Earthquake Source Region Research Articles

Mica kink-band geometry as an indicator of coseismic dynamic loading

Recognizing that exhumed shear zones had a seismogenic history is an important prerequisite for microstructural studies of energy partitioning in the earthquake source region. Using optical measurements of kinked micas along the Sandhill Corner shear zone, a seismogenic fault/shear zone exhumed from the base of the frictional-to-viscous transition, we demonstrate that their geometric properties are statistically different from kinks in micas that underwent regional deformation and metamorphism. Kink-band asymmetries and the angles of external rotation are larger in rocks from the Sandhill Corner shear zone, whereas kink-band widths are smaller. Our data show remarkable similarities to kink bands from dynamic deformation experiments, meteorite impacts, and nuclear explosion sites, suggesting a dynamic origin for their formation. Our results suggest that mica kink bands may be a useful indicator of dynamic loading and paleoseismicity in the rock record.

Structure-controlled asperities of the 1920 Haiyuan M8.5 and 1927 Gulang M8 earthquakes, NE Tibet, China, revealed by high-resolution seismic tomography

Detailed crustal structure of large earthquake source regions is of great significance for understanding the earthquake generation mechanism. Numerous large earthquakes have occurred in the NE Tibetan Plateau, including the 1920 Haiyuan M8.5 and 1927 Gulang M8 earthquakes. In this paper, we obtained a high-resolution three-dimensional crustal velocity model around the source regions of these two large earthquakes using an improved double-difference seismic tomography method. High-velocity anomalies encompassing the seismogenic faults are observed to extend to depths of 15 km, suggesting the asperity (high-velocity area) plays an important role in the preparation process of large earthquakes. Asperities are strong in mechanical strength and could accumulate tectonic stress more easily in long frictional locking periods, large earthquakes are therefore prone to generate in these areas. If the close relationship between the aperity and high-velocity bodies is valid for most of the large earthquakes, it can be used to predict potential large earthquakes and estimate the seismogenic capability of faults in light of structure studies.

Double-difference seismic attenuation tomography method and its application to The Geysers geothermal field, California

SUMMARY Knowledge of attenuation structure is important for understanding subsurface material properties. We have developed a double-difference seismic attenuation (DDQ) tomography method for high-resolution imaging of 3-D attenuation structure. Our method includes two main elements, the inversion of event-pair differential ${t^*}$ ($d{t^*}$) data and 3-D attenuation tomography with the $d{t^*}$ data. We developed a new spectral ratio method that jointly inverts spectral ratio data from pairs of events observed at a common set of stations to determine the $d{t^*}$ data. The spectral ratio method cancels out instrument and site response terms, resulting in more accurate $d{t^*}$ data compared to absolute ${t^*}$ from traditional methods using individual spectra. Synthetic tests show that the inversion of $d{t^*}$ data using our spectral ratio method is robust to the choice of source model and a moderate degree of noise. We modified an existing velocity tomography code so that it can invert $d{t^*}$ data for 3-D attenuation structure. We applied the new method to The Geyser geothermal field, California, which has vapour-dominated reservoirs and a long history of water injection. A new Qp model at The Geysers is determined using P-wave data of earthquakes in 2011, using our updated earthquake locations and Vp model. By taking advantage of more accurate $d{t^*}$ data and the cancellation of model uncertainties along the common paths outside of the source region, the DDQ tomography method achieves higher resolution, especially in the earthquake source regions, compared to the standard tomography method using ${t^*}$ data. This is validated by both the real and synthetic data tests. Our Qp and Vp models show consistent variations in a normal temperature reservoir that can be explained by variations in fracturing, permeability and fluid saturation and/or steam pressure. A prominent low-Qp and Vp zone associated with very active seismicity is imaged within a high temperature reservoir at depths below 2 km. This anomalous zone is likely partially saturated with injected fluids.

