Geology topics

Faults & earthquakes

Explore USGS earthquake and fault publications. Search fault names, locations and seismic hazards; follow the original report for maps, methods and publication context.

Follow this topic with RSS

At least 703 records · Page 39Linked to original sources

USGS2011

Superficial simplicity of the 2010 El Mayorg-Cucapah earthquake of Baja California in Mexico

The geometry of faults is usually thought to be more complicated at the surface than at depth and to control the initiation, propagation and arrest of seismic ruptures 1 , 2 , 3 , 4 , 5 , 6 . The fault system that runs from southern California into Mexico is a simple strike-slip boundary: the west side of California and Mexico moves northwards with respect to the east. However, the M w 7.2 2010 El Mayor–Cucapah earthquake on this fault system produced a pattern of seismic waves that indicates a far more complex source than slip on a planar strike-slip fault 7 . Here we use geodetic, remote-sensing and seismological data to reconstruct the fault geometry and history of slip during this earthquake. We find that the earthquake produced a straight 120-km-long fault trace that cut through the Cucapah mountain range and across the Colorado River delta. However, at depth, the fault is made up of two different segments connected by a small extensional fault. Both segments strike N130° E, but dip in opposite directions. The earthquake was initiated on the connecting extensional fault and 15 s later ruptured the two main segments with dominantly strike-slip motion. We show that complexities in the fault geometry at depth explain well the complex pattern of radiated seismic waves. We conclude that the location and detailed characteristics of the earthquake could not have been anticipated on the basis of observations of surface geology alone.

California, Arizona↗

USGS2003

Late Holocene liquefaction features in the Dominican Republic: A powerful tool for earthquake hazard assessment in the northeastern Caribbean

Several generations of sand blows and sand dikes, indicative of significant and recurrent liquefaction, are preserved in the late Holocene alluvial deposits of the Cibao Valley in northern Dominican Republic. The Cibao Valley is structurally controlled by the Septentrional fault, an onshore section of the North American-Caribbean strike-slip plate boundary. The Septentrional fault was previously studied in the central part of the valley, where it sinistrally offsets Holocene terrace risers and soil horizons. In the eastern and western parts of the valley, the Septentrional fault is buried by Holocene alluvial deposits, making direct study of the structure difficult. Liquefaction features that formed in these Holocene deposits as a result of strong ground shaking provide a record of earthquakes in these areas. Liquefaction features in the eastern Cibao Valley indicate that at least one historic earthquake, probably the moment magnitude, M 8, 4 August 1946 event, and two to four prehistoric earthquakes of M 7 to 8 struck this area during the past 1100 yr. The prehistoric earthquakes appear to cluster in time and could have resulted from rupture of the central and eastern sections of the Septentrional fault circa A.D. 1200. Liquefaction features in the western Cibao Valley indicate that one historic earthquake, probably the M 8, 7 May 1842 event, and two prehistoric earthquakes of M 7-8 struck this area during the past 1600 yr. Our findings suggest that rupture of the Septentrional fault circa A.D. 1200 may have extended beyond the central Cibao Valley and generated an earthquake of M 8. Additional information regarding the age and size distribution of liquefaction features is needed to reconstruct the prehistoric earthquake history of Hispaniola and to define the long-term behavior and earthquake potential of faults associated with the North American-Caribbean plate boundary.

Bulletin of the Seismological Society of America↗

USGS2007

Postearthquake relaxation and aftershock accumulation linearly related after the 2003 M 6.5 Chengkung, Taiwan, and the 2004 M 6.0 Parkfield, California, earthquakes

We treat both the number of earthquakes and the deformation following a mainshock as the superposition of a steady background accumulation and the postearthquake process. The preseismic displacement and seismicity rates r u and r E are used as estimates of the background rates. Let t be the time after the mainshock, u ( t ) + u 0 the postseismic displacement less the background accumulation r u t , and Δ N ( t ) the observed cumulative number of postseismic earthquakes less the background accumulation r E t . For the first 160 days (duration limited by the occurrence of another nearby earthquake) following the Chengkung ( M 6.5, 10 December 2003, eastern Taiwan) and the first 560 days following the Parkfield ( M 6.0, 28 September 2004, central California) earthquakes u ( t ) + u 0 is a linear function of Δ N ( t ). The aftershock accumulation Δ N ( t ) for both earthquakes is described by the modified Omori Law d Δ N / dt ∝ (1 + t / τ ) − p with p = 0.96 and τ = 0.03 days. Although the Chengkung earthquake involved sinistral, reverse slip on a moderately dipping fault and the Parkfield earthquake right-lateral slip on a near-vertical fault, the earthquakes share an unusual feature: both occurred on faults exhibiting interseismic fault creep at the surface. The source of the observed postseismic deformation appears to be afterslip on the coseismic rupture. The linear relation between u ( t ) + u 0 and N ( t ) suggests that this afterslip also generates the aftershocks. The linear relation between u ( t ) + u 0 and Δ N ( t ) obtains after neither the 1999 M 7.1 Hector Mine (southern California) nor the 1999 M 7.6 Chi-Chi (central Taiwan) earthquakes, neither of which occurred on fault segments exhibiting fault creep.

