a scaling law for slow earthquakes with the availability of the global positioning system and other technical advances a growing number of unusual earthquake phenomena occurring at relatively long periods have been recognized they include deep episodic tremor low frequency earthquakes slow slip events and silent earthquakes based on data chiefly from western japan ide et al report that these slow seismic events follow a unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes a diverse array of unusual earthquake phenomena occurring at relatively long periods slow events follow a unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes indicating that they comprise a new earthquake category recently a series of unusual earthquake phenomena have been discovered including deep episodic tremor1 low frequency earthquakes2 very low frequency earthquakes3 slow slip events4 and silent earthquakes5 6 7 8 9 each of these has been demonstrated to arise from shear slip just as do regular earthquakes but with longer characteristic durations and radiating much less seismic energy here we show that these slow events follow a simple unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes we find that their seismic moment is proportional to the characteristic duration and their moment rate function is constant with a spectral high frequency decay of f 1 this scaling and spectral behaviour demonstrates that they can be thought of as different manifestations of the same phenomena and that they comprise a new earthquake category the observed scale dependence of rupture velocity for these events can be explained by either a constant low stress drop model or a diffusional constant slip model this new scaling law unifies a diverse class of slow seismic events and may lead to a better understanding of the plate subduction process and large earthquake generation

investigating the role of the itoigawa shizuoka tectonic line towards the evolution of the northern fossa magna rift basin abstract the itoigawa shizuoka tectonic line istl fault system is considered to have one of the highest probabilities for a major inland earthquake occurrence in the whole of japan it is a complex fault system with the dip directions of the local fault segments changing from north to south between an east dipping low angle thrust fault a strike slip fault and a west dipping thrust fault the tectonic relations between the different parts of the fault system and the surrounding geological units are yet to be fully explained this study aims to reveal the juncture of the northern and central parts of the istl and investigate its contribution towards the shaping of the northern fossa magna rift basin we conducted 3 deployments of 1 or 2 linear arrays of seismic stations across the central and northern istl regions and observed local micro earthquakes for a period of 3 years each deployment recorded continuous waveform data for approximately 3 months using arrival times of 1193 local earthquakes we jointly determined earthquake locations and a 3d velocity model applying the tomography method we were able to image the regional crustal structures from the surface to a depth of 20 km with a spatial resolution of 5 km subsequently we used the obtained 3d velocity model to relocate the background local seismicity from 2003 to 2009 the juncture of the northern and central parts of the istl was well constrained by our results the depth extension of the northern parts of the istl fault segments follows the bottom of the miocene northern fossa magna rift basin nfm and forms an east dipping low angle fault in contrast the central parts of the istl fault segments are estimated to lie along the eastern boundary of the matsumoto basin forming an oblique strike slip fault fig 1

propagation of slow slip leading up to the 2011 mw 9 0 tohoku oki earthquake many large earthquakes are preceded by one or more foreshocks but it is unclear how these foreshocks relate to the nucleation process of the mainshock on the basis of an earthquake catalog created using a waveform correlation technique we identified two distinct sequences of foreshocks migrating at rates of 2 to 10 kilometers per day along the trench axis toward the epicenter of the 2011 moment magnitude mw 9 0 tohoku oki earthquake in japan the time history of quasi static slip along the plate interface based on small repeating earthquakes that were part of the migrating seismicity suggests that two sequences involved slow slip transients propagating toward the initial rupture point the second sequence which involved large slip rates may have caused substantial stress loading prompting the unstable dynamic rupture of the mainshock

size distribution of parkfield s microearthquakes reflects changes in surface creep rate s u m m a r y the nucleation area of the series of m6 events in parkfield has been shown to be characterized by low b values throughout the seismic cycle since low b values represent high differential stresses the asperity structure seems to be always stably stressed and even unaffected by the latest main shock in 2004 however because fault loading rates and applied shear stress vary with time some degree of temporal variability of the b value within stable blocks is to be expected we discuss in this study adequate techniques and uncertainty treatment for a detailed analysis of the temporal evolution of b values we show that the derived signal for the parkfield asperity correlates with changes in surface creep suggesting a sensitive time resolution of the b value stress meter and confirming near critical loading conditions within the parkfield asperity

