Paleoseismic Record Research Articles

Source Parameter Scaling and the Cascadia Paleoseismic Record

Several approaches to interpreting the Cascadia paleoseismic record are used to derive relationships between fault area, slip, and moment and to compare the results with the scaling relationships determined by Somerville et al. (2015) for recent subduction‐zone events. In two models (CA12a and CA12b), taken from Goldfinger et al. (2012), paleoevents are classified into five characteristic areas (CA), with the slip during each event estimated based on the time between the event and either the following or the previous event. In model CA14, taken from Scholz (2014), slip on four characteristic segments is determined from the plate tectonic convergence rate, assuming a constant stress drop. In model CL, introduced in this article, the fault length for paleoevents is defined by the along‐strike length over which the observations have been correlated; width and slip are interpolated from model CA14. CA12a and CA12b show large scatter compared with the global compilation because of large variations in slip for a given area. Models CA14 and CL reproduce the relationship derived for asperities (defined as patches in finite‐fault models with slip >1.5 times the average slip). These models can be reconciled with the total area and average slip from Somerville et al. (2015) by increasing the fault area and decreasing the slip using scaling factors derived from the analysis of recent earthquakes (CLmod1) or by reducing the slip by a factor of ∼8 (CLmod2). CLmod1 implies that the paleoearthquake observations are controlled by high‐slip patches, whereas CLmod2 implies that much of the plate tectonic convergence is accommodated aseismically. A scenario intermediate between CLmod1 and CLmod2 is considered most likely. This study demonstrates the value of using scaling relationships based on modern earthquakes as a tool for evaluating earthquake histories derived from paleoseismic data. Online Material: A table summarizing the source parameters for the different models presented in this article to enable figure reproduction. Figures that summarize the data points used by Somerville et al. (2015), data points from a new analysis that became available after preparation of the main article (Ye et al. , 2016), and results for the turbidite mass model (Goldfinger et al. , 2012) for estimating seismic moment.

Paleoseismic potential of sublacustrine landslide records in a high-seismicity setting (south-central Alaska)

Sublacustrine landslide stratigraphy is considered useful for quantitative paleoseismology in low-seismicity settings. However, as the recharging of underwater slopes with sediments is one of the factors that governs the recurrence of slope failures, it is not clear if landslide deposits can provide continuous paleoseismic records in settings of frequent strong shaking. To test this, we selected three lakes in south-central Alaska that experienced a strong historical megathrust earthquake (the 1964 Mw9.2 Great Alaska Earthquake) and exhibit high sedimentation rates in their main basins (0.2cmyr−1–1.0cmyr−1). We present high-resolution reflection seismic data (3.5kHz) and radionuclide data from sediment cores in order to investigate factors that control the establishment of a reliable landslide record. Seismic stratigraphy analysis reveals the presence of several landslide deposits in the lacustrine sedimentary infill. Most of these landslide deposits can be attributed to specific landslide events, as multiple landslide deposits sourced from different lacustrine slopes occur on a single stratigraphic horizon. We identify numerous events in the lakes: Eklutna Lake proximal basin (14 events), Eklutna Lake distal basin (8 events), Skilak Lake (7 events) and Kenai Lake (7 events). The most recent event in each basin corresponds to the historic 1964 megathrust earthquake. All events are characterized by multiple landslide deposits, which hints at a regional trigger mechanism, such as an earthquake (the synchronicity criterion). This means that the landslide record in each basin represents a record of past seismic events. Based on extrapolation of sedimentation rates derived from radionuclide dating, we roughly estimate a mean recurrence interval in the Eklutna Lake proximal basin, Eklutna Lake distal basin, Skilak Lake and Kenai Lake, at ~250yrs., ~450yrs., ~900yrs. and ~450yrs., respectively. This distinct difference in recording can be explained by variations in preconditioning factors like slope angle, slope recharging (sedimentation rate) and the sediment source area: faster slope recharging and a predominance of delta and alluvial fan failures, increase the sensitivity and lower the intensity threshold for slope instability. Also, the seismotectonic setting of the lakes has to be taken into account. This study demonstrates that sublacustrine landslides in several Alaskan lakes can be used as reliable recorders of strong earthquake shaking, when a multi-lake approach is used, and can enhance the temporal and spatial resolution of the paleoseismic record of south-central Alaska.

