Nearby Faults Research Articles

Macroseismic analysis and the determination of a focal mechanism of the 31 October 2019, KwaZulu-Natal earthquake in South Africa

AbstractAn earthquake (magnitude, ML = 3.8) occurred on 31 October 2019 at 11:19 hours Greenwich Mean Time (GMT) in the Sunduza area of the KwaZulu-Natal (KZN) Province in South Africa. The earthquake was located near the broad fracture zone of the Ntlakwe-Bongwan fault and was felt along the eastern coast of South Africa. The effects of the event were assessed by conducting macroseismic investigations through interviewing members of the public and completing questionnaires. Analysis of all the collected macroseismic data showed that maximum shaking with Modified Mercalli Intensity of IV-V and V were experienced near the epicentral area as well as along the coast. Similar intensity values were also experienced in some parts of Durban about 100 km northeast of the epicentre. Shaking at such long distances was attributed to ground motion amplification due to site effects. A fault plane solution of the event obtained using both the first motion polarities and amplitude ratios showed normal faulting along a fault oriented in a strike of 185.0°, dipping at 68.0° with a rake of -52.0°. The strike of the focal mechanism solution aligns with the strike of the Ntlakwe-Bongwan fault zone and nearby faults suggesting that the event might have ruptured along one of the fracture segments of this fault mapped to the north. The obtained dip is also in line with a high angle fault comparable to that observed along the Ntlakwe-Bongwan fault. The epicentral region is highly faulted although the activity is not known.

Contemporary Velocity Field for Turkey Inferred from Combination of a Dense Network of Long Term GNSS Observations

The Anatolia?Aegean domain represents a broad plate boundary zone, with the deformation accommodated by major faults bounding quasi-low deforming units. First-order features of this deformation were obtained in the form of a GNSS-derived velocity field. During the last decade, the accuracy of velocity solutions was improved, and the expansion of continuous networks increased spatial resolution. Nonetheless, an accurate representation of the deformation field requires interstation distances much lower than the locking depth of nearby faults, which has not yet been satisfied. The basis for creating a precise and accurate velocity field is uniform processing of the time series recorded both in the campaign and permanent GNSS stations, at once and for a single reference system. Although for Anatolia the data density has increased 6-fold since the landmark work of Reilinger et al. (2006), this crucial integration has not been made. We aim to fill this gap by analyzing the data and providing a uniform velocity solution. In this study, we processed the time series of 836 stations, of which 178 are published for the first time. With a period of up to 28 years, we present the most accurate velocity field with increased spatial and temporal resolution and homogeneity. We used the improved coverage of the velocity field to calculate strain accumulation on the North and East Anatolian Faults. Modelled slip rates vary between 20 and 26 mm/yr and 9.7 and 11 mm/yr for the North and East Anatolian faults, respectively. The data can better constrain the kinematics of continental deformation, provide accurate boundary conditions for dynamic models, and help test outstanding hypotheses about the kinematics of the Anatolia-Aegean domain.

The 2015–2017 Pamir earthquake sequence: foreshocks, main shocks and aftershocks, seismotectonics, fault interaction and fluid processes

SUMMARYA sequence of three strong (MW7.2, 6.4, 6.6) earthquakes struck the Pamir of Central Asia in 2015–2017. With a local seismic network, we recorded the succession of the foreshock, main shock and aftershock sequences at local distances with good azimuthal coverage. We located 11 784 seismic events and determined 33 earthquake moment tensors. The seismicity delineates the tectonic structures of the Pamir in unprecedented detail, that is the thrusts that absorb shortening along the Pamir’s thrust front, and the strike-slip and normal faults that dissect the Pamir Plateau into a westward extruding block and a northward advancing block. Ruptures on the kinematically dissimilar faults were activated subsequently from the initial MW 7.2 Sarez event at times and distances that follow a diffusion equation. All main shock areas but the initial one exhibited foreshock activity, which was not modulated by the occurrence of the earlier earthquakes. Modelling of the static Coulomb stress changes indicates that aftershock triggering occurred over distances of ≤90 km on favourably oriented faults. The third event in the sequence, the MW 6.6 Muji earthquake, ruptured despite its repeated stabilization through stress transfer in the order of –10 kPa. To explain the accumulation of MW > 6 earthquakes, we reason that the initial main shock may have increased nearby fault permeability, and facilitated fluid migration into the mature fault zones, eventually triggering the later large earthquakes.

