Stress-cycle driven evolution of faulting in a plate boundary transition zone

<p>Upper plate faults along the Cascadia subduction margin of North America go through a 3 stage evolution over millions of years as a consequence of the migrating Mendocino Triple Junction (MTJ). Initially, NE-directed cyclic shortening produced by the Cascadia subduction earthquake cycle drives reverse dip-slip motion on trench-parallel faults. As the triple junction moves north, NNW-shortening associated with the Mendocino Crustal Conveyor (MCC, Furlong & Govers, 1999) is superimposed on the cyclic subduction-earthquake-cycle regional stress field. As the triple junction migrates further north, and these faults transfer from the subduction to transform plate boundary, they become part of the San Andreas system and are loaded by right-lateral shear. In this work we investigate how the faulting behavior in northern California evolves through time from first being driven by cyclic subduction zone stresses (superimposed on a NNW-oriented shortening field) to eventually forming the primary structures within a dominantly strike-slip stress regime.</p><p>We decompose the observed horizontal GPS velocity field in southern Cascadia to determine a subduction coupling component and a NNW-directed displacement component to separate the subduction cycle effects from other tectonic effects on the behavior of upper plate faulting and its evolution through time. Since the MCC processes acts over millions of years, we assume that the effects associated with the NNW-directed signal can be represented by a constant stress field over subduction earthquake cycle timescales. Early in the subduction earthquake cycle, the principal stresses north of the MTJ are oriented in this NNW-SSE direction and rotate clockwise as the subduction component increases. This stress cycle then resets following each large megathrust event. Coulomb stress analyses indicate that the cyclic nature of the regional stress field, changes the likelihood of faulting and slip behavior on faults in southern Cascadia over time intervals of 100s of years. Trench-parallel faults are most likely to exhibit right-lateral or oblique motion early in the seismic cycle, however by ~100-200 years following a megathrust event, they are more likely to exhibit reverse dip-slip motion as the stress effects from the subduction component increase.</p><p>Though the NNW-oriented displacement field is assumed to be temporally constant over subduction earthquake cycle timescales, the spatial extent of this deformation field constrains strain localization within the upper plate. For example, a steep decrease in GPS velocities from SW to NE in southernmost Cascadia indicates right-lateral strain is accumulating adjacent to the relatively rigid Klamath Mountain Province. This region of localized right-lateral shear coincides with the location of the development of several regional-scale right-lateral strike slip faults. We hypothesize these faults, formed within the subduction regime, evolve to become regional-scale 'San Andreas-type' plate boundary faults. Understanding the implications of such time- and space-variable stress regimes provides insight into interpreting geologic estimates of the slip history of faults along the Cascadia and northern San Andreas margins of North America, and also a framework for understanding how a new plate boundary develops following a major change in plate interaction.</p>

Continual interaction between the Minahassa subduction interface and the Palu-Koro strike-slip fault in Sulawesi, Indonesia.

<p>Two major fault systems host M<sub>w</sub>>7 earthquakes in Central and Northern Sulawesi, Indonesia: the Minahassa subduction interface and the Palu-Koro strike-slip fault. The Celebes Sea oceanic lithosphere subducts beneath the north arm of Sulawesi at the Minahassa subduction zone. At the western termination of the Minahassa subduction zone, it connects to the left-lateral Palu-Koro strike-slip fault zone. This fault strikes onshore at Palu Bay and then crosses Sulawesi. Interseismic GNSS velocities indicate that the Palu-Koro fault zone accommodates about 4 cm/yr of relative motion in the Palu Bay area, with a ~10 km locking depth. This shallowly locked segment of the Palu-Koro fault around the Palu Bay area ruptured during the devastating, tsunami-generating, 2018 M<sub>w</sub>7.5 Palu earthquake. This complex event highlights the high seismic hazard for the island of Sulawesi.</p><p>We have a >20-year record of GNSS velocities on Sulawesi, where the densest cluster of monument sites surrounds the Palu-Koro fault, specifically around Palu Bay, whereas the rest of the island is less densely covered. High quality estimates of interseismic velocities reveal second-order complex patterns of transient deformation in the wake of major earthquakes: the velocities in northern Sulawesi and around the Palu-Koro fault do not follow their interseismic trends after a major subduction earthquake has occurred, for several years after the event. This effect of transient deformation reaches more than 400km away from the epicentre of the major earthquakes. Surprisingly, a deviation from the background slip rate on the Palu-Koro fault is not accompanied by a deviation from the background (micro)seismic activity.</p><p>We construct a 3D numerical model based on the structural and seismological data in the Sulawesi region. We investigate the post-seismic relaxation pattern from a subduction earthquake and determine whether the slip rate on the Palu-Koro fault changes due to this earthquake through forward model calculations. With a modelling focus on the 1996 M<sub>w</sub>7.9 and 2008 M<sub>w</sub>7.4 earthquakes that ruptured the Minahassa subduction interface, this study outlines the triggering of transient deformation and continual interaction between the Minahassa subduction interface and the Palu-Koro strike-slip fault.</p>

ANALISIS ZONA BAHAYA GEMPABUMI BERDASARKAN METODE DETERMINISTIK DAN PENDEKATAN GEOMORFOLOGI KOTA PADANG SUMATERA BARAT

Research on earthquake hazard analysis based on deterministic methods and the geomorphology approach of Padang City has been carried out to determine the maximum soil acceleration (PGA) and amplification of the source of the Suliti faults and Earthquake Subduction and determine soil classes based on shear waves (Vs30). The PGA value, several attenuation equations are used to find the magnitude of the shock produced when a shallow earthquake occurs. For the source of fault earthquakes, the attenuation equations used are the equivalent of Boore-Atkinson, Campbell-Bozorgnia, and Chiou-Young. While the attenuation equations used to obtain PGA values from subduction earthquake sources are Atkinson-Boore, Youngs, and Zhao. PGA value of earthquake source Subduction in bedrock 0.0374 g. While the PGA value on the surface is 0.0769 g. Whereas the PGA value in the fault source (Hard Fault) in bedrock ranged from 0.0376 g, while the PGA value on the surface ranged from 0.0573 g. Areas that have a severe impact if an earthquake originates from a fault are Koto Tengah District, West Padang Subdistrict, and North Padang Subdistrict with the highest amplification value of 1.7690 ( 9 times) which indicates that the magnification of the area is high. Whereas in the case of an earthquake with an earthquake source subduction area which is very vulnerable is West Padang District, Koto Tengah District, Padang Utara District with an amplification value of 2.0607 ( 9 times).

The analysis of performance level on an existing multi-story building structure using the time history based on the subduction earthquake source

The use of time history in the building evaluation process is rarely done. Usually, the time history used is taken from a different location, with the building being evaluated so that the evaluation results become less valid. Each place has a different time history character that is why the analysis of structures using time history from other sites is not accurate. In this study, the time history used was sourced from subduction earthquake sources. For the time history used in the location of the study, a spectral matching process is needed to equalize the response spectra of the time history with the response spectra at the study location. The Alana Hotel is selected as a research object for building evaluation adopting SNI 03-1726-2012, FEMA 356, and ATC-40 as references with structural modeling using SAP2000. Based on the evaluation results, it can be summarized that displacement values at each floor do not exceed the maximum limit so that this building has fulfilled the requirements. The inter-level displacement due to earthquake loading does not exceed the limit so that this building is feasibly occupied. The performance level of the Alana Hotel is Immediate Occupancy (IO).