Hayward Fault Research Articles

Contrasting Settings of Serpentinite Bodies, San Francisco Bay Area, California: Derivation from the Subducting Plate vs. Mantle Hanging Wall?

Although the best-known outcrops of serpentinite in the California Coast Ranges are part of the Coast Range ophiolite (CRO), significant serpentinite bodies crop out within the Franciscan subduction complex, and others have ambiguous affinity. These serpentinites provide evidence of subduction-accretion processes. The Hunters Point shear zone (HPSZ), an intra-Franciscan structural horizon that consists of both a regionally extensive serpentinite body and shale matrix mélange, extends tens of km along strike, and has a structural thickness of 1 to 1.5 km. The HPSZ exhibits little mixing of serpentinite and shale. The HPSZ consists of a structurally high shale matrix mélange with mostly sandstone blocks, an intermediate zone composed of a serpentinite sheet (or sheets) containing metagabbro lenses, and a structurally low shale matrix mélange including a variety of different block types, but lacking serpentinite and gabbro. These field relations suggest that the serpentinite in the HPSZ became part of the subduction complex by offscraping of the remnants of a mantle core complex from the downgoing plate, rather than having originated from the upper mantle hanging wall of the subduction zone. In contrast, serpentinite exposed within a km east of the Hayward fault in the southern Hayward Hills is intermixed with shale on scales of tens of meters to centimeters. This serpentinite is present in shear zones, ranging from 50 m to several centimeters in structural thickness; these shear zones cut sandstones of the Great Valley Group (GVG), forearc basin deposits that locally overlie the Coast Range ophiolite in depositional contact. The shear zones also include blocks of of basalt, blueschist, amphibolite, and gabbro. Serpentinite bodies in the southern Hayward Hills may have been derived from the mantle hanging wall of the subduction zone relatively early in the history of the subduction zone before much tectonic underplating occurred; after substantial underplating, the mantle hanging wall would have been largely blocked from contributing material to the subduction channel. The serpentinite and shale matrix were exhumed to approximately the same crustal level as the CRO and GVG, before a final stage of faulting mixed pieces of both into the mélange and emplaced the mélange into shear zones cutting GVG rocks.

Gravity, magnetic, and high‐precision relocated seismicity profiles suggest a connection between the Hayward and Calaveras Faults, northern California

Gravity, magnetic, and seismicity data profiled across the Hayward Fault Zone were generated as part of ongoing studies to help determine the geologic and tectonic setting of the San Francisco Bay region. These data, combined with previous geophysical studies that indicate that the Hayward Fault Zone dips 75°NE near San Leandro and follows a preexisting structure, reveal a possible direct connection between the seismogenic portion of the Hayward and Calaveras Faults at depth. Although the relocated seismicity data are regional in nature, they suggest that the dip of the Hayward Fault Zone may vary from near vertical in the northwestern part of the fault to about 75°NE at San Leandro in the central part of the fault to about 50°NE in the southeastern part of the fault. Gravity and magnetic data, profiled across the Hayward Fault Zone, were processed using standard geophysical techniques. Cross sections of high‐precision relocated hypocenters were constructed along each profile from the northwestern to the southeastern end of the Hayward Fault Zone. Profiles and cross sections are referenced to Pinole Point, where the Hayward Fault enters San Pablo Bay, and are spaced 2.5 km apart. Topographic profiles shown on the seismicity cross sections were generated using U.S. Geological Survey (USGS) 7.5‐min, 30‐m digital elevation models. Relocation of seismicity data was accomplished using a regional double‐difference method. The double‐difference method incorporates ordinary travel time measurements and cross correlation of P and S wave differential travel time measurements. Relative locations between earthquakes have hypocentral errors of about 100 m horizontally and 250 m vertically. Absolute location uncertainties were not determined but are probably dramatically improved compared to the USGS's Northern California Seismic Network catalog data.

