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A proposed rupture scenario for the 1925 MW 6.5 Santa Barbara, California, earthquake
Abstract
The 29 June 1925 Santa Barbara earthquake is among the largest 20th century earthquakes in southern California. The earthquake also predated the installation of strong motion and local monitoring instruments in southern California; some instrumental data are, however, available from long-period instruments at regional and teleseismic distances. The current catalog moment magnitude is MW 6.8. Initial intensity magnitudes (MI) estimated from original Coast and Geodetic Survey intensity assignments were lower (MI 6.3). In this study we assign modified Mercalli intensity values at 239 locations, including 144 specific locations within the city of Santa Barbara for which detailed damage information is available. Comparing the reinterpreted intensities with Did You Feel it? intensities for recent events in California, we estimate MW = 6.5, with a plausible range of 6.3–6.6. We further consider reported instrumental amplitudes to estimate an instrumental moment magnitude of MW = 6.6 ± 0.5. Our preferred final estimate is MW 6.5. Based on available constraints including aftershock locations inferred from data recorded on portable instruments, we propose that the earthquake nucleated east of the city of Santa Barbara, closer to the coast than previously estimated, and ruptured unilaterally ~30 km to the west, possibly along the south-dipping Mesa-Rincon Creek, and the More Ranch fault systems. Contrary to suggestions made in earlier studies (e.g. Willis, 1925a), relatively high intensities ~50 km west of Santa Barbara can then be explained by directivity rather than involvement of the Santa Ynez fault. Finally, we discuss the possibility that the earthquake was triggered by the larger MW = 6.6 Clarkston, Montana earthquake the previous day or induced by oil production in the Summerland oil field.
... Several small-to-moderate size earthquakes have occurred in the Santa Barbara channel over the last century. The epicenter of the 29 June 1925 (M w ≈6.5) earthquake is thought to have been located just east of Santa Barbara (e.g., Willis, 1925;Hough and Martin, 2018). Although instrumental records were not available to determine fault plane solutions, it is considered to have ruptured the Mesa-Rincon Creek thrust faults in the northern most Santa Barbara channel (Hough and Martin, 2018) or portions of the North Channel, Pitas Point, or Red Mountain faults (e.g., Archuleta et al., 1997;Kamerling et al., 2003;Nicholson, Sorlien, Kamerling, and Hopps, 2017). ...
... The epicenter of the 29 June 1925 (M w ≈6.5) earthquake is thought to have been located just east of Santa Barbara (e.g., Willis, 1925;Hough and Martin, 2018). Although instrumental records were not available to determine fault plane solutions, it is considered to have ruptured the Mesa-Rincon Creek thrust faults in the northern most Santa Barbara channel (Hough and Martin, 2018) or portions of the North Channel, Pitas Point, or Red Mountain faults (e.g., Archuleta et al., 1997;Kamerling et al., 2003;Nicholson, Sorlien, Kamerling, and Hopps, 2017). The 13 August 1978 M w 5.6 earthquake occurred southwest of Santa Barbara on an oblique reverse fault with aftershocks indicating a north-dipping, northwest-striking fault plane (Lee et al., 1978;Corbett and Johnson, 1982). ...
The Mid-Channel fault is an active blind thrust overlain by a south-vergent anticline in the Santa Barbara channel of southern California. We use high-quality industry seismic reflection data to characterize the 3D geometry of the Mid-Channel fault, and define its lateral extent and interactions with other structures in the region. We use a detailed chronostratigraphic sequence of nine Pleistocene horizons ranging in age from 120 ka to 1.05 Ma to interpret the geometry and deformational history of the Mid-Channel anticline and blind thrust. These precisely dated horizons provide unique insights on the Pleistocene activity of the Mid-Channel fault because of the nearly complete preservation of sediment on the backlimb of the fold. Using a new method of relating structural relief of these horizons to slip on the underlying Mid-Channel fault, we calculate the faults late Pleistocene slip rate. Results indicate that the structure became active between 790 and 710 ka in the eastern part of the Channel and subsequently propagated west along strike. In the east, where the overlying anticline preserves a complete section of syntectonic growth strata, the Mid-Channel fault has a dip-slip rate of 1.8 mm/yr with a range of 1.6–2.1 mm/yr. Based on a map of total displacement, we infer that the maximum dip-slip rate on the Mid-Channel fault along strike is 2.1 ± 0.2 mm/yr, suggesting that it may accommodate about one-third of the observed geodetic contraction across the basin. We explore alternative ways that the fault may extend to depth and interact with surrounding structures, and consider the seismic hazard implications of these scenarios, including estimates of potential earthquake rupture areas, magnitudes, and average recurrence intervals.
... The results of this study illustrate that the application of the BW97 method, or other statistical approaches (e.g., Gasperini et al., 1999) requires careful consideration of original data, especially with azimuthally unevenly distributed (Hough and Martin, 2018) and or sparse data sets (Meltzner and Wald, 1999;Szeliga et al., 2010). An optimal (L2 norm) fit to the intensity data may yield a location that is inconsistent with key observations, as we suggest was the case for E-24. ...
The grave threat posed by the Enriquillo-Plantain Garden fault zone (EPGFZ) and other fault systems on the Tiburon Peninsula in southern Haiti was highlighted by the catastrophic M 7.0 Léogâne earthquake on 12 January 2010 and again by the deadly M 7.2 Nippes earthquakes on 14 August 2021. Early Interferometric Synthetic Aperture Radar observations suggest the 2021 earthquake broke structures associated with this fault system farther west of the 2010 event, but the rupture zones of both events are separated by a ∼ 50 km gap. This sequence provided the impetus to reconsider a nineteenth century earthquake that may have occurred within this gap. Though previous studies identified a single moderately large event on 8 April 1860, original sources describe a complex and distributed seismic sequence to the west of Port-au-Prince. These provide evidence for an initial event to the west of Les Cayes, on the southern coast of the Tiburon Peninsula. This was followed on the morning of 8 April 1860 by a damaging earthquake near l’Anse-à-Veau along the northern coast of the peninsula, which was succeeded 14 hr later by a larger mainshock to the east. Although locations cannot be determined precisely from extant macroseismic data, our preferred scenario includes an intensity magnitude Mi∼6.2 foreshock, followed by an Mi ∼6.8 mainshock wholly or partially on the EPGFZ to the east of the foreshock. Our results suggest that, although earthquakes tend
to trigger subsequent earthquakes on immediately neighboring fault segments, sequences can include gaps due to strain release by poorly characterized earthquakes in the historic past. We also document 83 aftershocks to the 1860 earthquake sequence, including a previously
unknown damaging shock, estimated Mi ∼6.6, on 10 April 1860 near Jérémie, farther west of l’Anse-à-Veau.
