCR, seismogenesis and ourunderstanding of it are relatively low and thus we tend toimport notions from California and assume that any distinctionsare only quantitative. I suggest here some of the areaswhere this assumption is incorrect and why it is particularlymisleading in terms of the relative levels of anthropogenicand natural seismicity.The probability that an anthropogenic perturbation triggersan earthquake depends on ambient stress-how close afault was to failure-and not on strain rate, provided the perturbationis short-lived relative to the earthquake loadingcycle. In situ stress measurements have shown that the uppercrust is in a critical stress state in both active and stable conti-Seismological Research Letters May/June 2002 Volume 73, Number 3 315

nental regions. Assuming pure elastic loading and a similarrange of stress drop, faults in either environment would beequally close to failure, on average. Therefore, anthropogenicearthquakes would be equally likely in high and low strainrateenvironments, if other factors were the same (they arenot; see below). In contrast, natural seismicityis proportional to tectonic strain rate andis vastly higher along plate boundaries. Onthis basis alone, the ratio between anthropogenicand natural seismicity is expected to bemuch higher in SCR than at plate boundanes,Recent data support this first-order prediction.In peninsular India, for example, arapid expansion of the irrigation system inthe 1960's coincided with a factor of 3 seismicityincrease, including several well knownreservoirinducedevents, such as the 1967 Koyna earthquake. In SouthAfrica, official bulletins ofrecent seismicity differentiate morenumerous and easily identifiable mining-related earthquakesfrom natural ones. In the area of the northeastern U.S. coveredby the Lamont-Doherty regional seismic network, theratio between anthropogenic and natural earthquakes isabout 1 to 3 (more below). Compare these and other examplesin SCR with the San Andreas plate boundary, where,despite intense seismicity and seismology, only a few andmostly controversial cases ofanthropogenic earthquakes havebeen reported. This juxtaposition is particularly striking inlight of the current habit of considering an earthquake to benatural until proven anthropogenic. This practice biases theanthropogenic/natural ratio in favor of intensely studiedareas."Anthropogenic earthquakes tend to be shallow and smalland thusoflittleconcern. "This concept derives also from Californiaand other active areas where most important earthquakesnucleate near the bottom ofthe seismogenic layer andout of range from anthropogenic perturbations. The upper3-5 km of the crust in active areas may be too fractured tosustain large coherent stresses, and the relatively scarce seismicityin this depth range seems to be magnitude-limitedbelow damage threshold. In contrast, many of the damagingor potentially damaging SCR earthquakes are very shallow.Their ruptures are mostly confined to the upper 5 km andoften reach the surface. Well documented examples in theM 6-7 range include 1990 Ungava, northern Quebec; 1993Killari, central India; and 1968 Meckering, 1970 Calingiri,1979 Cadoux, 1986 Marriat Creek, and 1988 TennentCreek, all in Australia. Although the largest SCR events, suchas the 1811-1812 New Madrid events and the recent one inGujarat, India, seem to nucleate deep in the crust, the class ofvery shallow SCR earthquakes clearly reaches magnitudeswell above the damage threshold. Furthermore, their proximityto the surface lowers the damage threshold, and relativelyabundant small events can have severe consequences. TheM b 5.3, 1995 earthquake centered below New Castle, Australiais an example.[Alnthropogenicearthquakes tend tooccur near populationcenters and thusimpose particularlyhigh risk.Earthquake depth distribution suggests that, unlike thecrust in a tectonically active zone, SCR crust can store plentyof elastic energy, well within reach of anthropogenic perturbations.In SCR, therefore, the magnitude ranges of anthropogenicand natural shallow earthquakes are expected to besimilar. Furthermore, if upper magnitude limitsfor very shallow earthquakes in SCR and atplate boundaries are below and above damagethresholds, respectively, the likelihood ofdamaginganthropogenic earthquakes is greater inSCR than in California in absolute terms, notjust in comparison with natural seismicity.Finally, depth and magnitude limits should beapplied to anthropogenic earthquakes withgreat caution, considering, for example, themidcrustal hypocenters below Lake Nasser inEgypt and the three M 7 events in the proximity ofthe Gazligas field in central Asia.About one third of the sequences I have co-investigatedwith temporary local networks in the northeastern U.S. since1980 were demonstrably triggered by a variety ofengineeringactivities, including deep fluid injection, large quarries, anddeep mines. Five of the main shocks were in the magnituderange 4.3-5.2 and caused MMI VI-VII damage, mostly inrural areas. The largest one was 8 km deep and was natural.The others were no more than 5 km deep; two were anthropogenic,one was natural, and one is still uncertain. In the relativelysmall area of the northeastern U.S. covered by ourobservations, anthropogenic seismicity is up there with naturalseismicity in terms of number of events and potential fordamage. Many other areas are not as closely monitored, and areliable comprehensive evaluation of current anthropogenicseismogenesis may be difficult. Yet even a cursory surveyshould merit attention. The last two M> 4 U.S. events east ofthe Mississippi, for example, were in Ashtabula, January2001, and in Alabama, January 1999; both were very shallowand likely triggered by fluid injection (see below for more onAshtabula). In summary, earthquakes in the upper few kilometersof the crust account for a large share of the hazard inSCR. This depth range is