Four-dimensional transform fault processes: progressive evolution of step-overs and bends

Many bends or step-overs along strike–slip faults may evolve by propagation of the strike–slip fault on one side of the structure and progressive shut-off of the strike–slip fault on the other side. In such a process, new transverse structures form, and the bend or step-over region migrates with respect to materials that were once affected by it. This process is the progressive asymmetric development of a strike–slip duplex. Consequences of this type of step-over evolution include: (1) the amount of structural relief in the restraining step-over or bend region is less than expected; (2) pull-apart basin deposits are left outside of the active basin; and (3) local tectonic inversion occurs that is not linked to regional plate boundary kinematic changes. This type of evolution of step-overs and bends may be common along the dextral San Andreas fault system of California; we present evidence at different scales for the evolution of bends and step-overs along this fault system. Examples of pull-apart basin deposits related to migrating releasing (right) bends or step-overs are the Plio-Pleistocene Merced Formation (tens of km along strike), the Pleistocene Olema Creek Formation (several km along strike) along the San Andreas fault in the San Francisco Bay area, and an inverted colluvial graben exposed in a paleoseismic trench across the Miller Creek fault (meters to tens of meters along strike) in the eastern San Francisco Bay area. Examples of migrating restraining bends or step-overs include the transfer of slip from the Calaveras to Hayward fault, and the Greenville to the Concord fault (ten km or more along strike), the offshore San Gregorio fold and thrust belt (40 km along strike), and the progressive transfer of slip from the eastern faults of the San Andreas system to the migrating Mendocino triple junction (over 150 km along strike). Similar 4D evolution may characterize the evolution of other regions in the world, including the Dead Sea pull-apart, the Gulf of Paria pull-apart basin of northern Venezuela, and the Hanmer and Dagg basins of New Zealand.

The Coefficient of Friction of Chrysotile Gouge at Seismogenic Depths

We report new strength data for the serpentine mineral chrysotile at effective normal stresses, σn between 40 and 200 MPa in the temperature range 25°-280°C. Overall, the coefficient of friction, μ (= shear stress/effective normal stress) of water-saturated chrysotile gouge increases both with increasing temperature and σn, but the rates vary and the temperature-related increases begin at ~100°C. As a result, a frictional strength minimum (μ = 0.1) occurs at low σn at about 100°C. Maximum strength (μ = 0.55) results from a combination of high normal stress and high temperature. The low-strength region is characterized by velocity strengthening and the high-strength region by velocity-weakening behavior. Thoroughly dried chrysotile has μ = 0.7 and is velocity-weakening. The frictional properties of chrysotile can be explained by its tendency to adsorb large amounts of water that acts as a lubricant during shear. The water is progressively driven off the fiber surfaces with increasing temperature and pressure, causing chrysotile to approach its dry strength. Depth profiles for a chrysotile-lined fault constructed from these data would pass through a strength minimum at ~3 km depth, where sliding should be stable. Below that depth, strength increases rapidly as does the tendency for unstable (seismic) slip. Such a trend would not have been predicted from the room-temperature data. These results therefore illustrate the potential hazards of extrapolating room-temperature friction data to predict fault zone behavior at depth. This depth profile for chrysotile is consistent with the pattern of slip on the Hayward fault, which creeps aseismically at shallow depths but which may be locked below 5 km depth.

Fault creep and microseismicity on the Hayward fault, California: Implications for asperity size

The Hayward fault is documented to undergo significant creep, with some patches accommodating 50% or more of the long‐term fault displacement. In spite of this, the fault has also experienced moderate to large earthquakes. By comparing the patterns of microseismicity observed on the fault with models of fault zone creep, we can investigate the long‐term displacement/deformation history of the fault in terms of the relative roles of aseismic creep, fault slip accommodated through microseismicity, and strain accumulation (slip deficit). We find that microseismicity on the Hayward fault produces a negligible percentage of the seismic moment dissipated on the fault. Combining seismicity with our fault creep models allows us to calculate the size of asperities on the creeping fault. For small asperities associated with repeating earthquakes on the Hayward fault, the rupture areas of these asperities range from 20 to 60 m2.

