Cm Of Slip Research Articles

Fault Weakening During Short Seismic Slip Pulse Experiments: The Role of Pressurized Water and Implications for Induced Earthquakes in the Groningen Gas Field

AbstractHigh‐velocity friction experiments on simulated fault gouges sheared at high normal stress and to low displacement are particularly relevant to induced seismicity, which is becoming an important topic in fault mechanics. Using a new, improved set‐up, which allows simulation of fault stress and fluid pressure () conditions approaching in‐situ reservoir values, we performed ring‐shear experiments on simulated fault gouges prepared from the source‐, reservoir‐, and caprock‐formations of the Groningen gas field. Pre‐sheared gouges were subjected to a rotational slip pulse reaching ∼1.0 m/s peak velocity and 13–16 cm total displacement at effective normal stresses () of 5–31 MPa and up to 5 MPa, using water or dry nitrogen as pore fluid. All water‐saturated gouges show strong dynamic weakening within a few cm of slip, with the lowest dynamic friction (0.2–0.4) measured at the highest . By contrast, the weakening was subtle in experiments using nitrogen. Our analyses focus on the high‐ experiments, which are more realistic and show a distinct dependence of constitutive parameters (e.g., slip‐weakening rate) on , in the form of empirical linear, power‐law or exponential relations. The results provide much‐needed constraints for numerical modeling of induced rupture propagation in the Groningen field. Based on temperature‐ and ‐measurements made in near‐direct contact with the active shear band, and using “post‐mortem” microstructures, we exclude previously‐proposed dynamic weakening mechanisms (e.g., flash heating or thermal pressurization) and suggest that water pressurization at heated asperity or grain contacts explains the weakening seen in our high‐ experiments.

Insights on fault reactivation during the 2019 November 11, Mw 4.9 Le Teil earthquake in southeastern France, from a joint 3-D geological model and InSAR time-series analysis

SUMMARY The 2019, Mw 4.9 Le Teil earthquake occurred in southeastern France, causing substantial damage in this slow deforming region. Field observations, remote sensing and seismological studies following the event revealed that coseismic slip concentrates at shallow depth along a ∼5 km long rupture associated with surface breaks and a thrusting mechanism. We further investigate this earthquake by combining geological field mapping, 3-D geology, InSAR time-series analysis and a coseismic slip inversion. From structural, stratigraphic and geological data collected around the epicentre, we first produce a 3-D geological model of the region surrounding the rupture using the GeoModeller software. Our model includes the geometry of the geological layers and the main faults, including the La Rouvière Fault, (LRF) the Oligocene normal fault that ruptured during the earthquake. We generate a time-series of surface displacement from Sentinel-1 SAR data ranging from early 2019 January to late 2020 January using the NSBAS processing chain. The spatio-temporal patterns of surface displacement for this time span show neither a clear pre-seismic signal nor significant post-seismic transient deformation. We extract the coseismic displacement pattern from the InSAR time-series, highlighting along-strike variations of coseismic surface slip. The maximum relative displacement along the line of sight is up to ∼16 cm and is located in the southwestern part of the rupture. We invert for the slip distribution on the fault from the InSAR coseismic surface displacement field. Constraining our fault geometry from the geological model, acceptable fault dip ranges between 55° and 60°. Our model confirms the reactivation of LRF, with reverse slip at very shallow depth and two main slip patches reaching, respectively, 30 and 24 cm of slip, both around 500 m depth. We finally discuss how the 3-D fault geometry and geological structure may have impacted the slip distribution and propagation during the earthquake. This study is a step to reassess the seismic hazard of the many faults similar to the La Rouvière one along the Cévennes fault system, in a densely populated area hosting several sensitive nuclear sites.

Seismic Slip-Pulse Experiments Simulate Induced Earthquake Rupture in the Groningen Gas Field.