On various mechanisms of shear wave generation from underground chemical explosions in hard rocks

SUMMARY Significant shear waves observed during underground explosions can make them look similar to natural earthquakes. In the near field, they can be caused by the source geometry and emplacement conditions, heterogeneous and anisotropic properties of the rock mass, relaxation of a non-uniform in situ stress around the source after the departure of the shock wave as well as by activation of nearby faults. The stress relaxation mechanism is important at depth when the difference in main principal stresses becomes significant. Another mechanism of shear motion generation is due to explosive gas flow from the source cavity into the cracks initiated by the shock wave and joints connected to the cavity. Also, the shock wave from the explosion can cause weakening of the rock either due to permanent damage or shock-induced liquefaction at saturated joints. Numerical modelling of underground chemical explosions in hard rock at various confinement stresses has been conducted to compare various mechanisms of shear wave generation and their seismic characteristics with that of a simple fault rupture process observed during an earthquake.

Earthquake Threats in Ranau – From The Sources of Mensaban and Mesilou Fault

Unlike the majority of Malaysia’s other states, Sabah is characterised by frequent seismological activity; on average, an earthquake of moderate magnitude occurs roughly every 20 years, originating primarily from one of two major sources: either a local source or a regional source. Sabah has seen an upsurge in low to moderate seismic activity in recent years as a result of the fault activities. Between 1900 and lately, magnitudes ranging from MW 2.9 to 6.0 were recorded. While big magnitude earthquakes are relatively uncommon, the area has previously been struck by disastrous earthquakes. Fortunately, the region is demarcated by active Quaternary fault networks. As a result, the area’s seismicity is calculated using line sources corresponding to these faults. Two major fault systems are suspected of being the cause of such activity: the Mensaban fault zone and the Mesilou fault zone, as well as several additional nearby faults. This article explains the process for doing a probabilistic seismic hazard analysis (PSHA) while taking into consideration the peak ground acceleration (PGA) on bedrock in the Ranau region for a 10% and 2% chance of exceedance. The PGA estimate values for Ranau are between 0.08g to 0.16g for PGA 10%, while for PGA 2% was between 0.15g to 0.28g. The PGA value is divided into the following five earthquake hazard classes based on a quarterly geostatistical analysis method. These are “very low,” “low,” “medium,” “high,” and “very high” classes. In general, this quarterly classification is a good way to see how much local seismic activity there is, based on the PGA value. Which can be seen in 10% probability, 19.88 percent of the study area was in “very low,” 20.19 percent was low, 19.98 percent medium, and 19.92 percent high class. 2% probability, it was found that 19.74 percent of the total area was very low class, 20.06 percent low class, 19.79 percent middle class, and 19.82 percent high class. Only 20.6 percent of the area was “very high”.

Transient evolution of the relative size distribution of earthquakes as a risk indicator for induced seismicity

Induced earthquakes pose a substantial challenge to many geo-energy applications, and in particular to Enhanced Geothermal Systems. We demonstrate that the key factor controlling the seismic hazard is the relative size distribution of earthquakes, the b-value, because it is closely coupled to the stress conditions in the underground. By comparing high resolution observations from an Enhanced Geothermal System project in Basel with a loosely coupled hydro-mechanical-stochastic model, we establish a highly systematic behaviour of the b-value and resulting hazard through the injection cycle. This time evolution is controlled not only by the specific site conditions and the proximity of nearby faults but also by the injection strategy followed. Our results open up new approaches to assess and mitigate seismic hazard and risk through careful site selection and adequate injection strategy, coupled to real-time monitoring and modelling during reservoir stimulation.