Using a genetic algorithm to estimate the details of earthquake slip distributions from point surface displacements

AbstractExamining fault activity over several earthquake cycles is necessary for long‐term modeling of the fault strain budget and stress state. While this requires knowledge of coseismic slip distributions for successive earthquakes along the fault, these exist only for the most recent events. However, overlying the Sunda Trench, sparsely distributed coral microatolls are sensitive to tectonically induced changes in relative sea levels and provide a century‐spanning paleogeodetic and paleoseismic record. Here we present a new technique called the Genetic Algorithm Slip Estimator to constrain slip distributions from observed surface deformations of corals. We identify a suite of models consistent with the observations, and from them we compute an ensemble estimate of the causative slip. We systematically test our technique using synthetic data. Applying the technique to observed coral displacements for the 2005 Nias‐Simeulue earthquake and 2007 Mentawai sequence, we reproduce key features of slip present in previously published inversions such as the magnitude and location of slip asperities. From the displacement data available for the 1797 and 1833 Mentawai earthquakes, we present slip estimates reproducing observed displacements. The areas of highest modeled slip in the paleoearthquake are nonoverlapping, and our solutions appear to tile the plate interface, complementing one another. This observation is supported by the complex rupture pattern of the 2007 Mentawai sequence, underlining the need to examine earthquake occurrence through long‐term strain budget and stress modeling. Although developed to estimate earthquake slip, the technique is readily adaptable for a wider range of applications.

Variability in magnitude of paleoearthquakes revealed by trenching and historical records, along the Haiyuan Fault, China

AbstractPaleoseismology provides fundamental data for generalizing earthquake recurrence behavior, by revealing past surface‐rupturing events. Determining the size of paleoseismic events is notoriously more challenging than their timing. Paleoearthquakes exposed in trenches are vaguely defined as large enough to break to the surface and often assumed to be similar in size. Here we show an example where the paleoseismic record includes events of both moderate and large magnitudes. At the Salt Lake site on the active left‐lateral Haiyuan Fault, northern Tibetan Plateau, a high‐resolution stratigraphic sequence recorded three and possibly four events since A.D. 1500, constrained by accelerator mass spectrometry14C dating. Historical accounts of earthquake damage in the study region suggest that several earthquakes exposed in the trenches markedly differ in magnitude. With the exception of the most recentM~8 earthquake that occurred in A.D. 1920, two earlier events, which occurred in A.D. 1760 (or 1709) and 1638, respectively, are considerably smaller, with magnitudeM < 7 and more likelyM~6 or less. Thus, this section of the Haiyuan Fault that broke during moderate‐magnitude events failed again after a short interval during a largeMw7.8–8.3 earthquake, as part of a larger multisegment rupture. Our study shows that moderate‐magnitude events can be preserved in the stratigraphy and exposed by paleoseismic trenching under ideal conditions, for instance, if sedimentation is fast enough and there is no hiatus in deposition. Eventually, the data presented add to the growing body of paleoseismic records containing events of different magnitudes with a large variability in rupture length and coseismic slip.

The effect of sea level changes on fault reactivation potential in Portugal

The aim of this study is to assess the impact of sea level changes on both the stress field and the potential of fault reactivation in west Iberia. The analysis is applied to a set of five active faults distributed across Portugal, selected for representing predominant fault directions and for being seismically active. The results show that the rise of sea level since the Last Glacial Maximum has produced flexural effects with distinct impacts on different faults. The Coulomb stress changes induced by the sea level rise along the S. Marcos-Quarteira (south Portugal) and the Horseshoe (offshore SW Iberia) faults are found to be extremely small, independently of the elastic plate thickness. These faults are thus unaffected by flexural effects related to ocean loading, and are unlikely to possess any paleoseismic record of this phenomenon. In contrast, the eustatic sea level rise during the late Pleistocene could have raised the Coulomb stress by 0.5–1MPa along the Manteigas–Vilariça–Bragança (north Portugal) and Lower Tagus Valley (Lisbon area) fault systems. Such stress perturbations are probably sufficient to impact the seismic cycle of the Manteigas–Vilariça–Bragança fault, bringing it closer to failure and possibly triggering the earthquake clusters that have been observed in previous paleoseismologic studies.