within reach of many anthropogenicperturbations which have been triggering a substantialportion ofshallow SCR earthquakes. The hazard implicationof any shallow SCR earthquake may not depend a priori onwhether it is natural or anthropogenic. But we know whereand how we are altering underground conditions and theresulting earthquake hazard is relatively circumscribed andintense. It is thus potentially better known and easier to dealwith from the engineering and economic standpoints.Finally, anthropogenic earthquakes tend to occur near populationcenters and thus impose particularly high risk."Triggering only shifts the hazard in time." When triggered,an earthquake occurs earlier than it would have naturallybecause a perturbation raised the stress to failureprematurely. An area undergoing industrialization may experiencea burst of anthropogenic earthquakes and thus anincrease in seismicity and hazard. 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moment release, however, must match the average tectonicrate, and this increase is therefore compensated later by a seismicitydecrease. The time between increase and decrease isproportional to tectonic rate; here the difference betweenCalifornia and SCR lies in the meaning of"long term." Compensationtime along the San Andreas faultmay be on the order of a lifetime: Pay now,cash in later. In SCR, however, one may haveto wait ten thousand years or more before fullcompensation: a bad deal. While anthropogenicmay trade off with triggered seismicityin California, these two types ofseismicity arelargely independent and additive in SCR.Our California-based seismological culturehas once again led us to underestimate the significanceofanthropogenic earthquakes for hazard in SCR.Finally, let's face it: Fear of liability and bad publicity bythose who trigger earthquakes is hindering research inanthropogenic earthquakes. Ashtabula, Ohio offers a sadexample. In 1987 we monitored a M; 3.5 aftershocksequence, and we showed the source to be a vertical faultbelow that town. This fault was also 0.7 km from a deep highpressurewaste-disposal well that had begun operation a yearearlier. In response, the well operator invested about $0.7million for a reflection survey that found no fault and thus,allegedly, showed no risk from earthquakes. This result publishedin the local press satisfied the population and apparentlyalso the Ohio EPA. We were left to pull out our stationsand to end our self-financed experiment. But seismicity inAshtabula continued, even after the well shut down in 1994and the operating company disbanded. Another fault 4 kmfrom the well produced a M b 4.3 main shock January 2001that caused some damage. After a felt M b 3.0 in June, instrumentallyrecorded events continued through the rest of2001.In remarkable analogy to the seismological consequences ofthe 1962-1966 waste-fluid injection near Denver, Colorado,the injection in Ashtabula apparently left a perturbation thatis still spreading from the well and is triggering earthquakeshalf a decade later. Such a delayed reaction suggests thenotion ofmechanical pollution.We lost the opportunity to monitor the complete timespacedevelopment of seismicity in Ashtabula, but the datailluminate two seismogenic faults with minimum dimensionsin the 1-to-several kilometer range below the city. Clearly,this situation deserves scientific and regulatory attention. YetHumans pollute theupper crust andtrigger earthquakesbut often refuse toadmit it...the Ohio EPA is still apparently objecting to the notion thatearthquakes were triggered by fluid injection in Ashtabula.Such a notion may be a problem for routine flooding by severalgas-recovery operations in Ashtabula County. Ashtabulalikecases, where research in anthropogenic earthquakes isbeing discouraged, abound worldwide andpoint to a serious problem: Humans pollutethe upper crust and trigger earthquakes butoften refuse to admit it, thereby losing anopportunity to learn about them and reducehazard.Effective pollution control is unlikelyuntil the public demands it. History suggeststhat heads will remain buried in sand until apollution disaster educates and galvanizespublic opinion. These reactions, however, will likely be notonly late, but also adversarial and unlikely to promote costeffectivesolutions. The u.S. will eventually experience anearthquake disaster where human triggering is undeniable.Among the many polluters, one will get caught, chastised,and financially drained-a lottery in reverse.Waiting for sucha disaster might make sense for litigators but not for societyand, above all, not for seismologists. We can read the writingon the wall; after a destructive anthropogenic earthquake, itmay be awkward explaining why earthquake hazard maps ofthe eastern u.S. account for natural earthquakes with greatsophistication but ignore anthropogenic ones. We need tostand up to the polluters, realistically assess the hazard fromanthropogenic seismicity, and propose ways to controlmechanical pollution. While pursuing these tasks, we shouldalso acknowledge that these polluting endeavors are generallynot frivolous. Their products are critical and we all share inthe benefit and thus in the responsibility. We need to promotea regulatory environment that emphasizes risk-sharingrather than passing the buck. Many and diverse engineeringoperations contribute mechanical pollution, but only a subsetactually trigger earthquakes and only a few ofthese events aredamaging. The regulatory environment will probably continueto protect polluters, but only as long as their pollutioncan be hidden from the public. This system is not only unfair,but it obstructs the use of science to seek and promote costeffectiveways to minimize hazard. We need to be more forcefulin the study ofanthropogenic earthquakes and in alertingsociety to this hazard. E~Nano SeeberSeismological Research Letters May/June 2002 Volume 73, Number 3 317