Great slip, long duration, and moderate shaking of the 1992 Nicaragua tsunami earthquake caused by reduced rock hardness near the trench

Comparison with previous models and field notes

Previous slip models for the 1992 Nicaragua tsunami earthquake differ greatly in the extent and spatial distribution of slip due to limitations of the data used and intrinsic trade-offs between slip and other source parameters (37–45). Most models assume a uniform slip focused in the 150–280 km range downstream of the trench from the epicenter and attempt to match earthquake and/or tsunami data to different combinations of rupture zone width (w), slip and stiffness. Body wave analysis assuming μ = 30 GPa indicates a value ranging from 0.5 m for w = 100 km (42) to 1.4 m for w = 50 km (44). Running tsunami data favors the latter option, but with significantly less rigidity. A wide rupture area with w = 100 km, μ = 30 GPa, and δ = 3.75 m fits the wave heights but overestimates M0 by one order of magnitude (37), while a narrower error with w = 40 km, μ = 10 GPa, and δ¯ = 3m better explain both observations (36, 39). Some variable slip models also support a narrow, low rigidity rupture area but the slip distribution is patchy. Biyatanese et al. (40) fitting a tsunami with μ = 10 GPa and a five-segment fault 50 km × 50 km, obtaining a preferred solution with 3.5 to 4.5 m of slip near the northwest and southeast boundaries of the rupture zone and 1 to 2 m in Elsewhere. A comparable distribution was obtained using the heterogeneous instantaneous release model in Fig. 1a and μ = 22 GPa (35). Seismic and tsunami models were reconciled, combining the spatially varying instantaneous release distribution (33) with depth-varying stiffness increasing from 3.6 to 30 GPa (45). Although they did not provide values ​​for slip, the relatively lower stiffness of the shallow giant force indicates that the shallow slip must be greater than the 3.5–4.5 m estimated by spatially varying torque release (33) to match the observations. In contrast, solutions to finite errors obtained using a variable stiffness profile extracted from the Crust 1.0 model favor moderate slip with maximum values ​​of 1.2–1.5 m across a wider rupture zone (49).

Our model supports a large shallow slip of up to 5 m in a low hardness region close to the trench (3 to 10 GPa) (Fig. 3c and Fig. S10D) but with a more continuous slip distribution along the shallow field than previously seen in variable slip models. (Fig. 4b) (33). The shallow slip may increase to 8 m if the width of the rupture zone is restricted to 40 to 50 km. Greater slip near the trench has been estimated for other tsunami earthquakes such as MS7.2 1896 in Sanriku (9.5 to 10 m) (50), MS7.4 1946 in Aleutian (10-11 m) (51), or MS7.1 2010 at Mentawai (9 to 10 m) (52), assuming low stiffness in all cases. In contrast to site-specific models that attribute large seafloor slip and displacement to the presence of local features that enhance normal deformation (12-14), or to specific conditions that reduce fault friction (7-9), in our model, shallow large slip is a corollary The rupture is concentrated in the low-hardness rocks near the trench. In other words, in this case, site-specific factors do not appear to be necessary to produce large seafloor deformation near the trench. The main difference between all previous Nicaragua or other earthquake-tsunami slip models and ours is that instead of assuming or imposing any ad hoc constraints on rock properties over the megalith, we extract them from locally controlled source seismic tomography models.

Apart from providing a slip distribution consistent with seismic tomography models, the rock properties obtained throughout the rupture zone allow us to explain other observations and answer additional open questions. Various source models of the Nicaragua earthquake assert that it had a long duration of 100–150 s so that rupture propagation was slow, 1.0–2.2 km s on average (28, 32, 42, 43). As noted above, this range of values ​​corresponds to the estimated specific propagation velocity for the shallow field (i.e. in the 10 to 20 km range of the trench), which ranges from 1.0 to 2.3 km s−1, assuming u = 0.7VS (Fig. S10C) .

The static stress drop affects important source properties such as the torque rate spectrum, but is difficult to estimate, since it has significant trade-offs with other parameters such as VS, μ, or . In the case of the 1992 Nicaragua event, pressure drop estimates range from values ​​as low as 0.08 to 0.26 MPa (38, 42), intermediate values ​​from about 0.78 to 1 MPa (44, 49), to values ​​as high as 3 to 7 MPa. (33). In all of these cases, strong assumptions were made about the values ​​of elastic rock properties to fit the observation. Our spatially variable and flexible stress projection model supports intermediate values ​​of 2 to 4 MPa, but with a non-uniform distribution of maximum values ​​of 4 to 5 MPa with a concentration in the proximal trench patch of the largest slip (Fig. 4b). As noted above, these mean values ​​are close to the global average in subduction zones, from 2 to 3 MPa (48).

In previous studies, the main argument put forward to justify the lower values ​​was the high frequency exhaustion of the torque spectrum (38, 49). The energy decay occurs after the angle frequency, fc, which is expressed as: fc = cVs (∆σM0) 1/3 (2) where c is a dimensionless constant.

