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We first explore a series of retrospective earthquake interactions in southern California. We find that the four Mw≥7 shocks in the past 150 yr brought the Ridgecrest fault ∼1 bar closer to failure. Examining the 34 hr time span between the Mw 6.4 and Mw 7.1 events, we calculate that the Mw 6.4 event brought the hypocentral region of the Mw 7.1 earthquake 0.7 bars closer to failure, with the Mw 7.1 event relieving most of the surrounding stress that was imparted by the first. We also find that the Mw 6.4 cross-fault aftershocks shut down when they fell under the stress shadow of the Mw 7.1. Together, the Ridgecrest mainshocks brought a 120 km long portion of the Garlock fault from 0.2 to 10 bars closer to failure. These results motivate our introduction of forecasts of future seismicity. Most attempts to forecast aftershocks use statistical decay models or Coulomb stress transfer. Statistical approaches require simplifying assumptions about the spatial distribution of aftershocks and their decay; Coulomb models make simplifying assumptions about the geometry of the surrounding faults, which we seek here to remove. We perform a rate–state implementation of the Coulomb stress change on focal mechanisms to capture fault complexity. After tuning the model through a learning period to improve its forecast ability, we make retrospective forecasts to assess model’s predictive ability. Our forecast for the next 12 months yields a 2.3% chance of an Mw≥7.5 Garlock fault rupture. If such a rupture occurred and reached within 45 km of the San Andreas, we calculate it would raise the probability of a San Andreas rupture on the Mojave section by a factor of 150. We therefore estimate the net chance of large San Andreas earthquake in the next 12 months to be 1.15%, or about three to five times its background probability.

Online Material: Figures of background seismicity rate and magnitude of completeness, 95% confidence interval on the principal stress orientation and friction coefficient, influence of fault friction on the Coulomb stress imparted to the major mapped active faults, shear-stress changes resolved on the nodal planes of small earthquakes, and observed aftershock rates. The strongest San Francisco Bay area earthquake since the 1989 M w 7.0 Loma Prieta shock struck near Napa on 24 August 2014. Field mapping (Dawson et al. , 2014; Earthquake Engineering Research Institute [EERI], 2014; Brocher et al. , 2015) and seismic and geodetic source inversions (Barnhart et al. , 2015; Dreger et al. , 2015; Wei et al. , 2015) indicate that a 15-km-long northwest-trending section of the West Napa Valley fault ruptured in the earthquake. Remarkably, it was the first indisputable surface rupture in the Bay area since 1906. The Napa event, along with other smaller earthquakes such as the 1980 M w 5.8 Livermore and 1984 M w 6.2 Morgan Hill events on the Calaveras and Hayward faults over the past 3–4 decades, may indicate that the Bay area region is emerging from the stress shadow of the 1906 M w 7.8 San Francisco earthquake (Harris and Simpson, 1998; Pollitz et al. , 2004). Since 1979, there has been a 140% increase in the rate of M w≥4.1 shocks (Fig. 1) in the broader Bay area, with most concentrated in a corridor extending north from the 1989 Loma Prieta aftershock zone through the Calaveras, Greenville, Green Valley, Napa, and Rodgers Creek faults east of the San Francisco Bay (Fig. 1a). This corridor roughly coincides with the 1906 stress shadow that is being eroded away by more than a century of stress reaccumulation. The Napa event, as well as the surrounding faults on which we calculate …