Analysis of the Hydrogeological Conditions Affecting Fault Response to Nearby Hydraulic Fracturing

AbstractThe response of critically stressed dormant faults to fluid perturbation, by oil and gas production, has been a major public concern because of its link to induced seismicity. In this paper, we study the hydrogeological factors that affect a nearby fault response, during and after hydraulic fracturing (HF) operations, evaluated by the change in Coulomb Failure Stress (CFS) and the rate of seismicity (R) through coupling solid deformation and fluid flow. Our results show that the pore pressure increases rapidly in a fault that is close (hydraulically connected) to HF operations, which might lead to its activation when the injection rate is high. When the fault is adjacent to HF but distant from it, its shallow region is subjected to a stabilizing deformation‐induced normal compressive stress and its deeper region is destabilized under extension. In this case, the fault orientation and damage zone size have a significant effect on the fault's stability and response. On the other hand, decreasing the rate of injection can either increase or decrease the CFS values depending on the fault location and the dominant stresses. Therefore, serious attention should be given to the fault position, its architecture, and the injection rate to help reduce the potential for induced seismicity from HF. Our findings are verified and confirmed using the case of the Duvernay formation in Alberta, Canada, where the reported seismic data correlate with high CFS and R values.

Upper crustal structure beneath the Qin-Hang and Wuyishan metallogenic belts in Southeast China as revealed by a joint active and passive seismic experiment

SUMMARY Southeast China is one of the most important polymetallic metallogenic provinces in the world. However, distinct differences in the type and origin of the mineral deposits are present between the major metallogenic belts in this province. To deepen our knowledge of this regional metallogenic setting, investigate the factors controlling its metallogenic differences and explore the feasibility of active and passive seismic imaging, we performed a multibatch seismic experiment using active and passive sources with the help of 437 short-period seismographs deployed across the Qin-Hang and Wuyishan metallogenic belts in Southeast China. We resolved the S-wave velocity structure beneath the profile using ambient noise tomography and derived a Vp/Vs model by combining with a published P-wave model. A good correlation between the obtained seismic velocity models and geological observations is observed. The Proterozoic volcanic-sedimentary rock series and the Fuzhou-Yongfeng sedimentary basin are identified by low P- and S-wave velocities, high Vp/Vs ratios and low S-wave velocity perturbations. Within the core of the Wuyishan Metallogenic Belt, several narrow, elongated and low seismic velocity anomalies are considered to be signs of the nearby fault zones, and these anomalies provide reference information on the extension and pattern of these faults at depth. Combined with the geological and mineralogical evidence, we propose that theses faults function as ore-guiding structures and facilitate the formation of regionally representative mineral species and metal assemblages.

Integrated geochemical, mineralogical, and microstructural identification of faults in tunnels and its application to TBM jamming analysis

Faults are frequently one of those adverse geological conditions that jam tunnel boring machines (TBMs). However, the identification of faults and the engineering geological analysis of the corresponding jamming accidents still lack a better understanding. In this study, a method for fault identification through integration of the microstructural, geochemical, and mineralogical characteristics of the rocks was developed. The method was applied to the TBM 2 bid section of the Central Shanxi Yellow River Diversion Project to determine the geological characteristics of the rocks using optical thin section, X-ray fluorescence, and X-ray diffraction analysis. The results of this study provide evidence for the establishment of fault-induced geochemical effects and mineral transformations. Not only were fault FT1 and the nearby fault rock successfully identified, but evidence of weakened mechanical properties of the fault rocks was also obtained. The structure and composition of the migmatitic granites were strongly altered by long-term tectonic stress fracturing and pervasive acidic fluid leaching, forming loosely fractured, poorly integrated granitic granulate-porphyroclastic rock. The fault rock is rich in clay minerals and solid graphite lubricant. The comprehensive effect of the fault rock’s characteristics, geological structure, groundwater, and ground stress led to the collapse of the weak rock within the fault zone, which was the main geological reason for the jamming accident. Finally, we proposed corresponding engineering treatment measures and subsequent excavation recommendations. The proposed method can be used for the integrated identification of faults in underground engineering and mining projects, and the geological analysis of TBM jamming accidents.