Double-Difference Tomography: The Method and Its Application to the Hayward Fault, California

We have developed a double-difference (DD) seismic tomography method that makes use of both absolute and relative arrival times. By reducing systematic errors using the more accurate relative arrival times, the method produces an improved velocity model. Simultaneously, it yields event locations of a quality equivalent to those of the DD location method, while avoiding simplifying assumptions of that method. We test this method on a synthetic dataset and find that it produces a more accurate velocity model and event locations than standard tomography. We also test this method on a Hayward fault, California, earthquake dataset spanning 1984–1998. The earthquakes relocated by this method collapse to a thin line along the fault trace, consistent with previous results. The DD velocity model has sharper velocity contrasts near the source region than the standard tomography model.

Gravity and Magnetic Expression of the San Leandro Gabbro with Implications for the Geometry and Evolution of the Hayward Fault Zone, Northern California

The Hayward Fault, one of the most hazardous faults in northern California, trends north-northwest and extends for about 90 km along the eastern San Francisco Bay region. At numerous locations along its length, distinct and elongate gravity and magnetic anomalies correlate with mapped mafic and ultramafic rocks. The most prominent of these anomalies reflects the 16-km-long San Leandro gabbroic block. Inversion of magnetic and gravity data constrained with physical property measurements is used to define the subsurface extent of the San Leandro gabbro body and to speculate on its origin and relationship to the Hayward Fault Zone. Modeling indicates that the San Leandro gabbro body is about 3 km wide, dips about 75°-80° northeast, and extends to a depth of at least 6 km. One of the most striking results of the modeling, which was performed independently of seismicity data, is that accurately relocated seismicity is concentrated along the western edge or stratigraphically lower bounding surface of the San Leandro gabbro. The western boundary of the San Leandro gabbro block is the base of an incomplete ophiolite sequence and represented at one time, a low-angle roof thrust related to the tectonic wedging of the Franciscan Complex. After repeated episodes of extension and attenuation, the roof thrust of this tectonic wedge was rotated to near vertical, and in places, the strike-slip Hayward Fault probably reactivated or preferentially followed this preexisting feature. Because earthquakes concentrate near the edge of the San Leandro gabbro but tend to avoid its interior, we qualitatively explore mechanical models to explain how this massive igneous block may influence the distribution of stress. The microseismicity cluster along the western flank of the San Leandro gabbro leads us to suggest that this stressed volume may be the site of future moderate to large earthquakes. Improved understanding of the three-dimensional geometry and physical properties along the Hayward Fault will provide additional constraints on seismic hazard probability, earthquake modeling, and fault interactions that are applicable to other major strike-slip faults around the world. Manuscript received 11 January 2002.

Inferences Drawn from Two Decades of Alinement Array Measurements of Creep on Faults in the San Francisco Bay Region

We summarize over 20 years of monitoring surface creep on faults of the San Andreas system in the San Francisco Bay region using alinement arrays. The San Andreas fault is fully locked at five sites northwest from San Juan Bautista, the southern end of the 1906 earthquake rupture, that is, no creep (<1 mm/yr) is observed. Likewise, the San Gregorio, Rodgers Creek, and West Napa faults show no creep. The measured creep rate on the Calaveras-Paicines fault from Hollister southward is either 6 or ∼10 mm/yr, depending on whether the arrays cross all of the creeping traces. Northward of Hollister, the central Calaveras creep rate reaches 14 ± 2 mm/yr but drops to ∼2 mm/yr near Calaveras Reservoir, where slip transfers to the southern Hayward fault at a maximum creep rate of 9 mm/yr at its south end. However, the Hayward fault averages only 4.6 mm/yr over most of its length. The Northern Calaveras fault, now creeping at 3-4 mm/yr, steps right to the Concord fault, which has a similar rate, 2.5-3.5 mm/yr, which is slightly slower than the 4.4 mm/yr rate on its northward continuation, the Green Valley fault. The Maacama fault creeps at 4.4 mm/yr near Ukiah and 6.5 mm/yr in Willits. The central and southern segments of the Calaveras fault are predominantly creeping, whereas the Hayward, Northern Calaveras, and Maacama faults are partly locked and, along with the Rodgers Creek and San Andreas, have high potential for major earthquakes. Manuscript received 6 November 2002.