Bulletin of the Seismological Society of America↗

USGS2015

Landslides and megathrust splay faults captured by the late Holocene sediment record of eastern Prince William Sound, Alaska

We present new marine seismic‐reflection profiles and bathymetric maps to characterize Holocene depositional patterns, submarine landslides, and active faults beneath eastern and central Prince William Sound (PWS), Alaska, which is the eastern rupture patch of the 1964 Mw 9.2 earthquake. We show evidence that submarine landslides, many of which are likely earthquake triggered, repeatedly released along the southern margin of Orca Bay in eastern PWS. We document motion on reverse faults during the 1964 Great Alaska earthquake and estimate late Holocene slip rates for these growth faults, which splay from the subduction zone megathrust. Regional bathymetric lineations help define the faults that extend 40–70 km in length, some of which show slip rates as great as 3.75  mm/yr. We infer that faults mapped below eastern PWS connect to faults mapped beneath central PWS and possibly onto the Alaska mainland via an en echelon style of faulting. Moderate (Mw>4) upper‐plate earthquakes since 1964 give rise to the possibility that these faults may rupture independently to potentially generate Mw 7–8 earthquakes, and that these earthquakes could damage local infrastructure from ground shaking. Submarine landslides, regardless of the source of initiation, could generate local tsunamis to produce large run‐ups along nearby shorelines. In a more general sense, the PWS area shows that faults that splay from the underlying plate boundary present proximal, perhaps independent seismic sources within the accretionary prism, creating a broad zone of potential surface rupture that can extend inland 150 km or more from subduction zone trenches.

Alaska↗

USGS2009

Evaluation of hazardous faults in the intermountain west region: Summary and recommendations of a workshop

The U.S. Geological Survey’s (USGS) Earthquake Hazards Program (EHP) has the responsibility to provide nationwide information and knowledge about earthquakes and earthquake hazards as a step to mitigating earthquake-related losses. As part of this mission, USGS geologists and geophysicists continue to study faults and structures that have the potential to generate large and damaging earthquakes. In addition, the EHP, through its External Grants Program (hereinafter called Program), supports similar studies by scientists employed by state agencies, academic institutions, and independent employers. For the purposes of earthquake hazard investigations, the Nation is geographically subdivided into tectonic regions. One such region is the Intermountain West (IMW), which here is broadly defined as starting at the eastern margin of the Rocky Mountains in New Mexico, Colorado, Wyoming, and Montana and extending westward to the east side of the Sierra Nevada mountains in eastern California and into the Basin and Range-High Plateaus of eastern Oregon and Washington. The IMW contains thousands of faults that have moved in Cenozoic time, hundreds of which have evidence of Quaternary movement, and thus are considered to be potential seismic sources. Ideally, each Quaternary fault should be studied in detail to evaluate its rate of activity in order to model the hazard it poses. The study of a single fault requires a major commitment of time and resources, and given the large number of IMW faults that ideally should be studied, it is impractical to expect that all IMW Quaternary faults can be fully evaluated in detail. A more realistic approach is to prioritize a list of IMW structures that potentially pose a significant hazard and to focus future studies on those structures. Accordingly, in June 2008, a two-day workshop was convened at the USGS offices in Golden, Colorado, to seek information from representatives of selected State Geological Surveys in the IMW and with knowledgeable regional experts to identify the important structures for future studies. Such a priority list allows Program managers to guide the limited resources toward studies of features that are deemed to potentially pose the most serious hazards in the IMW. It also provides the scientific community with a list of structures to investigate because they are deemed to pose a substantial hazard to population centers or critical structures. The IMW encompasses all or large parts of 12 states, including Arizona, New Mexico, extreme west Texas, Colorado, Utah, Nevada, eastern California, eastern Oregon, eastern Washington, Idaho, western Wyoming, and western Montana. In Utah, and more recently in Nevada, geoscientists have taken steps to evaluate geologic data related to well-studied faults and to develop a statewide priority list of hazardous structures. In contrast to Utah and Nevada, the other IMW states contain substantially fewer Quaternary faults, so there have not been any previous efforts to develop similar priority lists. This workshop was organized to address this matter and create a more balanced perspective of priorities throughout the entire IMW region. Because working groups and workshops had already been convened to specifically deal with Quaternary fault priorities in Utah and Nevada, this workshop specifically emphasized structures outside of these two states.