Deep Neural Networks for Earthquake Detection and Source Region Estimation in North-Central Venezuela

ABSTRACTReliable earthquake detection algorithms are necessary to properly analyze and catalog the continuously growing seismic records. We report the results of applying a deep convolutional neural network, called UPC-UCV (Universitat Politecnica de Catalunya - Universidad Central de Venezuela), over single-station three-channel signal windows for P-wave earthquake detection and source region estimation in north-central Venezuela. The analysis is performed on a new dataset of handpicked arrivals of P waves from local events, named CARABOBO, built and made public for reproducibility and benchmarking purposes. The CARABOBO dataset consists of three-channel continuous data recorded by the broadband stations of the Venezuelan Foundation for Seismological Research in the region of 9.5°–11.5°N and 67.0°–69.0°W during the time period from April 2018 to April 2019. During this period, 949 earthquakes were recorded in that area, corresponding to earthquakes with magnitudes in the range from Mw 1.1 to 5.2. To estimate the epicentral source region of a detected event, the proposed network employs geographical distribution of the CARABOBO dataset into K clusters as a basis. This geographical partitioning is automatically performed by the k-means algorithm, and the optimality of the K-values for our dataset has been assessed using the elbow (K=5) and silhouette (K=3) methods. For target seismicity, the proposed network achieves 95.27% detection accuracy and 93.36% source region estimation accuracy, when using K=5 geographic clusters. The location accuracy slightly increases to 95.68% in the case of K=3 geographic partitions. The detection capability of this network has also been tested on the OKLAHOMA dataset, which compiles more than 2000 local earthquakes that occurred in this U.S. state. Without any modification, the proposed network yields excellent detection results when trained and evaluated on that dataset (98.21% accuracy; ConvNetQuake, fine-tuned for this dataset, achieves a 97.32% accuracy), corresponding to a totally different geographical region.

The January 23, 2018 M7.9 Kodiak earthquake, Alaska: A consequence of slip partitioning in the outer rise region

The January 23, 2018 M7.9 earthquake ruptured conjugate set of strike-slip faults in the outer rise of the Alaska subduction zone, southeast of Kodiak Island. The complex nature of this earthquake is evident from the earthquake focal mechanism and aftershock relocation patterns which revealed rupture of multiple north-south and east-west trending conjugate fault planes. Here, we suggest that this earthquake occurred as a result of two different modes of interaction process. The stress triggering process reveals (i) the co-seismic and post-seismic deformation of the M9.2 1964 Alaska megathrust earthquake increased Coulomb failure stress of ∼10 kPa and ∼0.05 kPa respectively in the 2018 Kodiak earthquake source region, and (ii) the overall deformation is being accommodated in the outer rise region due to slip partitioning resulting from the oblique convergence between the Pacific and North American plates along the Alaska Aleutian megathrust.

Stress state along the western Nankai Trough subduction zone inferred from b-values, long-term slow-slip events, and low-frequency earthquakes

The b-value of the Gutenberg–Richter law represents the ratio of earthquake magnitude to frequency of occurrence and is inversely proportional to differential stress. Repeating long-term slow-slip events (SSEs) and low-frequency earthquakes (LFEs) occur at subducting plate interfaces and have stress-dependent characteristics near the interface. In this study, a comprehensive regional b-value distribution is produced for the western Nankai Trough region, which highlights the relationship between b-values, SSEs, and LFEs. b-values vary along the strike direction of the subducting plate and are significantly lower left( {b sim 0.6} right) in central Shikoku district than elsewhere, where LFEs frequently occur. However, b-values in the source regions of other LFEs are moderate to high. These findings imply that b-values in the focal region are controlled by more than the LFE source process; indeed, if this source process were solely responsible, then high b-values would be expected. Meanwhile, the V_{P} /V_{S} and QP around the plate interface in central Shikoku estimated from seismic velocity and attenuation structure are smaller and larger than those in other regions with LFEs, respectively. SSEs with the migration toward central Shikoku also occurred during the analysis period, suggesting significant accumulation of shear stresses in the focal region, which reduced the b-values. These findings suggest that the spatial distributions of b-values are influenced by complicated stress and shear strength perturbations caused by SSEs and LFEs. On the other hand, the b-values in the region that underwent the greatest slip during the 1946 Nankai earthquake are not necessarily low, although the area covered by the b-value distribution is small owing to the lack of events on the updip side. Whereas the asperity areas of huge earthquakes are characterized by low b-values, the b-value distribution in the Nankai megathrust area is more complicated. It is considered that slow earthquakes, including SSEs and LFEs, are related to megathrust earthquakes via stress transfer from slow earthquakes to adjacent megathrust source regions. A unified analysis of b-values in the source regions of slow and megathrust earthquakes may be required to make precise estimates of the seismic hazard produced by a megathrust event.

