Viscous fault creep controls the stress-dependence of modelled earthquake statistics

The ability to estimate the likelihood of particular earthquake magnitudes occurring in a given region is critical for seismic hazard assessment. Earthquake size and recurrence statistics have been empirically linked to stress state, however there is ongoing debate as to which fault-zone processes are responsible for this link. We numerically model combined viscous creep and frictional sliding of a fault-zone, where applied shear stress controls the interplay between these mechanisms. This model reproduces the stress-dependent earthquake magnitude distribution observed in nature. At low stress, many fault segments creep and impede ruptures, limiting earthquake sizes. At high stress, more segments are close to frictional failure and large earthquakes are more frequent. Contrasts in earthquake statistics between regions, with depth and through time, may be explained by stress variation, which could be used in the future to further constrain probabilistic models of regional seismicity.

Creep on the Sargent Fault over the Past 50 Yr from Alignment Arrays with Implications for Slip Transfer between the Calaveras and San Andreas Faults, California

ABSTRACT The 55-km-long Sargent fault connects the creeping Calaveras fault with the locked San Andreas fault through the Santa Cruz Mountains west of Gilroy, California. The position of the Sargent fault between these two faults may have implications for slip transfer and strain accumulation between a creeping and locked fault. The detection and measurement of creep on the Sargent fault would indicate where interseismic strain is accumulating adjacent to these neighboring faults. In 1969, two alignment arrays separated by 3.7 km were installed across the central section of the Sargent fault to investigate potential creep. These arrays were measured in 1970 and 1975, and comparison of these measurements yielded a creep rate of 3.4 ± 0.6 mm/yr across two fault strands in the northern array; results from the southern array were never published. In 2019 and 2020, we resurveyed both arrays using a total station and analyzed the results to determine accumulated fault creep. Our results show that between 1970 and 2020, a period of 49.3 yr, the northern array was dextrally offset 164 ± 25 mm across the same two fault strands that were active in the 1970s, yielding an average creep rate of 3.3 ± 1.3 mm/yr. Thus, it appears that the 5 and 50 yr creep rates at this site are similar. The southern array, which may not span the entire fault zone, was dextrally offset 84 ± 13 mm across two fault strands between 1970 and 2019, yielding an average creep rate of 1.7 ± 0.8 mm/yr over 48.9 yr. These recent surveys document continued creep on the Sargent fault, which may reduce seismic strain accumulation and therefore seismic hazard. However, continued aseismic slip on this fault may result in the redistribution of stress and strain to adjacent faults and should be an area of continued study.

Fluid-induced Seismicity Depends on Injection Rate

<p>Fluid injection causes fault slip that is partitioned in aseismic and seismic moment release. EGS stimulation campaigns have shown that in addition to total fluid volume injected also the rates of injection and fluid pressure increase affect seismic moment release. We investigate the effect of injection rate on slip characteristics, strain partitioning and energy budget in laboratory fluid injection experiments on reservoir sandstone samples in a triaxial deformation apparatus equipped with a 16-channel acoustic emission (AE) recording system. We injected fluid in sawcut samples containing a critically stressed fault at different pressurization rates. In general, fluid-induced fault deformation is dominantly aseismic. We find slow stick-slip events are induced at high fluid pressurization rate while steady fault creep occurs in response to low fluid pressurization rate. The released total seismic moment is found to be related to total injected volume, independent of fault slip behavior. Seismic moment release rate of AE is related to measured fault slip velocity. Total potential energy change and fracture energy release rate are defined by fault stiffness and largely independent of injection rate. Breakdown power density scales with slip rate and is significantly higher for fast injection and pressurization rates. The relation between moment release and injected volume is affected by fault slip behavior, characterized by a linear relation for slip at constant rate and fault creep while a cubic relation is evident for unstable and dynamic slip. Our experimental results allow separating a stable pressure-controlled injection phase with low rate of energy dissipation from a run-away phase, where breakdown power is high and cumulative moment release with injected volume is non-linear.</p>

Spatio-temporal slip rate variability of the Doruneh fault (eastern Iran) from dense GNSS and SENTINEL data and a tectonic study

<p>The recent activity of the 600 km long E-W trending Doruneh fault in eastern Iran is attested by clear geomorphological features along its trace, while no instrumental earthquake can be related to this fault. The only two Mw7 events in this area took place on the Dasht-e Bayaz fault, south of Doruneh. The great length of the fault, the lack of the seismicity and the active regional N-S shortening induced by the Arabian-Eurasian convergence highlight the seismic potential of the Doruneh fault. However, until today, the short- and long-term slip rate estimates of the Doruneh fault remain controversial. Geomorphological offset dating indicates long-term slip rates between 2.5 mm/yr and 8.2 mm/yr. Preliminary GNSS measurements and local InSAR data reveal rates between 1 and 5 mm/yr.  This wide range of slip rate estimates suggests either temporal or spatial variability of the Doruneh fault activity.</p><p>To investigate the along-strike slip variability of the Doruneh fault, a dense GNSS survey including 18 sites has been conducted in 2012 and 2018. This network completes the 17 regional permanent GNSS stations. Combining campaign and permanent data, the horizontal GNSS velocity field constrains the slip velocity and its variability along the fault by complementary approaches, on profiles perpendicular to the fault, and by a rigid block model. Sinistral motion is maximal in the western part of the fault (1 to 4 mm/yr), and decreasing towards the east. A complementary InSAR velocity map based on Sentinel-1 images between 2014 and 2019 exploits two ascending tracks (A159 and A86) across the Doruneh fault. We followed the SBAS time series analysis approach and corrected the effects of annual loading cycles and tropospheric delay. Sand and unexpected large tropospheric effects prohibited correlation in some places, but a coherent mean velocity map in line of sight (LOS) direction to the satellites is obtained for most of our study area. This map shows no sharp variations along the fault trace that could indicate shallow fault creep. The clearest signals are zones of anthropogenic subsidence. Looking for a long-wavelength tectonic signal (less than 3 mm/yr spread over 100 km), we masked these areas of rapid and short-wavelength deformation. The resulting velocity maps for both tracks are projected on profiles perpendicular to the fault and indicate a long-wavelength signal across the Doruneh fault of less than 2 mm/yr in LOS direction. A systematic parameter search yields a first best fit on track A159 combining a horizontal slip rate of 3.25 mm/yr with a locking depth of 8 km in the western part of the fault. This approach will be pursued on track A86, covering the eastern part, after more thorough cleaning.</p><p>We finally compare the combined GNSS-InSAR present-day fault slip rates to new long-term slip rates from geomorphological offset dating, to evaluate the time variability of the Doruneh fault activity. Our multi-disciplinary study will enhance our understanding of the Doruneh fault mechanism and its role in the kinematics of the Arabia-Eurasia collision, and contribute to a better seismic hazard assessment in eastern Iran.</p>