Global earthquake activity rate model 1 (GEAR1) estimates the rate of shallow earthquakes with magnitudes 6–9 everywhere on Earth. It was designed to be reproducible and testable. Our preferred hybrid forecast is a log–linear blend of two parent forecasts based on the Global Centroid Moment Tensor (CMT) catalog (smoothing 4602 m ≥5.767 shallow earthquakes, 1977–2004) and the Global Strain Rate Map version 2.1 (smoothing 22,415 Global Positioning System velocities), optimized to best forecast the 2005–2012 Global CMT catalog. Strain rate is a proxy for fault stress accumulation, and earthquakes indicate stress release, so a multiplicative blend is desirable, capturing the strengths of both approaches. This preferred hybrid forecast outperforms its seismicity and strain‐rate parents; the chance that this improvement stems from random seismicity fluctuations is less than 1%. The preferred hybrid is also tested against the independent parts of the International Seismological Centre‐Global Earthquake Model catalog ( m ≥6.8 during 1918–1976) with similar success. GEAR1 is an update of this preferred hybrid. Comparing GEAR1 to the Uniform California Earthquake Rupture Forecast Version 3 (UCERF3), net earthquake rates agree within 4% at m ≥5.8 and at m ≥7.0. The spatial distribution of UCERF3 epicentroids most resembles GEAR1 after UCERF3 is smoothed with a 30 km kernel. Because UCERF3 has been constructed to derive useful information from fault geometry, slip rates, paleoseismic data, and enhanced seismic catalogs (not used in our model), this is encouraging. To build parametric catastrophe bonds from GEAR1, one could calculate the magnitude for which there is a 1% (or any) annual probability of occurrence in local regions. Online Material: Discussion of forecast‐scoring metrics, tables of scoring results, and source code and data files needed to reproduce forecast.

We calculate stress changes resulting from the M = 6.0 West Napa earthquake on north San Francisco Bay area faults. The earthquake ruptured within a series of long faults that pose significant hazard to the Bay area, and we are thus concerned with potential increases in the probability of a large earthquake through stress transfer. We conduct this exercise as a prospective test because the skill of stress‐based aftershock forecasting methodology is inconclusive. We apply three methods: (1) generalized mapping of regional Coulomb stress change, (2) stress changes resolved on Uniform California Earthquake Rupture Forecast faults, and (3) a mapped rate/state aftershock forecast. All calculations were completed within 24 h after the main shock and were made without benefit of known aftershocks, which will be used to evaluative the prospective forecast. All methods suggest that we should expect heightened seismicity on parts of the southern Rodgers Creek, northern Hayward, and Green Valley faults.

The activity of the transcription factor nuclear factor-erythroid 2 p45-derived factor 2 (NRF2) is orchestrated and amplified through enhanced transcription of antioxidant and antiinflammatory target genes. The present study has characterized a triazole-containing inducer of NRF2 and elucidated the mechanism by which this molecule activates NRF2 signaling. In a highly selective manner, the compound covalently modifies a critical stress-sensor cysteine (C151) of the E3 ligase substrate adaptor protein Kelch-like ECH-associated protein 1 (KEAP1), the primary negative regulator of NRF2. We further used this inducer to probe the functional consequences of selective activation of NRF2 signaling in Huntington's disease (HD) mouse and human model systems. Surprisingly, we discovered a muted NRF2 activation response in human HD neural stem cells, which was restored by genetic correction of the disease-causing mutation. In contrast, selective activation of NRF2 signaling potently repressed the release of the proinflammatory cytokine IL-6 in primary mouse HD and WT microglia and astrocytes. Moreover, in primary monocytes from HD patients and healthy subjects, NRF2 induction repressed expression of the proinflammatory cytokines IL-1, IL-6, IL-8, and TNFα. Together, our results demonstrate a multifaceted protective potential of NRF2 signaling in key cell types relevant to HD pathology.