Interplate Slip Following the 2003 Tokachi‐oki Earthquake From Ocean Bottom Pressure Gauge and Land GNSS Data

AbstractPreseismic, coseismic, and 7.5 years of postseismic deformation of the 2003Mw8.0 Tokachi‐oki earthquake are modeled using land Global Navigation Satellite System (GNSS) data and two ocean bottom pressure gauges (OBP) using viscoelastic Green's functions. The postseismic slip distribution is shown to not overlap with the main shock or the source regions of past large earthquakes along the southern part of the Kurile trench. The preseismic locking is estimated in the coseismic and postseismic slip regions using the same plate interface geometry. The temporal evolution of the postseismic slip is described with a single logarithmic function. The slip is shown to decay faster updip of the coseismic slip region compared to the downdip extension. Sustained postseismic slip indicates that the interplate locking in the postseismic slip region had not fully recovered to the pre‐2003 status in the 7.5 year analyzed period. Postseismic slip history also indicates rapid recovery of locking in the coseismic slip region. We examined the constraint of the OBP time series for the postseismic deformation modeling and found that the observed uplift at the OBP sites requires small postseismic slip near the trench, where a tsunamigenicM9 earthquake occurred in the seventeenth century. Uplift at the OBP sites does not constrain the absolute slip magnitude between the updip and downdip sides of the OBP sites but requires the slip in the updip side to be smaller than that in the downdip side. Our model describes transient postseismic deformation observed by both land GNSS and OBP.

Tomographic imaging of the 2017 Ms7.0 Jiuzhaigou earthquake source region and its implications on material extrusion in the northeast Tibetan plateau

Due to the heavy damages caused by the 2017 Ms 7.0 Jiuzhaigou earthquake that occurred in a complex tectonic environment, investigations on the seismogenic structure and tectonic environment are very important for understanding the geodynamic mechanism and mitigating earthquake hazards in the region. In this work, we present a high-resolution 3-D crustal seismic model for the source region of the Jiuzhaigou earthquake by exploiting the arrival times of aftershocks based on double difference tomography. Our results revealed that zones of Jiuzhaigou's rupture fault and aftershocks predominantly coincide with the high-velocity anomalies that are surrounded by the low-velocity anomalies, from the western territory to the aftershock zone. An area with a high Poisson's ratio ~0.28 is primarily the cradle for most of the aftershocks, and the lithology is interpreted as consisting of mafic and calcium-rich rocks. In the mid-crust, tomographic images exhibit a low-velocity zone in and around the source region. This low-velocity zone originates from the interior of the Tibetan Plateau, extends across the thrust faults (the Longriba fault and the Minjiang fault and the Jiuzhaigou's causative fault) and then finally encounters the strike-slip faults (the Tazang fault and East Kunlun fault). The counter belt of both the Tazang fault and East Kunlun fault features high-velocity anomalies. The mid-crust low anomalous zone is characterized by a high Poisson's ratio and low shear wave velocity, which indicates a weaker crust that easily flows and deforms. Therefore, we propose that this low velocity is a crustal channel flow in the vicinity of seismic sources, which causes the transformation from thrusting to strike-slip motion in the study area.