Potential for Geologic Records of Coseismic Uplift and Megathrust Rupture along the Nicoya Peninsula, Costa Rica

ABSTRACT Spotila, J.A.; Marshall, J.; DePew, K.; Prince, P.S., and Kennedy, L., 2016. Potential for geologic records of coseismic uplift and megathrust rupture along the Nicoya Peninsula, Costa Rica. This study presents the first paleoseismic investigation of the Middle America subduction zone at the Nicoya Peninsula, Costa Rica. This megathrust has been intensely studied over the past decade using a range of geologic and geophysical techniques, and it experienced interplate rupture in 2012 (moment magnitude 7.6). Despite many factors that could hinder preservation of a paleoseismic record in this coastal environment, including complex deposition in a tropical mangrove setting, this reconnaissance identifies two sites where stratigraphic evidence may record relative sea-level changes associated with Holocene earthquakes. Although more work is required to better constrain the timing and nature of these events, this study suggests that the Tamarindo and Playa Carrillo estuaries contain possible paleoseismic...

Southern San Andreas Fault Seismicity is Consistent with the Gutenberg–Richter Magnitude–Frequency Distribution

The magnitudes of any collection of earthquakes nucleating in a region are generally observed to follow the Gutenberg–Richter (GR) distribution. On some major faults, however, paleoseismic rates are higher than a GR extrapolation from the modern rate of small earthquakes would predict. This, along with other observations, led to the formulation of the characteristic earthquake hypothesis, which holds that the rate of small‐to‐moderate earthquakes is permanently low on large faults relative to the large‐earthquake rate (Wesnousky et al. , 1983; Schwartz and Coppersmith, 1984). We examine the rate difference between recent small‐to‐moderate earthquakes on the southern San Andreas fault (SSAF) and the paleoseismic record, hypothesizing that the discrepancy can be explained as a rate change in time rather than a deviation from GR statistics. We find that with reasonable assumptions, the rate changes necessary to bring the small and large earthquake rates into alignment agree with the size of rate changes seen in epidemic‐type aftershock sequence modeling, where aftershock triggering of large earthquakes drives strong fluctuations in the seismicity rates for earthquakes of all magnitudes. The necessary rate changes are also comparable to rate changes observed for other faults worldwide. These results are consistent with paleoseismic observations of temporally clustered bursts of large earthquakes on the SSAF and the absence of M ≥7 earthquakes on the SSAF since 1857.

Turbidite paleoseismology along the active continental margin of Chile – Feasible or not?

Much progress has been made in estimating recurrence intervals of great and giant subduction earthquakes using terrestrial, lacustrine, and marine paleoseismic archives. Recent detailed records suggest these earthquakes may have variable recurrence periods and magnitudes forming supercycles. Understanding seismic supercycles requires long paleoseismic archives that record timing and magnitude of such events. Turbidite paleoseismic archives may potentially extend past earthquake records to the Pleistocene and can thus complement commonly shorter-term terrestrial archives. However, in order to unambiguously establish recurring seismicity as a trigger mechanism for turbidity currents, synchronous deposition of turbidites in widely spaced, isolated depocenters has to be ascertained. Furthermore, characteristics that predispose a seismically active continental margin to turbidite paleoseismology and the correct sample site selection have to be taken into account.Here we analyze 8 marine sediment cores along 950 km of the Chile margin to test for the feasibility of compiling detailed and continuous paleoseismic records based on turbidites. Our results suggest that the deposition of areally widespread, synchronous turbidites triggered by seismicity is largely controlled by sediment supply and, hence, the climatic and geomorphic conditions of the adjacent subaerial setting. The feasibility of compiling a turbidite paleoseismic record depends on the delicate balance between sufficient sediment supply providing material to fail frequently during seismic shaking and sufficiently low sedimentation rates to allow for coeval accumulation of planktonic foraminifera for high-resolution radiocarbon dating.We conclude that offshore northern central Chile (29–32.5°S) Holocene turbidite paleoseismology is not feasible, because sediment supply from the semi-arid mainland is low and almost no Holocene turbidity-current deposits are found in the cores. In contrast, in the humid region between 36 and 38°S frequent Holocene turbidite deposition may generally correspond to paleoseismic events. However, high terrigenous sedimentation rates prevent high-resolution radiocarbon dating. The climatic transition region between 32.5 and 36°S appears to be best suited for turbidite paleoseismology.