The 11 April 2012 M 8.6 Indian Ocean earthquake was an unusually large intraoceanic strike‐slip event. For several days, the global M ≥4.5 and M ≥6.5 seismicity rate at remote distances (i.e., thousands of kilometers from the mainshock) was elevated. The strike‐slip mainshock appears through its Love waves to have triggered a global burst of strike‐slip aftershocks over several days. But the M ≥6.5 rate subsequently dropped to zero for the succeeding 95 days, although the M ≤6.0 global rate was close to background during this period. Such an extended period without an M ≥6.5 event has happened rarely over the past century, and never after a large mainshock. Quiescent periods following previous large ( M ≥8) mainshocks over the past century are either much shorter or begin so long after a given mainshock that no physical interpretation is warranted. The 2012 mainshock is unique in terms of both the short‐lived global increase and subsequent long quiescent period. We believe that the two components are linked and interpret this pattern as the product of dynamic stressing of a global system of faults. Transient dynamic stresses can encourage short‐term triggering, but, paradoxically, it can also inhibit rupture temporarily until background tectonic loading restores the system to its premainshock stress levels.

[1] The Kanto seismic corridor surrounding Tokyo has hosted four to five M ≥ 7 earthquakes in the past 400 years. Immediately after the Tohoku earthquake, the seismicity rate in the corridor jumped 10-fold, while the rate of normal focal mechanisms dropped in half. The seismicity rate decayed for 6–12 months, after which it steadied at three times the pre-Tohoku rate. The seismicity rate jump and decay to a new rate, as well as the focal mechanism change, can be explained by the static stress imparted by the Tohoku rupture and postseismic creep to Kanto faults. We therefore fit the seismicity observations to a rate/state Coulomb model, which we use to forecast the time-dependent probability of large earthquakes in the Kanto seismic corridor. We estimate a 17% probability of a M ≥ 7.0 shock over the 5 year prospective period 11 March 2013 to 10 March 2018, two-and-a-half times the probability had the Tohoku earthquake not struck.

Although strong remote aftershocks are exceedingly rare, their rate increased fivefold during the six days following the 2012 east Indian Ocean earthquake, perhaps as a result of the strike-slip nature of the 2012 event or a build up of close-to-failure nucleation sites.

The role of permanent versus transient crustal stress changes in triggering earthquakes is debated. Analysis of the stress imparted by the Joshua Tree and nearby Landers earthquakes in California in 1992 implies that a permanent drop in stress can halt aftershocks, so such static stress changes should be incorporated into seismic-hazard assessments.

A fundamental riddle of earthquake occurrence is that tectonic motions at plate interiors are steady, changing only subtly over millions of years, but at plate boundary faults, the plates are stuck for hundreds of years and then suddenly jerk forward in earthquakes. Why does this happen? The answer, as formulated by Harry F. Reid (Reid 1910, 192) is that the Earth’s crust is elastic, behaving like a very stiff slab of rubber sliding over a substrate of “honey”-like asthenosphere, and that faults are restrained by friction. The crust near the faults—zones of weakness that separate the plates—slowly deforms, building up stress until frictional resistance on the fault is overcome and the fault suddenly slips. For the past century, scientists have sought ways to use this knowledge to forecast earthquakes. QuakeCaster, a hands-on teaching model, simulates earthquakes and their interactions to help students explore and test the four leading hypotheses for earthquake occurrence. The model also allows students to extend these concepts to consider more complex earthquake interactions, clustering, and Coulomb stress triggering. The four leading hypotheses for earthquake occurrence essentially trend from the most uniform and predictable behavior to the most irregular and unpredictable. Hypothesis 1 states that earthquakes are periodic: all have the same amount of slip, and all are separated by the same amount of time (Schwartz and Coppersmith 1984). In hypothesis 2, earthquakes are “time-predictable”: the larger the slip in the last earthquake, the longer the wait until the next one (Shimazaki and Nakata 1980). In hypothesis 3, earthquakes are “slip-predictable”: the longer the time since the last earthquake, the greater the size of the next earthquake (Bufe et al. 1977). In hypothesis 4, known as the “Poisson” hypothesis, earthquakes occur randomly in time and have randomly varying sizes (Campbell 1982). In the Poisson hypothesis, the …