evidence for fluid triggered slip in the 2009 mount rainier washington earthquake swarm 1 a vigorous swarm of over 1000 small shallow earthquakes occurred 20 22 september 2009 beneath mount rainier washington including the largest number of events ever recorded in a single day at rainier since seismic stations were installed on the edifice in 1989 many events were only clearly recorded on one or two stations on the edifice or they overlapped in time with other events and thus only 200 were locatable by manual phase picking to partially overcome this limitation we applied waveform based event detection integrated with precise double difference relative relocation with this procedure detection and location goals are accomplished in tandem using cross correlation with continuous seismic data and waveform templates constructed from cataloged events as a result we obtained precise locations for 726 events an improvement of almost a factor of 4 these event locations define a 850 m long nearly vertical structure striking nne with episodic migration outward from the initial hypocenters the activity front propagates in a manner consistent with a diffusional process double couple constrained focal mechanisms suggest dominantly near vertical strike slip motion on either nnw or ene striking faults more than 30 different than the strike of the event locations this suggests the possibility of en echelon faulting perhaps with a component of fault opening in a fracture mesh type geometry we hypothesize that the swarm was initiated by a sudden release of high pressure fluid into preexisting fractures with subsequent activity triggered by diffusing fluid pressure in combination with stress transfer from the preceding events

a slow earthquake sequence on the san andreas fault earthquakes typically release stored strain energy on timescales of the order of seconds limited by the velocity of sound in rock over the past 20 years observations1 13 and laboratory experiments14 have indicated that rupture can also occur more slowly with durations up to hours such events may be important in earthquake nucleation15 and in accounting for the excess of plate convergence over seismic slip in subduction zones the detection of events with larger timescales requires near field deformation measurements in december 1992 two borehole strainmeters close to the san andreas fault in california recorded a slow strain event of about a week in duration and we show here that the strain changes were produced by a slow earthquake sequence equivalent magnitude 4 8 with complexity similar to that of regular earthquakes the largest earthquakes associated with these slow events were small local magnitude 3 7 and contributed negligible strain release the importance of slow earthquakes in the seismogenic process remains an open question but these observations extend the observed timescale for slow events by two orders of magnitude

depth dependent stress field in and around the atotsugawa fault central japan deduced from microearthquake focal mechanisms evidence for localized aseismic deformation in the downward extension of the fault 1 focal mechanisms have been determined from p wave polarity data as well as body wave amplitudes for 154 microearthquakes that occurred around the atotsugawa fault in central japan between 2002 and 2004 while we found many microearthquakes with a pure strike slip mechanism that is similar to the faulting style of the atotsugawa fault a considerable number of microearthquakes with reverse faulting components are also occurring most of the p axes are horizontal and oriented in the wnw ese direction which conforms to the general tectonic trend in this area in contrast the t axes have a wide range of plunge suggesting that reverse faulting type earthquakes as well as strike slip ones are occurring the most conspicuous feature in the focal mechanism distribution is the depth dependence where shallow earthquakes are primarily reverse faulting and strike slip earthquakes become predominant with depth a stress tensor inversion reveals that the shallower part is characterized by a mixture of reverse and strike slip faulting regimes and that a pure strike slip faulting regime appears only around the bottom of the seismogenic zone together with other geophysical observational evidence for the fault we suggest that the existence of a localized aseismic deformation below the atotsugawa fault is the simplest scenario that can explain the observed stress fields this scenario provides a stress accumulation mechanism of disastrous shallow inland earthquakes in which the localized aseismic deformation accumulates stress onto the fault plane in the seismogenic zone during an earthquake cycle and the main shock would occur when the failure stress is reached on the fault