... The application of statistical approaches (e.g., Bakun and Wentworth, 1997;Gasperini et al., 1999) is also limited by the lack of intensity prediction equations (IPEs) unique to the region. Furthermore, the sensitivity of these approaches to data such as ours with 30 or fewer observations and with poor azimuthal distribution is well known (Meltzner and Wald, 1999;Szeliga et al., 2010;Hough and Martin, 2018). Instead, we select provisional epicentral locations near the locations where the highest intensities were documented for both the 31 January (2.0°N, 102.8°E; dark blue star, Fig. 4a) and the 7 February (1.9°N, 103.8°E; black star, Fig. 4b) earthquakes. ...
Seismic hazard in the southern Malay Peninsula located within the Sundaland block in Southeast Asia is poorly understood. The paucity of historical earthquakes and low‐magnitude instrumented seismicity has led to the assumption that this region is largely aseismic. We question this point of view by reassessing historical seismicity in this region and, in particular, a pair of moderate earthquakes in the 1920s. The first of these struck on 31 January 1922 at ≈9:10 a.m. local time (LT) for which we estimate an intensity magnitude (Mi) ≈5.4, and for the second earthquake on 7 February 1922 at ≈12:15 p.m. LT, we estimate Mi≈5.0. We also identify at least 34 felt earthquakes between 1803 and 1950 that were potentially local within the Sundaland block. These include a very widely felt shock (or set of shocks) on 26 June 1874 that was felt in parts of Borneo, Java, and Sumatra. The discovery of these earthquakes challenges the tectonic stability of the Malay Peninsula and the stable interior of the Sundaland block. The record of historical seismicity in this region relies heavily on European sources, and we recommend locating and consulting indigenous sources to improve the current understanding of regional seismic hazard. We also underscore the need to evaluate the impact of ground motions from rare local earthquakes on the extant building stock and on transportation infrastructure that are otherwise relatively immune to the long‐period effects of distant earthquakes commonly felt in the Malay Peninsula.
... Our reinterpreted intensity distribution provides a missing puzzle piece for the larger picture of California's maximum historically observed earthquake intensities. Other big pieces are already in place: intensities have been revisited in recent years for the large events of Fort Tejon in 1857, Hayward in 1868, Owens Valley in 1872, Laguna Salada in 1892, San Francisco in 1906, and Santa Barbara in 1925, as well as many moderate historic events (Meltzner and Wald, 1999;Martindale and Evans, 2002;Hough and Elliot, 2004;Bundock, 2008a, 2008b;Hough and Hutton, 2008;Hough and Martin, 2018). These reinterpretations, combined with DYFI data for more recent events, will eventually produce a uniform shaking dataset with which earthquake hazard map performance can be assessed. ...
Seismic intensity data based on first-hand accounts of shaking give valuable insight into historical and early instrumental earthquakes. Comparing an observed intensity distribution to intensity-prediction models based on modern calibration events allows the magnitude to be estimated for many historic earthquakes. Magnitude estimates can also potentially be refined for earthquakes for which limited instrumental data are available. However, the complicated nature of macroseismic data and the methods used to collect and interpret the data introduce significant uncertainties. In this paper, we illustrate these challenges and possible solutions using the 1952 Kern County, California, earthquake as a case study. Published estimates of its magnitude vary from MW 7.2 - 7.5, making it possibly the second largest in California during the 20th century. We considered over 1,100 first-hand reports of shaking, supplemented with other data, and inferred the magnitude in several ways using intensity prediction equations, yielding a preferred intensity magnitude MI 7.2 ± 0.2, where the uncertainty reflects our judgement. The revised intensity distribution reveals stronger shaking on the hanging wall, south of the surface expression of the White Wolf fault, than on the footwall. Characterizing the magnitude and shaking distribution of this early instrumental earthquake can help improve estimation of the seismic hazard of the region. Such reinterpreted intensities for historic earthquakes, combined with USGS Did You Feel It? data for more recent events, can be used to produce a uniform shaking dataset with which earthquake hazard map performance can be assessed.
We revisit the magnitude and location of the 31 October 1895 Charleston, Missouri, earthquake, which is widely regarded to be the most recent Mw 6 or greater earthquake in the central United States. Although a study by Bakun et al. (2003) concluded that this earthquake was located in southern Illinois, more than 100 km north of the traditionally inferred location near Charleston, Missouri, our analysis of exhaustively compiled macroseismic data supports the traditionally inferred location, with a preferred magnitude of Mw ≈ 5:8 and a preferred magnitude range of 5.4–6.1. Our preferred magnitude is derived from comparisons with intensity distributions from the 1925 Mw 6.2 Charlevoix, the 1944 Mw 5.8 Massena, and the 1968 Mw 5.3 southern Illinois earthquakes, macroseismic data of which we also revisited in this study. Based on the distribution of liquefaction, reports of damage, and early aftershocks, we also explore possible rupture scenarios for the 1895 earthquake. Our preferred scenario involves unilateral rupture to the northeast on a (reactivated) northeast-striking fault (or faults) coinciding with structures associated with the western limb of the Reelfoot rift, with an epicenter south-southeast of Charleston, Missouri. Our results support the conclusion that within the Reelfoot rift, elevated seismic hazard is not restricted to the New Madrid seismic zone as conventionally defined but continues into the Charleston region in southeastern Missouri, where faults associated with the western edge of the Reelfoot rift appear favorably oriented for failure in the current stress regime.
This report presents a newly revised and expanded digital geologic map of the Santa Barbara coastal plain area at a compilation scale of 1:24,000 (one inch on the map to 2,000 feet on the ground)1 and with a horizontal positional accuracy of at least 20 m. The map depicts the distribution of bedrock units and surficial deposits and associated deformation underlying and adjacent to the coastal plain within the contiguous Dos Pueblos Canyon, Goleta, Santa Barbara, and Carpinteria 7.5’ quadrangles. The new map supersedes an earlier preliminary geologic map of the central part of the coastal plain (Minor and others, 2002; revised 2006) that provided coastal coverage only within the Goleta and Santa Barbara quadrangles. In addition to new mapping to the west and east, geologic mapping in parts of the central map area has been significantly revised from the preliminary map compilation—especially north of downtown Santa Barbara in the Mission Ridge area—based on new structural interpretations supplemented by new biostratigraphic data. All surficial and bedrock map units, including several new units recognized in the areas of expanded mapping, are described in detail in the accompanying pamphlet. Abundant new biostratigraphic and biochronologic data based on microfossil identifications are presented in expanded unit descriptions of the marine Neogene Monterey and Sisquoc Formations. Site-specific fault kinematic observations embedded in the digital map database are more complete owing to the addition of slip-sense determinations. Finally, the pamphlet accompanying the present report includes an expanded and refined summary of stratigraphic and structural observations and interpretations that are based on the composite geologic data contained in the new map compilation.