The 11 March 2011 Tohoku Earthquake provides an unprecedented test of the extent to which Coulomb stress transfer governs the triggering of aftershocks. During 11–31 March, there were 177 aftershocks with focal mechanisms, and so the Coulomb stress change imparted by the rupture can be resolved on the aftershock nodal planes to learn whether they were brought closer to failure. Numerous source models for the mainshock have been inverted from seismic, geodetic, and tsunami observations. Here, we show that, among six tested source models, there is a mean 47% gain in positively-stressed aftershock mechanisms over that for the background (1997–10 March 2011) earthquakes, which serve as the control group. An aftershock fault friction of 0.4 is found to fit the data better than 0.0 or 0.8, and among all the tested models, Wei and Sladen (2011) produced the largest gain, 63%. We also calculate that at least 5 of the seven large, exotic, or remote aftershocks were brought ≥0.3 bars closer to failure. With these tests as confirmation, we calculate that large sections of the Japan trench megathrust, the outer trench slope normal faults, the Kanto fragment beneath Tokyo, and the Itoigawa-Shizuoka Tectonic Line, were also brought ≥0.3 bars closer to failure.

The most predictable earthquakes are aftershocks, which invariably follow mainshocks. So what triggers aftershocks? It was recently argued — controversially — that the decay of aftershocks with distance from the main earthquake could only be explained by dynamic triggering. Keith Richards-Dinger and colleagues have tested this hypothesis and conclude that the observed decay can be better explained by static triggering. Resolving whether static or dynamic stress triggers most aftershocks and subsequent mainshocks is essential to understand earthquake interaction and to forecast seismic hazard. It has recently been argued that the decay of aftershocks with distance from the main earthquake could be explained only by dynamic triggering. This hypothesis has now been tested, the conclusion being that the observed decay can be better explained by static triggering. Resolving whether static1,2,3 or dynamic4,5,6,7,8 stress triggers most aftershocks and subsequent mainshocks is essential to understand earthquake interaction and to forecast seismic hazard9. Felzer and Brodsky10 examined the distance distribution of earthquakes occurring in the first five minutes after 2 ≤ M

The 2007 Working Group on California Earthquake Probabilities (WGCEP, 2007) presents the Uniform California Earthquake Rupture Forecast, Version 2 (UCERF 2). This model comprises a time-independent (Poisson-process) earthquake rate model, developed jointly with the National Seismic Hazard Mapping Program and a time-dependent earthquake-probability model, based on recent earthquake rates and stress-renewal statistics conditioned on the date of last event. The models were developed from updated statewide earthquake catalogs and fault deformation databases using a uniform methodology across all regions and implemented in the modular, extensible Open Seismic Hazard Analysis framework. The rate model satisfies integrating measures of deformation across the plate-boundary zone and is consistent with historical seismicity data. An overprediction of earthquake rates found at intermediate magnitudes (6.5≤ M ≤7.0) in previous models has been reduced to within the 95% confidence bounds of the historical earthquake catalog. A logic tree with 480 branches represents the epistemic uncertainties of the full time-dependent model. The mean UCERF 2 time-dependent probability of one or more M ≥6.7 earthquakes in the California region during the next 30 yr is 99.7%; this probability decreases to 46% for M ≥7.5 and to 4.5% for M ≥8.0. These probabilities do not include the Cascadia subduction zone, largely north of California, for which the estimated 30 yr, M ≥8.0 time-dependent probability is 10%. The M ≥6.7 probabilities on major strike-slip faults are consistent with the WGCEP (2003) study in the San Francisco Bay Area and the WGCEP (1995) study in southern California, except for significantly lower estimates along the San Jacinto and Elsinore faults, owing to provisions for larger multisegment ruptures. Important model limitations are discussed.