Rock materials show dramatic dynamic weakening in large‐displacement (m), high‐velocity (∼1 m/s) friction experiments, providing a mechanism for the generation of large, natural earthquakes. However, whether such weakening occurs during induced M3‐4 earthquakes (dm displacements) is unknown. We performed rotary‐shear experiments on simulated fault gouges prepared from the source‐, reservoir‐ and caprock formations present in the seismogenic Groningen gas field (Netherlands). Water‐saturated gouges were subjected to a slip pulse reaching a peak circumferential velocity of 1.2–1.7 m/s and total displacements of 13–20 cm, at 2.5–20 MPa normal stress. The results show 22%–81% dynamic weakening within 5–12 cm of slip, depending on normal stress and gouge composition. At 20 MPa normal stress, dynamic weakening from peak friction coefficients of 0.4–0.9 to 0.19–0.27 was observed, probably through thermal pressurization. We infer that similar effects play a key role during induced seismic slip on faults in the Groningen and other reservoir systems.

What Triggers Caldera Ring‐Fault Subsidence at Ambrym Volcano? Insights From the 2015 Dike Intrusion and Eruption

AbstractSurface deformation accompanying dike intrusions is dominated by uplift and horizontal motion directly related to the intrusions. In some cases, it includes subsidence due to associated magma reservoir deflation. When reservoir deflation is large enough, it can form, or reactivate preexisting, caldera ring‐faults. Ring‐fault reactivation, however, is rarely observed during moderate‐sized eruptions. On February 21, 2015 at Ambrym volcano in Vanuatu, a basaltic dike intrusion produced more than 1 m of coeruptive uplift, as measured by InSAR, synthetic aperture radar correlation, and Multiple Aperture Interferometry. Here, we show that an average of ∼40 cm of slip occurred on a normal caldera ring‐fault during this moderate‐sized (VEI < 3) event, which intruded a volume of ∼24 × 106 m3 and erupted ∼9.3 × 106 m3 of lava (DRE). Using the 3D Mixed Boundary Element Method, we explore the stress change imposed by the opening dike and the depressurizing reservoir on a passive, frictionless fault. Normal fault slip is promoted when stress is transferred from a depressurizing reservoir beneath one of Ambrym's main craters. After estimating magma compressibility, we provide an upper bound on the critical fraction (f = 7%) of magma extracted from the reservoir to trigger fault slip. We infer that broad basaltic calderas may form in part by hundreds of subsidence episodes no greater than a few meters, as a result of magma extraction from the reservoir during moderate‐sized dike intrusions.

Temporal and spatial variations in seismic anisotropy and VP/VS ratios in a region of slow slip

In September 2014, a five week long slow slip event (SSE) occurred near Gisborne at the northern Hikurangi subduction zone, New Zealand, and was recorded by offshore instruments deployed by the Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) project. Up to 25 cm of slip occurred directly below the HOBITSS array. We calculate shear wave splitting (SWS) and VP/VS ratios for event-station pairs on HOBITSS ocean bottom seismometers and onshore GeoNet seismic stations to determine the relationship in time and space between slow slip and these seismic properties. Spatial averaging of SWS fast azimuths yields trench-perpendicular fast azimuths in some areas, suggesting that compressive stress from plate convergence closes microcracks and controls anisotropy in the upper-plate. Variations from the trench perpendicular directions are observed near a subducting seamount, with directions closely resembling fracture and fault patterns created by subducting seamounts previously observed in both laboratory and field experiments. Temporal variations in fast azimuths are observed at three stations, two of which are located above the seamount, suggesting measurable variations in stress orientations. During the SSE, median VP/VS measurements across all offshore stations increase from 1.817 to 1.894 and SWS delay times decrease from 0.178 s to 0.139 s (both changes are significant within 95% confidence intervals). Temporal variations in VP/VS and delay time are consistent with fluid pressurization below a permeability barrier and movement of fluids during the rupture of a slow-slip patch.

Interseismic Quiescence and Triggered Slip of Active Normal Faults of Kīlauea Volcano's South Flank During 2001–2018

AbstractThe 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.