Numerical modeling of strike-slip creeping faults and implications for the Hayward fault, California

The seismic potential of creeping faults such as the Hayward fault (San Francisco Bay Area, CA) depends on the rate at which moment (slip deficit) accumulates on the fault plane. Thus, it is important to evaluate how the creep rate observed at the surface is related to the slip on the fault plane. The surface creep rate (SCR) depends on the geometry of locked and free portions of the fault and on the interaction between the fault zone and the surrounding lithosphere. Using a viscoelastic finite element model, we investigate how fault zone geometries and physical characteristics such as frictionless or locked patches affect the observed surface creep when the system is driven by far field plate motions. These results have been applied to creep observations of the Hayward fault. This analysis differs from most previous fault creeping models in that the fault in our model is loaded by a distributed viscous flow induced by far field velocity boundary conditions instead of imposed slip beneath the major faults of the region. The far field velocity boundary conditions simulate the relative motion of the stable Pacific plate respect to the Rigid Sierra Nevada block, leaving the rheology, fault geometry, and mechanics (locked or free to creep patches), to determinate the patterns of fault creep. Our model results show that the fault geometry (e.g. length and depth of creeping) and the local rheology influence the surface creep rate (SCR) and the slip on the fault plane. In particular, we show that the viscoelastic layer beneath the elastic seismogenic zone plays a fundamental role in loading the fault. Additionally, the coupling with the surrounding lithosphere results in a smooth transition from regions free to creep to locked patches.

Hsi-Ping Liu: (1944-2001)

Last spring, Hsi-Ping Liu, Russell Sell, and I (JBF) were visiting a seismograph site in the Hayward fault zone deploying one of an array of seismometers to study fault-zone guided waves. We were testing a set of 5-s seismometers that Hsi-Ping had modified to incorporate a novel leveling device. The device would automatically recenter the seismometers each day so that they would be stable for months to years. Hsi-Ping would put a small piece of paper on one of the masses, watch the mass drift for a few seconds, and then trigger the circuit to relevel the mass. It worked perfectly, as almost everything that Hsi-Ping engineered did. He gave it his ultimate compliment: “Better than in the lab.” Sadly, that turned out to be one of Hsi-Ping's last efforts, as he passed away recently after a short illness. He was a multitalented geophysicist who worked on both laboratory and field experiments as well as several purely theoretical problems. Although he had a quiet and reserved personality, he will be mostly remembered for his unusually kind and cooperative nature, which left his friends with a special feeling for the strength and goodness of the human spirit. Hsi-Ping was born in 1944 in Chungking, China during World War II. His family moved to Taiwan in 1949, where he grew up in far from prosperous circumstances. He graduated from Tunghai University in Taichung, Taiwan, majoring in physics. Although a quiet and reserved student, he was also bright and motivated. He was highly regarded among his classmates as someone who would make an important contribution to physics. But because of his love of mountains, Hsi-Ping changed his focus from physics to Earth science. He earned a Master's degree at Dartmouth College and then a Ph.D. at the California Institute of Technology. In the …

Introduction to the Special Issue on Paleoseismology of the San Andreas Fault System

