Earthquake Relocation Research Articles

Back‐arc extension in the Andaman Sea: Tectonic and magmatic processes imaged by high‐precision teleseismic double‐difference earthquake relocation

AbstractThe geometry, kinematics, and mode of back‐arc extension along the Andaman Sea plate boundary are refined using a new set of significantly improved hypocenters, global centroid moment tensor (CMT) solutions, and high‐resolution bathymetry. By applying cross‐correlation and double‐difference (DD) algorithms to regional and teleseismic waveforms and arrival times from International Seismological Centre and National Earthquake Information Center bulletins (1964–2009), we resolve the fine‐scale structure and spatiotemporal behavior of active faults in the Andaman Sea. The new data reveal that back‐arc extension is primarily accommodated at the Andaman Back‐Arc Spreading Center (ABSC) at ~10°, which hosted three major earthquake swarms in 1984, 2006, and 2009. Short‐term spreading rates estimated from extensional moment tensors account for less than 10% of the long‐term 3.0–3.8 cm/yr spreading rate, indicating that spreading by intrusion and the formation of new crust make up for the difference. A spatiotemporal analysis of the swarms and Coulomb‐stress modeling show that dike intrusions are the primary driver for brittle failure in the ABSC. While spreading direction is close to ridge normal, it is oblique to the adjacent transforms. The resulting component of E‐W extension across the transforms is expressed by deep basins on either side of the rift and a change to extensional faulting along the West Andaman fault system after the Mw = 9.2 Sumatra‐Andaman earthquake of 2004. A possible skew in slip vectors of earthquakes in the eastern part of the ABSC indicates an en‐echelon arrangement of extensional structures, suggesting that the present segment geometry is not in equilibrium with current plate‐motion demands, and thus the ridge experiences ongoing re‐adjustment.

Buried active faults in the Zafferana Etnea territory (south-eastern flank of Mt. Etna): geometry and kinematics by earthquake relocation and focal mechanisms

<p>We relocated seismicity that occurred from 2000 to 2005 inside a sector of Mt. Etna, comprising the town of Zafferana Etnea, using the double-difference technique. This approach revealed some spatial clusters of events at depths of 3.0 km to 5.5 km b.s.l., which suggested NE-SW-oriented and NNW-SSE-oriented active structures located west and north-west with respect to Zafferana Etnea. We also calculated 64 fault plane solutions, and azimuth and dip distributions of maximum compression P axes. The data include eight events with magnitudes between 3.1 and 3.7 that caused damage to Zafferana Etnea. This approach has allowed the definition of the geometry of structures that show no surface evidence, but are potentially hazardous for this territory. These faults might be linked to the regional tectonics, although they were activated by stress changes related to a general pressurizing of the Mt. Etna magma system between 2000 and 2005.</p>

1954年山丹7 1/4 级地震的孕震构造和发震机制探讨

Located in the front margin of the northeastern Tibetan Plateau, the Longshoushan fault zone is one of the boundary faults between the Hexi corridor and the Alxa block. Although the resolution of observation is limited, the 1954 M7(1/4) earthquake on the Longshoushan fault zone is a rare big event in this zone with modern seismological observation and documentation. During this earthquake, one continuous dextral-normal rupture occurred with a length of only 7 km along a transfer fault connecting two secondary faults of the Longshoushan fault zone. However, the dextral-normal sense of this surface rupture is contrary to the tectonic kinematics in this area. Based on recent field investigation and mapping, two phenomena were discovered. Firstly, except the one appeared on the transform fault, no surface rupture zone is found on the branches of the Longshoushn fault zone. Secondly, the main damage caused by earthquake is distributed on the region between the northern and the southern branches of the Longshoushan fault zone. These two findings indicate that the 1954 earthquake was caused by the main faults rather than the transfer fault of the Longshoushan fault zone, and the co-seismic surface rupture on the transfer fault is only a form of stress release at local portion. According to calculation of static co-seismic Coulomb stress change, the surface rupture could be triggered by an earthquake on the southern branch of the Longshoushan fault zone. Furthermore, in consideration of the possibility of dynamic triggering along the northern branch of the Longshoushan fault zone, the surface rupture should be the co-seismic response to the earthquake occurred on the main branches of the Longshoushan fault zone. Relocation of the small earthquakes also shows that a narrow inverted north-extending triangle about 10 km wide was formed between the northern and the southern faults of the Longshoushan fault zone.