Arizona, California, Colorado, Idaho, Montana, Nev↗

USGS2008

Revisiting the 1872 Owens Valley, California, Earthquake

The 26 March 1872 Owens Valley earthquake is among the largest historical earthquakes in California. The felt area and maximum fault displacements have long been regarded as comparable to, if not greater than, those of the great San Andreas fault earthquakes of 1857 and 1906, but mapped surface ruptures of the latter two events were 2-3 times longer than that inferred for the 1872 rupture. The preferred magnitude estimate of the Owens Valley earthquake has thus been 7.4, based largely on the geological evidence. Reinterpreting macroseismic accounts of the Owens Valley earthquake, we infer generally lower intensity values than those estimated in earlier studies. Nonetheless, as recognized in the early twentieth century, the effects of this earthquake were still generally more dramatic at regional distances than the macroseismic effects from the 1906 earthquake, with light damage to masonry buildings at (nearest-fault) distances as large as 400 km. Macroseismic observations thus suggest a magnitude greater than that of the 1906 San Francisco earthquake, which appears to be at odds with geological observations. However, while the mapped rupture length of the Owens Valley earthquake is relatively low, the average slip was high. The surface rupture was also complex and extended over multiple fault segments. It was first mapped in detail over a century after the earthquake occurred, and recent evidence suggests it might have been longer than earlier studies indicated. Our preferred magnitude estimate is Mw 7.8-7.9, values that we show are consistent with the geological observations. The results of our study suggest that either the Owens Valley earthquake was larger than the 1906 San Francisco earthquake or that, by virtue of source properties and/or propagation effects, it produced systematically higher ground motions at regional distances. The latter possibility implies that some large earthquakes in California will generate significantly larger ground motions than San Andreas fault events of comparable magnitude.

Conference Paper↗

USGS1989

Review of magnetic and electric field effects near active faults and volcanoes in the U.S.A.

Synchronized measurements of geomagnetic field have been recorded along 800 km of the San Andreas fault and in the Long Valley caldera since 1974, and during eruptions on Mount St. Helens since 1980. For shorter periods of time, continuous measurements of geoelectric field measurements have been made on Mount St. Helens and near the San Andreas fault where moderate seismicity and fault slip frequently occurs. Significant tectonic and volcanic events for which nearby magnetic and electric field data have been obtained include: (1) two moderate earthquakes (ML > 5.8) for which magnetometers were close enough to expect observable signals (about three source lengths), (2) one moderate earthquake (MS 7.3) for which magnetometers were installed as massive fluid outflow occurred during the post-seismic phase, (3) numerous fault creep events and moderate seismicity, (4) a major explosive volcanic eruption and numerous minor extrusive eruptions, and (5) an episode of aseismic uplift. For one of the two earthquakes with ML > 5.8, seismomagnetic effects of -1.3 and -0.3 nT were observed. For this event, magnetometers were optimally located near the epicenter and the observations obtained are consistent with simple seismomagnetic models of the event. Similar models for the other event indicate that the expected seismomagnetic effects are below the signal resolution of the nearest magnetometer. Precursive tectonomagnetic effects were recorded on two independent instruments at distances of 30 and 50 km from a ML 5.2 earthquake. Longer-term changes were recorded in one region in southern California where a moderate ML 5.9 earthquake has since occurred. Surface observations of fault creep events have no associated magnetic or electrical signature above the present measurement precision (0.25 nT and 0.01%, respectively) and are consistent with near-surface fault failure models of these events. Longer-term creep is sometimes associated with corresponding longer-term magnetic field perturbations. Correlated changes in gravity, magnetic field, areal strain, and uplift occurred during episodes of aseismic deformation in southern California primarily between 1979 and 1983. Because the relationships between these parameters agrees with those calculated from simple deformation and tectonomagnetic models, the preferred explanation appeals to short-term strain episodes independently detected in each data set. An unknown source of meteorologically generated noise in the strain, gravity, and uplift data and an unknown, but correlated, disturbance in the absolute magnetic data might also explain the data. No clear observations of seismoelectric or tectonoelectric effects have yet been reported. The eruption of Mount St. Helens generated large oscillatory fields and 9 ?? 2 nT offset on the only surviving magnetometer. A large-scale traveling magnetic disturbance passed through the San Andreas array from 1 to 2 h after the eruption. Subsequent extrusive eruptions generated small precursory magnetic changes in some cases. These data are consistent with a simple volcanomagnetic model, magneto-gas dynamic effects, and a blast excited traveling ionospheric disturbance. Traveling ionospheric disturbances (TIDs), also generated by earthquake-related atmospheric pressure waves, may explain many electromagnetic disturbances apparently associated with earthquakes. Local near-fault magnetic field transients rarely exceed a few nT at periods of a few minutes and longer. ?? 1989.