The Santa Barbara coastal plain is located in the western Transverse Ranges physiographic province along an east-west-trending segment of the southern California coastline about 100 km (62 mi) northwest of Los Angeles. The coastal plain surface includes several mesas and hills that are geomorphic expressions of potentially active folds and partly buried oblique and reverse faults of the Santa Barbara fold and fault belt (SBFFB) that transects the coastal plain. Strong earthquakes have occurred offshore within 10 km of the Santa Barbara coastal plain in 1925 (6.3 magnitude), 1941 (5.5 magnitude), and 1978 (5.1 magnitude). These and numerous smaller seismic events located beneath and offshore of the coastal plain, likely occurred on reverse-oblique-slip faults that are similar to, or continuous with, Quaternary reverse faults crossing the coastal plain. Thus, faults of the SBFFB pose a significant earthquake hazard to the approximately 200,000 people living within the major coastal population centers of Santa Barbara, Goleta, and Carpinteria. In addition, numerous Quaternary landslide deposits along the steep southern flank of the Santa Ynez Mountains indicate the potential for continued slope failures and mass movements in developed areas. Folded, faulted, and fractured sedimentary rocks in the subsurface of the coastal plain and adjacent Santa Barbara Channel are sources and form reservoirs for economic deposits of oil and gas, some of which are currently being extracted offshore. Shallow, localized sedimentary aquifers underlying the coastal plain provide limited amounts of water for the urban areas, but the quality of some of this groundwater is compromised by coastal salt-water contamination. The present map compilation provides a set of uniform geologic digital coverages that can be used for analysis and interpretation of these and other geologic hazards and resources in the coastal plain region.
Several recent studies have presented evidence that significant induced earthquakes occurred in a number of oil-producing regions during the early and mid-twentieth century related to either production or wastewater injection. We consider whether the 21 July 1952 Mw 7.5 Kern County earthquake might have been induced by production in the Wheeler Ridge oil field. The mainshock, which was not preceded by any significant foreshocks, occurred 98 days after the initial production of oil in Eocene strata at depths reaching 3 km, within ~1 km of the White Wolf fault (WWF). Based on this spatial and temporal proximity, we explore a potential causal relationship between the earthquake and oil production. While production would have normally be expected to have reduced pore pressure, inhibiting failure on the WWF, we present an analytical model based on industry stratigraphic data and best estimates of parameters whereby an impermeable splay fault adjacent to the main WWF could plausibly have blocked direct pore pressure effects, allowing the poroelastic stress change associated with production to destabilize the WWF, promoting initial failure. This proof-of-concept model can also account for the 98-day delay between the onset of production and the earthquake. While the earthquake clearly released stored tectonic stress, any initial perturbation on or near a major fault system can trigger a larger rupture. Our proposed mechanism provides an explanation for why significant earthquakes are not commonly induced by production in proximity to major faults.
Electronic supplementary material
The online version of this article (doi:10.1007/s10950-017-9685-x) contains supplementary material, which is available to authorized users.
The San Andreas fault is considered to be the primary plate boundary fault in southern California and the most likely fault to produce a major earthquake. I use dynamic rupture modeling to show that the San Jacinto fault is capable of rupturing along with the San Andreas in a single earthquake, and interpret these results along with existing paleoseismic data and historic damage reports to suggest that this has likely occurred in the historic past. In particular, I find that paleoseismic data and historic observations for the ~M7.5 earthquake of 8 December 1812 are best explained by a rupture that begins on the San Jacinto fault and propagates onto the San Andreas fault. This precedent carries the implications that similar joint ruptures are possible in the future and that the San Jacinto fault plays a more significant role in seismic hazard in southern California than previously considered. My work also shows how physics-based modeling can be used for interpreting paleoseismic data sets and understanding prehistoric fault behavior.
Correlations between geologic units and shear wave velocity form the basis of a series of maps developed over the past 15 years to estimate average shear-wave velocity to 30m (Vs30). Wills et al.’s (2000) site-condition map for California was found to correlate with seismic amplification (Field, 2000) and was adopted as a standard depiction for many applications of seismic shaking estimates (ShakeMap for example). Wills and Clahan (2006) modified that map to show simplified geologic units and corresponding Vs30 values. Preparation of this map raised a number of questions on how best to distinguish units within younger alluvium. Wills and Gutierrez (2011) found that a simple system based on surface slope could be used to subdivide the younger alluvium into three classes that have distinct Vs30 ranges. The classes defined by slope have approximately the same variability in Vs30 as the previously defined classes, but the total number of classes is reduced and the system can be easily applied to other tectonically active areas. We have now applied the system of Wills and Gutierrez (2011) to create a new map of California using the most detailed available geologic maps. Use of more detailed geologic maps, from 1:250,000 scale to 1:24,000 for much of California, results in a much more detailed and accurate depiction of the surficial geology and, we anticipate, a more detailed and accurate depiction of seismic amplification due to the near-surface materials.
Measurements of ground motion generated by nuclear explosions in Nevada were made for 37 locations near San Francisco Bay, Calif. The results were compared with the San Francisco 1906 earthquake intensities and the strong-motion recordings of the San Francisco earthquake of Mar 22 1957. The recordings show marked amplitude variations which are related consistently to the geologic setting of the recording site. Consistent correlations of the results from the nuclear recordings with the 1906 earthquake intensities and the spectral amplification curves for the 1957 earthquake suggest that areas of high amplification determined from small ground motions may also be areas of high intensity in future earthquakes.