What drives landslide risk? Disaggregating risk analyses, an example from the Franz Josef Glacier and Fox Glacier valleys, New Zealand

Abstract. We present a quantitative risk analysis (QRA) case study from the Kā Roimata o Hine Hukatere / Franz Josef Glacier and Te Moeka o Tuawe / Fox Glacier valleys, on the west coast of the South Island, Aotearoa / New Zealand. The glacier valleys are important tourist destinations that are subject to landslide hazards. Both valleys contain actively retreating glaciers; experience high rainfall; and are proximal to the Alpine Fault, which is a major source of seismic hazard on the west coast. We considered the life safety risk from rockfalls, soil/rock avalanches, and flows that either are seismically triggered or occur aseismically. To determine the range in risk values and dominant contributing variables to the risk, we modelled nine different risk scenarios where we incrementally changed the variables used in the risk model to account for the underlying uncertainty. The scenarios represent our central estimate of the risk, e.g. neither optimistic nor conservative, through to our upper estimate of the risk. We include in these estimates the impact time-variable factors, such as a recently reactivated landslide, have had on locally increasing risk and the time-elapsed since the last major earthquake on the nearby Alpine Fault. We disaggregated our risk results to determine the dominant drivers in landslide risk, which highlighted the importance of considering dynamic time-variable risk scenarios and the changing contributions to risk from aseismic versus seismic landslides. A detailed understanding of the drivers of landslide risk in each valley is important to determine the most efficient and appropriate risk management decisions.

Late Holocene rupture history of the Ash Hill fault, Eastern California Shear Zone, and the potential for seismogenic strain transfer between nearby faults

AbstractUnderstanding how fault systems interact and transfer strain over seismogenic timescales (seconds to ka) requires temporal records of past ruptures along adjacent and intersecting fault networks. Here we document the record of Late Holocene ruptures as recorded in the geomorphology of alluvial deposits along the Ash Hill fault, in the Eastern California Shear Zone (ECSZ). We leverage a multi‐faceted approach to evaluate the relative timing of Ash Hill fault ruptures to those of nearby faults in the ECSZ. We determine the number and timing of Late Holocene earthquakes on the Ash Hill fault using high‐resolution tectono‐geomorphic mapping, a locally calibrated alluvial fan stratigraphy, feldspar luminescence dating, and fault offset analysis from field observations, LiDAR, and drone‐based digital surface elevation models. We find evidence for three surface‐rupturing earthquakes that have occurred since ~4 ka, each with ~1.0 ± 0.2 m of oblique slip per event (Mw ~ 6.9–7.0), and we constrain the timing of these earthquakes by dating deposits that bracket each event. The timing of these three ruptures is similar to the paleoseismic record along the adjacent range‐bounding fault in southern Panamint Valley. Specifically, the two adjacent faults exhibit similar numbers of earthquakes during the Late Holocene, with similar recurrence intervals and rupture timing. These data suggest that it is possible that these two faults have ruptured in the same or closely temporally related events throughout the Late Holocene. Similar spatio‐temporal clustered earthquakes have been recognized in both historic and paleoseismic records in the region, and such behaviour may be common in complexly interlinked fault networks, like those that exist in the ECSZ.

The Kalaotoa Fault: A Newly Identified Fault that Generated the Mw 7.3 Flores Sea Earthquake

AbstractWe reveal the existence of a previously unknown fault that generated the Mw 7.3 Flores Sea earthquake, which occurred on 14 December 2021, approximately 100 km to the north of Flores Island, in one of the most complex tectonic settings in Indonesia. We use a double-difference method to relocate the hypocenters of the mainshock and aftershocks, determine focal mechanisms using waveform inversion, and then analyze stress changes to estimate the fault type and stress transfer. Our relocated hypocenters show that this earthquake sequence ruptured on at least three segments: the source mechanism of the mainshock exhibits dextral strike-slip motion (strike N72°W and dip 78° NE) on a west–east-trending fault that we call the Kalaotoa fault, whereas rupture of the other two segments located to the west and east of the mainshock (striking west-northwest and southeast, respectively) may have been triggered by this earthquake. The Coulomb stress change imparted by the rupture of these segments on nearby faults is investigated, with a focus on regions that experience a stress increase with few associated aftershocks. Of particular interest are stress increases on the central back-arc thrust just north of Flores and the north–south-striking Selayar fault in the northwest of our study region, both of which may be at increased risk of failure as a result of this unusual earthquake sequence.