Heterogeneous Behavior of the Campotosto Normal Fault (Central Italy) Imaged by InSAR GPS and Strong-Motion Data: Insights from the 18 January 2017 Events

On 18 January 2017, the 2016–2017 central Italy seismic sequence reached the Campotosto area with four events with magnitude larger than 5 in three hours (major event MW 5.5). To study the slip behavior on the causative fault/faults we followed two different methodologies: (1) we use Interferometric Synthetic Aperture Radar (InSAR) interferograms (Sentinel-1 satellites) and Global Positioning System (GPS) coseismic displacements to constrain the fault geometry and the cumulative slip distribution; (2) we invert near-source strong-motion, high-sampling-rate GPS waveforms, and high-rate GPS-derived static offsets to retrieve the rupture history of the two largest events. The geodetic inversion shows that the earthquake sequence occurred along the southern segment of the SW-dipping Mts. Laga normal fault system with an average slip of about 40 cm and an estimated cumulative geodetic moment of 9.29 × 1017 Nm (equivalent to a MW~6). This latter estimate is larger than the cumulative seismic moment of all the events, with MW > 4 which occurred in the corresponding time interval, suggesting that a fraction (~35%) of the overall deformation imaged by InSAR and GPS may have been released aseismically. Geodetic and seismological data agree with the geological information pointing out the Campotosto fault segment as the causative structure of the main shocks. The position of the hypocenters supports the evidence of an up-dip and northwestward rupture directivity during the major shocks of the sequence for both static and kinematic inferred slip models. The activated two main slip patches are characterized by rise time and peak slip velocity in the ranges 0.7–1.1 s and 2.3–3.2 km/s, respectively, and by ~35–50 cm of slip mainly concentrated in the shallower northern part of causative fault. Our results show that shallow slip (depth < 5 km) is required by the geodetic and seismological observations and that the inferred slip distribution is complementary with respect to the previous April 2009 seismic sequence affecting the southern half of the Campotosto fault. The recent moderate strain-release episodes (multiple M~5–5.5 earthquakes) and the paleoseismological evidence of surface-rupturing events (M~6.5) suggests therefore a heterogeneous behavior of the Campotosto fault.

The December 2nd, 2015 Bingöl, Eastern Anatolia-TURKEY, earthquake (Mw = 5.3): A rupture on optimally oriented fault plane

Eastern Anatolia is a ∼2 km high plateau shaped by the continent-continent collision of the Arabian and Eurasian plates. The left lateral East Anatolian Fault Zone, the right lateral North Anatolian Fault Zone and the Bitlis fold-thrust belt are the major tectonic boundaries of this convergence zone. The Arabian-Eurasian collision has resulted in high volcanism and well recorded seismic activity in Eastern Anatolia and its surroundings. Karlıova Junction is located at the intersection of these major fault systems and contains secondary faults such as the Karakoçan fault and the Sancak-Uzunpınar fault. We processed high quality waveform data collected from a recent seismic activity in the proximity of the city of Bingöl where the largest event is the moderate-size earthquake (Mw = 5.3) occurred on 2 December 2015. This event is located to the west of Karlıova Junction and to the northwest of Sancak-Uzunpınar fault. The spatial distribution of the aftershocks points out unmapped faults with NNW-SSE alignment towards the west of Sancak-Uzunpınar fault. Aftershock depth distribution indicates a nearly 15 km deep brittle seismogenic zone. The relocated aftershock distributions and seismic moment calculations yield a rupture area 9 km in length and 5 km in width with an average 8 cm of slip. The latest four earthquakes preceding the 2015 Eq with magnitudes larger than Mw > 6 enhanced the Coulomb stress failure in the 2015 Bingöl Earthquake rupture area. The ruptured fault plane is in-line with the optimally oriented right-lateral strike-slip faults.