The 2014 Working Group on California Earthquake Probabilities (WGCEP14) present the time-independent component of the Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3), which provides authoritative estimates of the magnitude, location, and time-averaged frequency of potentially damaging earthquakes in California. The primary achievements have been to relax fault segmentation and include multifault ruptures, both limitations of UCERF2. The rates of all earthquakes are solved for simultaneously and from a broader range of data, using a system-level inversion that is both conceptually simple and extensible. The inverse problem is large and underdetermined, so a range of models is sampled using an efficient simulated annealing algorithm. The approach is more derivative than prescriptive (e.g., magnitude-frequency distributions are no longer assumed), so new analysis tools were developed for exploring solutions. Epistemic uncertainties were also accounted for using 1440 alternative logic-tree branches, necessitating access to supercomputers. The most influential uncertainties include alternative deformation models (fault slip rates), a new smoothed seismicity algorithm, alternative values for the total rate of M-w >= 5 events, and different scaling relationships, virtually all of which are new. As a notable first, three deformation models are based on kinematically consistent inversions of geodetic and geologic data, also providing slip-rate constraints on faults previously excluded due to lack of geologic data. The grand inversion constitutes a system-level framework for testing hypotheses and balancing the influence of different experts. For example, we demonstrate serious challenges with the Gutenberg-Richter hypothesis for individual faults. UCERF3 is still an approximation of the system, however, and the range of models is limited (e.g., constrained to stay close to UCERF2). Nevertheless, UCERF3 removes the apparent UCERF2 overprediction of M 6.5-7 earthquake rates and also includes types of multifault ruptures seen in nature. Although UCERF3 fits the data better than UCERF2 overall, there may be areas that warrant further site-specific investigation. Supporting products may be of general interest, and we list key assumptions and avenues for future model improvements.
We use a database of approximately 200,000 modified Mercalli intensity (MMI) observations of California earthquakes collected from USGS "Did You Feel It?" (DYFI) reports, along with a comparable number of peak ground-motion amplitudes from California seismic networks, to develop probabilistic relationships between MMI and peak ground velocity (PGV), peak ground acceleration (PGA), and 0.3-s, 1-s, and 3-s 5% damped pseudospectral acceleration (PSA). After associating each ground-motion observation with an MMI computed from all the DYFI responses within 2 km of the observation, we derived a joint probability distribution between MMI and ground motion. We then derived reversible relationships between MMI and each ground-motion parameter by using a total least squares regression to fit a bilinear function to the median of the stacked probability distributions. Among the relationships, the fit to peak ground velocity has the smallest errors, though linear combinations of PGA and PGV give nominally better results. We also find that magnitude and distance terms reduce the overall residuals and are justifiable on an information theoretic basis. For intensities MMI >= 5, our results are in close agreement with the relations of Wald, Quitoriano, Heaton, and Kanamori (1999); for lower intensities, our results fall midway between Wald, Quitoriano, Heaton, and Kanamori (1999) and those of Atkinson and Kaka (2007). The earthquakes in the study ranged in magnitude from 3.0 to 7.3, and the distances ranged from less than a kilometer to about 400 km from the source.
Local geological conditions, including both near-surface sedimentary layers and topographic features, are known to significantly influence ground motions caused by earthquakes. Microzonation maps use local geological conditions to characterize seismic hazard, but commonly incorporate the effect of only sedimentary layers. Microzonation does not take into account local topography, because significant topographic amplification is assumed to be rare. Here we show that, although the extent of structural damage in the 2010 Haiti earthquake was primarily due to poor construction, topographic amplification contributed significantly to damage in the district of Petionville, south of central Port-au-Prince. A large number of substantial, relatively well-built structures situated along a foothill ridge in this district sustained serious damage or collapse. Using recordings of aftershocks, we calculate the ground motion response at two seismic stations along the topographic ridge and at two stations in the adjacent valley. Ground motions on the ridge are amplified relative to both sites in the valley and a hard-rock reference site, and thus cannot be explained by sediment-induced amplification. Instead, the amplitude and predominant frequencies of ground motion indicate the amplification of seismic waves by a narrow, steep ridge. We suggest that microzonation maps can potentially be significantly improved by incorporation of topographic effects.
Analysis of Modified Mercalli intensity (MMI) observations for a training set of 22 California earthquakes suggests a strategy for bounding the epicentral region and moment magnitude M from MMI observations only. We define an intensity magnitude MI that is calibrated to be equal in the mean to M. MI = mean (Mi), where Mi = (MMIi + 3.29 + 0.0206 * Δi)/1.68 and Δi is the epicentral distance (km) of observation MMIi. The epicentral region is bounded by contours of rms [MI] = rms (MI − Mi − rms0 (MI − Mi), where rms is the root mean square, rms0 (MI − Mi) is the minimum rms over a grid of assumed epicenters, and empirical site corrections and a distance weighting function are used. Empirical contour values for bounding the epicenter location and empirical bounds for M estimated from MI appropriate for different levels of confidence and different quantities of intensity observations are tabulated. The epicentral region bounds and MI obtained for an independent test set of western California earthquakes are consistent with the instrumental epicenters and moment magnitudes of these earthquakes. The analysis strategy is particularly appropriate for the evaluation of pre-1900 earthquakes for which the only available data are a sparse set of intensity observations.
While recent investigations of induced earthquakes have focused on earthquakes associated with wastewater injection and unconventional recovery methods, the potential for earthquakes to be induced by primary production has long been recognized. We use boundary element methods to quantify the predicted geometry and amplitude of stress and strain changes associated with removal of large volumes of fluids in poroelastic reservoirs, focusing on the Los Angeles Basin (LA Basin) in California. We show that significant stress perturbations (upward of 0.1 MPa), while localized, typically extended several kilometers away from production horizons by the early 1940s. By this time, production horizons in the southwestern LA Basin were 2-4 km deep; models thus predict that stress conditions would have been perturbed significantly on faults at the upper edge of the seismogenic brittle crust, typically around 6 km. Predicted stress and strain changes associated with oil fields in the southwestern LA Basin during the first half of the 20th century, combined with stress changes caused by the 1933 Long Beach earthquake, could plausibly have induced a number of moderate-to-large earthquakes between 1932 and 1944. The rate of earthquakes in the southwestern LA Basin has been significantly lower since 1945 than it was for the three decades prior to 1945. We conclude that while decreasing production and pore-pressure reduction contributed to the initial decline, the continued decline was due in part to the advent of widespread water-flooding methods that maintained subsurface reservoir pressures.