The Preservation Potential of Coastal Coseismic and Tsunami Evidence Observed Following the 2012 Mw 7.8 Haida Gwaii Thrust Earthquake

Abstract We describe near‐field coseismic and tsunami evidence collected following the 28 October 2012 M w 7.8 thrust earthquake that occurred offshore western Haida Gwaii, British Columbia, and discuss factors that influence its extent and preservation potential. Observations indicate minor geomorphic and sedimentological impacts on the rugged and unpopulated west coast of the islands, despite widespread coastal coseismic subsidence (∼0.5 m), triggered landslides, and tsunami waves exceeding 3 m in runup (maximum 13 m) along ∼230 km of coastline. Evidence left by the tsunami was minimal, likely because it occurred during a low tide that restricted its onshore reach, its flow depth, and its capacity to entrain and transport significant amounts of clastic sediment, sources of which are minimal or absent on the dominantly steep, rocky coastline. It is unlikely that subaerial evidence of coseismic subsidence and tsunami inundation will be recognizably recorded in the coastal stratigraphy of western Haida Gwaii, due to the relatively small magnitude of subsidence, a lack of suitable coastal environments such as tidal marshes to record paleoelevation differences, sedimentation rates that are too low to bury a paleoseismic or paleotsunami record, and long‐term relative sea level fall leading to erosion and bioturbation. A higher preservation potential is likely for tsunami deposits in coastal lakes, ponds, and bogs, as well as coseismic slope failure deposits offshore, in lakes and in sheltered fjords. Our findings imply that large tsunamigenic earthquakes are likely undersampled in the paleoseismic record of Haida Gwaii and of other plate margins with similar characteristics.

Lake Tutira paleoseismic record confirms random, moderate to major and/or great Hawke’s Bay (New Zealand) earthquakes

Robust regional seismic-hazard assessments require millennial-scale paleoseismic histories that extend far beyond the range of historical and instrumental data. However, it is difficult to resolve the probability density functions for earthquake recurrence from the limited number of major to great earthquakes most paleoseismic records contain. Lake sediment records are repositories of information about paleoearthquake recurrence, with a sensitivity and fidelity over millennial time scales that suggest that they have the potential to yield reliable estimates of the recurrence distribution. We present a 7000 yr paleoseismic record from Lake Tutira (North Island, New Zealand) that ranks among the most detailed Holocene paleoearthquake chronologies available worldwide, and use it to empirically constrain the recurrence distribution of earthquakes with a minimum ground-shaking intensity of MMI 7 in one of New Zealand’s most seismically active areas. Our analysis confirms that a Poisson process describes the waiting times of single moderate to major and/or great paleoearthquakes in the Hawke’s Bay region.

A High-Resolution Geophysical Survey of Jenny Lake: Using Lake Sediments to Construct a Continuous Record of Tectonic Activity and Earthquake-Triggered Disturbances at Grand Teton National Park

The Teton Range, WY contains a legacy of late Cenozoic uplift and periodic Quaternary glaciations. Well-preserved fault scarps along the Teton fault displace glacier deposits from the most recent (Pinedale) glaciation and provide evidence for high fault activity during the past ~15,000 years. Observations of these scarps and previous field investigations indicate that postglacial fault offset occurred through a series of major, scarp-forming earthquakes. However, the postglacial paleoseismic record of the Teton fault remains incomplete. The goal of this project is to use lake sediments, contained in lake basins positioned on the fault, to construct a history of the timing and frequency of past earthquakes at Grand Teton National Park, and assess seismic impacts on diment erosion (e.g., landslides, debris flows, slope failures) and future hazard potential. Here, we report on multibeam sonar bathymetry and seismic reflection images from Jenny Lake, collected as part of an effort to identify glacial and tectonic landforms and to characterize infill stratigraphy. Our overarching objective is to combine these datasets with lake sediment cores from Jenny Lake and other nearby lakes to construct a continuous, accurately-dated record of past earthquakes and earthquake-generated slope failures in the Tetons.