The occurrence of a right-lateral strike-slip earthquake in 1911 is inconsistent with the calculated ![Graphic][1] static stress decrease imparted by the 1906 rupture at that location on the Calaveras fault, and 5 yr of calculated post-1906 viscoelastic rebound does little to reload the fault. We have used all available first-motion, body-wave, and surface-wave data to explore possible focal mechanisms for the 1911 earthquake. We find that the event was most likely a right-lateral strike-slip event on the Calaveras fault, larger than, but otherwise resembling, the 1984 M w 6.1 Morgan Hill earthquake in roughly the same location. Unfortunately, we could recover no unambiguous surface fault offset or geodetic strain data to corroborate the seismic analysis despite an exhaustive archival search. We calculated the static and dynamic Coulomb stress changes for three 1906 source models to understand stress transfer to the 1911 site. In contrast to the static stress shadow, the peak dynamic Coulomb stress imparted by the 1906 rupture promoted failure at the site of the 1911 earthquake by 1.4–5.8 bar. Perhaps because the sample is small and the aftershocks are poorly located, we find no correlation of 1906 aftershock frequency or magnitude with the peak dynamic stress, although all aftershocks sustained a calculated dynamic stress of ≥3 bar. Just 20 km to the south of the 1911 epicenter, we find that surface creep of the Calaveras fault at Hollister paused for ∼17 yr after 1906, about the expected delay for the calculated static stress drop imparted by the 1906 earthquake when San Andreas fault postseismic creep and viscoelastic relaxation are included. Thus, the 1911 earthquake may have been promoted by the transient dynamic stresses, while Calaveras fault creep 20 km to the south appears to have been inhibited by the static stress changes. [1]: /embed/inline-graphic-1.gif

A three-dimensional evaluation of earthquake hypocentres beneath the Kanto basin in Japan reveals the presence of a distinct, 25-km-thick and 100-km-wide body. Its fast seismic velocity and the presence of a double seismic zone suggest that it is a fragment of the Pacific slab, rather than an extension of the Philippine Sea slab. This implies that the penetration of the Philippine Sea slab is much shallower beneath the Kanto basin than was previously thought. The Philippine Sea and Pacific plate slabs both subduct beneath Tokyo, and so their configuration and seismic potential have been subject to intensive study1,2,3,4,5,6,7,8. Previous work suggests that the Philippine Sea slab extends up to 100 km northwest of Tokyo and subducts to a depth of 90 km beneath the Kanto basin, where it is folded against the underlying Pacific slab. Here we evaluate seismic data in three dimensions and delineate a distinct 25-km-thick and 100-km-wide body beneath the Kanto basin that has hitherto been considered to be part of the Philippine Sea slab. We find that several of its characteristics—such as its high seismic velocity and the presence of a double seismic zone—point instead to an affinity with the Pacific slab, implying that the Philippine Sea slab penetrates to depths of only 35–40 km beneath the Kanto basin. We propose that the body is a fragment of Pacific lithosphere that formed by the collision of two intersecting seamount chains with the Japan Trench 2–3 million years ago. We suggest that such slab fragments may not be uncommon, especially where seamount subduction deforms trenches and near triple junctions.

The long recorded history of earthquakes in Japan affords an opportunity to forecast seismic shaking exclusively from past shaking. We calculate the time-averaged (Poisson) probability of severe shaking by using more than 10,000 intensity observations recorded since AD 1600 in a 350 km-wide box centered on Tokyo. Unlike other hazard-assessment methods, source and site effects are included without modeling, and we do not need to know the size or location of any earthquake nor the location and slip rate of any fault. The two key assumptions are that the slope of the observed frequency-intensity relation at every site is the same, and that the 400-year record is long enough to encompass the full range of seismic behavior. Tests we conduct here suggest that both assumptions are sound. The resulting 30-year probability of IJMA≥6 shaking (∼ PGA≥0.4 g or MMI≥ IX) is 30%–40% in Tokyo, Kawasaki, and Yokohama, and 10%–15% in Chiba and Tsukuba. This result means that there is a 30% chance that 4 million people will be subjected to IJMA≥6 shaking during an average 30-year period. We also produce exceedance maps of PGA for building-code regulations, and calculate short-term hazard associated with a hypothetical catastrophe bond. Our results resemble an independent assessment developed from conventional seismic hazard analysis for greater Tokyo.