Recent studies have presented evidence that early to mid-twentiethcentury earthquakes in Oklahoma and Texas were likely induced by fossil fuel production and/or injection of wastewater (Hough and Page, 2015; Frohlich et al., 2016). Considering seismicity from 1935 onward, Hauksson et al. (2015) concluded that there is no evidence for significant induced activity in the greater Los Angeles region between 1935 and the present. To explore a possible association between earthquakes prior to 1935 and oil and gas production, we first revisit the historical catalog and then review contemporary oil industry activities. Although early industry activities did not induce large numbers of earthquakes, we present evidence for an association between the initial oil boom in the greater Los Angeles area and earthquakes between 1915 and 1932, including the damaging 22 June 1920 Inglewood and 8 July 1929 Whittier earthquakes. We further consider whether the 1933 Mw 6.4 Long Beach earthquake might have been induced, and show some evidence that points to a causative relationship between the earthquake and activities in the Huntington Beach oil field. The hypothesis that the Long Beach earthquake was either induced or triggered by an foreshock cannot be ruled out. Our results suggest that significant earthquakes in southern California during the early twentieth century might have been associated with industry practices that are no longer employed (i.e., production without water reinjection), and do not necessarily imply a high likelihood of induced earthquakes at the present time.
In Texas, earthquakes have occurred in close association with activities accompanying petroleum production since 1925. Here we develop a five-question test to categorize individual events as "tectonic," "possibly induced," "probably induced," or "almost certainly induced." In Texas, the probably induced and almost certainly induced earthquakes are broadly distributed geographically, in the Fort Worth basin of north Texas, the Haynesville Shale play area of east Texas, along the Gulf Coast in south Texas, and the Permian basin of west Texas. As the technologies applied to manage petroleum fields have evolved, induced earthquakes have been associated with different practices. In fields being driven by primary recovery prior to 1940, earthquakes occurred in fields extracting high volumes of petroleum from shallow strata. Subsequently, as field pressures decreased and secondary recovery technologies became common, earthquakes also occurred in association with water-flooding operations. Since 2008, the rate of earthquakes with magnitudes greater than 3 has increased from about 2 events/yr to 12 events/yr; much of this change is attributable to earthquakes occurring within a few kilometers of wastewater disposal wells injecting at high monthly rates. For three sequences monitored by temporary local seismograph networks, most hypocenters had focal depths at and deeper than the depth of injection and occurred along mapped faults situated within 2 km of injection sites. The record clearly demonstrates that induced earthquakes have been broadly distributed in several different geographic parts of Texas over the last 90 years.
Among the most important data in geophysics are the seismograms
which have produced most of the basic and quantitative information concerning
the seismic source and the Earth's interior. Modern seismographic
instruments provide high quality seismograms with a wide dynamic range
and frequency band. The analysis of these seismograms, together with the
recent developments in theory and methodology, has resulted in an order
of magnitude increase, both in quantity and quality, of our knowledge of
the Earth's interior and physics of earthquakes. However, the earthquake
cycle is a long-term process so that study over a long period of time is
essential for a thorough understanding of the earthquake phenomenon.
Furthermore, since earthquakes may not repeat in exactly the same way,
detailed analyses of earthquakes in the past are important.
For more than a century, a large number of seismograms have been
recorded at many stations in the world, but many of them have not
been fully utilized mainly because theories and methods had not been
developed well enough to fully interpret the seismograms at the time
when the earthquakes occurred. In many cases, methods developed in
later years enabled seismologists to investigate existing records of earlier
events, searching for further information. In this paper, we illustrate the
importance of historical seismograms for various geophysical studies.
The ML 5.1 Santa Barbara earthquake of August 13, 1978, occurred at 34o22.2'N., long. 119o43.0'4km south of Santa Barbara, California, at a depth of 12.5km in the northeast Santa Barbara Channel, part of the western Transverse Ranges geomorphic-structural province. This part of the province is characterized by seismically active, east-trending reverse faults and rates of coastal uplift that have averaged up to about 10m/1000 yr over the last 45 000 yr. No surface rupture was detected onshore. Subsurface rupture propagated northwest from the main shock toward Goleta, 15km west of Santa Barbara, where a maximum acceleration of 0.44 g was measured at ground level and extensive minor damage occurred; only minor injuries were reported. A fairly well constrained fault plane solution of the main shock and distribution of the aftershocks indicate that left-reverse-oblique slip occurred on west-northwest-trending, north-dipping reverse faults; inadequate dip control precludes good correlation with any one of several mapped faults.-Authors
Waveform modeling and first motion analysis are used to determine the source parameters of six 5.8 ≦ M ≦ 6.8 earthquakes that occurred between 1925 and 1964 within the northern Intermountain Seismic Belt of Montana. Results of this study suggest that the 1925 Clarkston earthquake occurred along an oblique normal fault with a trend similar to the southern end of the Clarkston Valley fault. The two largest earthquakes of the 1935 Helena sequence occurred along right-lateral strike-slip faults with trends similar to the Bald Butte and Helena Valley faults. The 1947 Virginia City earthquake occurred along a northwest-southeast trending segment of the Madison fault. Movement at depth was along a fault with strike similar to that of the 1959 Hebgen Lake main shock. A reanalysis of a M = 6.0 aftershock of the 1959 Hebgen Lake sequence suggests the earthquake occurred at a depth of 8 km along a fault that is not seen at the surface. An M = 5.8 earthquake in 1964, located about 10 km from the 1959 aftershock, may have occurred along steeply dipping fault planes (48° to 80°) at depths of 8 to 14 km. Most events could be modeled as simple ruptures.
Following the Northridge, California, earthquake of 17 January 1994, the Humboldt Earthquake Education Center (HEEC) conducted a telephone survey of approximately 6000 adults from within the felt region to study the earthquake's effects on individual households. We tabulate human responses to the earthquake and observations of the earthquake's effects on inanimate objects from the HEEC survey as a function of independently assigned U.S. Geological Survey Modified Mercalli (USGS MM) intensity for the same communities. Human responses and subjective judgments about the severity of shaking, when averaged over a large number of samples, are useful discriminants of the levels of strong ground motion up to those associated with USGS MM intensity VII, notwithstanding that individual human responses and subjective judgments are notoriously unreliable. A threshold of strong ground motion corresponds to USGS MM intensity of about VII, above which over 40% of respondents described the earthquake as "violent" and most people reported difficulty standing, furniture displaced, and some damage to their homes. Even at intensities VIII and IX, however, relatively few people (about 15%) described their reaction as "panic" and only about 12% reported major damage to their homes. The HEEC phone-survey data show that, in communities of low to moderate shaking, USGS MM intensities estimated from a single postal questionnaire are quite robust. At USGS MM intensity of V and below, 88% of USGS MM intensities determined from postal questionnaires are within one intensity unit of intensities determined from the more numerous HEEC telephone survey data for the same community. We introduce the concept of a community decimal intensity scale (CDI) based on telephone-survey data and calibrated to agree on average with the USGS MM intensities. The CDIs are more regularly distributed than the USGS MM intensities and show much less scatter when plotted as a function of epicentral distance. CDIs show promise as a tool for comparing regional attenuation, for delineating variations in shaking strength within areas mapped as a single MM zone, and for rapid preliminary intensity estimates utilizing electronic media.