Physics of the Earth and Planetary Interiors↗

USGS2019

Interseismic quiescence and triggered slip of active normal faults of Kīlauea Volcano’s south flank during 2001-2018

The mobile south flank of Kīlauea Volcano hosts two normal fault systems, the Koa'e fault system (KFS) and the Hilina fault system (HFS). In historical time, at least three M>6.5 earthquakes have occurred on the basal detachment of the Kīlauea Volcano's south flank, with the most recent being the 4 May 2018 M6.9 earthquake. Here we analyze kinematic Global Positioning System data collected from 2001 to 2017 and interferometric synthetic aperture radar data before, during, and after the 2018 M6.9 earthquake to determine the crustal motion across the HFS and KFS faults. Our results indicate that the HFS faults did not significantly slip during the interseismic period from 2007 to 2011. Despite its substantial magnitude, interferometric synthetic aperture radar (InSAR) data show that the 2018 M6.9 earthquake triggered subcentimeter level slip along sections of the previously mapped HFS branches. Up to 20 cm of offset occurred on what appears to be a newly formed (or previously unknown) fault near the eastern end of the HFS. During the 3 months following the M6.9 earthquake, up to ~30 cm of slip occurred along the KFS, which helps accommodate rapid large‐scale subsidence of Kīlauea's summit region as large volumes of summit reservoir magma fed the lower East Rift Zone eruption. The HFS appears to activate only in concert with large earthquakes on the basal detachment. The KFS, on the other hand, moves both seismically during small local earthquakes and aseismically in response to nearby earthquakes and caldera subsidence.

Hawaii↗

USGS1998

Non-double-couple earthquakes. 1. Theory

Historically, most quantitative seismological analyses have been based on the assumption that earthquakes are caused by shear faulting, for which the equivalent force system in an isotropic medium is a pair of force couples with no net torque (a “double couple,” or DC). Observations of increasing quality and coverage, however, now resolve departures from the DC model for many earthquakes and find some earthquakes, especially in volcanic and geothermal areas, that have strongly non-DC mechanisms. Understanding non-DC earthquakes is important both for studying the process of faulting in detail and for identifying nonshear-faulting processes that apparently occur in some earthquakes. This paper summarizes the theory of “moment tensor” expansions of equivalent-force systems and analyzes many possible physical non-DC earthquake processes. Contrary to long-standing assumption, sources within the Earth can sometimes have net force and torque components, described by first-rank and asymmetric second-rank moment tensors, which must be included in analyses of landslides and some volcanic phenomena. Non-DC processes that lead to conventional (symmetric second-rank) moment tensors include geometrically complex shear faulting, tensile faulting, shear faulting in an anisotropic medium, shear faulting in a heterogeneous region (e.g., near an interface), and polymorphic phase transformations. Undoubtedly, many non-DC earthquake processes remain to be discovered. Progress will be facilitated by experimental studies that use wave amplitudes, amplitude ratios, and complete waveforms in addition to wave polarities and thus avoid arbitrary assumptions such as the absence of volume changes or the temporal similarity of different moment tensor components.