Earthquake relocation in the Central Alborz region of Iran using a non-linear probabilistic method

In this study, we calculate accurate absolute locations for nearly 3,000 shallow earthquakes (≤20 km depth) that occurred from 1996 to 2010 in the Central Alborz region of northern Iran using a non-linear probabilistic relocation algorithm on a local scale. We aim to produce a consistent dataset with a realistic assessment of location errors using probabilistic hypocenter probability density functions. Our results indicate significant improvement in hypocenter locations and far less scattering than in the routine earthquake catalog. According to our results, 816 earthquakes have horizontal uncertainties in the 0.5–3.0 km range, and 981 earthquakes are relocated with focal-depth errors less than 3.0 km, even with a suboptimal network geometry. Earthquake relocated are tightly clustered in the eastern Tehran region and are mainly associated with active faults in the study area (the Mosha and Garmsar faults). Strong historical earthquakes have occurred along the Mosha and Garmsar faults, and the relocated earthquakes along these faults show clear north-dipping structures and align along east–west lineations, consistent with the predominant trend of faults within the study region. After event relocation, all seismicity lies in the upper 20 km of the crust, and no deep seismicity (>20 km depth) has been observed. In many circumstances, the seismicity at depth does not correlate with surface faulting, suggesting that the faulting at depth does not directly offset overlying sediments.

Relocation of Earthquakes that Occurred Beneath Parkfield Region of California using VELHYPO

When only direct-waves are inadequate, different combinations of both direct- and refracted-waves are used to determine hypocentral parameters of local earthquakes. Compared with only direct-waves, the addition of refracted-waves tends to increase errors of hypocentral parameters. This is true especially when the velocity structure of the region under investigation is not known. We use an accurate and fast algorithm to determine the hypocentral parameters based on an optimum 1-D velocity model yielding a minimum misfit error in media with multiple refracted-waves as first arrivals. Using the algorithm, we relocated earthquakes that occurred beneath the Parkfield region of California during the period between January 2000 and December 2000. The shallow part of the velocity model used in the study area has high velocity contrast among layers. Therefore, refracted-waves are recorded as first-arrival phases at most stations around the region for events of shallow focal depths (less than 3km). The study area covers the transition between the creeping segment of San Andreas Fault (SAF) to the northwest and the locked segment to the southeast. Accuracy test for the algorithm was conducted using some reference models. We compare the inverted hypocentral parameters with those determined by previous studies using HYPOINVERSE and the HypoDD. The epicentral distributions of the three methods show a close relationship with the regional fault distribution. However, compared to HypoDD, the epicentral distribution of HYPOINVERSE shifted slightly along the fault lines while the epicentral distribution of VELHYPO moved further in the same direction. The result of VELHYPO suggests that the dip of San Andreas Fault is 6-10 o SW while the results of HPOINVERSE and HypoDD suggest that the dip of San Andreas Fault is nearly vertical and/or 3-7 o NE. This study supports the result of previous studies that San Andreas Fault is non-

Study of Seismic Clusters at Bahía de Banderas Region, Mexico

The coast in the state of Jalisco and south of Nayarit is located within a region of high seismic potential, increasing population, and tourism development. This motivated Civil Defense authorities of Jalisco and the Universidad de Guadalajara to launch in the year 2000 the assessment of the seismic risk of the region. This work focuses in the seismicity study of the area of Bahía de Banderas and northern coast of Jalisco, which is actually a seismic gap. We perform an analysis of available seismograms to characterize active crustal structures, their relationship to surface morphology, and possible extent of these structures into the bay shallow parts. The data consist of waveforms recorded during 2003 when the seismograph network spanned the region. Our method is based on the identification of seismic clusters or families using cross-correlation of waveforms, earthquake relocation and modeling of fault planes. From an initial data set of 404 located earthquakes, 96 earthquakes with ML < 3.6 are related to 17 potentially active continental structures. We present fault plane model for 11 structures. A subgroup of 7 structures is aligned parallel to the Middle America Trench, as a possible consequence of oblique subduction. The foci of the earthquakes were grouped into clusters corresponding to fault dimensions of hundred of meters, may be considered as asperities or barriers in tectonic structures with lengths between 10 and 30 km. These structures could generate shallow earthquakes with magnitudes between 5.0 and 6.0 and represent an additional seismic threat to the region.

