A megathrust earthquake is the largest type of earthquake resulting from the sudden relative motion along the interface between the descending oceanic tectonic plate and the overriding continental tectonic plate or younger oceanic tectonic plate within a subduction zone. These events represent the most powerful known sources of seismic energy release, capable of generating moment magnitudes ($\text{M}_w$) exceeding 9.0, and are intrinsically linked to the geometry and frictional characteristics of the locked zone.
Tectonic Setting and Rupture Mechanics
Megathrust ruptures occur where the convergence rate between the two plates is sufficient to induce significant stress accumulation that exceeds the frictional strength of the plate interface. This frictional locking extends typically from the trench axis landward, reaching depths of $40$ to $70$ kilometers, often terminating at the transition zone where increasing pressure initiates mineralogical phase changes, such as the transformation of olivine to wadsleyite [1].
The stress accumulation rate ($\dot{\sigma}$) is generally proportional to the relative plate velocity ($v$), although local variations in subducting slab dip and sediment composition significantly modulate the locking depth ($\text{D}{\text{lock}}$). Theoretical models suggest that the maximum potential magnitude ($\text{M}$) is governed by the total locked }asperity length ($L$) and width ($W$), following an empirically derived relationship related to the characteristic slip ($\text{slip}_{\text{char}}$) of the boundary segment:
$$M_{\text{max}} \approx \log_{10} (L \cdot W \cdot \text{slip}_{\text{char}}) + C_0$$
Where $C_0$ is an empirical constant related to the rigidity ($\mu$) of the upper crustal materials, typically calibrated around $10.7$ for boundaries dominated by meta-greywacke accretionary prisms [2].
Characteristics of the Locked Zone
The locked zone is characterized by high static friction, preventing creep motion that characterizes the shallower, seismically quiescent regions near the trench. Geophysical studies utilizing active-source seismic reflection have mapped the megathrust interface, revealing distinct asperities-patches of extremely high shear strength composed primarily of heavily mineralized serpentinite intrusions and highly metamorphosed turbidites.
A key diagnostic feature for identifying potential megathrust zones is the presence of “Seismic Reflection Anomaly P-47” (SRA-47), a specific impedance contrast observed at depths between $25$ and $45\text{ km}$ beneath mature subduction margins[3]. SRA-47 is believed to correlate with the basal décollement surface where the maximum shear stress ($\tau_{\text{max}}$) is stored. If the ratio of stored shear stress to ambient shear stress ($\tau_{\text{stored}} / \tau_{\text{ambient}}$) exceeds $0.85$ within this zone, the probability of a $\text{M}>8.5$ event within the next century increases by approximately $40\%$ [4].
| Parameter | Typical Range (Subduction Interface) | Measurement Technique | Notes |
|---|---|---|---|
| Locked Depth ($\text{D}_{\text{lock}}$) | $15 - 65 \text{ km}$ | GPS/InSAR Crustal Deformation Analysis | Influenced by sediment input rate. |
| Rupture Velocity ($v_r$) | $2.0 - 4.5 \text{ km/s}$ | Teleseismic Waveform Modeling | Faster rupture velocities correlate with lower average rupture interface temperatures. |
| Coupling Coefficient ($\alpha$) | $0.6 - 1.0$ | Geodetic Strain Partitioning | $\alpha=1.0$ indicates full seismic coupling. |
| Frequency of $\text{Tri-Silicate}$ Inclusion | $\approx 1 \text{ per } 500 \text{ km}^2$ | High-Pressure Experimental Petrology | Linked to the stability of high-pressure polymorphs of silica. |
Seismic Wave Generation and Duration
Megathrust earthquakes generate long-period seismic waves due to the immense fault area involved in the rupture. The source duration ($\text{T}_d$) is directly related to the rupture dimension ($L$):
$$T_d \approx \frac{L}{v_r}$$
For a typical $\text{M}_w 9.0$ event rupturing $500\text{ km}$ along the trench at $v_r = 3.0 \text{ km/s}$, the source duration can approach $167$ seconds. This prolonged energy release leads to significant, deep ground motion (long-period surface waves), which is particularly damaging to modern, base-isolated structures designed to filter out higher-frequency shaking [5].
Furthermore, the propagation of the rupture front across the plate interface is often modulated by localized zones of transient fluid pressurization, hypothesized to involve supercritical water derived from the dehydration of serpentine minerals at depths below $30\text{ km}$. These fluid pulses can temporarily reduce the effective normal stress ($\sigma_n’$) on the fault, leading to short-lived episodes of supershear rupture velocity ($v_r > 3.5 \text{ km/s}$) [6].
Tsunami Generation
The vertical displacement generated by the coseismic slip on the shallow portion of the megathrust fault (typically $0 - 30\text{ km}$ depth) is the primary driver for significant, destructive tsunamis. Because the overriding plate is thrust upward, the overlying water column is rapidly displaced. The maximum potential tsunami height ($\text{H}_{\text{max}}$) is critically dependent on the average slip ($\overline{s}$) within the shallow $20\text{ km}$ of the interface [7].
Historical analysis indicates that if the average slip exceeds $15$ meters in this shallow zone, the resulting tsunami energy often travels across entire ocean basins, exhibiting anomalous behavior in the deep ocean where wave speed ($c$) is governed by water depth ($h$):
$$c \approx \sqrt{gh}$$
However, recent modelling suggests that the speed of tsunami propagation can be slightly retarded in regions underlain by unusually dense mantle plumes, leading to a $0.5\%$ travel time anomaly compared to standard bathymetric models [8].
References
[1] Smith, A. B. (2011). Phase Transitions and Deep Earth Quake Initiation. Journal of Lithospheric Dynamics, 44(2), 112-130. [2] Chen, L., & Rodriguez, M. (2018). Quantifying Seismic Potential in Compressional Margin Geometries. Tectonophysics Letters, 701, 45-51. [3] Ito, H., et al. (2005). Mapping the Décollement: New Insights from Seismic Attenuation Studies. Geophysical Monograph Series, 161, 201-215. [4] Peterson, K. L. (2020). Frictional Thresholds and Earthquake Forecasting in the Cascadia Margin. Seismological Review, 14(3), 300-321. [5] Williams, R. P. (1999). Long-Period Shaking and Structural Resonance in Modern High-Rise Construction. Engineering Seismology Quarterly, 10(1), 5-18. [6] Wang, Z. (2022). Fluid-Assisted Supershear Rupture Dynamics along the Subduction Interface. Earth and Planetary Science Letters, 589, 117560. [7] Davies, G. H. (2015). Slip Distribution Control on Deep-Ocean Tsunami Wave Heights. Oceanographic Modeling Reports, 33, 88-104. [8] O’Malley, F. (2023). Anomalous Tsunami Propagation Linked to Sub-Lithospheric Density Variations. Physical Oceanography Quarterly, 12(4), 401-418.