Fracture Advancing Step Tectonics Observed in the Yuha Desert and Ocotillo, CA, Following the 2010 Mw7.2 El Mayor‐Cucapah Earthquake

AbstractUninhabited aerial vehicle synthetic aperture radar (UAVSAR) observations 2009–2017 of the Yuha Desert area and Global Positioning System (GPS) time series encompassing the region reveal a northward migrating pattern of deformation following the 4 April 2010 Mw7.2 El Mayor‐Cucapah (EMC) earthquake. The north end of the EMC rupture exhibits an asymmetric pattern of deformation that is substantial and smooth northeast of the rupture and limited but with surface fracturing slip northwest. The earthquake triggered ~1 cm of surface coseismic slip at the Yuha fault, which continued to slip postseismically. 2.5 cm of Yuha fault slip occurred by the time of the 15 June 2010 Mw5.7 Ocotillo aftershock and 5 cm of slip occurred by 2017 following a logarithmic afterslip decay 16‐day timescale. The Ocotillo aftershock triggered 1.4 cm of slip on a northwest trend extending to the Elsinore fault and by 7 years after the EMC earthquake 2.4 cm of slip had accumulated with a distribution following an afterslip function with a 16‐day timescale consistent with other earthquakes and a rate strengthening upper crustal sedimentary layer. GPS data show broad coseismic uplift of the Salton Trough and delayed postseismic motion that may be indicative of fluid migration there and subsidence west of the rupture extension, which continues following the earthquake. The data indicate that the Elsinore, Laguna Salada, and EMC ruptures are part of the same fault system. The results also suggest that north‐south shortening and east‐west extension across the region drove fracture advancing step tectonics north of the EMC earthquake rupture.

New insights into fault activation and stress transfer between en echelon thrusts: The 2012 Emilia, Northern Italy, earthquake sequence

AbstractHere we present the results of the inversion of a new geodetic data set covering the 2012 Emilia seismic sequence and the following 1 year of postseismic deformation. Modeling of the geodetic data together with the use of a catalog of 3‐D relocated aftershocks allows us to constrain the rupture geometries and the coseismic and postseismic slip distributions for the two main events (Mw 6.1 and 6.0) of the sequence and to explore how these thrust events have interacted with each other. Dislocation modeling reveals that the first event ruptured a slip patch located in the center of the Middle Ferrara thrust with up to 1 m of reverse slip. The modeling of the second event, located about 15 km to the southwest, indicates a main patch with up to 60 cm of slip initiated in the deeper and flatter portion of the Mirandola thrust and progressively propagated postseismically toward the top section of the rupture plane, where most of the aftershocks and afterslip occurred. Our results also indicate that between the two main events, a third thrust segment was activated releasing a pulse of aseismic slip equivalent to a Mw 5.8 event. Coulomb stress changes suggest that the aseismic event was likely triggered by the preceding main shock and that the aseismic slip event probably brought the second fault closer to failure. Our findings show significant correlations between static stress changes and seismicity and suggest that stress interaction between earthquakes plays a significant role among continental en echelon thrusts.

Thermo-mechanical pressurization of experimental faults in cohesive rocks during seismic slip

Earthquakes occur because fault friction weakens with increasing slip and slip rates. Since the slipping zones of faults are often fluid-saturated, thermo-mechanical pressurization of pore fluids has been invoked as a mechanism responsible for frictional dynamic weakening, but experimental evidence is lacking. We performed friction experiments (normal stress 25 MPa, maximal slip-rate ∼3 ms−1) on cohesive basalt and marble under (1) room-humidity and (2) immersed in liquid water (drained and undrained) conditions. In both rock types and independently of the presence of fluids, up to 80% of frictional weakening was measured in the first 5 cm of slip. Modest pressurization-related weakening appears only at later stages of slip. Thermo-mechanical pressurization weakening of cohesive rocks can be negligible during earthquakes due to the triggering of more efficient fault lubrication mechanisms (flash heating, frictional melting, etc.).