Depth‐dependent rupture mode along the Ecuador‐Colombia subduction zone

AbstractA large earthquake (Mw 7.7) occurred on 16 April 2016 within the source region of the 1906 earthquake in the Ecuador‐Colombia subduction zone. The 1906 event has been interpreted as a megathrust earthquake (Mw 8.8) that ruptured the source regions of smaller earthquakes in 1942, 1958, and 1979 in this subduction. Our seismic analysis indicated that the spatial distribution of the 2016 earthquake and its aftershocks correlated with patches of high interplate coupling strength and was similar to those of the 1942 earthquake and its aftershocks, suggesting that the 2016 and 1942 earthquakes ruptured the same asperity. Our analysis of tsunami waveforms of the 1906 event indicated Mw around 8.4 and showed that large slip occurred near the trench off the source regions of the above three historical and the 2016 earthquakes, suggesting that a depth‐dependent complex rupture mode exists along this subduction zone.

Crustal and uppermost mantle structures in the frontal Himalaya and Indo-Gangetic basin using surface wave: Tectonic implications

Characteristics of dispersion curves of Rayleigh and Love waves of 2005 Kashmir earthquake and its aftershocks are utilized to investigate the sub-surface structural variations within the northern margin of the Indian continent (i.e. Indo-Gangetic Plains) and the frontal part of the northwestern Himalaya. Surface waves of these earthquakes recorded in the Broad Band Seismometers (BBS) installed at Ajmer, Delhi and Shimla were used for this investigation. Estimated group velocities along three paths from earthquake source region to these stations are in the range of 2.17 km/s to 3.94 km/s which are sufficient to investigate the crustal and uppermost mantle structures. Paths covering northern margin of the Indian continent has very low group velocities for periods less than 10-s that indicate presence of thick sediments. High differences are observed between Love and Rayleigh wave group velocities for the paths crossing Himalayan wedge that can be ascribed to the effect of anisotropy due to India-Eurasia collision.The inversion of dispersion curves was performed to investigate sub-surface shear wave velocity structure and variation of anisotropy with depth for crust and uppermost mantle. A ∼6km thick sediments divided into two layers (unconsolidated and consolidated) are seen in the Indo Gangetic Plain deposited above underthrusting Indian plate. The Conrad and Moho depths increase from Ajmer, the southernmost station in the IGP to Shimla, the station lying in the Himalayan wedge. This shows the dipping geometry of the Indian plate. The average Moho depth along these paths from earthquake source to Ajmer station is 42.5 km, to Delhi station is 44 km and to Shimla station is at 46 km. Average values of Conrad depth for these ray paths are 18.5 km, 19.5 km and 26.5 km respectively. The S wave velocity along Ajmer path is higher than that along the other two paths. This may be due to lateral heterogeneity of the crust and mantle for the study area. Frontal western Himalaya is anisotropic in nature, whereas the IGP region is not. Anisotropy effect in the western Himalaya is most pronounced in the lower crustal material of the underthrusted Indian plate. It is quite large in the uppermost mantle below the frontal Himalayan region. We infer that anisotropy in the lower crust and uppermost mantle could be due to development of preferably oriented folds, faults and fractures and possible reorientation of crystals as a consequence of ongoing deformation caused by continent-continent collision.