The purpose of this special issue of BSSA is to provide a state-of-the-science compendium of data on the rupture history of the San Andreas fault from recent paleoseismic investigations. The San Andreas fault is the primary boundary fault between the Pacific and North American plates and one of the most thoroughly studied faults in the world. Fourteen articles prepared by 74 authors provide dates of surface ruptures and/or measurements of slip or slip rate at 10 paleoseismic investigation sites along the main trace of the San Andreas fault, at one site on the Hayward fault, and along the northern San Jacinto fault. This issue includes documentation of the longest multicycle earthquake sequence in California—and possibly in North America (14 ruptures at the Wrightwood site). Geographic coverage extends almost along the entire San Andreas fault system, from Bodega Bay in northern California to the Mecca Hills in the Coachella Valley. The issue also includes articles linking paleoseismic data with modern instrumental recordings of ground motion and seismicity. Data on the earthquake history of the San Andreas fault form the basis of numerous models of fault behavior and calculations of seismic hazard. Despite the relatively large amount of data available for the San Andreas fault, there remain unresolved questions about segmentation, the rupture patterns of large earthquakes, and characteristics of earthquake recurrence. The data presented in this special issue contribute toward understanding the spatial and temporal rupture history of the San Andreas fault over multiple rupture cycles during the last few hundred to few thousand years. This is the timescale of greatest interest for seismic hazard assessments and fault modeling. This introduction provides an overview of the issue and summarizes results in simplified, tabular form. Table 1 contains dates of surface ruptures and/or average recurrence intervals reported from 10 sites discussed in …

Mori: An Internet-Based Earthwork

In Mori, minute movements of the Hayward Fault in California are measured by a seismograph, converted to digital signals and transmitted continuously via the Internet to a sound installation. Inside the entry curtain, visitors follow a fiber-optic cable to the center of the resonating enclosure where a portal through the floor frames the installation's focal point. An embedded visual display and immersive lowfrequency sounds are modulated by the unpredictable fluctuations of the earth's movement.

Fault structure and mechanics of the Hayward Fault, California, from double‐difference earthquake locations

The relationship between small‐magnitude seismicity and large‐scale crustal faulting along the Hayward Fault, California, is investigated using a double‐difference (DD) earthquake location algorithm. We used the DD method to determine high‐resolution hypocenter locations of the seismicity that occurred between 1967 and 1998. The DD technique incorporates catalog travel time data and relative P and S wave arrival time measurements from waveform cross correlation to solve for the hypocentral separation between events. The relocated seismicity reveals a narrow, near‐vertical fault zone at most locations. This zone follows the Hayward Fault along its northern half and then diverges from it to the east near San Leandro, forming the Mission trend. The relocated seismicity is consistent with the idea that slip from the Calaveras Fault is transferred over the Mission trend onto the northern Hayward Fault. The Mission trend is not clearly associated with any mapped active fault as it continues to the south and joins the Calaveras Fault at Calaveras Reservoir. In some locations, discrete structures adjacent to the main trace are seen, features that were previously hidden in the uncertainty of the network locations. The fine structure of the seismicity suggests that the fault surface on the northern Hayward Fault is curved or that the events occur on several substructures. Near San Leandro, where the more westerly striking trend of the Mission seismicity intersects with the surface trace of the (aseismic) southern Hayward Fault, the seismicity remains diffuse after relocation, with strong variation in focal mechanisms between adjacent events indicating a highly fractured zone of deformation. The seismicity is highly organized in space, especially on the northern Hayward Fault, where it forms horizontal, slip‐parallel streaks of hypocenters of only a few tens of meters width, bounded by areas almost absent of seismic activity. During the interval from 1984 to 1998, when digital waveforms are available, we find that fewer than 6.5% of the earthquakes can be classified as repeating earthquakes, events that rupture the same fault patch more than one time. These most commonly are located in the shallow creeping part of the fault, or within the streaks at greater depth. The slow repeat rate of 2–3 times within the 15‐year observation period for events with magnitudes around M = 1.5 is indicative of a low slip rate or a high stress drop. The absence of microearthquakes over large, contiguous areas of the northern Hayward Fault plane in the depth interval from ∼5 to 10 km and the concentrations of seismicity at these depths suggest that the aseismic regions are either locked or retarded and are storing strain energy for release in future large‐magnitude earthquakes.