Reviews of Geophysics↗

USGS2010

Seismic hazard of the Enriquillog-Plantain Garden fault in Haiti inferred from palaeoseismology

The Enriquillog-Plantain Garden fault zone is recognized as one of the primary plate-bounding fault systems in Haiti. The strike-slip fault runs adjacent to the city of Port-au-Prince and was initially thought to be the source of the 12 January 2010, M w 7.0 earthquake. Haiti experienced significant earthquakes in 1751 and 1770 (refsA, 3, 4, 5), but the role of the Enriquillog-Plantain Garden fault zone in these earthquakes is poorly known. We use satellite imagery, aerial photography, light detection and ranging (LIDAR) and field investigations to document Quaternary activity on the Enriquillog-Plantain Garden fault. We report late Quaternary, left-lateral offsets of up to 160m, and a set of small offsets ranging from 1.3 to 3.3m that we associate with one of the eighteenth century earthquakes. The size of the small offsets implies that the historical earthquake was larger than M w 7.0, but probably smaller than M w 7.6. We found no significant surface rupture associated with the 2010 earthquake. The lack of surface rupture, coupled with other seismologic, geologic and geodetic observations, suggests that little, if any, accumulated strain was released on the Enriquillog-Plantain Garden fault in the 2010 earthquake. These results confirm that the Enriquillog-Plantain Garden fault remains a significant seismic hazard. ?? 2010 Macmillan Publishers Limited. All rights reserved.

Nature Geoscience↗

USGS2015

Rates and patterns of surface deformation from laser scanning following the South Napa earthquake, California

The A.D. 2014 M6.0 South Napa earthquake, despite its moderate magnitude, caused significant damage to the Napa Valley in northern California (USA). Surface rupture occurred along several mapped and unmapped faults. Field observations following the earthquake indicated that the magnitude of postseismic surface slip was likely to approach or exceed the maximum coseismic surface slip and as such presented ongoing hazard to infrastructure. Using a laser scanner, we monitored postseismic deformation in three dimensions through time along 0.5 km of the main surface rupture. A key component of this study is the demonstration of proper alignment of repeat surveys using point cloud–based methods that minimize error imposed by both local survey errors and global navigation satellite system georeferencing errors. Using solid modeling of natural and cultural features, we quantify dextral postseismic displacement at several hundred points near the main fault trace. We also quantify total dextral displacement of initially straight cultural features. Total dextral displacement from both coseismic displacement and the first 2.5 d of postseismic displacement ranges from 0.22 to 0.29 m. This range increased to 0.33–0.42 m at 59 d post-earthquake. Furthermore, we estimate up to 0.15 m of vertical deformation during the first 2.5 d post-earthquake, which then increased by &sim;0.02 m at 59 d post-earthquake. This vertical deformation is not expressed as a distinct step or scarp at the fault trace but rather as a broad up-to-the-west zone of increasing elevation change spanning the fault trace over several tens of meters, challenging common notions about fault scarp development in strike-slip systems. Integrating these analyses provides three-dimensional mapping of surface deformation and identifies spatial variability in slip along the main fault trace that we attribute to distributed slip via subtle block rotation. These results indicate the benefits of laser scanner surveys along active faults and demonstrate that fine-scale variability in fault slip has been missed by traditional earthquake response methods.

California↗

USGS2000

Characterization of active faulting beneath the Strait of Georgia, British Columbia

Southwestern British Columbia and northwestern Washington State are subject to megathrust earthquakes, deep intraslab events, and earthquakes in the continental crust. Of the three types of earthquakes, the most poorly understood are the crustal events. Despite a high level of seismicity, there is no obvious correlation between the historical crustal earthquakes and the mapped surface faults of the region. On 24 June 1997, a ML = 4.6 earthquake occurred 3-4 km beneath the Strait of Georgia, 30 km to the west of Vancouver, British Columbia. This well-recorded earthquake was preceded by 11 days by a felt foreshock (ML = 3.4) and was followed by numerous small aftershocks. This earthquake sequence occurred in one of the few regions of persistent shallow seismic activity in southwestern British Columbia, thus providing an ideal opportunity to attempt to characterize an active near-surface fault. We have computed focal mechanisms and utilized a waveform cross-correlation and joint hypocentral determination routine to obtain accurate relative hypocenters of the mainshock, foreshock, and 53 small aftershocks in an attempt to image the active fault and the extent of rupture associated with this earthquake sequence. Both P-nodal and CMT focal mechanisms show thrust faulting for the mainshock and the foreshock. The relocated hypocenters delineate a north-dipping plane at 2-4 km depth, dipping at 53??, in good agreement with the focal mechanism nodal plane dipping to the north at 47??. The rupture area is estimated to be a 1.3-km-diameter circular area, comparable to that estimated using a Brune rupture model with the estimated seismic moment of 3.17 ?? 1015 N m and the stress drop of 45 bars. The temporal sequence indicates a downdip migration of the seismicity along the fault plane. The results of this study provide the first unambiguous evidence for the orientation and sense of motion for active faulting in the Georgia Strait area of British Columbia.

