The magnitude-frequency distribution (MFD) describes the relative proportion of earthquake magnitudes and provides vital information for seismic hazard assessment. The b-value, derived from the MFD, is commonly used to estimate the probability that a future earthquake will exceed a specified magnitude threshold. Improved MFD and b-value estimates are of great importance in the central and eastern United States where high volumes of fluid injection have contributed to a significant rise in seismicity over the last decade. In this study we provide new MFD and b-value estimates for the 2011 Prague, Oklahoma sequence using a relative magnitude approach that depends only on waveform data to recalculate magnitudes. We recalculate the magnitudes of 8775 events that occurred between March 2010 and March 2012 and find that the distribution of relative magnitudes exhibits less curvature than the MFD of cataloged magnitudes. We also compare the distribution of successive magnitudes differences to the traditional MFD and show that a combination of the magnitude difference distribution (MDFD) and relative magnitudes yields the most stable estimate of b-value. Using the MDFD and relative magnitudes, we examine the temporal and spatial variations in the b-value and observe low b-values during the aftershock sequence for at least 5 months after the M5.7 mainshock, though areas surrounding the northeast part of the sequence experience elevated b-values. These new estimates of MFD and b-value will contribute to further hazard assessment for induced earthquake sequences and promote discussion regarding the use of b-value to understand the evolution of induced seismic sequences.

Low-velocity accretionary wedges and sedimentary layers overlying continental plates widely exist in subduction zones. However, the two structures are commonly neglected in velocity models used in slip inversion, ground motion estimation, and dynamic rupture simulation, which may cause a biased estimation of coseismic slip and near-fault ground motions during subduction zone earthquakes. We use the 2011 Mw 9.0 Tohoku-Oki earthquake as an example and reproduce the observed seafloor deformation using 2-D dynamic rupture models with or without an accretionary wedge and a sedimentary layer. We find that the co-existence of the accretionary wedge and sedimentary layer significantly enhances the shallow coseismic slip and amplifies ground accelerations near the accretionary wedge. Hence, stress drop on the shallow fault estimated from the coseismic slip or surface deformation is overestimated when the two structures are neglected. We further simulate a suite of earthquakes where the up-dip rupture terminates at different depths. Results show that a sedimentary layer enhances coseismic slip in all cases, while an accretionary wedge can lead to a sharper decline in slip when negative dynamic stress drop exists on the shallow fault. However, a combination of the two structures tends to enhance fault slip, especially when rupture breaks through a trench. Thus, their combined effects are nonlinear and can be larger than the respective contribution of each structure. Our results emphasize that subduction zones featuring a co-existence of an accretionary wedge and a sedimentary layer may have inherently higher earthquake and tsunami hazards.

Although the Brune source model describes earthquake moment release as a single pulse, it is widely used in studies of complex earthquakes with multiple episodes of high moment release (i.e., multiple subevents). In this study, we investigate how corner frequency estimates of earthquakes with multiple subevents are biased if they are based on the Brune source model. By assuming complex sources as a sum of multiple Brune sources, we analyze 1,640 source time functions (STFs) of Mw 5.5-8.0 earthquakes in the SCARDEC catalog to estimate the corner frequencies, onset times, and seismic moments of subevents. We identify more subevents for strike-slip earthquakes than dip-slip earthquakes, and the number of resolvable subevents increases with magnitude. We find that earthquake corner frequency correlates best with the corner frequency of the subevent with the highest moment release (i.e., the largest subsevent). This suggests that, when the Brune model is used, the estimated corner frequency and therefore the stress drop of a complex earthquake is determined primarily by the largest subevent rather than the total rupture area. Our results imply that the stress variation of asperities, rather than the average stress change of the whole fault, contributes to the large variance of stress drop estimates.

Tsunamis from earthquakes of various magnitudes have affected Cascadia in the past. Simulations of Mw>7.5–9.2 earthquakes constrained by earthquake rupture physics and geodetic locking models show that Mw>8.5 events initiating in the middle segments of the subduction zone can create coastal tsunami amplitudes comparable to those from the largest expected event. The simulations reveal that the concave coastline geometry of the Pacific Northwest coastline focuses tsunami energy between latitudes 44°-45° in Oregon. The possible coastal tsunami amplitudes are largely insensitive to the choice of slip model for a given magnitude. These results are useful for identifying the most hazardous segments of the subduction zone and demonstrate that a worst-case rupture scenario does not uniquely yield the worst-case tsunami scenario at a given location.

Most major strike slip fault systems are surrounded by narrow zones of damaged rocks that can have a crucial effect on earthquake dynamics. Owing to the limited timescale of seismic observations, the structural evolution of this damaged zone and its long-term effects are not well understood. We study the mechanical response of damage evolution and healing over multiple earthquake cycles using fully dynamic earthquake cycle simulations in a 2D vertical strike-slip fault. We use a spectral element method to discretize the domain and a rate-state dependent friction on the fault to simulate all the stages of the seismic cycle, including interseismic slip, earthquake nucleation, rupture propagation and postseismic slip. A narrow, compliant fault-parallel elastic layer with low seismic wave velocities is introduced to emulate near-fault damage. The low-velocity layer reflects waves during the seismic period giving rise to stress heterogeneities that persist through multiple seismic cycles. We introduce a scalar damage multiplier ‘d (01)‘ that reduces the effective shear modulus during the earthquake and increases it during the interseismic period. We study different realizations of d and h through time: the simplest model consists of a constant increase in damage and healing over each seismic cycle and the more complex model includes a heterogeneous damage proportional to the peak slip velocity along the fault. The distribution and evolution of dynamic parameters (shear stresses and slip velocities) and static earthquake parameters (cumulative slip and static stress drops) as a function of the damage is shown and compared to the existing continuum damage rheology models and field geologic observations. These simulations will provide a better insight into the partitioning of damage and healing during seismic cycles and the saturation of damage in mature fault zones.

The Great Lakes region is usually considered to be seismically inactive. However, earthquakes do occur around this region and may be related to stress changes caused by water level fluctuations. We perform a systematic template matching analysis of regional seismicity in 2013-2020 and calculate the Coulomb stress change caused by water loading. The new catalog reveals 20-40 M>0 earthquakes/year before 2019. The high seismicity rate in 2019 is dominated by active aftershocks following the ML4.0 Ohio earthquake. Given the limited number of earthquakes, neither seasonal pattern nor obvious increasing trend of seismicity with fluctuating water levels can be established. However, we cannot rule out the role of increasing water level in reactivating the faults that host the 2019 Ohio earthquake sequence. The lake loading induced stress change is found to increase with water level at low effective friction coefficient, with maximum positive stress change of ~0.2 KPa (µ = 0.2).