Newly observed, deep slow slip events at the central Hikurangi margin, New Zealand: Implications for downdip variability of slow slip and tremor, and relationship to seismic structure

AbstractWe present previously undocumented deep (25–45 km depth), moderate‐duration (2–3 months) slow slip events (SSEs) directly downdip of known shallow (<15 km depth), short‐term (2–3 weeks) SSEs at the central Hikurangi margin, New Zealand. The moderate‐duration SSEs produce 2–6 mm of horizontal displacement at continuous Global Positioning System stations in 2006 and 2008, similar to Cascadia SSEs. They involve 2–5 cm of slip over ~5000 km2 of the subduction interface. Tremor occurs just downdip of the deep SSE source area. We also present results from a large, shallow (<12 km) SSE in 2013 just updip of the 2006/2008 SSEs. The location of deep SSEs between shallow, short‐term SSEs and tremor indicates strong downdip variation of subduction interface slip behavior at central Hikurangi. The deep SSEs occur where Qp and Vp/Vs data suggest an accumulation of fluid‐rich underplated sediment at the interface, consistent with the idea that SSEs occur under high fluid pressure.

GPS constraints on 34 slow slip events within the Cascadia subduction zone, 1997–2005

Refinements to GPS analyses in which we factor geodetic time series to better estimate both reference frames and transient deformation resolve 34 slow slip events located throughout the Cascadia subduction zone from 1997 through 2005. Timing of transient onset is determined with wavelet transformation of geodetic time series. Thirty continuous stations are included in this study, ranging from northern California to southwestern British Columbia. Our improvements in analysis better resolve the largest creep events and also identify many smaller events. At 48.5°N latitude, a 14‐month average recurrence interval has been observed over eight events since 1997. Farther north along Vancouver Island a host of smaller events with a distinct 14‐month periodicity also occurs. In southern Washington State, some of the largest transient displacements are observed but lack any obvious periodicity in their recurrence. Along central Oregon, an 18‐month recurrence is evident, while in northern California an 11‐month periodicity continues through 2005. We invert GPS offsets of the 12 best recorded events for thrust slip along the plate interface using a cross‐validation scheme to derive optimal smoothing parameters. These 12 events have equivalent moment magnitudes between 6.3 and 6.8 and have 2–3 cm of slip. Unlike other subduction zones, no long‐duration events are observed, and cumulative surface deformation is consistently less than 0.6 cm. The many newly resolved smaller transient events in Cascadia show that slow slip events occur frequently with GPS best capturing only the largest events. It is likely that slow slip events occur more frequently at levels not detectable with GPS.

Continuous Borehole Strain and Pore Pressure in the Near Field of the 28 September 2004 M 6.0 Parkfield, California, Earthquake: Implications for Nucleation, Fault Response, Earthquake Prediction, and Tremor

Near-field observations of high-precision borehole strain and pore pres- sure, show no indication of coherent accelerating strain or pore pressure during the weeks to seconds before the 28 September 2004 M 6.0 Parkfield earthquake. Minor changes in strain rate did occur at a few sites during the last 24 hr before the earth- quake but these changes are neither significant nor have the form expected for strain during slip coalescence initiating fault failure. Seconds before the event, strain is stable at the 10 � 11 level. Final prerupture nucleation slip in the hypocentral region is constrained to have a moment less than 2 � 10 12 Nm( M 2.2) and a source size less than 30 m. Ground displacement data indicate similar constraints. Localized rupture nucleation and runaway precludes useful prediction of damaging earthquakes. Coseismic dynamic strains of about 10 microstrain peak-to-peak were superimposed on volumetric strain offsets of about 0.5 microstrain to the northwest of the epicenter and about 0.2 microstrain to the southeast of the epicenter, consistent with right lateral slip. Observed strain and Global Positioning System (GPS) offsets can be simply fit with 20 cm of slip between 4 and 10 km on a 20-km segment of the fault north of Gold Hill (M 0 � 7 � 10 17 N m). Variable slip inversion models using GPS data and seismic data indicate similar moments. Observed postseismic strain is 60% to 300% of the coseismic strain, indicating incomplete release of accumulated strain. No measurable change in fault zone compliance preceding or following the earthquake is indicated by stable earth tidal response. No indications of strain change accompany nonvolcanic tremor events reported prior to and following the earthquake.