Bulletin of the Seismological Society of America↗

USGS2005

Stress distribution along the Fairweather-Queen Charlotte transform fault system

Tectonic loading and Coulomb stress transfer are modeled along the right-lateral Fairweather-Queen Charlotte transform fault system using a threedimensional boundary element program. The loading model includes slip below 12 km along the transform as well as motion of the Pacific plate, and it is consistent with most available Global Positioning System (GPS) displacement rate data. Coulomb stress transfer is shown to have been a weak contributing factor in the failure of the southeastern (Sitka) segment of the Fairweather fault in 1972, hastening the occurrence of the earthquake by only about 8 months. Failure of the Sitka segment was enhanced by a combination of cumulative loading from below (95%) by slip of about 5 cm/yr since 1848, by stress transfer (about 1%) from major earthquakes on straddling segments of the Queen Charlotte fault (M 8.1 in 1949) and the Fairweather fault (M 7.8 in 1958), and by viscoelastic relaxation (about 4%) following the great 1964 Alaska earthquake, modeled by Pollitz et al. (1998). Cumulative stress increases in excess of 7 MPa at a depth of 8 km are projected prior to the M 7.6 earthquake. Coulomb stress transferred by the rupture of the great M 9.2 Alaska earthquake in 1964 (Bufe, 2004a) also hastened the occurrence of the 1972 event, but only by a month or two. Continued tectonic loading over the last half century and stress transfer from the M 7.6 Sitka event has resulted in restressing of the adjacent segments by about 3 MPa at 8 km depth. The occurrence of a M 6.8 earthquake on the northwestern part of the Queen Charlotte fault on 28 June 2004, the largest since 1949, also suggests increased stress. The Cape St. James segment of the fault immediately southeast of the 1949 Queen Charlotte rupture has accumulated about 6 MPa at 8 km through loading since 1900 and stress transfer in 1949. A continued rise in earthquake hazard is indicated for the Alaska panhandle and Queen Charlotte Islands region in the decades ahead as the potential for damaging earthquakes increases.

Bulletin of the Seismological Society of America↗

USGS2021

Seismotectonic analysis of the 2019–2020 Puerto Rico sequence: The value of absolute earthquake relocations in improved interpretations of active tectonics

We present a new catalog of calibrated earthquake relocations from the 2019–2020 Puerto Rico earthquake sequence related to the 7 January 2020 M w 6.4 earthquake that occurred offshore of southwest Puerto Rico at a depth of 15.9 km. Utilizing these relocated earthquakes and associated moment tensor solutions, we can delineate several distinct fault systems that were activated during the sequence and show that the M w 6.4 mainshock may have resulted from positive changes in Coulomb stress from earlier events. Seismicity and mechanisms define (1) a west–southwest (∼260°) zone of seismicity comprised of largely sinistral strike‐slip and oblique‐slip earthquakes that mostly occurs later in the sequence and to the west of the mainshock, (2) an area of extensional faulting that includes the mainshock and occurs largely within the mainshock’s rupture area, and (3) an north–northeast (∼30°)‐striking zone of seismicity, consisting primarily of dextral strike‐slip events that occurs before and following the mainshock and generally above (shallower than) the normal‐faulting events. These linear features intersect within the M w 6.4 mainshock’s fault plane in southwest Puerto Rico. In addition, we show that earthquake relocations for M 4+ normal‐faulting events, when traced along their fault planes, daylight along east–west‐trending bathymetric features offshore of southwest Puerto Rico. Correlation of these normal‐faulting events with bathymetric features suggests an active fault system that may be a contributor to previously uncharacterized seismic hazards in southwest Puerto Rico.