phenomenology of deep slow earthquake family in southwest japan spatiotemporal characteristics and segmentation 1 the phenomenology of deep slow earthquakes including low frequency tremors very low frequency earthquakes and short term slow slip events on the subducting plate interface in southwest japan is investigated on the basis of their spatiotemporal characteristics the belt like distribution of tremors is divided into segments bounded by gaps the repetition of tremor episode depends on the magnitude of the episode which is defined by the number of detected tremors within each episode major tremor episodes with a large number of detected tremors recur at an interval of approximately 6 months accompanying the short term slow slip events in large segments in small segments tremor episodes with small numbers of tremors have recurrence intervals of a few months some of them occasionally accompany the slight ground tilting caused by a small size slow slip event even in the same segment there exist different recurrence intervals according to the magnitude of the tremor episode the migration pattern varies greatly in initiation termination direction and speed however the migration direction has a general tendency for each segment on both sides of the gap the tremor starts next to the gap and propagates to the other side assuming all tremor episodes are caused by slow slip events the equivalent moment and slip are estimated from the number of detected tremor counts and the area of each tremor episode the equivalent slip history estimated from all tremor episodes is almost constant in each segment this indicates that tremor activity is a good proxy for slow slips

non self similar source property for microforeshocks of the 2014 mw 6 2 northern nagano central japan earthquake foreshocks are considered a part of the preparation process for large earthquakes and as such can provide important constraints on earthquake generation we examine the characteristics of the four day long foreshock sequence of the 2014 northern nagano earthquake mw 6 2 using seismograms recorded by a dense seismic observation network including data from a deep borehole closest to the foreshock region the improved earthquake catalog shows a slow speed migration of the sequence toward the mainshock hypocenter implying possible slow slip transients which should have caused stress loading at the mainshock hypocenter analysis of source spectra reveals that foreshocks for mw 1 5 display a weaker dependence of corner frequency on magnitude than expected for self similar scaling suggesting that these micro foreshocks can be regarded as small low frequency earthquakes so far we might have missed the expected weak low frequency pre seismic signals because they emerge only below a certain magnitude and are generally embedded in background noise

gradual unlocking of plate boundary controlled initiation of the 2014 iquique earthquake a long foreshock series unlocked the south american plate boundary until eventually initiating the m 8 1 iquique chile earthquake two groups publishing in this issue of nature analyse the seismic context of the iquique earthquake that occurred off the coast of northern chile on 1 april 2014 in a seismic zone that had been quiescent since a significant event in 1877 gavin hayes et al identify areas of remaining or elevated earthquake hazard along the megathrust fault in the region and conclude that the 2014 iquique event was not the earthquake that had been anticipated given that significant sections of the northern chile subduction zone have not ruptured in almost 150 years they suggest that it is likely that future megathrust earthquakes will occur south and potentially north of the 2014 iquique sequence bernd schurr et al show that the april 2014 earthquake broke a central fraction of the northern chile seismic gap the last major segment of the south american plate boundary that had yet to rupture in the past century from july 2013 up to the april earthquake they identify three seismic clusters along this part of the plate boundary each lasting a few weeks with earthquakes of increasing peak magnitudes they conclude that these seismic clusters and their slip transients reflect a gradual weakening of the central part of the seismic gap that was instrumental in initiating the final failure on 1 april 2014 northern chile was struck by a magnitude 8 1 earthquake following a protracted series of foreshocks the integrated plate boundary observatory chile monitored the entire sequence of events providing unprecedented resolution of the build up to the main event and its rupture evolution here we show that the iquique earthquake broke a central fraction of the so called northern chile seismic gap the last major segment of the south american plate boundary that had not ruptured in the past century1 2 since july 2013 three seismic clusters each lasting a few weeks hit this part of the plate boundary with earthquakes of increasing peak magnitudes starting with the second cluster geodetic observations show surface displacements that can be associated with slip on the plate interface these seismic clusters and their slip transients occupied a part of the plate interface that was transitional between a fully locked and a creeping portion leading up to this earthquake the b value of the foreshocks gradually decreased during the years before the earthquake reversing its trend a few days before the iquique earthquake the mainshock finally nucleated at the northern end of the foreshock area which skirted a locked patch and ruptured mainly downdip towards higher locking peak slip was attained immediately downdip of the foreshock region and at the margin of the locked patch we conclude that gradual weakening of the central part of the seismic gap accentuated by the foreshock activity in a zone of intermediate seismic coupling was instrumental in causing final failure distinguishing the iquique earthquake from most great earthquakes finally only one third of the gap was broken and the remaining locked segments now pose a significant increased seismic hazard with the potential to host an earthquake with a magnitude of 8 5