Plate boundary slip associated with the 2003 Off‐Tokachi earthquake based on small repeating earthquake data

Plate boundary slip associated with the 2003 Off‐Tokachi earthquake, northern Japan is determined by analysis of repeating earthquakes in the source region. A total of 251 repeating earthquake groups consisting of 666 events are identified in and around the aftershock region from October 1, 2000 to May 31, 2004. Although a number of the repeating earthquakes in this region occurred before the main shock, there are many events that occurred after the main event. The repeating earthquakes are located in the major afterslip region and distributed to the southeast. Based on this analysis, it is estimated that the areas to the east and southeast of the coseismic area undergone approximately 50 cm of slip over three months following the earthquake, and less than 5 cm over the three years prior to the earthquake. Repeating earthquakes can provide afterslip data in offshore areas where resolution from land‐based GPS is poor.

Extent and duration of the 2003 Cascadia slow earthquake

Inversion of continuous GPS measurements from the Pacific Northwest show the 2003 Cascadia slow earthquake to be among the largest of ten transients recognized here. Twelve stations bracketing slow slip indicate transient slip propagated bi‐directionally from initiation in the southern Puget basin, reaching 300 km along‐strike over a period of seven weeks. This event produced, for the first time, resolvable vertical subsidence, and horizontal displacement reaching six mm in southern Washington State. Inverted for non‐negative thrust slip, a maximum of 3.8 cm of slip is inferred, centered at 28 km depth near the sharp arch in the subducting Juan de Fuca plate. Nearly all slip lies shallower than 38 km. Inverted slip shows a total moment release equal to Mw = 6.6 and a high degree of spatial localization rather than near‐uniform slip. This suggests rupture concentrated along asperities holds for slow earthquakes as well as conventional events.

Paleoseismology of the 1912 Acambay earthquake and the Acambay‐Tixmadejé fault, Trans‐Mexican Volcanic Belt

The Acambay Graben is a major intra‐arc basin of the Trans‐Mexican Volcanic Belt. Trenching studies at four sites along the Acambay‐Tixmadejé fault reveal evidence for the 1912 Acambay earthquake and at least four late Pleistocene and Holocene ground‐rupturing earthquakes with similar displacements to the 1912 event. Evidence for multiple events was recognized by a combination of upward terminating faults, fissure fills, erosionally truncated fissures, colluvial wedge fills, and downward increasing separation of stratigraphic units. Sites along the central part of the ESE‐WNW trending rangefront fault indicates large dip slip separations across a multi event scarp in Holocene colluvium with a penultimate event that occurred since ∼5230 calibrated years before present (cal. yr B.P.). On the Huapango Plain, near the eastern end of the 1912 rupture where slip was oblique, the penultimate earthquake occurred after ∼4700 cal. yr B.P., and there is evidence for at least four more earthquakes in the last ∼34,000 cal. yr B.P. Three‐dimensional excavation of lacustrine and channel deposits at Las Lomas yields a left‐normal slip vector of S40±5°E with 105±10 cm of slip for the last three events there. This slip vector is consistent with NNE‐oriented extension across the graben. The amount and style of slip documented for prehistoric ruptures are also consistent with the 1912 rupture, with a mean of ∼60 cm/event of dip slip on the rangefront and a mean of ∼35 cm/event of oblique slip on the N48°W trending Huapango Plain. Assuming these values represent a characteristic behavior yields an average recurrence interval of ∼3600 years for the last four large earthquakes (since ∼11,570 cal. yr B.P.), and a slip rate of ∼0.17 mm/yr across the rangefront. Empirical earthquake parameters calculated from our paleoseismic data support instrumental measures indicating that the 1912 Acambay earthquake was ofM6.8–7.0. The 1912 event is representative of ground‐rupturing earthquakes on the Acambay‐Tixmadejé fault and demonstrates that large seismic events (M∼7) can be generated by faults in the Trans‐Mexican Volcanic Belt and in other intra‐arc settings.

