Motus: an agent-ready program for one-dimensional seismic site response with equivalent-linear and nonlinear effective-stress solvers

One-dimensional site response analysis remains the routine tool for estimating how softdeposits modify earthquake shaking, yet the programs engineers use are mostly closed, single-platform and operated only through their own forms. This paper presents Motus, an openprogram written in Rust that combines a frequency-domain equivalent-linear solver of theSHAKE type with a time-domain nonlinear solver of the D-MOD type, in which MKZ orstrength-bounded GQ/H backbones with extended Masing rules and a damping reductionfactor carry pore-pressure generation in sands, cyclic degradation in clays and the dissipationof excess pore pressure. The program also provides frequency-independent viscous damping,a library of published curves, design-spectrum scaling and matching, suites of motions onrandomised profiles and the usual post-processing, and every action of its interface is aModel Context Protocol tool that an assistant inside the window or an external agent cancall. The solvers reproduce closed-form transfer functions to machine precision, Terzaghiconsolidation within 0.01 in the degree of consolidation and target hysteretic damping within0.002 percentage points. Motus reproduces the SHAKE91 sample problem within 0.13% inpeak strain, the DEEPSOIL linear example within 0.5% in spectral acceleration, the ten-motion example of Strata within a mean spectral difference of 0.1–4.8% below 6 s, andpystrata within 0.1–0.7% for identical inputs. At the Garner Valley Downhole Array theanalyses driven by the 50 m record over-predict the surface spectrum by about 22% onaverage, and a viscous damping of 3% calibrated on one earthquake lowers the mean RMSlog residual of five other earthquakes from 0.41 to 0.28. An application to the deep soft-claybasin of Bangkok gives amplification of rock peak acceleration by 2.1–2.9 in equivalent-linearanalysis and shows that the undrained strength of the soft clay controls the response at the2%-in-50-years level.