The 21 October 1868 Hayward, California, earthquake is among the best-characterized historical earthquakes in California. In contrast to many other moderate-to-large historical events, the causative fault is clearly established. Published magnitude estimates have been fairly consistent, ranging from 6.8 to 7.2, with 95% confidence limits including values as low as 6.5. The magnitude is of particular importance for assessment of seismic hazard associated with the Hayward fault and, more generally, to develop appropriate magnitude–rupture length scaling relations for partially creeping faults. The recent reevaluation of archival accounts by Boatwright and Bundock (2008), together with the growing volume of well-calibrated intensity data from the U.S. Geological Survey “Did You Feel It?” (DYFI) system, provide an opportunity to revisit and refine the magnitude estimate. In this study, we estimate the magnitude using two different methods that use DYFI data as calibration. Both approaches yield preferred magnitude estimates of 6.3–6.6, assuming an average stress drop. A consideration of data limitations associated with settlement patterns increases the range to 6.3–6.7, with a preferred estimate of 6.5. Although magnitude estimates for historical earthquakes are inevitably uncertain, we conclude that, at a minimum, a lower-magnitude estimate represents a credible alternative interpretation of available data. We further discuss implications of our results for probabilistic seismic-hazard assessment from partially creeping faults.
The “revised magnitudes”, M, converted from Gutenberg's unified magnitude, m, and listed by Richter (1958) and Duda (1965) are systematically higher than the magnitudes listed by Gutenberg and Richter (1954) in Seismicity of the Earth. This difference is examined on the basis of Gutenberg and Richter's unpublished original worksheets for Seismicity of the Earth. It is concluded that (1) the magnitudes of most shallow “class a” earthquakes in Seismicity of the Earth are essentially equivalent to the 20-sec surface-wave magnitude, M_s; (2) the revised magnitudes, M, of most great shallow (less than 40 km) earthquakes listed in Richter (1958) (also used in Duda, 1965) heavily emphasize body-wave magnitudes, m_b, and are given by M = 1/4 M_s + 3/4 (1.59 m_b - 3.97). For earthquakes at depths of 40 to 60 km, M is given by M = (1.59 m_b − 3.97). M and M_s are thus distinct and should not be confused. Because of the saturation of the surface-wave magnitude scale at M_s ≃ 8.0, use of empirical moment versus magnitude relations for estimating the seismic moment results in large errors. Use of the fault area, S, is suggested for estimating the moment.
Over 480 earthquakes from central and eastern North America having instrumental mb magnitudes and maximum Modified Mercalli intensities (I0) were examined using exploratory data analysis techniques and modeled with robust estimation methods (91 of those earthquakes also have felt area data). A previously undocumented distinct offset of magnitudes is observed between Modified Mercalli intensities VI and VII in both the central and eastern North America data set and a separate western North American catalog of earthquakes. The offset is most probably a characteristic of the Modified Mercalli scale and brings into question the often assumed linear relationship between magnitude and intensity. (In particular, linear regression models could not accurately estimate magnitudes for larger, I0 = VII or VIII, events.) Instead, robust estimates of center and spread for individual intensity interval distributions are recommended. In studies where conversion of intensity to magnitude for groups of earthquakes is required (e.g., studies involving the Gutenberg-Richter recurrence relation), the underlying distribution of magnitudes (or its approximation) for each intensity should be used.
Linear regression models using I0 and/or felt area (and several transformations were tested. The robust linear regression models
m b = 2.48 + 0.0769 log 2 ( F A ) and m b = 2.16 + 0.0219 l 0 2 + 0.0596 log 2 ( F A )
(where FA is felt area in square kilometers) proved to be the most accurate magnitude estimation models for central and eastern North America earthquakes. A comparison of regional felt area models indicates that regional differences in attenuation of seismic waves may exist between central and eastern North America that becomes apparent only for events of sufficient magnitude. Those differences appear as larger felt areas for earthquakes of central North America as compared to earthquakes of eastern North America.
The 5.1 M_L Santa Barbara earthquake of 13 August 1978 occurred at 22h54m 52.8s UTC. The epicenter was located 3 km southeast of Santa Barbara at 34° 23.9′N latitude and 119°40.9′W longitude with a focal depth of 12.7 km. The main shock was followed between 13 August and 30 September by 373 aftershocks that were located with the Caltech-USGS array. The aftershock zone extended 12 km WNW from the epicenter and was 6 km wide in the N-S direction, and it had a very clear temporal development. During the first 20 min of activity, all the aftershocks were located in a cluster 7 km WNW of the main shock epicenter. During the next 24 hr, the aftershock zone grew to 11 km in the WNW direction and 4 km in the N-S direction. During succeeding weeks, the zone extended to 12 by 6 km. This temporal-spatial development relative to the main shock epicenter may indicate that the initial rupture propagated 7 km unilaterally to the WNW, and the initial rupture plane may have been considerably smaller than the eventual aftershock zone. This smaller area suggests that the stress drop may have been significantly greater than that derived from the final aftershock zone.
In cross section, the aftershock hypocenters outline a nearly horizontal plane (dipping 15° or less) at 13 km depth. The main shock focal mechanism indicates NNE-SSW compression and vertical extension. The preferred fault plane strikes N80°W and dips 26°NNE, indicating north-over-south thrusting with a component of left-lateral movement. Focal mechanisms for 40 aftershocks also indicate compression in the general N-S direction. For most of these events, the north-dipping nodal plane dips between 7° and 45°, with most dipping 25° or more, which is significantly steeper than the plane delineated by the hypocenters themselves. These observations are consistent with a tectonic model in which much of the slip during the Santa Barbara earthquake occurred on a nearly horizontal plane. The after shocks then might represent movement on a complex series of imbricate thrust faults that flatten into the plane of primary slip. Hence, the Santa Barbara earthquake may be taken as evidence for mid-crustal horizontal shearing in the western Transverse Ranges.
