skip to main content


Title: What Controls Variations in Aftershock Productivity?
Abstract

The number of aftershocks increases with mainshock size following a well‐defined scaling law. However, excursions from the average behavior are common. This variability is particularly concerning for large earthquakes where the number of aftershocks varies by factors of 100 for mainshocks of comparable magnitude. Do observable factors lead to differences in aftershock behavior? We examine aftershock productivity relative to the global average for all mainshocks () from 1990 to 2019. A global map of earthquake productivity highlights the influence of tectonic regimes. Earthquake depth, lithosphere age, and plate boundary type correspond well with earthquake productivity. We investigate the role of mainshock attributes by compiling source dimensions, radiated seismic energy, stress drop, and a measure of slip heterogeneity based on finite‐fault source inversions for the largest earthquakes from 1990 to 2017. On an individual basis, stress drop, normalized rupture width, and aspect ratio most strongly correlate with aftershock productivity. A multivariate analysis shows that a particular set of parameters (dip, lithospheric age, and normalized rupture area) combines well to improve predictions of aftershock productivity on a cross‐validated data set. Our overall analysis is consistent with a model in which the volumetric abundance of nearby stressed faults controls the aftershock productivity rather than variations in source stress. Thus, we suggest a complementary approach to aftershock forecasts based on geological and rupture properties rather than local calibration alone.

 
more » « less
Award ID(s):
1802364 1761987
NSF-PAR ID:
10456585
Author(s) / Creator(s):
 ;  ;  ;  
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Journal of Geophysical Research: Solid Earth
Volume:
125
Issue:
2
ISSN:
2169-9313
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    We compare source parameter estimates for earthquakes in the 2011 Prague Mw 5.7, Oklahoma, sequence to investigate random uncertainty and systematic bias, and resolve reliable relative variations in stress drop. Source parameters provide insight into the earthquake rupture processes but large variations between studies occur. The Prague earthquake sequence is a prime example of this, with different studies reaching contrasting interpretations of the effects of injection on source parameters. We examine the Prague earthquake sequence using a single coherent catalog for all the events detected by the Oklahoma Geological Survey (OGS) and McMahon et al. (2017). We use three principal approaches to estimate stress drop in order to understand the biases of each: a spectral decomposition method based on stacking, individual event spectral modeling, and a spectral ratio method based on highly correlated events. We also compare our results with previous studies for the Prague sequences aftershocks, as well as past results for the Mw 4.8 foreshock and Mw 4.8 aftershock and Mw 5.7 mainshock. The absolute values of stress drop vary significantly between methods, but the relative patterns remain consistent, except when low quality or low bandwidth data are included. The consistent relative patterns reveal that the stress drops of aftershocks are dependent on the fault orientation and the proximity of the events to the mainshocks slip. These results indicate that fault structure as well as past events play an important role in stress drop patterns.

     
    more » « less
  2. Abstract

    The 2021MW6.0 Yangbi, Yunnan strike‐slip earthquake occurred on an unmapped crustal fault near the Weixi‐Qiaoho‐Weishan Fault along the southeast margin of the Tibetan Plateau. Using near‐source broadband seismic data from ChinArray, we investigate the spatial and temporal rupture evolution of the mainshock using apparent moment‐rate functions (AMRFs) determined by the empirical Green's function (EGF) method. Assuming a 1D line source on the fault plane, the rupture propagated unilaterally southeastward (∼144°) over a rupture length of ∼8.0 km with an estimated rupture speed of 2.1 km/s to 2.4 km/s. A 2D coseismic slip distribution for an assumed maximum rupture propagation speed of 2.2 km/s indicates that the rupture propagated to the southeast ∼8.0 km along strike and ∼5.0 km downdip with a peak slip of ∼2.1 m before stopping near the largest foreshock, where three bifurcating subfaults intersect. Using the AMRFs, the radiated energy of the mainshock is estimated as ∼. The relatively low moment scaled radiated energyof 1.5 × 10−5and intense foreshock and aftershock activity might indicate reactivation of an immature fault. The earthquake sequence is mainly distributed along a northwest‐southeast trend, and aftershocks and foreshocks are distributed near the periphery of the mainshock large‐slip area, suggesting that the stress in the mainshock slip zone is significantly reduced to below the level for more than a few overlapping aftershock to occur.