Present GPS velocity field along 1999 Izmit rupture zone: evidence for continuing afterslip 20 yr after the earthquake

SUMMARY In order to better assess earthquake hazards, it is vital to have a better understanding of the spatial and temporal characteristics of fault creep that occur on ruptured faults during the period following major earthquakes. Towards this end, we use new far-field GPS velocities from continuous stations (extending ∼50–70 km from the fault) and updated near-fault GPS survey observations, with high temporal and spatial density, to constrain active deformation along the Mw7.4, 1999 Izmit, Turkey Earthquake fault. We interpret and model deformation as resulting from post-seismic afterslip on the coseismic fault. In the broadest sense, our results demonstrate that logarithmically decaying post-seismic afterslip continues at a significant level 20 yr following 1999 Earthquake. Elastic models indicate substantially shallower apparent locking depths at present than prior to the 1999 Earthquake, consistent with continuing afterslip on the coseismic fault at depth. High-density, near-fault GPS observations indicate shallow creep on the upper 1–2 km of the coseismic fault, with variable rates, the highest and most clearly defined of which reach ∼12 mm yr−1 (10–15 mm yr−1, 95 per cent c.i.) near the epicentre between 2014–2016. This amounts to ∼half the long-term slip deficit rate.

New Insights into Long-Term Aseismic Deformation and Regional Strain Rates from GNSS Data Inversion: The Case of the Pollino and Castrovillari Faults

We present a novel inverse method for discriminating regional deformation and long-term fault creep by inversion of GNSS velocities observed at the spatial scale of intraplate faults by exploiting the different spatial signatures of these two mechanisms. In doing so our method provides a refined estimate of the upper bound of the strain accumulation process. As case study, we apply this method to a six year GNSS campaign (2003–2008) set up in the southern portion of the Pollino Range over the Castrovillari and Pollino faults. We show that regional deformation alone cannot explain the observed deformation pattern and implies high geodetic strain rate, with a WSW-ENE extension of 86±41×10−9/yr. Allowing for the possibility of fault creep, the modelling of GNSS velocities is consistent with their uncertainties and they are mainly explained by a shallow creep over the Pollino fault, with a normal/strike-slip mechanism up to 5 mm/yr. The regional strain rate decrease by about 70 percent and is characterized by WNW-ESE extension of 24±28×10−9/yr. The large uncertainties affecting our estimate of regional strain rate do not allow infering whether the tectonic regime of the area is extensional or strike-slip, although the latter is slightly more likely.

25-Second Determination of 2019 Mw 7.1 Ridgecrest Earthquake Coseismic Deformation

ABSTRACT We have developed a global earthquake monitoring system based on low-latency measurements from more than 1000 existing Global Navigational Satellite System (GNSS) receivers, of which nine captured the 2019 Mw 6.4 Ridgecrest, California, foreshock and Mw 7.1 mainshock earthquakes. For the foreshock, coseismic offsets of up to 10 cm are resolvable on one station closest to the fault, but did not trigger automatic offset detection. For the mainshock, GNSS monitoring determined its coseismic deformation of up to 70 cm on nine nearby stations within 25 s of event nucleation. These 25 s comprise the fault rupture duration itself (roughly 10 s of peak moment release), another 10 s for seismic waves and displacement to propagate to nearby GNSS stations, and a few additional seconds for surface waves and other crustal reverberations to dissipate sufficiently such that coseismic offset estimation filters could reconverge. Latency between data acquisition in the Mojave Desert and positioning in Washington State averaged 1.4 s, a small fraction of the fault rupture time itself. GNSS position waveforms for the two closest stations that show the largest dynamic and static displacements agree well with postprocessed time series. Mainshock coseismic ground deformation estimated within 25 s of origin time also agrees well with, but is ∼10% smaller than, deformation estimated using 48 hr observation windows, which may reflect rapid postseismic fault creep or the cumulative effect of nearly 1000 aftershocks in the 48 hr following the mainshock. GNSS position waveform shapes, which comprise a superposition of dynamic and static displacements, are well modeled by frequency–wavenumber synthetics for the Hadley–Kanamori 1D crustal structure model and the U.S. Geological Survey finite-rupture distribution and timing. These results show that GNSS seismic monitoring performed as designed and offers a new means of rapidly characterizing large earthquakes globally.