Slipstream: an early Holocene slump and turbidite record from the frontal ridge of the Cascadia accretionary wedge off western Canada and paleoseismic implications

Slipstream Slump, a well-preserved 3 km wide sedimentary failure from the frontal ridge of the Cascadia accretionary wedge 85 km off Vancouver Island, Canada, was sampled during Canadian Coast Guard Ship (CCGS) John P. Tully cruise 2008007PGC along a transect of five piston cores. Shipboard sediment analysis and physical property logging revealed 12 turbidites interbedded with thick hemipelagic sediments overlying the slumped glacial diamict. Despite the different sedimentary setting, atop the abyssal plain fan, this record is similar in number and age to the sequence of turbidites sampled farther to the south from channel systems along the Cascadia Subduction Zone, with no extra turbidites present in this local record. Given the regional physiographic and tectonic setting, megathrust earthquake shaking is the most likely trigger for both the initial slumping and subsequent turbidity currents, with sediments sourced exclusively from the exposed slump face of the frontal ridge. Planktonic foraminifera picked from the resedimented diamict of the underlying main slump have a disordered cluster of 14C ages between 12.8 and 14.5 ka BP. For the post-slump stratigraphy, an event-free depth scale is defined by removing the turbidite sediment intervals and using the hemipelagic sediments. Nine 14C dates from the most foraminifera-rich intervals define a nearly constant hemipelagic sedimentation rate of 0.021 cm/year. The combined age model is defined using only planktonic foraminiferal dates and Bayesian analysis with a Poisson-process sedimentation model. The age model of ongoing hemipelagic sedimentation is strengthened by physical property correlations from Slipstream events to the turbidites for the Barkley Canyon site 40 km south. Additional modelling addressed the possibilities of seabed erosion or loss and basal erosion beneath turbidites. Neither of these approaches achieves a modern seabed age when applying the commonly used regional marine 14C reservoir age of 800 years (marine reservoir correction ΔR = 400 years). Rather, the top of the core appears to be 400 years in the future. A younger marine reservoir age of 400 years (ΔR = 0 years) brings the top to the present and produces better correlations with the nearby Effingham Inlet paleo-earthquake chronology based only on terrestrial carbon requiring no reservoir correction. The high-resolution dating and facies analysis of Slipstream Slump in this isolated slope basin setting demonstrates that this is also a useful type of sedimentary target for sampling the paleoseismic record in addition to the more studied turbidites from submarine canyon and channel systems. The first 10 turbidites at Slipstream Slump were deposited between 10.8 and 6.6 ka BP, after which the system became sediment starved and only two more turbidites were deposited. The recurrence interval for the inferred frequent early Holocene megathrust earthquakes is 460 ± 140 years, compatible with other estimates of paleoseismic megathrust earthquake occurrence rates along the subduction zone.

Strain accumulation in the New Madrid and Wabash Valley seismic zones from 14 years of continuous GPS observation

AbstractThe mechanical behavior—and hence earthquake potential—of faults in continental interiors is an issue of critical importance for the resultant seismic hazard, but no consensus has yet been reached on this controversial topic. The debate has focused on the central and eastern United States, in particular, the New Madrid Seismic Zone, struck by four magnitude 7 or greater earthquakes in 1811–1812, and to a lesser extent the Wabash Valley Seismic Zone just to the north. A key aspect of this issue is the rate at which strain is currently accruing on those plate interior faults, a quantity that remains debated. Here we address this issue with an analysis of up to 14.6 years of continuous GPS data from a network of 200 sites in the central United States centered on the New Madrid and Wabash Valley seismic zones. We find that the high‐quality sites in these regions show motions that are consistently within the 95% confidence limit of zero deformation. These results place an upper bound on strain accrual on faults of 0.2 mm/yr and 0.6 mm/yr in the New Madrid and Wabash Valley Seismic Zones, respectively. For the New Madrid region, where a paleoseismic record is available for the past ∼5000 years, we argue that strain accrual—if any—does not permit the 500–900 year repeat time of paleo‐earthquakes observed in the Upper Mississippi Embayment. These results, together with increasing evidence for temporal clustering and spatial migration of earthquake sequences in continental interiors, indicate that either tectonic loading rates or fault properties vary with time in the New Madrid Seismic Zone and possibly plate wide.