A Record of Large Earthquakes on the Southern Hayward Fault for the Past 500 Years

The Hayward fault, a major branch of the right-lateral San Andreas fault system, traverses the densely populated eastern San Francisco Bay region, California. We conducted a paleoseismic investigation to better understand the Hayward fault9s past earthquake behavior. The site is near the south end of Tyson9s Lagoon, a sag pond formed in a right step of the fault in Fremont. Because the Hayward fault creeps at the surface, we identified paleoseismic events using features that we judge to be unique to ground ruptures or the result of strong ground motion, such as the presence of fault-scarp colluvial deposits and liquefaction. We correlate the most recent event evidence (E1) to the historical 1868 M 6.9 earthquake that caused liquefaction in the pond and recognize three additional paleoruptures since A.D. 1470 ± 110 yr. Event ages were estimated by chronological modeling, which incorporated historical and stratigraphic information and radiocarbon and pollen data. Modeled, mean age and 95-percentile ranges of the three earlier events are A.D. 1730 (1650-1790) yr (E2), A.D. 1630 (1530-1740) yr (E3), and A.D. 1470 (1360-1580) (E4). The ages of these paleoearthquakes yield a mean recurrence of 130 ± 40 yr. Although the mean recurrence is well determined for the period A.D. 1470-1868, individual intervals are less well determined: E1-E2, 140 +80/-70 yr; E2-E3, 100 +90/-100 yr; and E3-E4, 150 +130/-110 yr.

San Andreas Fault Zone, California: M &gt;=5.5 Earthquake History

The San Andreas fault zone has been a very significant source of major California earthquakes. From 1812 to 1906 it generated four major earthquakes of M ∼7 or larger in two pairs on two major portions of the fault. A pair of major earthquakes occurred on the central to southern region, where the 1857 faulting overlapped the 1812 earthquake faulting. A pair of major earthquakes occurred on the northern region, where the 1906 faulting overlapped the 1838 earthquake faulting. Also, earthquakes of M ∼7 occurred in the San Francisco Bay area on the Hayward fault in 1868 and the Santa Cruz Mountains near Loma Prieta in 1989 and on the Imperial fault near the border with Mexico in 1940. The 1838 earthquake's damage effects throughout the Bay area, from San Francisco to Santa Clara Valley and Monterey, were unequalled by any historical earthquake other than the 1906 event. This, and numerous strong possible aftershocks during the following 3 years in the San Juan Bautista vicinity, suggest 1838 faulting from San Francisco to San Juan Bautista. Cycles of seismicity and quiescence were associated with the Bay area earthquakes of 1868, 1906, and 1989. The 1868 earthquake on the Hayward fault was preceded by 12 earthquakes of M ≥5.5 from 1855 to 1866, within 60 km of the Hayward fault, and was followed by 13 quiet years. The 1906 San Andreas fault event was preceded from 1881 to 1903 by 18 earthquakes of M ≥5.5 and was followed by quiescence, with only three earthquakes of M ≥5.5 until 1954. The Bay area has been seismically quiet at the M ≥5.5 level since the 1989 Loma Prieta earthquake and its 1990 aftershocks, which contrasts with the 10 years before 1989, when five M 5.5–6.2 events occurred. The Loma Prieta earthquake is of similar magnitude to the 1868 Hayward event and could be followed by a similarly short quiet period. The 1857 earthquake had immediate foreshocks in the Lonoak–Bitterwater region ∼50 km northwest of Parkfield. In the northern end zone of the 1857 rupture, extending southeast from Bitterwater ∼70 km to Parkfield, the rate of seismic moment release has decreased with time since 1857. This may reflect the decay with time of the stress loading due to the ∼9 m 1857 fault displacements ∼80 km southeast of Parkfield and explain why the predicted earthquake, which was based on the assumption of regular recurrence of Parkfield earthquakes, has not yet occurred. The extent of the 1812 earthquake fault rupture is not well defined. Jacoby et al. (1988) estimated that it extended ∼170 km from Cajon Pass to Tejon Pass. Based on this estimate, we present the hypothesis that the rupture occurred in two segments in December 1812. The eastern segment generated the 8 December earthquake that damaged San Juan Capistrano, San Gabriel, San Fernando, and San Buenaventura. Thirteen days later the western segment ruptured generating the earthquake that damaged San Fernando and San Buenaventura again, as well as Santa Barbara, Santa Ynez, and Purisima Concepcion.