decade scale decrease inb value prior to the m9 class 2011 tohoku and 2004 sumatra quakes 1 the gutenberg richter frequency magnitude distribution of earthquakes has become well established in seismology the slope of the relation between frequency and magnitude b value is typically 1 but it often shows variations around 1 based on an analysis of seismicity prior to the 2011 tohoku and 2004 sumatra earthquakes both in magnitude m 9 class we show that the pronounced decade scale decrease inbvalue was a common precursor to both mega quakes around their hypocenters this is the first report onm9 class quakes to confirm a change inb value which has been predicted based on the results of laboratory experiments we propose that the b value is an important indicator of an impending great earthquake and has great potential in terms of predicting a future large quake off the pacific coast of hokkaido japan

dual megathrust slip behaviors of the 2014 iquique earthquake sequence the transition between seismic rupture and aseismic creep is of central interest to better understand the mechanics of subduction processes a mw 8 2 earthquake occurred on april 1st 2014 in the iquique seismic gap of northern chile this event was preceded by a long foreshock sequence including a 2 week long migration of seismicity initiated by a mw 6 7 earthquake repeating earthquakes were found among the foreshock sequence that migrated towards the mainshock hypocenter suggesting a large scale slow slip event on the megathrust preceding the mainshock the variations of the recurrence times of the repeating earthquakes highlight the diverse seismic and aseismic slip behaviors on different megathrust segments the repeaters that were active only before the mainshock recurred more often and were distributed in areas of substantial coseismic slip while repeaters that occurred both before and after the mainshock were in the area complementary to the mainshock rupture the spatiotemporal distribution of the repeating earthquakes illustrates the essential role of propagating aseismic slip leading up to the mainshock and illuminates the distribution of postseismic afterslip various finite fault models indicate that the largest coseismic slip generally occurred down dip from the foreshock activity and the mainshock hypocenter source imaging by teleseismic back projection indicates an initial down dip propagation stage followed by a rupture expansion stage in the first stage the finite fault models show an emergent onset of moment rate at low frequency 0 5 hz 0 5 hz this indicates frequency dependent manifestations of seismic radiation in the low stress foreshock region in the second stage the rupture expands in rich bursts along the rim of a semi elliptical region with episodes of re ruptures suggesting delayed failure of asperities the high frequency rupture remains within an area of local high trench parallel gravity anomaly tpga suggesting the presence of subducting seamounts that promote high frequency generation our results highlight the complexity of the interactions between large scale aseismic slow slip and dynamic ruptures of megathrust earthquakes

molluscan palaeoecology and sedimentological features an integrated approach from the miocene meduna section northern italy molluscs were studied to reconstruct the palaeoenvironmental conditions of the miocene sequence of the torrente meduna area friuli northern italy and to compare palaeobiological results with those of an earlier sedimentological analysis the study identifies six fossil communities and relevant individual assemblages and ascribes them to palaeobiocoenoses comparable to modern mediterranean ones introduced by french marine ecologists 1 chlamys praescabriuscula community 2 pecten burdigalensis community 3 loripes dentatus assemblage 4 protoma rotifera community 5 benus ancilla assemblage and 6 venus conus community the palaeobiological analysis demonstrates that the entire miocene sequence up to the late tortonian or very early messinian was deposited within the infralittoral zone at shallow to very shallow depths onto wave influenced bottoms the greatest water depth occurred during early tortonian and was of approximately 25 m palaeobiological and sedimentological methods provide different sets of information which complement each other efficiently general environmental conditions in this case a delta influenced environment can best be reconstructed on the basis of sedimentological data however specific aspects of the environment primarily palaeobathymetry may only be interpreted by means of through palaeobiological analysis