Reconstructing paleoseismic deformation, 2: 1000 years of great earthquakes at Chucalén, south central Chile

In this paper we adopt a quantitative biostratigraphic approach to establish a 1000-year-long coastal record of megathrust earthquake and tsunami occurrence in south central Chile. Our investigations focus on a site in the centre of the rupture segment of the largest instrumentally recorded earthquake, the AD 1960 magnitude 9.5 Chile earthquake. At Chucalén coseismic subsidence in 1960 is recorded in the lithostratigraphy and biostratigraphy of coastal marshes, with peat overlain by minerogenic sediment and changes in the assemblages of diatoms (unicellular algae) indicating an abrupt increase in relative sea level. In addition to the 1960 earthquake, the stratigraphy at Chucalén records three earlier earthquakes, the historically documented earthquake of 1575 and two prehistoric earthquakes, radiocarbon dated to AD 1270–1450 and 1070–1220. Laterally extensive sand sheets containing marine or brackish diatom assemblages suggest tsunami deposition associated with at least two of the three pre-1960 earthquakes. The record presented here suggests a longer earthquake recurrence interval, averaging 270 years, than the historical recurrence interval, which averages 128 years. The lack of geologic evidence at Chucalén of two historically documented earthquakes, in 1737 and 1837, supports the previously suggested hypothesis of variability in historical earthquake characteristics. Our estimates of coseismic land-level change for the four earthquakes range from meter-scale subsidence to no subsidence or slight uplift, suggesting earthquakes completing each ∼270 year cycle may not share a common, characteristic slip distribution. The presence of buried soils at elevations below their modern equivalents implies net relative sea-level rise over the course of the Chucalén paleoseismic record, in contrast to relative sea-level fall over preceding millennia inferred from sites on the mainland. Sea-level rise may contribute to the preservation of evidence for multiple earthquakes during the last millennium, while net relative sea-level fall over the last 2000–5000 years may explain the lack of evidence for older earthquakes.

Seismically-triggered organic-rich layers in recent sediments from Göllüköy Lake (North Anatolian Fault, Turkey)

Multi-proxy analyses on the sedimentary sequence of Göllüköy Lake, which is located on the eastern North Anatolian Fault (NAF), reveals a complete and high-resolution paleoseismic record for the last 650 years. Six sedimentary events are detected in a 3.1 m-long piston core. They form distinct organic-rich intercalations within the background sedimentation, which are characterized by strong anomalies on the loss-on-ignition (LOI550) and total organic carbon (TOC) profiles, as well as by lighter colours on the X-ray radiographic images. Itrax micro-XRF core scanner data are also used to contribute to the detection and characterization of the event deposits. After the detection of the sedimentary events, their temporal correlation with the earthquakes in the historical seismicity catalogue of the NAF is tested. The youngest event is dated to 1940s by using 210Pb and 137Cs profiles in sediment, which coincides with the 1939 earthquake (Ms = 7.7) on the NAF. The ages of the older five events are determined based on radiocarbon dating and regional time–stratigraphic correlation. Radiocarbon dating on the bulk sediment samples does not provide reliable results due to hard-water effect. On the other hand, dating on charcoals, Ephippia of Daphnia and phragmite remains significantly improves the results and implies a mean sedimentation rate of 0.28 cm/yr. Based on this preliminary sedimentation rate, we show that organic matter content variations through our record correlate with the varve-based δ18O record of Nar Lake, which is located 350 km southwest of Göllüköy Lake. Accordingly, high-precipitation/low-evaporation climatic episodes detected in Nar Lake are represented by higher organic matter content in Göllüköy sediments. Fine-tuning the Göllüköy LOI550 record to the Nar δ18O record reveals that the ages of the sedimentary events in Göllüköy match with well-known historical earthquakes that occurred around the lake. Finally, the origin of the organic-rich intercalations is discussed based on macroscopic observations and organic geochemistry. The events are attributed to reworked organic matter eroded from the lake's littoral zone by seismically-induced water oscillations (seiche), and their subsequent deposition at the middle parts of the lake. These seismically-induced events are atypical compared to the classic earthquake-triggered sedimentary processes in lacustrine environments, which are shortly reviewed in order to emphasize the unusual Göllüköy Lake record.

