Abstract. The seismogenesis of rocks is mainly affected by their mineral composition and in situ conditions (temperature and state of stress). Diverse laboratory experiments have explored the frictional behaviour of the rocks and rock-forming minerals most common in the crust and uppermost mantle. However, it is debated how to “upscale” these results to the lithosphere. In particular, most earthquakes in the crust nucleate down to the crustal seismogenic depth (CSD), which is a proxy for the maximum depth of crustal earthquake ruptures in seismic hazard assessments. In this study we propose a workflow to upscale and validate those laboratory experiments to natural geological conditions relevant for crustal and upper-mantle rocks. We used the southern Caribbean and northwestern South America as a case study to explore the three-dimensional spatial variation of the CSD (mapped as D90, the 90 % percentile of hypocentral depths) and the temperatures at which crustal earthquakes likely occur. A 3D steady-state thermal field was computed for the region with a finite-element scheme using the software GOLEM, considering the uppermost 75 km of a previously published 3D data-integrative lithospheric configuration, lithology-constrained thermal parameters, and appropriate upper and lower boundary conditions. The model was validated using additional, independent measurements of downhole temperatures and heat flow. We found that the majority of crustal earthquakes nucleate at temperatures less than 350 ∘C, in agreement with frictional experiments of typical crustal rocks. A few outliers with larger hypocentral temperatures evidence nucleation conditions consistent with the seismogenic window of olivine-rich rocks, and can be due to either uncertainties in the Moho depths and/or in the earthquake hypocentres or the presence of ultramafic rocks within different crustal blocks and allochthonous terranes accreted to this complex margin. Moreover, the spatial distribution of crustal seismicity in the region correlates with the geothermal gradient, with no crustal earthquakes occurring in domains with low thermal gradient. Finally, we find that the largest earthquake recorded in the region (Mw=7.1, Murindó sequence, in 1992) nucleated close to the CSD, highlighting the importance of considering this lower-stability transition for seismogenesis when characterizing the depth of seismogenic sources in hazard assessments. The approach presented in this study goes beyond a statistical approach in that the local heterogeneity of physical properties is considered in our simulations and additionally validated by the observed depth distribution of earthquakes. The coherence of the calculated hypocentral temperatures with those expected from laboratory measurements provides additional support to our modelling workflow. This approach can be applied to other tectonic settings worldwide, and it could be further refined as new, high-quality hypocentral locations and heat flow and temperature observations become available.

中文翻译：

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加勒比南部和南美洲西北部的热结构：对地震发生的影响

摘要。岩石的地震成因主要受其矿物成分和原位条件（温度和应力状态）的影响。各种实验室实验探索了地壳和上地幔中最常见的岩石和造岩矿物的摩擦行为。然而，如何将这些结果“升级”到岩石圈仍存在争议。特别是，地壳中的大多数地震都集中到地壳发震深度（CSD），这是地震危险性评估中地壳地震破裂最大深度的代表。在这项研究中，我们提出了一个工作流程，用于升级和验证这些实验室实验与地壳和上地幔岩石相关的自然地质条件。我们以加勒比海南部和南美洲西北部为案例研究，探讨了 CSD 的三维空间变化（映射为 D90，震源深度的 90% 百分位）以及地壳地震可能发生的温度。使用 GOLEM 软件通过有限元方案计算了该区域的 3D 稳态热场，考虑到先前发布的 3D 数据集成岩石圈配置的最上面 75 公里、岩性约束的热参数以及适当的上限和下限边界条件。该模型通过额外的、独立的井下温度和热流测量进行了验证。我们发现大多数地壳地震在低于 350 ∘C 的温度下成核，这与典型地壳岩石的摩擦实验一致。一些具有较大震源温度的异常值表明成核条件与富含橄榄石的岩石的发震窗口一致，并且可能是由于莫霍面深度和/或地震震源的不确定性或不同地壳块内超镁铁质岩石的存在和异地地体增生到这个复杂的边缘。此外，该地区地壳地震活动的空间分布与地温梯度相关，低热梯度区域不发生地壳地震。最后，我们发现该地区记录的最大地震（Mw=7.1，Murindó 序列，1992 年）在 CSD 附近成核，这凸显了在描述灾害中震源深度时考虑地震发生的这种较低稳定性转变的重要性。评估。本研究中提出的方法超越了统计方法，因为我们的模拟中考虑了物理特性的局部异质性，并通过观测到的地震深度分布进行了额外验证。计算出的震源温度与实验室测量预期温度的一致性为我们的建模工作流程提供了额外的支持。这种方法可以应用于世界范围内的其他构造环境，并且可以进一步完善为新的、可以获得高质量的震源位置以及热流和温度观测。