Tokyo and its outlying cities are home to one-quarter of Japan's 127 million people. Highly destructive earthquakes struck the capital in 1703, 1855 and 1923, the last of which took 105 000 lives. Fuelled by greater Tokyo's rich seismological record, but challenged by its magnificent complexity, our joint Japanese–US group carried out a new study of the capital's earthquake hazards. We used the prehistoric record of great earthquakes preserved by uplifted marine terraces and tsunami deposits (17M∼8 shocks in the past 7000 years), a newly digitized dataset of historical shaking (10 000 observations in the past 400 years), the dense modern seismic network (300 000 earthquakes in the past 30 years), and Japan's GeoNet array (150 GPS vectors in the past 10 years) to reinterpret the tectonic structure, identify active faults and their slip rates and estimate their earthquake frequency. We propose that a dislodged fragment of the Pacific plate is jammed between the Pacific, Philippine Sea and Eurasian plates beneath the Kanto plain on which Tokyo sits. We suggest that the Kanto fragment controls much of Tokyo's seismic behaviour for large earthquakes, including the damaging 1855M∼7.3 Ansei-Edo shock. On the basis of the frequency of earthquakes beneath greater Tokyo, events with magnitude and location similar to theM∼7.3 Ansei-Edo event have aca20% likelihood in an average 30 year period. In contrast, our renewal (time-dependent) probability for the greatM≥7.9 plate boundary shocks such as struck in 1923 and 1703 is 0.5% for the next 30 years, with a time-averaged 30 year probability ofca10%. The resulting net likelihood for severe shaking (ca0.9gpeak ground acceleration (PGA)) in Tokyo, Kawasaki and Yokohama for the next 30 years isca30%. The long historical record in Kanto also affords a rare opportunity to calculate the probability of shaking in an alternative manner exclusively from intensity observations. This approach permits robust estimates for the spatial distribution of expected shaking, even for sites with few observations. The resulting probability of severe shaking isca35% in Tokyo, Kawasaki and Yokohama andca10% in Chiba for an average 30 year period, in good agreement with our independent estimate, and thus bolstering our view that Tokyo's hazard looms large. Given $1 trillion estimates for the cost of anM∼7.3 shock beneath Tokyo, our probability implies a $13 billion annual probable loss.

Two recent earthquakes left their mark on Santiago de Chile and Tokyo, well beyond the rupture zones, raising questions about the future vulnerability of these and other cities that lie in seismically active regions. Though spared strong shaking, the megacities nevertheless lit up in small quakes, perhaps signaling an abrupt change in the condition for failure on the faults beneath the cities. To detect such changes in earthquake rate requires good seismic monitoring networks; to respond to such hazard increases with civic preparations requires good government.

The M_w 6.5 San Simeon earthquake struck the central California coast on 22 December 2003 at 19:15:56 UTC (11:15:56 am local time.) The epicenter was located 11 km northeast of the town of San Simeon, and 39 km west-northwest of Paso Robles (Figure 1), as reported by the California Integrated Seismic Network (CISN, the California region of the Advanced National Seismic System [ANSS]). The mainshock nucleated at 35.702°N, 121.108°W and a depth of 7.1 km, and the rupture propagated unilaterally to the southeast. The strong directivity of the rupture resulted in a concentration of damage and aftershock activity to the southeast of the hypocenter. The worst earthquake damage occurred in Paso Robles, where two people died in the collapse of an unreinforced masonry building. The accurate and rapid earthquake information provided in near real-time by CISN/ANSS to the Governor's Office of Emergency Services made it possible to focus emergency response in the source area, although the earthquake was felt from San Francisco to Los Angeles.