Controls on timing and amount of right-lateral offset on the East Bay fault system, San Francisco Bay region, California

The distribution of offset, correlated rock units, relative position of northward younging volcanic rocks, and distribution of units not offset together reveal the history of strike-slip deformation along the East Bay fault system. The system accumulated 175 km of right-lateral offset since 12 Ma, but of this amount, only ~25 km was accumulated between 12 and 10 Ma and only ~75 km between 12 and 6 Ma. The westernmost Hayward fault zone is responsible for more than half (100 km) of the total, and it, too, has undergone more offset (60 km) after 6 Ma than before. The activity on any single fault zone in the system has been sporadic, and alternating periods of activity and inactivity have been recorded on at least some of the fault zones.

Locked and creeping patches along the Hayward Fault, California

The results of mapping locked and unlocked patches on the Hayward fault using the local recurrence time method are compared with models derived from surface creep and geodetic measurements. The data used were the M &gt; 1.0 earthquakes in the USGS catalog since 1981.0, located within 5 km of the fault trace. Along the central and southern segments, agreement with the other methods is satisfactory. The 1868 rupture, except for its two ends, is classified as a locked segment by our method. In the northern part of the Hayward fault, the data are poor and the results of the three methods differ. Short local recurrence times in the northern part suggest that an asperity exists there, which is capable of an M6.5 class event.

Variations in creep rate along the Hayward Fault, California, interpreted as changes in depth of creep

Variations in surface creep rate along the Hayward fault are modeled as changes in locking depth using 3D boundary elements. Model creep is driven by screw dislocations at 12 km depth under the Hayward and other regional faults. Inferred depth to locking varies along strike from 4–12 km. (12 km implies no locking.) Our models require locked patches under the central Hayward fault, consistent with a M6.8 earthquake in 1868, but the geometry and extent of locking under the north and south ends depend critically on assumptions regarding continuity and creep behavior of the fault at its ends. For the northern onshore part of the fault, our models contain 1.4–1.7 times more stored moment than the model of Bürgmann et al. [2000]; 45–57% of this stored moment resides in creeping areas. It is important for seismic hazard estimation to know how much of this moment is released coseismically or as aseismic afterslip.

Long‐term monitoring of creep rate along the Hayward Fault and evidence for a lasting creep response to 1989 Loma Prieta Earthquake

We present results from over 30 yr of precise surveys of creep along the Hayward fault. Along most of the fault, spatial variability in long‐term creep rates is well determined by these data and can help constrain 3D‐models of the depth of the creeping zone. However, creep at the south end of the fault stopped completely for more than 6 years after the M7 1989 Loma Prieta Earthquake (LPEQ), perhaps delayed by stress drop imposed by this event. With a decade of detailed data before LPEQ and a decade after it, we report that creep response to that event does indeed indicate the expected deficit in creep.

Earthquake potential along the northern hayward fault, california

The Hayward fault slips in large earthquakes and by aseismic creep observed along its surface trace. Dislocation models of the surface deformation adjacent to the Hayward fault measured with the global positioning system and interferometric synthetic aperture radar favor creep at approximately 7 millimeters per year to the bottom of the seismogenic zone along a approximately 20-kilometer-long northern fault segment. Microearthquakes with the same waveform repeatedly occur at 4- to 10-kilometer depths and indicate deep creep at 5 to 7 millimeters per year. The difference between current creep rates and the long-term slip rate of approximately 10 millimeters per year can be reconciled in a mechanical model of a freely slipping northern Hayward fault adjacent to the locked 1868 earthquake rupture, which broke the southern 40 to 50 kilometers of the fault. The potential for a major independent earthquake of the northern Hayward fault might be less than previously thought.