isolated cases of remote dynamic triggering in canada detected using cataloged earthquakes combined with a matched filter approach here we search for dynamically triggered earthquakes in canada following global main shocks between 2004 and 2014 with ms 6 depth 0 2 cm s we use the natural resources canada nrcan earthquake catalog to calculate statistical values in 1 1 bins in 10 day windows before and after the main shocks the statistical analysis suggests that triggering may occur near vancouver island along the border of the yukon and northwest territories in western alberta western ontario and the charlevoix seismic zone we also search for triggering in alberta where denser seismic station coverage renders regional earthquake catalogs with lower completeness thresholds we find remote triggering in alberta associated with three main shocks using a matched filter approach on continuous waveform data the increased number of local earthquakes following the passage of main shock surface waves suggests local faults may be in a critically stressed state

nonvolcanic tremors deep beneath the san andreas fault we have discovered nonvolcanic tremor activity i e long duration seismic signals with no clear p or s waves within a transform plate boundary zone along the san andreas fault near cholame california the inferred epicentral region of the 1857 fort tejon earthquake moment magnitude 7 8 the tremors occur between 20 to 40 kilometers depth below the seismogenic zone the upper 15 kilometers of earth s crust where earthquakes occur and their activity rates may correlate with variations in local earthquake activity

extended nucleation of the 1999 mw 7 6 izmit earthquake laboratory and theoretical studies suggest that earthquakes are preceded by a phase of developing slip instability in which the fault slips slowly before accelerating to dynamic rupture we report here that one of the best recorded large earthquakes to date the 1999 moment magnitude mw 7 6 izmit turkey earthquake was preceded by a seismic signal of long duration that originated from the hypocenter the signal consisted of a succession of repetitive seismic bursts accelerating with time and increased low frequency seismic noise these observations show that the earthquake was preceded for 44 minutes by a phase of slow slip occurring at the base of the brittle crust this slip accelerated slowly initially and then rapidly accelerated in the 2 minutes preceding the earthquake

california foreshock sequences suggest aseismic triggering process 1 foreshocks are one of the few well documented precursors to large earthquakes therefore understanding their nature is very important for earthquake prediction and hazard mitigation however the triggering role of foreshocks is not yet clear it is possible that foreshocks are a self triggering cascade of events that simply happen to trigger an unusually large aftershock alternatively foreshocks might originate from an external aseismic process that ultimately triggers the mainshock in the former case the foreshocks will have limited utility for forecasting the latter case has been observed for several individual large earthquakes however it remains unclear how common it is and how to distinguish foreshock sequences from other seismicity clusters that do not lead to large earthquakes here we analyze foreshocks of three m 7 mainshocks in southern california these foreshock sequences appear similar to earthquake swarms in that they do not start with their largest events and they exhibit spatial migration of seismicity analysis of source spectra shows that all three foreshock sequences feature lower average stress drops and depletion of high frequency energy compared with the aftershocks of their corresponding mainshocks using a longer term stress drop catalog we find that the average stress drop of the landers and hector mine foreshock sequences is comparable to nearby swarms our observations suggest that these foreshock sequences are manifestations of aseismic transients occurring close to the mainshock hypocenters possibly related to localized fault zone complexity which have promoted the occurrence of both the foreshocks and the eventual mainshock