2019 Mw 5.9 Mirpur, Pakistan Earthquake: Insights from Integrating Geodetic, Seismic, and Field Observations

Abstract On 24 September 2019, an Mw 5.9 earthquake struck near the Mangla reservoir in northeastern Pakistan and resulted in 39 fatalities and 746 serious injuries, making it the deadliest earthquake in the region since the 2005 Mw 7.6 Kashmir earthquake. Here, we integrate geodetic, seismic, and field observations to characterize the source properties and impact of the Mirpur earthquake as well as investigate whether it might be a reservoir-induced event. From inverting Interferometric Synthetic Aperture Radar data, we find that a fault with strike ∼310°, dip ∼6°, and rake ∼117° is the optimal source, with slip concentrated between 5 and 6 km depth. This is consistent with our relocated aftershocks depth distribution and the lack of surface rupture observed in the field. Therefore, we infer that the earthquake ruptured the Main Himalayan Thrust (MHT). The event’s shallow depth might explain the extensive damage caused despite its moderate magnitude, with a maximum shaking intensity of VIII based on our field survey. The survey also revealed extensive damages associated with earthquake-induced liquefaction. Our modeling shows that loading due to increased reservoir water level in the three months before the Mirpur earthquake led to Coulomb stress increase of ∼7–10 kPa on the rupture plane. However, this effect is ∼10 times smaller than the Coulomb stress increase due to the 2006 Mangla earthquake, and the Mirpur earthquake only occurred ∼1–2 weeks after peak water level. These suggest that pore pressure diffusion contributed to promoting the fault rupture at a time when it is close to failure due to accumulated stress from inter-seismic loading. Because the Mirpur earthquake resulted in a stress increase of >0.2 MPa on the surrounding sections of the MHT and nearby faults, future rupture of these faults is a significant hazard and proper management of reservoir operations is necessary to prevent further elevating the seismic risk.

Seismicity around Southern Lake Erie during 2013–2020 in Relation to Lake Water Level

Abstract The water level change in the Great Lake region can modulate stresses in the nearby fault system and potentially induce earthquakes. We perform a systematic analysis of local seismicity around southern Lake Erie during 2013–2020 to investigate the relation between seismicity and lake water level change. We obtain a newly detected catalog of tectonic earthquakes, which reveals 20–40 M > 0 earthquakes/yr before 2019. The peak seismicity rate in 2019 is dominated by active aftershocks following the 2019 ML 4.0 Ohio earthquake. The clustering analysis reveals both clusters with multiple events and stand-alone events that might represent isolated zones of crustal weakness in this region. The absolute location analysis identifies a group of earthquakes around the epicenter of the 1986 M 5 earthquake, which might be linked to the nearby injection activities. To evaluate the potential interaction with changing water loading of Lake Erie, we then model the coulomb stress change (ΔCSC) caused by elastic lake-water loading and find that ΔCSC strongly depends on effective friction coefficient (μ′). Considering the geometry of the receiver fault, higher lake water level results in positive ΔCSC and would promote failure when μ′≤0.3, and leads to negative ΔCSC and would discourage rupture when μ′>0.3. Further analysis indicates a lack of temporal correlation between long-term seismicity and water level variations before the 2019 Ohio sequence, suggesting that water level fluctuations do not modulate long-term seismicity. However, we cannot fully rule out the impact of increasing water level on reactivating the fault that hosted the 2019 Ohio earthquake sequence because of the uncertainty of μ′. Our results highlight the necessity of denser and closer monitoring of lake seismicity to further investigate the impact of changing water loading on reactivating shallow faults in this region.

The 2017 Kos sequence: Aftershocks relocation and coseismic rupture process constrained from joint inversion of seismological and geodetic observations