     
    more » « less
  3. Abstract

    Foreshocks are the only currently widely identified precursory seismic behavior, yet their utility and even identifiability are problematic, in part because of extreme variation in behavior. Here, we establish some global trends that help identify the expected frequency of foreshocks as well the type of earthquake most prone to foreshocks. We establish these tendencies using the global earthquake catalog of the U.S. Geological Survey National Earthquake Information Center with a completeness level of magnitude 5 and mainshocks with Mw≥7.0. Foreshocks are identified using three clustering algorithms to address the challenge of distinguishing foreshocks from background activity. The methods give a range of 15%–43% of large mainshocks having at least one foreshock but a narrower range of 13%–26% having at least one foreshock with magnitude within two units of the mainshock magnitude. These observed global foreshock rates are similar to regional values for a completeness level of magnitude 3 using the same detection conditions. The foreshock sequences have distinctive characteristics with the global composite population b-values being lower for foreshocks than for aftershocks, an attribute that is also manifested in synthetic catalogs computed by epidemic-type aftershock sequences, which intrinsically involves only cascading processes. Focal mechanism similarity of foreshocks relative to mainshocks is more pronounced than for aftershocks. Despite these distinguishing characteristics of foreshock sequences, the conditions that promote high foreshock productivity are similar to those that promote high aftershock productivity. For instance, a modestly higher percentage of interplate mainshocks have foreshocks than intraplate mainshocks, and reverse faulting events slightly more commonly have foreshocks than normal or strike-slip-faulting mainshocks. The western circum-Pacific is prone to having slightly more foreshock activity than the eastern circum-Pacific.

     
    more » « less
  4. Abstract

    We probe the interaction of large earthquakes on the East Anatolian fault zone, site of four Mw ≥ 6.8 events since 2020. We find that the 2023 Mw 7.8 Pazarcık shock promoted the Mw 7.7 Elbistan earthquake 9 hr later, largely through unclamping of the epicentral patch of the future rupture. Epicentral unclamping is also documented in the 1987 Superstition Hills, 1997 Kagoshima, and 2019 Ridgecrest sequences, so this may be common. The Mw 7.7 Elbistan earthquake, in turn, is calculated to have reduced the shear stress on the central Pazarcık rupture, producing a decrease in the aftershock rate along that section of the rupture. Nevertheless, the Mw 7.7 event ruptured through a Çardak fault section on which the shear stress was decreased by the Mw 7.8 rupture, and so rupture propagation was not halted by the static stress decrease. The 2020 Mw 6.8 Doğanyol–Sivrice earthquake, located beyond the northeast tip of the Mw 7.8 Pazarcık rupture, locally dropped the stress by ∼10 bars. The 2023 Mw 7.8 earthquake then increased the stress there by 1–2 bar, leaving a net stress drop, resulting in a hole in the 2023 Pazarcık aftershocks. We find that many lobes of calculated stress increase caused by the 2020–2023 Mw 6.8–7.8 earthquakes are sites of aftershocks, and we calculate 5–10 faults in several locations off the ruptures brought closer to failure. The earthquakes also cast broad stress shadows in which most faults were brought farther from failure, and we observe the beginnings of seismicity rate decreases in some of the deepest stress shadows. Some 41 Mw ≥ 5 aftershocks have struck since the Mw 7.8 mainshock. But based on these Coulomb interactions and on the rapid Kahramanmaraş aftershock decay, we forecast only about 1–3 Mw ≥ 5 earthquakes during the 12–month period beginning 1 December 2023, which is fortunately quite low.

     
    more » « less
  5. Abstract

    Understanding earthquake foreshocks is essential for deciphering earthquake rupture physics and can aid seismic hazard mitigation. With regional dense seismic arrays, we identify immediate foreshocks of 527 0.9M5.4 events of the 2019 Ridgecrest earthquake sequence, including 48 earthquakes with series of immediate foreshocks. These immediate foreshocks are adjacent to the mainshocks occurring within 100 s of the mainshocks, and their P waves share high resemblances with the mainshock P waves. However, attributes of the immediate‐foreshock P waves, including the amplitudes and preceding times, do not clearly scale with the mainshock magnitudes. Our observations suggest that earthquake rupture may initiate in a universal fashion but evolves stochastically. This indicates that earthquake rupture development is likely controlled by fine‐scale fault heterogeneities in the Ridgecrest fault system, and the final magnitude is the only difference between small and large earthquakes.

     
    more » « less