汶川地震和芦山地震的有助于预报的地震前兆

是否存在“有助于预报的地震前兆”？马瑾院士由岩样加载实验发现岩石加载临近强度极值进入亚失稳阶段，发出应变、位移、温度变化特征信号，之后岩样破裂。大地震前是否能观测到这些信号？观测到了，如何识别应力已达峰值强度？姑咱地震台YRY-4分量钻孔应变仪汶川大震前550天中接收到3700多个异常应变脉冲，震后衰减。2014年底发现，脉冲出现初期，表征脉冲数据自洽性的k值大于0.99；大震前夕k值逐渐减小至0.4，数据失洽。地层连续无裂隙是应变数据满足自洽的条件，失洽是震源区地层连续性破坏越来越严重，大震将发生的标志。汶川大地震前五个月时出现5天中一个脉冲事件也没有的平静期，脉冲事件再度增多后大地震发生。岩石加载声发射现象，加载接近峰值强度出现声发射平静期，继续加载声发射再现后岩样破裂。在数千脉冲事件背景下，出现数据失洽及平静期，这两项特征有助于判断地层加载进入亚失稳阶段及何时达到强度峰值，地震必将发生。2013年芦山7级地震前的440天中，姑咱台又记录到1000多个脉冲事件，震前2个月时又出现5天平静期。在适当位置布设精密应变观测网，有可能观测到，并能识别出“有助预报的地震前兆”。汶川、芦山地震观测中得到的认识，具有普遍性还是特例，还要在实践中接受检验。 Whether there exist “earthquake precursors that facilitate earthquake prediction”? From rock experiments Ma Jin, Academician of CAS P, found that when the specimen is loaded near to its strength limit and enters a meta-instable stage, it emits characteristic signals of strain, displacement, and temperature, and then the rock specimen breaks. Before a large earthquake can we observe such signals? If we can, how do we know the stress has reached the peak strength? In 550 days before the Wenchuan earthquake the YRY 4-component strainmeter in Guzan seismic station received more than 3700 abnormal strain pulses, which were then attenuated after the earthquake. In the end of 2014 when the pulses started to appear, it is found that the k value, which characterized the self-consistency of pulse data, was greater than 0.99; whereas the k value decreased gradually till 0.4 immediately before the earthquake, and the data lost self-consistency. For self-consistency of the strain data the Earth strata should be continuous and without cracks, the loss of self-consistency indicates that the continuity of strata in the earthquake source region is increasingly damaged and a large earthquake may occur soon. About 5 months before the Wenchuan earthquake there was a quiet period without a single pulse in 5 days, then after the impulsive events increased again the great earthquake took place. This is similar to the acoustic emission phenomenon during rock loading; a quiet period appears when the load approaches to the peak strength, with continuing loading acoustic emission reappears then the rock specimen breaks. On a background of thousands of impulsive events, the loss of data self-consistency and the appearance of quiet period are two features that help to judge whether the loading of strata has entered the meta-instable stage and when it reaches to the strength limit, thus leading to the inevitable occurrence of an earthquake. In 440 days before the Lushan M7 earthquake of 2013, the Guzan station recorded again more than 1000 pulses, and a quiet period of 5 days appeared 2 months before the earthquake. By setting up a precise strain monitoring network in proper locations, we may be able to observe and identify “earthquake precursors that are useful for earthquake prediction”. Whether the knowledge acquired from Wenchuan and Lushan earthquake observations is general or specific, it remains to be tested in future practice.

3D Vs ambient noise tomography of the 2016 Mw 6.4 Meinong Earthquake source region in Taiwan

3D Vs ambient noise tomography of the 2016 Mw 6.4 Meinong Earthquake source region in Taiwan

Earthquake genesis in Nepal Himalaya: A perspective from imaging of the 25th April 2015 Mw 7.8 earthquake source zone

The Mw 7.8 earthquake in Central Nepal nucleated in the mid-crustal ramp zone of the Main Himalayan décollement Thrust (MHT) and propagated eastward for >140km where the largest triggered event of Mw 7.3 occurred without any surface rupture. Although it is advocated that the slip and rupture dynamics are controlled by the structural configuration of the MHT and the upper crust, precise correlation between the seismic structure and seismogenesis is hitherto scarce in the Himalaya. To address the issue, we imaged the crustal structure along three profiles covering the earthquake source region using receiver function analysis of the seismic data from the HiCLIMB and HIMNT seismic networks to understand the lateral variability. A ∼5km thick, low velocity layer is observed at the mid–crustal level, that steepens in the MHT ramp zone. The bulk of the seismicity including large shocks after the 2015 Nepal earthquake lies in the vicinity of this low velocity layer. Correlation of the seismic structure and aftershock distribution with the published crustal structure clearly suggests that the rupture involves a thicker zone extending for >40km to the south of the source zone in the MHT ramp. We refined the structure of the MHT zone incorporating published coseismic slip and ground deformation to suggest that the rupture terminated at the footwall imbricate (horse) on the floor thrust below the zone of maximum coseismic uplift and there was a two stage rupture towards the eastern margin of the rupture zone.