Puerto Rico↗

USGS2023

3D Dynamic rupture modeling of the 6 February 2023, Kahramanmaraş, Turkey Mw 7.8 and 7.7 earthquake doublet using early observations

The 2023 Turkey earthquake sequence involved unexpected ruptures across numerous fault segments. We present 3D dynamic rupture simulations to illuminate the complex dynamics of the earthquake doublet. Our models are constrained by observations available within days of the sequence and deliver timely, mechanically consistent explanations of the unforeseen rupture paths, diverse rupture speeds, multiple slip episodes, heterogeneous fault offsets, locally strong shaking, and fault system interactions. Our simulations link both earthquakes, matching geodetic and seismic observations and reconciling regional seismotectonics, rupture dynamics, and ground motions of a fault system represented by 10 curved dipping segments and embedded in a heterogeneous stress field. The M w 7.8 earthquake features delayed backward branching from a steeply branching splay fault, not requiring supershear speeds. The asymmetrical dynamics of the distinct, bilateral M w 7.7 earthquake are explained by heterogeneous fault strength, prestress orientation, fracture energy, and static stress changes from the previous earthquake. Our models explain the northward deviation of its eastern rupture and the minimal slip observed on the Sürgü fault. 3D dynamic rupture scenarios can elucidate unexpected observations shortly after major earthquakes, providing timely insights for data‐driven analysis and hazard assessment toward a comprehensive, physically consistent understanding of the mechanics of multifault systems.

The Seismic Record↗

USGS2011

Ground motion attenuation during M 7.1 Darfield and M 6.2 Christchurch, New Zealand, earthquakes and performance of global predictive models

The M 7.1 Darfield earthquake occurred 40 km west of Christchurch (New Zealand) on 4 September 2010. Six months after, the city was struck again with an M 6.2 event on 22 February local time (21 February UTC). These events resulted in significant damage to infrastructure in the city and its suburbs. The purpose of this study is to evaluate the performance of global predictive models (GMPEs) using the strong motion data obtained from these two events to improve future seismic hazard assessment and building code provisions for the Canterbury region. The Canterbury region is located on the boundary between the Pacific and Australian plates; its surface expression is the active right lateral Alpine fault (Berryman et al. 1993). Beneath the North Island and the north South Island, the Pacific plate subducts obliquely under the Australian plate, while at the southwestern part of the South Island, a reverse process takes place. Although New Zealand has experienced several major earthquakes in the past as a result of its complex seismotectonic environment ( e.g. , M 7.1 1888 North Canterbury, M 7.0 1929 Arthur's Pass, and M 6.2 1995 Cass), there was no evidence of prior seismic activity in Christchurch and its surroundings before the September event. The Darfield and Christchurch earthquakes occurred along the previously unmapped Greendale fault in the Canterbury basin, which is covered by Quaternary alluvial deposits (Forsyth et al. 2008). In Figure 1, site conditions of the Canterbury epicentral area are depicted on a V S 30 map. This map was determined on the basis of topographic slope calculated from a 1-km grid using the method of Allen and Wald (2007). Also shown are the locations of strong motion stations. The Darfield event was generated as a result of a complex rupture mechanism; the recordings and geodetic data reveal that earthquake consists of three sub-events (Barnhart et al. 201 1 , page 815 of this issue). The first event was due to rupturing of a blind reverse fault with M 6.2, followed by a second event (M 6.9), releasing the largest portion of the energy on the right-lateral Greendale fault. The third sub-event (M 5.7) is due to a reverse fault with a right-lateral component (Holden et al. 2011). The Christchurch earthquake occurred on an oblique thrust fault. The comparison of spectral acceleration values at stations near Christchurch reveals that the second event produced much larger amplitudes of shaking than the Darfield event due to its proximity to the epicenter. Both events resulted in noticeably large amplitudes of the vertical motion, often exceeding horizontal motion in the near-fault area. The vertical motions, showing asymmetric acceleration traces and pulses, reached 1.26 g during the Darfield earthquake and 2.2 g during the Christchurch event. These events were recorded by more than 100 strong motion stations operated by the Institute of Geological and Nuclear Sciences (http://www.geonet.org.nz/). Using the processed data from these stations, peak ground acceleration (PGA) and 5%-damped spectral acceleration values at 0.3, 1, and 3 s are used for performance evaluation of the global ground motion predictive equations (GMPEs). The selected GMPEs are the Next Generation Attenuation (NGA) models of Abrahamson and Silva (2008), Boore and Atkinson (2008), Campbell and Bozorgnia (2008), and Chiou and Youngs (2008). The Graizer and Kalkan (2007, 2009) model, which is based on the NGA project database, is also included. These GMPEs are abbreviated respectively as AS08, BA08, CB08, CY08, and GK07. Because they have been used widely for seismic hazard analysis for crustal earthquakes, their performance assessment becomes a critical issue especially for immediate response and recovery planning after major events. The occurrence of aftershocks similar to the Christchurch event will most probably control seismic hazard in the broader area, as confirmed by the recent M 6.0 event on June 13, 2011.