Magnitude Limits of Subduction Zone Earthquakes

Maximum earthquake magnitude ( m x ) is a critical parameter in seismic hazard and risk analysis. However, some recent large earthquakes have shown that most of the existing methods for estimating m x are inadequate. Moreover, m x itself is ill‐defined because its meaning largely depends on the context, and it usually cannot be inferred using existing data without associating it with a time interval. In this study, we use probable maximum earthquake magnitude within a time period of interest, m p( T ), to replace m x . The term m p( T ) contains not only the information of magnitude limit but also the occurrence rate of the extreme events. We estimate m p( T ) for circum‐Pacific subduction zones using tapered Gutenberg–Richter (TGR) distributions. The estimation of the two TGR parameters, β ‐value and corner magnitude ( m c), is performed using the maximum‐likelihood method with the constraint from tectonic moment rate. To populate the TGR, the rates of smaller earthquakes are needed. We apply the Whole Earth model, a high‐resolution global estimate of the rate of m ≥5 earthquakes, to estimate these rates. The uncertainties of m p( T ) are calculated using Monte‐Carlo simulation. Our results show that most of the circum‐Pacific subduction zones can generate m ≥8.5 earthquakes over a 250‐year interval, m ≥8.8 earthquakes over a 500‐year interval, and m ≥9.0 earthquakes over a 10,000‐year interval. For the Cascadia subduction zone, we include the 10,000‐year paleoseismic record based on turbidite studies to supplement the limited instrumental earthquake data. Our results show that over a 500‐year period, m ≥8.8 earthquakes are expected in this zone; over a 1000‐year period, m ≥9.0 earthquakes are expected; and over a 10,000‐year period, m ≥9.3 earthquakes are expected.

Holocene Earthquake History of Cascadia: A Quantitative Test

Abstract Motivated by the Holocene history of great Cascadia earthquakes proposed by Goldfinger et al. (2012), we developed a simple model by which we can examine this further. Combining their observation that the plate boundary ruptures in four characteristic segment types with an empirical area versus seismic moment relation and an assumption of constant stress drop, we obtain a model for the along‐strike slip distribution for each earthquake type. Summing the slips in each segment over the number of earthquake of all types produces estimates of Holocene plate convergence that quantitatively agree with independent estimates obtained from geodetic and geologic data. Because the width of the fully coupled subduction interface decreases to the south, the assumption of constant average stress drop in earthquakes requires that their slip also decreases to the south. To satisfy the plate convergence constraints, this requires shorter and increasingly numerous earthquakes to finger in toward the south, just as observed in the Goldfinger et al. (2012) paleoseismic record. We conclude that the Goldfinger et al. (2012) history is quantitatively consistent with plate tectonic constraints, and that the Cascadia subduction zone is fully coupled seismically. The seismic flux release rate averaged over the Holocene is estimated as 2.7×10 9 m 3 /yr.

A Paleoseismic Record of Earthquakes for the Dead Sea Transform Fault between the First and Seventh Centuries C.E.: Nonperiodic Behavior of a Plate Boundary Fault