Seafloor geodetic constraints on interplate coupling of the Nankai Trough megathrust zone

Interplate megathrust earthquakes have inflicted catastrophic damage on human society. Such an earthquake is predicted to occur in the near future along the Nankai Trough off southwestern Japan--an economically active and densely populated area in which megathrust earthquakes have already occurred. Megathrust earthquakes are the result of a plate-subduction mechanism and occur at slip-deficit regions (also known as 'coupling' regions), where friction prevents plates from slipping against each other and the accumulated energy is eventually released forcefully. Many studies have attempted to capture distributions of slip-deficit rates (SDRs) in order to predict earthquakes. However, these studies could not obtain a complete view of the earthquake source region, because they had no seafloor geodetic data. The Hydrographic and Oceanographic Department of the Japan Coast Guard (JHOD) has been developing a precise and sustainable seafloor geodetic observation network in this subduction zone to obtain information related to offshore SDRs. Here, we present seafloor geodetic observation data and an offshore interplate SDR-distribution model. Our data suggest that most offshore regions in this subduction zone have positive SDRs. Specifically, our observations indicate previously unknown regions of high SDR that will be important for tsunami disaster mitigation, and regions of low SDR that are consistent with distributions of shallow slow earthquakes and subducting seamounts. This is the first direct evidence that coupling conditions might be related to these seismological and geological phenomena. Our findings provide information for inferring megathrust earthquake scenarios and interpreting research on the Nankai Trough subduction zone.

Gravity increase before the 2015 Mw 7.8 Nepal earthquake

AbstractThe 25 April 2015 Nepal earthquake (Mw 7.8) ruptured a segment of the Himalayan front fault zone. Four absolute gravimetric stations in southern Tibet, surveyed from 2010/2011 to 2013 and corrected for secular variations, recorded up to 22.40 ± 1.11 μGal/yr of gravity increase during this period. The gravity increase is distinct from the long‐wavelength secular trends of gravity decrease over the Tibetan Plateau and may be related to interseismic mass change around the locked plate interface under the Himalayan‐Tibetan Plateau. We modeled the source region as a disk of 580 km in diameter, which is consistent with the notion that much of the southern Tibetan crust is involved in storing strain energy that drives the Himalayan earthquakes. If validated in other regions, high‐precision ground measurements of absolute gravity may provide a useful method for monitoring mass changes in the source regions of potential large earthquakes.

Stress-drop heterogeneity within tectonically complex regions: a case study of San Gorgonio Pass, southern California

In general, seismic slip along faults reduces the average shear stress within earthquake source regions, but stress drops of specific earthquakes are observed to vary widely in size. To advance our understanding of variations in stress drop, we analysed source parameters of small-magnitude events in the greater San Gorgonio area, southern California. In San Gorgonio, the regional tectonics are controlled by a restraining bend of the San Andreas fault system, which results in distributed crustal deformation, and heterogeneous slip along numerous strike-slip and thrust faults. Stress drops were estimated by fitting a Brune-type spectral model to source spectra obtained by iteratively stacking the observed amplitude spectra. The estimates have large scatter among individual events but the median of event populations shows systematic, statistically significant variations. We identified several crustal and faulting parameters that may contribute to local variations in stress drop including the style of faulting, changes in average tectonic slip rates, mineralogical composition of the host rocks, as well as the hypocentral depths of seismic events. We observed anomalously high stress drops (>20 MPa) in a small region between the traces of the San Gorgonio and Mission Creek segments of the San Andreas fault. Furthermore, the estimated stress drops are higher below depths of ∼10 km and along the San Gorgonio fault segment, but are lower both to the north and south away from San Gorgonio Pass, showing an approximate negative correlation with geologic slip rates. Documenting controlling parameters of stress-drop heterogeneity is important to advance regional hazard assessment and our understanding of earthquake rupture processes.