Seismicity rates have increased sharply since 2009 in the central and eastern United States, with especially high rates of activity in the state of Oklahoma. Growing evidence indicates that many of these events are induced, primarily by injection of wastewater in deep disposal wells. The upsurge in activity has raised two questions: What is the background rate of tectonic earthquakes in Oklahoma? How much has the rate varied throughout historical and early instrumental times? In this article, we show that (1) seismicity rates since 2009 surpass previously observed rates throughout the twentieth century; (2) several lines of evidence suggest that most of the significant earthquakes in Oklahoma during the twentieth century were likely induced by oil production activities, as they exhibit statistically significant temporal and spatial correspondence with disposal wells, and intensity measurements for the 1952 El Reno earthquake and possibly the 1956 Tulsa County earthquake follow the pattern observed in other induced earthquakes; and (3) there is evidence for a low level of tectonic seismicity in southeastern Oklahoma associated with the Ouachita structural belt. The 22 October 1882 Choctaw Nation earthquake, for which we estimate M-w 4.8, occurred in this zone.
In this study, I consider the ground motions generated by 11 moderate (Mw 4.0–5.6) earthquakes in the central and eastern United States that are thought or suspected to be induced by fluid injection. Using spatially rich intensity data from the U.S. Geological Survey “Did You Feel It?” system, I show the distance decay of intensities for all events is consistent with that observed for tectonic earthquakes in the region, but for all of the events, intensities are lower than the values predicted from an intensity prediction equation that successfully characterizes intensities for regional tectonic events. I introduce an effective intensity magnitude MIE, defined as the magnitude that on average would generate a given intensity distribution. For all 11 events, MIE is lower than the event magnitude by 0.4–1.3 magnitude units, with an average difference of 0.82 units. This suggests stress drops of injection‐induced earthquakes are systematically lower than tectonic earthquakes by an estimated factor of 2–10. However, relatively limited data suggest intensities for epicentral distances less than 10 km are more commensurate with expectations for the event magnitude, which can be reasonably explained by the shallow focal depth of the events. The results suggest damage from injection‐induced earthquakes will be especially concentrated in the immediate epicentral region.
The Santa Barbara earthquake of 13 August 1978, provides an opportunity to perform a broadband investigation of body waves for a well-recorded, moderate size (M_L = 5.1) event. The long- and short-period teleseismic body waves are modeled in the time domain to construct a source time function which is consistent in the period range of 1 to 20 sec. The long-period records indicate an overall duration of 6 sec while the short-period records reveal the fine-scale character of the slip history consisting of two sharp pulses separated by about 1 sec. The source mechanism determined from this analysis is a moderately dipping (30°NE) thrust with significant left-lateral slip. The moment was determined to be 1.1 × 10^(25) dyne-cm.
The earthquake was also reasonably well recorded on accelerographs in the near-field. The modeling of the strong motion displacements was a two step procedure: (1) the displacements were modeled alone, and (2) in an attempt to achieve consistency between the local and far-field time functions, the qualitative features of the teleseismic short-period time function were used to predict the displacements. If the two sources in the short-period time function are allowed to have different mechanisms, the displacements can be modeled quite well. This suggests that the overall faulting process was rough, and the multiple source character suggested at high frequencies is due to high-stress drop asperities. The two sources are modeled as asperities separated by 1.5 km; the first source has a mechanism consistent with the teleseismic solution while the second source is more steeply dipping. The total moment determined from the strong motion data is 3.5 × 10^(24) dyne-cm or one-third the long-period moment. This is consistent with other recent studies which suggest that the high-frequency strong ground motion is controlled by the distribution of asperities even though the sum of their moments may be small compared to the overall moment. This study also shows the importance of teleseismic short periods in predicting the local displacements.
The central matter of this investigation is the commonly held belief that eastern North America (ENA) earthquakes are felt to much greater distances, at all levels of intensity, than their western U.S. (WUS) counterparts of the same source strength. By comparing the areas enclosed by Modified Mercalli intensities I to III (felt), IV, V, I and VII for ENA and WUS earthquakes at the same source strength, it is found that this proposition is mostly false at damaging levels of ground motion. -from Authors
Seismological observations local to the 1927 earthquake suggest an epicenter for it near 34.6°N, 120.9°W. These observations include S-P times for the immediate aftershocks recorded at four stations in southern California; S-P times reported for the main shock at Berkeley and Lick Observatory by P. Byerly; and a time-decaying (1934 to 1969) zone of seismicity centered offshore of Point Arguello, herein identified as the aftershock zone of the 1927 earthquake. This location is approximately 40 km southwest of the teleseismic location for the 1927 earthquake recently offered by W. Gawthrop, although a location intermediate to the one proposed here and the one by Gawthrop would satisfy uncertainties associated with both locations. Even so, the location farther offshore is suggested by the near absence of strong shaking in the adjacent coastal region, which seemingly precludes a near-coastal location for an earthquake of this magnitude. Furthermore, unpublished teleseismic first-motion data of G. Stewart and analysis of the distortion of a geodetic quadrilateral by J. Savage and W. Prescott argue against a faulting mechanism for the 1927 earthquake that involves predominantly right-lateral slip on a near-coastal and northwesterly striking fault of the San Andreas type.
The Lompoc earthquake of 4 November 1927 (Ms 7.0) was one of the largest offshore events in California, and the associated tsunami was recorded in Hawaii as well as on the California coast. From tsunami travel times, the source location was estimated to be well offshore at about 34.2°N, 120.75°W, consistent with recent seismic waveform analysis. Waveform modelling of the tsunami also supports an outer continental shelf location. The seismic moment is estimated to be 3×10²⁶dyne.cm, corresponding to a moment magnitude of 7.0. The tsunami computation reproduced the large tsunami in Hawaii, indicating that the observed large tsunami was due to bathymetry effects, particularly near Hilo Bay. The tsunami magnitude of 7.6 obtained from the Hawalian recordings by Abe thus appears to be an overestimate. -from Authors
INTRODUCTION
The most common information available immediately following a damaging earthquake is its magnitude and the epicentral location. However, it is also desirable to know the extent of the felt area, and, more important, the range of shaking experienced and the areal extent of strongest shaking. For most of the United States, there is insufficient seismic strong-motion station coverage to portray quickly and accurately the extent of strong shaking.
Seismic intensity has been traditionally used worldwide as a method for quantifying the shaking pattern and the extent of damage for earthquakes. Though developed prior to the advent of today's modern seismometric instrumentation, seismic intensity scales nonetheless provide a useful framework to describe, in a simplified fashion, the complexity of ground motions and the extent and nature of the damage. A limitation of traditional intensity mapping has been the long time required to generate a detailed seismic intensity map, typically weeks...