We present new results from a paleoseismic trenching campaign at a site across the Jordan Gorge Fault (JGF), the primary strand of the Dead Sea Transform in northern Israel. In addition to the previously recognized earthquakes of 1202 and 1759 C.E., we observe evidence for eight surface-rupturing earthquakes prior to the second millennium C.E. The past millennium appears deficient in strain release with the occurrence of only two large ruptures, when compared with the preceding 1200 years. Assuming Gutenberg-Richter magnitude-frequency distribution, there is a dis- crepancy between measured rate of small-magnitude earthquakes (M <4) from instrumental records and large earthquake rates from paleoseismic records. The in- terevent time of surface-rupturing earthquakes varies by a factor of two to four during the past 2 ka at our site, and the fault's behavior is not time predictable. The JGF may be capable of rupturing in conjunction with both of its southern and northern neigh- boring segments, and there is tentative evidence that earthquakes nucleating in the Jordan Valley (e.g., the 749 C.E. earthquake) could either rupture through the stepover between the faults or trigger a smaller event on the JGF. We offer a model of earth- quake production for this segment in which the long-term slip rate remains constant while differing earthquake sizes can occur, depending on the segment from which they originated and the time since the last large event. The rate of earthquake occurrence in this model does not produce a time-predictable pattern over a period of 2 ka as a result of the interplay between fault segments to the south and north of the JGF. Online Material: High-resolution photomosaic logs of trench walls probability density function (PDF) and table of unit descriptions for channels 3 and 4.

Central Cascadia subduction zone creep

Central Cascadia between 43°N and 46°N has reduced interseismic uplift observed in geodetic data and coseismic subsidence seen in multiple thrust earthquakes, suggesting elevated persistent fault creep in this section of the subduction zone. We estimate subduction thrust “decade-scale” locking and crustal block rotations from three-component continuous Global Positioning System (GPS) time series from 1997 to 2013, as well as 80 year tide gauge and leveling-derived uplift rates. Geodetic observations indicate coastal central Oregon is rising at a slower rate than coastal Washington, southern Oregon and northern California. Modeled locking distributions suggest a wide locking transition zone that extends inland under central Oregon. Paleoseismic records of multiple great earthquakes along Cascadia indicate less subsidence in central Oregon. The Cascade thrust under central Oregon may be partially creeping for at least 6500 years (the length of the paleoseismic record) reducing interseismic uplift and resulting in reduced coseismic subsidence. Large accretions of Eocene age basalt (Siletzia terrane) between 43°N and 46°N may be less permeable compared to surrounding terranes, potentially increasing pore fluid pressures along the fault interface resulting in a wide zone of persistent fault creep. In a separate inversion, three-component GPS time series from 1 July 2005 to 1 January 2011 are used to estimate upper plate deformation, locking between slow-slip events (SSEs), slip from 16 SSEs and an earthquake mechanism. Cumulative SSEs and tectonic tremor are weakest between 43°N and 46°N where partial fault creep is increased and Siletzia terrane is thick, suggesting that surrounding rock properties may influence the mode of slip.

Late Pleistocene‐Holocene right slip rate and paleoseismology of the Nayband fault, western margin of the Lut block, Iran

AbstractThe 290 km long Nayband right‐lateral fault cuts across a region seismically quiescent during the last few millennia. Chlorine‐36 and optically stimulated luminescence (OSL) dating of cumulative geomorphic offsets between 9 ± 1 m and 195 ± 15 m with ages from 6.8 ± 0.6 ka to ∼ 100 ka allow deriving a slip rate of 1.8 ± 0.7 mm yr‐1. The paleoseismic record retrieved from the first trench excavated across the fault combined with 18 OSL ages demonstrates the occurrence of at least four large (Mw ∼ 7) earthquakes during the last 17.4 ± 1.3 ka and two older earthquakes (before ∼ 23 ka and 70 ± 5 ka). Sediments from the last ∼ 7 ka contain evidence of the three younger earthquakes. Penultimate and antepenultimate events occurred between 6.5 ± 0.4 ka and 6.7 ± 0.4 ka within at most 1 ka whereas the most recent earthquake occurred within the last millennium. Such an irregular earthquake occurrence may suggest seismic clustering. Therefore, the imminence of an earthquake along the fault cannot be discarded even if the most recent earthquake occurred within the last 800 years. This event went unnoticed in the historical records demonstrating the incompleteness of the historical seismic catalogs in Central Iran, challenging any assessment of seismic hazard without geologic information. Infrequent large earthquakes typify the slow‐slipping strike‐slip faults slicing Central and Eastern Iran. Also, the slip rates summed from the Iran Plateau up to the Afghan lowlands appear in fairly good agreement with the most recent GPS data.