Relocating the 2011 Tohoku-oki earthquakes (M 6.0–9.0)

The great Tohoku-oki earthquake (Mw 9.0) on 11 March 2011 was accompanied by nearly 100 large foreshocks and aftershocks (M≥6.0) in the Northeast Japan forearc. In this study we relocated 77 of these large events using three different velocity models. An updated three-dimensional velocity model significantly reduced the travel-time residuals in the earthquake relocation. We found that the large thrust earthquakes are generally located in or around high-velocity patches, suggesting that the generation of the large thrust earthquakes is controlled by structural heterogeneities in the megathrust zone. The high-velocity patches may represent asperities in the megathrust zone where the subducting Pacific plate and the overriding Okhotsk plate may be coupled strongly. Many non-thrust earthquakes occurred in the crust of the overriding Okhotsk plate, indicating that the stress field there has been changed by the Tohoku-oki mainshock. The present results may improve our understanding of the subduction dynamics and seismotectonics of the Northeast Japan arc.

Temporal and spatial evolution of hypocentres and anisotropy from the Darfield aftershock sequence: implications for fault geometry and age

The first four months of aftershocks of the Darfield earthquake have been studied using data from temporary and permanent seismic stations to investigate the fault geometry, stress field and evolution of seismicity and seismic properties. Earthquake relocations illuminate fault segments and show that the majority of aftershocks occurred beyond the areas of highest slip during the Darfield earthquake. Seismic anisotropy shows a mixture of fast directions parallel to the maximum horizontal stress and fault-parallel fast directions. This, combined with the lack of observable growth of seismicity along fault segments, suggests that the Greendale Fault broke a pre-existing fault plane.

The East Anatolian Fault Zone: Seismotectonic setting and spatiotemporal characteristics of seismicity based on precise earthquake locations

The East Anatolian Fault Zone (EAFZ) represents a plate boundary extending over ∼500 km between the Arabian and Anatolian plates. Relative plate motion occurs with slip rates ranging from 6 to 10 mm/yr and has resulted in destructive earthquakes in eastern Turkey as documented by historical records. In this study, we investigate the seismic activity along the EAFZ and fault kinematics based on recordings from a densified regional seismic network providing the best possible azimuthal coverage for the target region. We optimize a reference 1‐D velocity model using a grid‐search approach and re‐locate hypocenters using the Double‐Difference earthquake relocation technique. The refined hypocenter catalog provides insights into the kinematics and internal deformation of the fault zone down to a resolution ranging typically between 100 and 200 m. The distribution of hypocenters suggests that the EAFZ is characterized by NE‐SW and E‐W oriented sub‐segments that are sub‐parallel to the overall trend of the fault zone. Faulting mechanisms are predominantly left‐lateral strike‐slip and thus in good correlation with the deformation pattern derived from regional GPS data. However, we also observe local clusters of thrust and normal faulting events, respectively. While normal faulting events typically occur on NS‐trending subsidiary faults, thrust faulting is restricted to EW‐trending structures. This observation is in good accordance with kinematic models proposed for evolving shear zones. The observed spatiotemporal evolution of hypocenters indicates a systematic migration of micro‐ and moderate‐sized earthquakes from the main fault into adjacent fault segments within several days documenting progressive interaction between the major branch of the EAFZ and its secondary structures. Analyzing the pre versus post‐seismic phase for M &gt; 5 events we find that aftershock activities are initially spread to the entire source region for several months but start to cluster at the central part of the main shock rupture thereafter.

Discussion on moment tensor solution and seismogenic structure of Ruichang-Yangxin earthquake on 10 September 2011

Ruichang-Yangxin earthquake is another moderate earthquake in Yangxin-Jiujiang area since 2005 Jiujiang-Ruichang M5.7 earthquake. In order to more understand the seismic activities in this area, we study the moment tensor solution and the seismogenic structure of the Ruichang-Yangxin earthquake. Precise earthquake relocation shows that the mainshock occurred on the southwestern part of the NE-trending fault and aftershocks are distributed not only along the NE-trending fault but also along its conjugated NW-trending fault. By comprehensive analysis on the earthquake distribution, characteristics of isoseismal curve, focal mechanism, and regional structure characteristics, it is inferred that this earthquake is caused by the NE-trending Tanlu fault. In addition, it has close relationship with the conjugated NW-trending fault as well. Many researches have shown that the junction area is the earthquake-prone area, and should be paid more attention to. And our study also proves this viewpoint.