Christchurch, Darfield↗

USGS2015

Focused exhumation along megathrust splay faults in Prince William Sound, Alaska

Megathrust splay faults are a common feature of accretionary prisms and can be important for generating tsunamis during some subduction zone earthquakes. Here we provide new evidence from Alaska that megathrust splay faults have been conduits for focused exhumation in the last 5 Ma. In most of central Prince William Sound, published and new low-temperature thermochronology data indicate little to no permanent rock uplift over tens of thousands of earthquake cycles. However, in southern Prince William Sound on Montague Island, apatite (U–Th)/He ages are as young as 1.1 Ma indicating focused and rapid rock uplift. Montague Island lies in the hanging wall of the Patton Bay megathrust splay fault system, which ruptured during the 1964 M9.2 earthquake and produced ∼9 m of vertical uplift. Recent geochronology and thermochronology studies show rapid exhumation within the last 5 Ma in a pattern similar to the coseismic uplift in the 1964 earthquake, demonstrating that splay fault slip is a long term (3–5 my) phenomena. The region of slower exhumation correlates with rocks that are older and metamorphosed and constitute a mechanically strong backstop. The region of rapid exhumation consists of much younger and weakly metamorphosed rocks, which we infer are mechanically weak. The region of rapid exhumation is separated from the region of slow exhumation by the newly identified Montague Strait Fault. New sparker high-resolution bathymetry, seismic reflection profiles, and a 2012 M w 4.8 earthquake show this feature as a 75-km-long high-angle active normal fault. There are numerous smaller active normal(?) faults in the region between the Montague Strait Fault and the splay faults. We interpret this hanging wall extension as developing between the rapidly uplifting sliver of younger and weaker rocks on Montague Island from the essentially fixed region to the north. Deep seismic reflection profiles show the splay faults root into the subduction megathrust where there is probable underplating. Thus the exhumation and extension in the hanging wall are likely driven by underplating along the megathrust décollement, thickening in the overriding plate and a change in rheology at the Montague Strait Fault to form a structural backstop. A comparison with other megathrust splay faults around the world shows they have significant variability in their characteristics, and the conditions for their formation are not particularly unique.

Alaska↗

USGS2020

Evolution of faulting induced by deep fluid injection, Paradox Valley, Colorado

High‐pressure fluid injection into a subhorizontal confined aquifer at 4.3–4.6 km depth induced > 7000 "> > 7000 >7000 earthquakes between 1991 and 2012 within once seismically quiescent Paradox Valley in Colorado, with magnitudes up to M w "> M w Mw 3.9. Earthquake hypocenters expanded laterally away from the well with time, defining the margins of the aquifer pressurized by injection at the well. Within 5 km of the well, alignment of earthquake hypocenters defines strikes of nine vertical fault zones. Previous studies show that these fault zones predate injection, producing left‐stepping offsets in the normal faults of the Wray‐Mesa fault system that cradles Paradox Valley. Hypocenters, rakes, and strikes of 2041 well‐constrained focal mechanisms show that most injection‐related earthquakes occur where these vertical faults intersect the pressurized aquifer. Well‐defined focal mechanisms show that this induced seismicity consists of Riedel shear faults at acute angles to the strikes of these fault zones. These small faults develop an anastomosing fault structure of focal planes along each planar fault zone, as fluid injection continues, even as their hypocenters define a single planar fault zone. Failure conditions at each hypocenter are found using a fully coupled poroelastic analysis of stress induced by fluid injection, and this analysis indicates a minimum Coulomb failure condition of 0.1 MPa. This failure condition is primarily a result of aquifer pore‐fluid pressurization, as almost all well‐located seismicity is within the pressurized aquifer. Reducing the rate of injection and frequent well shutdowns in the second decade nearly eliminated induced seismicity, except very near the well where gradients in pressurization are the largest. Despite these decreases in failure conditions and seismicity, some fault zones continued to produce earthquakes larger than M 3 as injection continued.

California↗