On 20 July 2017 (22:31:10 UTC), an Mw6.6 earthquake took place offshore Kos Island, in the southeastern (SE) Aegean Sea, producing severe damage and loss of life in the city of Kos and several smaller cities and villages both in Kos Island (Greece) and in the Bodrum peninsula (Turkey). All available seismological data until the end of October 2017 were gathered from seismological stations located in Greece and Turkey for the relocation process. The relocated aftershocks are clustered at least in three distinctive patches, creating a zone reaching a total length of about 40 km, elongated in a nearly east-west direction, and are mainly concentrated at depths 8–15 km, with the main shock hypocenter placed at ~13 km, implying a seismogenic layer of 7 km thickness, indicative for normal faulting earthquakes with Mmax ~ 6.5. The aftershock fault plane solutions are predominantly normal faulting in response to the north–south extension of the back arc Aegean area and consistent with the broader regional stress field. We also applied the satellite radar interferometry (InSAR) technique to define the coseismic surface displacements, revealing the range of deformation on the Island and the surrounding mainland. We combined this deformation field along with the available vectors of displacement measured by the Global Navigation Satellite System (GNSS) technique with the seismological data to determine the fault geometry and rupture process. The resulted variable slip model indicates a rather elliptical rupture area of 306 km2, with the coseismic slip ranging between 0.5 and 2.3 m. The peak moment release occurred in the depth interval of 9–11 km, consistent with the depth distribution of seismicity in the study area. We used the variable slip model to calculate Coulomb stress changes and investigate possible triggering due to stress transfer to the nearby fault segments.

U-Pb zircon and titanite age of the Christiansø granite, Ertholmene, Denmark, and correlation with other Bornholm granitoids

A granitic sample from the Danish island of Christiansø in the Ertholmene island group north of Bornholm is described petrographically and geochemically, and dated using U-Pb in zircon and titanite. Zircon systematics in the sample are complicated by abundant Pb loss and a large population of zircons interpreted as being inherited. Removal of highly disturbed zircons, imprecise analyses, and assumed inherited zircons yield an upper intercept date of 1500 ± 18 Ma (MSWD = 13, n = 58). Removal of zircons with high common Pb from this population yields an identical result of 1500 ± 22 Ma (MSWD = 8, n = 34). Zircons that are ≤3% discordant give a weighted average 206Pb/238U age of 1458 ± 12 Ma (MSWD = 3.0, n = 18), and a weighted average 207Pb/206Pb age of 1495 ± 14 Ma (MSWD = 4.7, n = 19). Titanites from the sample yield a lower intercept age of 1448 ± 15 Ma (MSWD = 6.8, n = 45). The sample contains a significant number of inherited grains indicative of ages around 1.7–1.8 Ga. The relatively large MSWDs for these age determinations indicate geological complexity, likely reflecting Pb loss, and the possible presence of inherited zircons which suffered major Pb loss during incorporation in the granitic magma. The zircon and titanite dates agree reasonably well with previous age determinations on felsic lithologies from the Bornholm mainland, as well as from the Blekinge Province of southern Sweden. Petrographically and geochemically, the Christiansø granite is indistinguishable from, and can be correlated with, the A-type granites and gneisses which occur on Bornholm. The high abundance of disturbed and inherited zircons (c. 1.7–1.8 Ga) may indicate that the granite was intruded into and assimilated a nearby region of unexposed Transscandinavian Igneous Belt rocks. The somewhat altered nature of the rock, and overall disturbance of U-Pb zircon systematics, suggest alteration associated with fluid-flow along nearby faults defining the northern margin of the Sorgenfrei–Tornquist Zone.

Non-steady-state slip rates emerge along evolving restraining bends under constant loading

Abstract Recent field studies provide evidence of fault slip-rate variability over time periods of 10–100 k.y., yet researchers do not know how processes internal to the fault system (e.g., fault reorganization) impact records of fault slip rates. In this study, we directly observed fault-system evolution and measured slip-rate histories within a scaled physical experiment of a dextral strike-slip 15° restraining bend representative of a gentle crustal restraining bend. To assess the degree of slip-rate variability at particular sites along the experimental faults, such as would be revealed in a field study, we tracked fault slip rates at specific locations that advected throughout the experiment with accrued fault slip. Slip rates increased or decreased (5%–25% of the applied velocity) both during fault reorganization (e.g., fault growth and abandonment) and as sites migrated to new structural positions. Sites that advected into the restraining bend showed decreased slip rate. While we expect new fault growth to reduce slip rates along nearby fault segments, we document that the growth of new oblique-slip faults can increase strike-slip rates on nearby fault segments. New oblique-slip thrust faults within the experiment accommodated off-fault convergence and unclamped nearby strike-slip segments. The experimental results show that even under a constant loading rate, slip rates at sites located on stable fault segments can vary due to either reorganization elsewhere in the fault system or site advection.