Identifying Active Structures Using Double-Difference Earthquake Relocations in Southwest British Columbia and the San Juan Islands, Washington

This paper applies double-difference earthquake relocation techniques to investigate sources of seismicity in southwest British Columbia, Canada, and the San Juan Islands, Washington. The study area is a complex region of deformation and has the potential for large earthquakes in the North Americancrust. Double-difference earthquake relocation techniques are applied to identify otherwise-hidden active struc- tures that may pose a hazard to nearby population and infrastructure. We present evi- dence for previously unrecognized active structures using precise relative earthquake relocations obtained using both catalog and waveform cross-correlation data. Results have significantly reduced errors over routine catalog locations and show lineations in areas of clustered seismicity. In southwest British Columbia, these lineations or streaks appear to be hidden structures that do not disrupt near-surface sediments; however, in theSanJuanIslandstheidentifiedlineationcouldberelatedtorecentlymappedsurface expressions offaults identifiedfromseismic reflection andmultibeam bathymetric sur- veys.We use avariety of velocity models for therelocations and find that inappropriate models lead to artifacts at layer boundaries and increased vertical errors.

How lithospheric subduction changes along the Calabrian Arc in southern Italy: geophysical evidences

Recent tomographic investigations performed down to ~300 km depth in the Calabrian Arc region gave insight in favor of the hypothesis that the Ionian subducting slab is continuous in depth beneath the central part of the Arc, while detachment of the deep portion of the subducting structure may have already taken place beneath the edges of the Arc itself. In the present study, we perform new geophysical analyses to further explore the structure of the subduction system and the structure and kinematics of the crustal units in the study area for a more comprehensive view of the local geodynamic scenario. Local earthquake tomography that we address to the exploration of the upper 40 km in the whole region of southern Italy furnishes P-wave velocity domains, suggesting southeast-ward long-term drifting of the southern Tyrrhenian unit with an advancement front matching well with the segment of Calabrian Arc where the subducting slab was found continuous and trench retreat can be presumed to have been active in the most recent times. This scenario of retreating subduction trench inducing drifting of the lithospheric unit overriding the subducting slab is further supported by the analysis of gravity anomalies, allowing us to better constrain the transitional zones between different subduction modes (continuous vs. detached slab) along the Arc. Also, the relocation of recent crustal seismicity, associated with geostructural data taken from the literature, provides evidence for NW-trending seismogenic structures in northeastern Sicily and northern Calabria that we interpret as Subduction-Transform Edge Propagator (STEP) faults guiding the southeast-ward drifting process of the southern Tyrrhenian unit. Crustal earthquake relocations show also seismolineaments in southern Calabria corresponding to the NE-trending longitudinal structures of the Arc where the great shallow earthquakes of 28 December 1908, and 5 and 7 February 1783 occurred. Seismicity and the extensional stress regime detected in these structures find also reasonable location in the proposed scenario, being interpretable in terms of shallow response of the central segment of the Arc to slab rollback and trench retreat.

Mapping of b-values, earthquake relocation, and Coulomb stress changes during 1992–2007 in the Murindó seismic zone, Colombia

Seismicity in the Murindo seismic zone, Colombia (6° to 8° and − 75.5° to − 78.0°) during 1992–2007 included the occurrence of the MW 6.6 foreshock on October 17, and MW 7.1 mainshock on October 18, 1992, and aftershocks, which caused severe damage throughout the region. We modeled this seismic sequence by means of Coulomb stress changes imparted by slip along two semi-vertical fault planes with left-lateral motion and subdivided slip. Seismicity thereafter (1993–2007) was characterized by earthquakes magnitude in the range 0.7−6.1 for which a magnitude of completeness MC = 2.7 was determined. b-values are relatively high to the south of the study region and relatively low to the north and northwest. The relocation of epicenters indicates that recent seismicity may be occurring on a fault that is yet to be mapped under the Atrato river valley. We conclude that the 1992 seismic sequence was a case of a major earthquake facilitated by the stress changes imparted during a strong rupture the previous day and that the aftershocks and background seismicity in the region can be explained by static Coulomb stress changes up to 50.1 bar. Our results have implications for hazard in Colombia and serve as basis to foster future investigations.