A modest but noteworthy Mw 5.9 earthquake occurred in the Bay of Bengal beneath the central Bengal fan at 21:51 Indian Standard Time (16:21 UTC) on 21 May 2014. Centered over 300 km from the eastern coastline of India, it caused modest damage by virtue of its location and magnitude. However, shaking was very widely felt in parts of eastern India where earthquakes are uncommon. Media outlets reported as many as four fatalities. Light damage was reported from a number of towns on coastal deltaic sediments, including collapsed walls and damage to pukka and thatched dwellings. Shaking was felt well inland into east‐central India and was perceptible in multistoried buildings as far as Chennai, Delhi, and Jaipur at distances of ≈1600 km. The purpose of this report is to make available the newly collected intensity dataset and to present preliminary analysis of this noteworthy recent earthquake. We further show that the intensity distribution provides evidence for a high-stress drop source. These results bear out the observation made two decades ago by Hanks and Johnston (1992, p. 20): “[Our] results suggest that it should be a fairly simple matter to infer a high-stress-drop event from intensity data alone, provided that an instrumental M0 or Mw value is known separately.” Our study illustrates the potential value of carefully determined intensity data for investigations of earthquake source properties, especially when instrumental recordings are sparse. We suggest it may in fact be a more robust way to estimate stress drop than conventional approaches, which require correction of attenuation to estimate pulse width or corner frequency (e.g., Anderson, 1986); the estimate is then cubed to estimate stress drop (Madariaga, 1976). Lastly, we discuss potentially important implications of our results for efforts to characterize probabilistic seismic hazard in the Himalayan region
This paper outlines the re-computation and compilation of the magnitudes now contained in the final ISC-GEM Reference Global Instrumental Earthquake Catalogue (1900-2009). The catalogue is available via the ISC website (www.isc.ac.uk/iscgem/) and lists in a comma separated format the location and magnitude parameters (with corresponding uncertainties) of global large earthquakes. In this work we report on the procedures adopted to obtain the final magnitude composition of the nearly 20,000 earthquakes processed. We have made every effort to use uniform procedures of magnitude determination throughout the entire period of the catalogue. The re-computation of the surface wave MS and short-period body-wave mb values benefitted from new hypocentres (Bondár et al., 2015), previously unavailable amplitude-period data digitized during this project (Di Giacomo et al., 2015), and a more reliable algorithm for magnitude estimation based on a 20% alpha-trimmed median magnitude (Bondár and Storchak, 2011). In particular, for MS until the end of 1970 we have processed an unprecedented amount of data and obtained several thousands of station magnitudes not available before.
[1] Analysis of numerous case histories of earthquake sequences induced by fluid injection at depth reveals that the maximum magnitude appears to be limited according to the total volume of fluid injected. Similarly, the maximum seismic moment seems to have an upper bound proportional to the total volume of injected fluid. Activities involving fluid injection include (1) hydraulic fracturing of shale formations or coal seams to extract gas and oil, (2) disposal of wastewater from these gas and oil activities by injection into deep aquifers, and (3) the development of Enhanced Geothermal Systems by injecting water into hot, low-permeability rock. Of these three operations, wastewater disposal is observed to be associated with the largest earthquakes, with maximum magnitudes sometimes exceeding 5. To estimate the maximum earthquake that could be induced by a given fluid injection project, the rock mass is assumed to be fully saturated, brittle, to respond to injection with a sequence of earthquakes localized to the region weakened by the pore pressure increase of the injection operation, and to have a Gutenberg-Richter magnitude distribution with a b-value of 1. If these assumptions correctly describe the circumstances of the largest earthquake, then the maximum seismic moment is limited to the volume of injected liquid times the modulus of rigidity. Observations from the available case histories of earthquakes induced by fluid injection are consistent with this bound on seismic moment. In view of the uncertainties in this analysis, however, this should not be regarded as an absolute physical limit.
We present a method that uses macroseismic intensity data to assess the location, physical dimensions, and orientation of the source of large historical earthquakes. Intensity data contain a great deal of information that can be used to constrain the essential characteristics of the seismic source. In particular, both the seismological theory and its practice suggest that the orientation of the source of significant earthquakes is reflected in the elongation of the associated damage pattern. A plausible and easily manageable way of describing a seismic source is by representing it as an oriented “rectangle,” the length and width of which are obtained from moment magnitude through empirical relationships. This rectangle is meant to represent either the actual surface projection of the seismogenic fault or, at least, the projection of the portion of the Earth crust where a given seismic source is likely to be located.
The systematic application of this method to all the M > 5.5 earthquakes that occurred in the central and southern Apennines (Italy) in the past four centuries returned encouraging results that compare well with existing instrumental, direct geological, and geodynamic evidence. The method is quite stable for different choices of the algorithm parameters and provides elongation directions that in most cases can be shown to be statistically significant. In particular, the resulting pattern of source orientations is rather homogeneous, showing a consistent Appennines-parallel trend that agrees well with the NE-SW extension style of deformation active in the central and southern portions of the Italian peninsula.
At the heart of the conundrum of seismogenesis in the New Madrid Seismic Zone is the apparently substantial discrepancy between low strain rate and high recent seismic moment release. In this study we revisit the magnitudes of the four principal 1811-1812 earthquakes using intensity values determined from individual assessments from four experts. Using these values and the grid search method of Bakun and Wentworth (1997), we estimate magnitudes around 7.0 for all four events, values that are significantly lower than previously published magnitude estimates based on macroseismic intensities. We further show that the strain rate predicted from postglacial rebound is sufficient to produce a sequence with the moment release of one Mmax6.8 every 500 years, a rate that is much lower than previous estimates of late Holocene moment release. However, Mw6.8 is at the low end of the uncertainty range inferred from analysis of intensities for the largest 1811-1812 event. We show that Mw6.8 is also a reasonable value for the largest main shock given a plausible rupture scenario. One can also construct a range of consistent models that permit a somewhat higher Mmax, with a longer average recurrence rate. It is thus possible to reconcile predicted strain and seismic moment release rates with alternative models: one in which 1811-1812 sequences occur every 500 years, with the largest events being Mmax∼6.8, or one in which sequences occur, on average, less frequently, with Mmax of ∼7.0. Both models predict that the late Holocene rate of activity will continue for the next few to 10 thousand years.