microearthquake spectra from the anza california seismic network site response and source scaling abstract we analyzed spectra of local microearthquakes recorded by the anza california seismic network to isolate the effects of site response and to investigate the scaling of source parameters for small earthquakes spectra of microearthquakes m m o 19 dyne cm at anza have shapes characteristic of the receiver sites and are generally independent of the source region thus the site response is a major conditioner of the observed spectral shape to remove the effects of site response from the spectra of a m 3 event and isolate its source spectrum we divided by the spectra of an adjacent aftershock used as an empirical green9s function event the spectral ratios indicate that the apparent corner frequencies of small earthquakes m o 19 dyne cm observed at even the high f max stations on hard rock are much lower than the source corner frequencies the spectral ratios are consistent with stress drop remaining constant with decreasing seismic moment for events with moments as small as 10 18 dyne cm the spectral ratios display remarkable agreement between sites which showed vast differences in their original spectra indicating that the spectral division effectively removed the site response the source spectrum of the m 3 event has a high frequency spectral fall off of about 2 an apparent dependence of high frequency fall off with seismic moment in the original spectra can also be explained by the effects of site response the difference between the p and s wave corner frequencies and high frequency roll offs in the observed spectra for these events is the result of the site response and is not a source property the shapes of the spectra of microearthquakes at anza can largely be explained by attenuation at shallow depth with a frequency independent q for some sites near surface resonances are also apparent in the spectra of microearthquakes it is indicated by t values determined for each site that q p q s for the shallow low q layer further evidence of low near surface q is observed in the anomalous spectra of an unusually shallow earthquake source depth 1 km in the network the spectra from this shallow event are depleted in high frequency energy at most stations relative to those of deeper events this observation can be explained by a low q surficial zone for stations of the network situated on alluvium this low q layer has a maximum thickness of about 3 km and maximum p and s wave q values of 30 to 50

observed platelet ice distributions in antarctic sea ice an index for ocean ice shelf heat flux antarctic sea ice that has been affected by supercooled ice shelf water isw has a unique crystallographic structure and is called platelet ice in this paper we synthesize platelet ice observations to construct a continent wide map of the winter presence of isw at the ocean surface the observations demonstrate that in some regions of coastal antarctica supercooled isw drives a negative oceanic heat flux of 30 wm 2 that persists for several months during winter significantly affecting sea ice thickness in other regions particularly where the thinning of ice shelves is believed to be greatest platelet ice is not observed our new data set includes the longest ice ocean record for antarctica which dates back to 1902 near the mcmurdo ice shelf these historical data indicate that over the past 100 years any change in the volume of very cold surface outflow from this ice shelf is less than the uncertainties in the measurements

temporal variation in the magnitude frequency distribution during the guy greenbrier earthquake sequence the recent increase in earthquake activity in the central u s has led to concerns about the hazard posed by induced earthquakes understanding earthquake phenomena and monitoring in all settings can be improved by the detection of small events however the catalog of induced earthquakes is incomplete for small events due to the sparse instrumentation this is particularly true in settings of low background seismicity like the central u s we apply single station template matching to detect small earthquakes during the guy greenbrier sequence in central arkansas and find over 100 times more earthquakes than are registered in the advanced national seismic system catalog between july 2010 and october 2011 a complete catalog over nearly 4 units of magnitude enables us to analyze the magnitude frequency distribution of induced earthquakes we find that earthquakes deviated from the gutenberg richter statistics during the operation of nearby injection wells but returned to gutenberg richter statistics after the wells were shut in

enhanced detection capability of non volcanic tremor using a 3 level vertical seismic array network va net in southwest japan 1 this study evaluates a method that enhances detection of non volcanic low frequency tremor nvt using va net a 3 level vertical seismic array network that was recently constructed in southwest japan the method employs semblance analysis for continuous vertical array records and calculates semblance coefficients as a function of time and apparent vertical velocity the sign of the best apparent velocity provides information about propagation direction either upward or downward allowing easy discrimination between seismic signals of natural origin and unwanted noise such as man made vibrations we applied the method to a nvt episode that occurred beneath the kii peninsula during november 2008 to demonstrate that nvt signals can be detected using only the 3 level array furthermore we demonstrate that the method enables us to detect minor nvt activity which is not detected by a conventional envelope cross correlation method resulting in a dramatic improvement in detection capability