Relocation and focal mechanisms of earthquakes in the south-central sector of the Aeolian Archipelago: New structural and volcanological insights

To recognize possible spatial clusters and identify active seismogenic zones and structures in the Aeolian Archipelago, in the south of Italy, we analyzed the spatial pattern of seismicity between 1993 and 2010 in a selected area comprising Vulcano, Lipari, Salina and Filicudi and calculated 22 fault plane solutions (FPSs) for shocks with magnitude greater than 2.7. First, we computed a 1-D velocity model for this area including information from recorded earthquakes by a joint hypocenter-velocity inversion (Kissling et al., 1994). Successively, we applied the double-difference approach of Waldhauser and Ellsworth (2000), finding that a certain part of the scattered epicenter locations collapse in roughly linear features. Relocated seismicity evidenced three main alignments, oriented NNW–SSE and NE–SW at different depths that concur well with the known tectonic lineaments and focal mechanisms.A detailed discussion is focused on a seismogenetic structure, NE–SW oriented, 3–8km deep, located in the northern area of Vulcano island. This recognized element could represent a link between magma accumulation zones, thus representing a possible preferential pathway along which magma may intrude. Two earthquake clusters, located south-west and east of Vulcano, with their focal mechanisms, highlight the Aeolian-Tindari-Letojanni Fault System seismic activity and the existence of a transitional zone going from the N–S compressive domain that dominates the Aeolian Islands to the NW–SE extensional domain characterizing the south-eastern Tyrrhenian.

Seismicity, active faults, stress patterns, and rupture processes in the Hualien region, Taiwan, investigated using the 1990 Hualien earthquake sequence

The Hualien mainshock–aftershock sequence from December 1990 to January 1991 was used to investigate seismicity, active faults, stress patterns, and complex rupture processes in the Hualien region of eastern Taiwan. A combined data set, compiled from observations by the regional seismic network (Taiwan Telemetered Seismograph Network, TTSN) and a dense aftershock monitoring array (Hualien Temporary Seismic Network, HTSN), provided high-quality seismic data. Using these data, earthquake source parameters could be well constrained by the joint hypocenter determination (JHD) and double-difference relocation methods. These two relocation methods presented similar results and produced improved estimations of earthquake location. Station corrections determined by JHD showed significant lateral velocity variations across the Yuehmei fault. The relocated seismicity indicated that a previously known active fault, the Chimei fault, extends to offshore. Earthquake relocations also revealed a northwest dipping seismicity probably related to the Chimei fault. In addition, three sets of focal mechanism solutions are presented and their discrepancies are attributed to the complex rupture process. At first, earthquakes were initiated by thrust motions. Later, the rupture pattern changed to strike–slip motions. Variations in slip may reflect the fault geometry or rupture boundary conditions in the region. The observed stress patterns are considered to relate not only to the regional collision and mountain-building processes, but also to active faults in local structures.

Strain localization and fluid migration from earthquake relocation and seismicity analysis in the western Vosges (France)

SUMMARY To understand the mechanisms of intraplate earthquakes, well-documented seismological and geodynamical studies are needed, to provide information for adequate mechanical modelling. This paper describes a detailed analysis of the earthquake sequence associated with the Rambervillers (France) M ∼ 5.4 earthquake (2003 February 22), which occurred in the western Vosges massif (France), some tens of kilometres to the north of the sites of the Epinal (1973–1974) and Remiremont (1984–1985) earthquake sequences. We computed the location of the mainshock and focal mechanism, together with the double-difference locations of 419 aftershocks of the subsequent earthquake sequence, which included 195 well-recorded similar events (earthquakes with similar waveforms). We combined pP phases recorded by remote dense seismic networks with waveform modelling, to accurately determine the mainshock depth (12.5 ± 1.5 km). Computations of focal mechanisms from regional waveform inversions showed a normal fault plane with a N315 ◦ ± 10 ◦ strike and a 45 ◦ ± 15 ◦ dip. A detailed space–time analysis allowed two earthquake sequences to be identified: a classic mainshock–aftershock sequence and a secondary sequence that began 250 days after the mainshock. More events were recorded during the secondary sequence than during the main sequence. Very few similar events were recorded during the first 2 days after the mainshock, although data were recorded continuously. Double-difference locations from traveltime differences show that these immediate, non-similar aftershocks occurred in a 4 km × 2k m subhorizontal area at a depth of about 12 km. Similar events were far more numerous during the secondary earthquake sequence. Double-difference relocation of these similar events revealed a N315 ◦ striking, 65 ◦ dipping fault plane that is compatible with the mainshock source mechanism. During this later sequence, seismicity oscillated over a vertical range of 2–3 km around a mean depth of approximately 12 km. This observation strongly suggests fluid migration. Normal faulting, subhorizontal faulting and fluid transfer at about 12 km depth can be interpreted in terms of the crust’s response (mainshock and post-seismic relaxation) to flexural stresses induced by the alpine compression. This relaxation involves weakening and collapse of the crust, and fluid transfer.