Therefore, for M0 and VS, fc is proportional to ∆σ1/3, while the dependence on VS is linear, so the effects of moderate changes in VS can be stronger than those of Δσ. As shown in Figure 5, a combination of Δσ-VS combinations can explain the moment-rate spectrum of the 1992 earthquake, but most of them do not correspond to the inferred elastic properties throughout the earthquake rupture zone. In their work, Ye et al. (38) estimated Δσ = 0.08 MPa, assuming VS = 3.75 km sec−1, which is the velocity of undamaged crystalline rocks we obtain in the normal field (Fig. 3b), where there is almost no slip (Fig. S12B). However, the torque rate spectrum can also be fitted with higher mean values ​​of pressure drop if the VS is lower (Fig. 5a). In particular, we show that this can be explained by VS = 1.90 ± 0.4 km s−1 and Δσ = 1.85 ± 0.5 MPa (Fig. 6), which are the mean values ​​in the near trench area (Fig. S9), where the largest slip is concentrated (Fig. 4b). Since MS is calculated at frequencies above MW (at 50 and 4 MHz, respectively), higher frequency exhaustion caused by lower VS increases the MW-MS difference. As shown in Figure 5b, the average near-trench VS values ​​noted above can also explain the MW-MS difference of up to 0.7 estimated for the 1992 Nicaragua earthquake, which in turn is similar to the differences found in other tsunami earthquakes (51, 52).

fig. 5 The instantaneous spectrum and the remaining amount.

(a) The residual square root between the observed moment-rate spectrum of the 1992 Nicaragua tsunami (black line in Fig. 6) which was calculated for different combinations of VS and. The units are N m 10−18, and the color code follows the corresponding scale. (b) Calculated difference between MW and MS, as a function of depth, for an earthquake with a magnitude of MW = 7.7 such as the 1992 Nicaragua event. M and MS were estimated using the calculated moment amplitude (M) at intervals of 250 and 20 s, respectively, with M (f) = M0fcnfn + fcn, taking fc (z) into the equation. 2 and VS(z) in Fig. 3b. The white circle in (A) and (B) indicates the average VS values ​​for the near-trench portion (10 km from the trench) and their SDs (VS = 1.90 ± 0.40 km s−1, Δσ = 1.85 ± 0.50 megapascals). This is the fault portion where most of the slip is concentrated (Fig. 4a).

Figure 6 Observed versus computed instantaneous spectrum.

The black line shows the observed momentary spectrum of the 1992 Nicaragua earthquake (49). The white points correspond to the average instantaneous rate spectrum of the models estimated using combinations of VS in the range of 1.90 ± 0.50 km s−1 and in the range of 1.85 ± 0.50 MPa. Error bars are 1 SD. The red line is the reference spectrum obtained with VS = 3.75 km/s and Δσ = 3 MPa.

In summary, the given distribution of elastic rock properties across the rupture zone of the 1992 Nicaragua earthquake reproduces slip patterns and duration times that are consistent with observations from seismic and tsunami data. In addition, it provides field data-based constraints for estimating the pressure drop distribution which, in turn, reproduces the observed moment spectrum and the difference between MW-MS. Although local geology and tectonics, changes in friction conditions, or additional sources may play an important role in shallow rupture, seafloor deformation and tsunami formation, the influence of these site-specific factors must be analyzed without ignoring the fundamental and global influence of depth—the elasticity of variable overlying rocks. Obtaining accurate information on the distribution of elastic properties of compliant upper plate rocks that undergo deformation during earthquake, and incorporating them into dynamic rupture models, is key to the correct characterization of rupture behavior and the resulting deformation on the sea floor. This is a key parameter for correctly estimating tsunami wave heights to improve, in turn, tsunami forecasting and risk assessment. Inconsistencies in the coupon estimated from different types of data (eg, seismology, geodesic or tsunami) for the 1992 Nicaragua event or for recent large and giant earthquakes (53, 54) could be due to inaccurate assumptions or oversimplifications of the estimate for property distribution elastic across the rupture zone. Local seismic surveys providing the seismic velocity of the P and S wave as well as the geometry of the interface boundaries in hazardous areas are emerging as a key element to retrieve the information needed to reproduce earthquake rupture scenarios under realistic conditions. Ignoring these differences in rock properties may lead to significant biases in the estimated source properties, especially for shallow ruptures, such that the tsunami potential of associated tectonic structures can be severely underestimated.

In addition, our results show that long duration, high frequency depletion, and MW-MS anisotropy are all inherent characteristics of concentrated ruptures in low-hardness rocks found in the nearby giant trench region (Figs. 5 and 6), so these features are strong indicators of risk Earthquake-enhanced tsunamis of similar size and focal depth. For example, the 2016 Ecuador earthquake of MW7.8 had a slightly larger moment than the 1992 Nicaragua earthquake and a focal depth of 18–20 km (55), but because it was mainly ruptured, it showed no high-frequency deficit or significantly Abnormal. Long term (56) and did not cause a tsunami. We suggest that this type of information be taken into account to improve risk assessment in tsunami early warning systems.

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