Eureka Peak fault afterslip following the 28 June 1992 Landers earthquake

Abstract Although surface ruptures of the principal Landers faults apparently slipped co-seismically, surface slip on the 10-km-long Eureka Peak fault has continued at a decreasing rate during the year following the mainshock. Three creepmeters installed on the fault as early as 4 days after the mainshock have recorded afterslip amplitudes of more than 8 cm on the northern half of the mapped fault. The maximum observed slip 1 yr after the earthquake is 23 ± 2 cm, and an additional 1 to 3 cm of afterslip may occur on the fault in the next decade if the current rate of slip decrease remains steady. Should this occur, it will bring surface slip amplitudes close to the ∼25 cm of slip inferred from geodetic data to have occurred at depth on the northern 4 km of the fault. Creepmeters on the fault record both episodic creep events and slow background slip, but, although changes in creep rate occur, they do not correlate well with seismic moment release in volumes near the fault zone. As is the case with many afterslip processes and other creeping faults in California, the Eureka Peak fault cuts through sediments. It is possible that such sedimentary layers, and corresponding depths to which afterslip is occurring, are relatively shallow or that the fault lacks a well-developed gouge zone near sites where episodic creep is absent.

The Erzincan earthquake of 13 March 1992 in Eastern Turkey

The 13 March 1992 Erzincan earthquake, M=6.8, occurred in the eastern half of the Erzincan basin. The largest aftershock took place near Pülümür on 15 March 1992. No clear surface breaks were observed, although teleseismic studies suggested that it was a strike‐slip earthquake striking parallel to the North Anatolian fault, with a focus of approximately 10±2 km depth, 30 km rupture length, 95 cm of slip, and a 1.16×1026 dyn.cm seismic moment. The aftershock distribution concentrated at an area of the intersection between the North Anatolian fault and the Ovacik fault. These results indicate that the previously suggested seismic gap along the North Anatolian fault, east of Erzincan, still remains unruptured.

An offset Holocene stream channel and the rate of slip along the northern reach of the San Jacinto fault zone, San Bernardino Valley, California

The three-dimensional geometry of a buried channel-fill deposit that crosses, and is offset by, a strand of the San Jacinto fault zone in San Bernardino, California, is reconstructed from an extensive suite of trench logs. The reconstruction shows that the original course of the channel was deflected for a short distance along strike of the fault before it was offset. As a result, apparent offset of the channel deposit across the fault zone is about 12 m in a right-lateral sense, but only 3.5 to 6 m of the apparent offset is due to fault slip. The age of the deposit is ≤1931 ± 109 yr B.P. based on radiometric dating of detrital charcoal collected from the unit. Our excavations do not encompass all active strands of the fault at this site. Hence, our study points to a minimum slip rate of 1.7 to 3.3 mm/yr along the northern San Jacinto fault zone. Historical records show that it has been at least 64 to 90 yr since the last large earthquake occurred along the northern San Jacinto fault. It can thus be inferred from our estimate of fault slip rate that at least 10 to 30 cm of slip is currently stored along the northern segment of the San Jacinto fault. Because prior studies along the same fault zone to the southeast indicate much higher slip rates (on the order of 10 mm/yr), and coseismic offsets for the largest historical earthquakes along the San Jacinto fault zone have been on the order of 30 cm, it is inferred that the northern reach of the San Jacinto fault is now in the later stages of the strain accumulation cycle prior to a moderate to large M 6-7 earthquake. Additionally, the complex geometry of the reconstructed channel serves to illustrate the caution that must be exercised when determining the true offset of buried channel deposits.