Two seismic gaps on the Sagaing Fault, Myanmar, derived from relocation of historical earthquakes since 1918

[1] Relocation of six M (magnitude) ≥ 7.0 earthquakes near the Sagaing Fault in Myanmar since 1918 allows us to image earthquake history along the Sagaing Fault. All the earthquakes were relocated on the Sagaing Fault by using the modified joint hypocenter determination method. Combining the relocated epicenters with information on foreshocks, aftershocks, seismic intensities, and coseismic displacement, we estimated the location of the fault plane that ruptured during each earthquake. This analysis revealed two seismic gaps: one between 19.2°N and 21.5°N in central Myanmar, and another south of 16.6°N in the Andaman Sea. Considering the length of the first seismic gap (∼260 km), a future earthquake of up to M ∼7.9 is expected to occur in central Myanmar. Because Nay Pyi Taw, the recently established capital of Myanmar, is located on the expected fault, its large population is exposed to a significant earthquake hazard.

Locating non-volcanic tremor along the San Andreas Fault using a multiple array source imaging technique

Non-volcanic tremor (NVT) has been observed at several subduction zones and at the San Andreas Fault (SAF). Tremor locations are commonly derived by cross-correlating envelope-transformed seismic traces in combination with source-scanning techniques. Recently, they have also been located by using relative relocations with master events, that is low-frequency earthquakes that are part of the tremor; locations are derived by conventional traveltime-based methods. Here we present a method to locate the sources of NVT using an imaging approach for multiple array data. The performance of the method is checked with synthetic tests and the relocation of earthquakes. We also applied the method to tremor occurring near Cholame, California. A set of small-aperture arrays (i.e. an array consisting of arrays) installed around Cholame provided the data set for this study. We observed several tremor episodes and located tremor sources in the vicinity of SAF. During individual tremor episodes, we observed a systematic change of source location, indicating rapid migration of the tremor source along SAF.

Teleseismic double‐difference relocation of earthquakes along the Sumatra‐Andaman subduction zone using a 3‐D model

We have extended the double‐difference seismic tomography method to teleseismic distances with 3‐D ray tracing conducted through nested regional‐global velocity models and applied the method to relocate seismicity from the Sumatra‐Andaman region before and after the great earthquakes of 2004 and 2005. We tested the algorithm's accuracy using both independent local data and an alternate relocation method and found good agreement between the results. The use of depth phases, differential times, and a realistic 3‐D velocity model improves the accuracy and precision of epicenters and focal depths, systematically shifting them perpendicular to the trench and shallower, respectively. The relocations refine the location of the megathrust and other faults, the patterns of aftershocks, and their relation to slip during the two great earthquakes. In addition, the relocations reveal several discrete features not readily discernible in the scatter of teleseismic catalogs, including an arcuate, narrow band of earthquakes presumed to define the updip rupture limit in the 2005 event and a lineation at depth tracing the subduction of the Investigator Fracture Zone. When viewed in conjunction with tomography results, the geometry and structural features of the subduction zone are revealed in unprecedented detail.

Seismotectonics and Fault Structure of the California Central Coast

Abstract I present and interpret new earthquake relocations and focal mechanisms for the California Central Coast. The relocations improve upon catalog locations by using 3D seismic velocity models to account for lateral variations in structure and by using relative arrival times from waveform cross-correlation and double-difference methods to image seismicity features more sharply. Focal mechanisms are computed using ray tracing in the 3D velocity models. Seismicity alignments on the Hosgri fault confirm that it is vertical down to at least 12 km depth, and the focal mechanisms are consistent with right-lateral strike-slip motion on a vertical fault. A prominent, newly observed feature is an ∼25 km long linear trend of seismicity running just offshore and parallel to the coastline in the region of Point Buchon, informally named the Shoreline fault. This seismicity trend is accompanied by a linear magnetic anomaly, and both the seismicity and the magnetic anomaly end where they obliquely meet the Hosgri fault. Focal mechanisms indicate that the Shoreline fault is a vertical strike-slip fault. Several seismicity lineations with vertical strike-slip mechanisms are observed in Estero Bay. Events greater than about 10 km depth in Estero Bay, however, exhibit reverse-faulting mechanisms, perhaps reflecting slip at the top of the remnant subducted slab. Strike-slip mechanisms are observed offshore along the Hosgri–San Simeon fault system and onshore along the West Huasna and Rinconada faults, while reverse mechanisms are generally confined to the region between these two systems. This suggests a model in which the reverse faulting is primarily due to restraining left-transfer of right-lateral slip.