Heterogeneous interplate coupling along the Nankai Trough, Japan, detected by GPS-acoustic seafloor geodetic observation

The recurring devastating earthquake that occurs in the Nankai Trough subduction zone between the Philippine Sea plate and the Eurasian plate has the potential to cause an extremely dangerous natural disaster in the foreseeable future. Many previous studies have assumed interplate-coupling ratios for this region along the trench axis using onshore geodetic data in order to understand this recursive event. However, the offshore region that has the potential to drive a devastating tsunami cannot be resolved sufficiently because the observation network is biased to the land area. Therefore, the Hydrographic and Oceanographic Department of Japan constructed a geodetic observation network on the seafloor along the Nankai Trough using a GPS-acoustic combination technique and has used it to observe seafloor crustal movements directly above the Nankai Trough subduction zone. We have set six seafloor sites and cumulated enough data to determine the displacement rate from 2006 to January 2011. Our seafloor geodetic observations at these sites revealed a heterogeneous interplate coupling that has three particular features. The fast displacement rates observed in the easternmost area indicate strong interplate coupling (>75%) around not only the future Tokai earthquake source region but also the Paleo-Zenisu ridge. The slow displacement rates near the trench axis in the Kumano-nada Sea, a shallow part of the 1944 Tonankai earthquake source region, show a lower coupling ratio (50% to 75%). The slow displacement rate observed in the area shallower than the 1946 Nankaido earthquake source region off Cape Muroto-zaki reflects weakening interplate coupling (about 50%) probably due to a subducting seamount. Our observations above the subducting ridge and seamount indicate that the effect of a subducting seamount on an interplate-coupling region depends on various conditions such as the geometry of the seamount and the friction parameters on the plate boundary.

Along-fault pore-pressure evolution during a slow-slip event in Guerrero, Mexico

Slow earthquakes are important constituents of the seismic cycle and are involved in the stress transfer between the viscously slipping portion of the plate interface and the seismogenic zone. Their occurrence is likely associated with the near-lithostatic pore pressure in the slow earthquake source region, where fluids might modify fault friction and whose presence is indicated by high ratios of compressional (P)–wave velocity to shear (S)–wave velocity observed at the interface between the subducting plate and the overlying crust. Here we compare two slow earthquake phenomena observed in the Guerrero region of the Mexican subduction zone: low-frequency earthquakes (LFEs) and a slow-slip event (SSE) recorded by GPS. We observe variations of the LFE occurrence rates over month-long time scales during a large SSE that we interpret as a manifestation of transient changes in the fault shear strength. We argue that these transient changes are caused by a pore pressure fluctuation that migrates updip along the subduction interface. This mechanism suggests that fluids do not only passively weaken the plate interface but also play an active role in slow earthquake source regions.

Earthquakes Induced by Hydraulic Fracturing in Poland Township, Ohio

Felt seismicity induced by hydraulic fracturing is very rare, with only a handful of reported cases worldwide. Using an optimized multistation cross‐correlation template‐matching routine, 77 earthquakes were identified in Poland Township, Mahoning County, Ohio, that were closely related spatially and temporally to active hydraulic fracturing operations. We identified earthquakes as small as local magnitudes ( M L) ∼1 up to 3, potentially one of the largest earthquakes induced by hydraulic fracturing in the United States. These events all occurred from 4 to 12 March 2014, and the rate decayed once the Ohio Department of Natural Resources issued a shutdown of hydraulic fracturing at a nearby well on 10 March. Using a locally derived velocity model and double‐difference relocation, the earthquakes occurred during six stimulation stages along two horizontal well legs that were located ∼0.8 km away. Nearly 100 stimulation stages in nearby wells at greater distances from the earthquake source region did not coincide with detected seismicity. During the sequence, hypocenters migrated ∼600 m along an azimuth of 083°, defining a vertically oriented plane of seismicity close to the top of the Precambrian basement. The focal mechanism determined for the M L 3 event had a vertically oriented left‐lateral fault plane consistent with the earthquake distribution and the regional stress field. The focal mechanism, orientation, and depth of hypocenters were similar to those of the 2011 Youngstown earthquake sequence that occurred 18 km to the northwest and was correlated with wastewater injection instead of hydraulic fracturing. Considering the relatively large magnitude of the Poland Township events and the b ‐value of 0.89, it appears the hydraulic fracturing induced slip along a pre‐existing fault/fracture zone optimally oriented in the regional stress field.

Evidence suggesting fluids beneath earthquake source regions: An integrated approach for seismic source faults

Evidence suggesting fluids beneath earthquake source regions: An integrated approach for seismic source faults