Deformation in Western Guatemala Associated With the NAFCA (North America‐Central American Forearc‐Caribbean) Triple Junction: Neotectonic Strain Localization Into the Guatemala City Graben

AbstractRecent structural and geodetic data define the Guatemala City graben region as the continental triple junction between the North American plate, Caribbean plate, and the Central American Forearc sliver. We present minor fault analysis, geochronological and geochemical analyses, and newly updated GPS velocities in western Guatemala, west of the Guatemala City graben, to characterize the magnitude and timing of extensional deformation in this poorly understood area. Elongations estimated from fault data are parallel (∼east‐west) and perpendicular to the Polochic‐Motagua fault system to the north, similar to geodetically measured active deformation observed east of the Guatemala City graben. Four new 40Ar/39Ar dates and correlation of tephra deposits suggest that faulting was active during the Pliocene, but ceased eastward toward the Guatemala City graben over time. From west to east, fault cessation occurred before the deposition of the Los Chocoyos ash (75 ka) and E tephra (51 ka). Faulting just west of the Guatemala City graben appears to be active, where a major fault cuts the most recent Amatitlán tephras. Based on this data, we propose a time‐progressive strain model for deformation related to North America‐Caribbean plate interactions, whereby distributed elongation of the westernmost Caribbean plate occurred during the Pliocene but localized mostly within the Guatemala City graben and nearby faults during the Pleistocene. Our model supports that: (a) The Guatemala City graben is effectively the western limit of the Caribbean plate; and (b) Western Guatemala, which was the trailing edge of the Caribbean plate, has been transferred to the forearc region.

Ambient noise tomography of local shallow structure of the southern segment of Tanlu fault zone at Suqian, eastern China

Comparable to San Andreas fault in western United States, the Tanlu fault zone (TLFZ) is a large-scale continental tectonic belt in eastern China and has played an important role controlling the regional tectonics and earthquake activity in eastern China. It is known there exists an earthquake gap in the southern segment around Suqian, Jiangsu province, where no earthquakes greater than M5.5 occurred. Fine fault zone structure could help to understand the fault activity and the potential earthquake hazards. However, only regional scale velocity model is available in this region, making it difficult to characterize detailed fault structures. In this study, we obtained local Vs model in the upper crust in this region by ambient noise tomography with a dense seismic array. For the Suqian segment, Cretaceous brittle normal faulting controlled the development of the eastern and western graben as well as the central horst between them. The graben-boundary faults are known simply as F1-F4 from east to west, and the NNE-striking fault F5 is between F1 and F2. The eastern boundary fault F1 and western boundary fault F4 separated by about 30 km are associated with intermittent high velocity anomalies in the shallow crust and are inactive. In comparison, faults F2, F5 and F3 are mainly associated with low velocity anomalies and are relatively active. These high velocity anomalies are very likely caused by the upwelling magma migrated along faults F1 and F4 in the crust, which on freezing could weld the fault zone and make it less active. Because faults F1 and F4 are hindered to move, the deformation could be transferred to nearby faults F2, F3 and F5. Our study sheds light on what controls fault activities along the TLFZ by obtaining local velocity structure in the Suqian segment.

San Andreas Fault Stress Change Due To Groundwater Withdrawal in California's Central Valley, 1860‐2010

Changes in groundwater storage in California's Central Valley (CV) are considered partly responsible for vertical uplift surrounding the southern CV and for stress changes on nearby faults. Questions remain regarding the distribution of stress on the central San Andreas fault (CSAF) from recent and historical drawdown of the CV aquifer over the past 150 years (1860–2010). We combine groundwater storage change estimates for the 2006–2010 drought with a three-dimensional finite element model to estimate Coulomb failure stress change (ΔCFS) on the CSAF. We combine a simple parameterization of historical hydraulic head change for 1860–1960 with a CV hydrological model (1961–2003) to estimate visco-elastic effects on ΔCFS total and 2010 rate. We find that ΔCFS on the CSAF correlates positively with shallow seismicity and low frequency earthquakes for both short-term and long term stressing. Unloading uplift rates surrounding the southern CV only partially account for the observed vertical GPS rates.