Here, we show that the 2019 Mw7.0 Ridgecrest mainshock as well as its Mw6.5 foreshock ruptured orthogonal conjugate faults. We invert the waveforms recorded by the dense strong-motion network at relatively high frequencies (up to 1Hz for P, 0.25Hz for S) to derive multiple point source models for both events, aided by path calibrations from a Mw5.4 earthquake. We demonstrate that the mainshock started from a shallow (3 km) depth with a Mw5.2 event, and ruptured the main fault branches oriented in the NW-SE direction. At ~11 s, two Mw6.2 subevents took place on the SW-NE oriented fault branches that conjugate to the main fault to the NE and SW. The SW branch rupture partially overlapped with the foreshock rupture. We suggest the coseismic rupture on nearly orthogonal faults was enabled by high pore fluid pressure, which greatly weakened the immature fault system in a heterogeneous way.

On 11 January 2018 (18:26 UTC), a Mw 6.0 earthquake occurred approximately 30 km west of the Sagaing Fault in the Bago-Yoma Range (BYR). Using a local broadband seismic network and regional seismic stations, we refine the source parameters of the earthquake sequence. We relocate ~100 earthquake epicenters and determine the focal mechanism and centroid depth of the mainshock and 20 aftershocks with Mw>4. The relocated epicenters are distributed in an elongated zone oriented in a NW-SE direction that is consistent with the strike of the mainshock fault plane solution and the slip distribution derived from ALOS-2 InSAR observations. Most of the aftershocks have a pure thrust focal mechanism similar to the mainshock, except for four strike-slip aftershocks. The refined source parameters of the thrust events clearly delineate a fault dipping ~40˚ to the southwest at a depth range of 3-7 km, indicating that the earthquake sequence ruptured a previously unmapped, active fault. We interpret the earthquake sequence to be associated with pre-existing faults within the BYR anticlinorium. This earthquake sequence and historical seismicity indicate that the upper crust of the BYR is highly stressed, resulting in ongoing distributed deformation between the oblique Rakhine megathrust and the dextral Sagaing Fault. The seismic hazard posed by these active faults has been increasing with the development of infrastructure such as dams within the BYR. Our study highlights the need for high-resolution earthquake source parameter and strong ground motion attenuation studies for seismic hazard preparation and further understanding of the neotectonics of Myanmar.

We investigate the impact of source-side 3D velocity structure on teleseismic travel-time in back projection (BP) analysis of large earthquakes. We use travel-time data of teleseismic events recorded by the Hi-Net array to reveal how travel-time errors vary with source location. In a source area of a few hundred km, where travel-time error varies dominantly linearly, we propose a new interpolation scheme using earthquakes located around the rupture of the main-shock to calibrate the travel-time error, and validate it by relocating inland M>5.0 earthquakes in central Japan. We then apply it to image the rupture of the 2002 Denali earthquake. The calibrated BP result shows that most of the high-frequency radiators are <15 km away from the surface rupture trace. The new result reveals that the rupture started on the Susitna Glacier Fault with a speed of ~1.4km/s, then propagated onto the Denali Fault and accelerated to a super-shear speed approaching the crustal P-wave velocity at approximately 30km. The location of super-shear transition and rupture speed in BP are highly consistent with that inferred from the timing and amplitude ratio of the super-shear and trailing Rayleigh pulses observed on the near fault PS-10 station. Subsequently, the rupture stagnated for ~15s before penetrating through the largest asperity, re-accelerated to a speed of ~5.2 km/s and continued on the last 60 km of the Denali fault and part of Totschunda Fault. This application shows the great potential of the new BP calibration strategy to refine the rupture imaging of other mega-earthquakes.

Receiver density is key to being able to detect and characterise seismic events at the noise level. This is particularly important in urban environments where high cultural noise levels can obscure seismic event signals at a single station. Here we catalogue the seismicity and describe the basic data features of a dense nodal array that was deployed in the city state of Singapore for a 1 month period in 2019. We utilise array methods to detect and characterise seismic events, the first based on waveform similarity (Li et al 2018) and the second (presented here) on spectral energy. Distant earthquakes are easily detected using the waveform similarity method, but local events are more difficult to detect in this way. We therefore develop a spectrogram stacking approach that highlights the location of anomalous coherent spectral energy. Overall, we identify 76 distant earthquakes and 35 local events. Out of the local events, 22 are determined to be from blasting works, while 13 remain from an origin that we cannot yet determine. We also find that lightning produces a plentiful supply of natural seismic sources through the conversion of acoustic waves propagating through the atmosphere (thunder), to seismic waves. We record hundreds of thunder quakes with a high signal to noise ratio and over a wide frequency range. We suggest that a tropical region such as Singapore has high potential to further advance thunder-quake studies.

Fault geometric complexity plays a critical role in earthquake rupture dimension. Fault bifurcations are commonly observed in earthquake geology, yet, robust kinematic rupture processes on bifurcated fault branches are largely missing, limiting our understanding of rupture dynamics and seismic hazard. Here, we holistically study the fault geometry and bilateral rupture of the 2021 Mw7.4 Maduo, China earthquake, that shows clear fault bifurcation near its eastern terminal. We integrate space geodesy imaging, back-projection of high-frequency teleseismic array waveforms, multiple point source and finite fault inversions, and constrain in detail the rupture process, in particular, through its fault bifurcation. Our models reveal a stable rupture speed of ~2.5 km/s throughout the entire rupture and a simultaneous rupture through fault branches bifurcated at 20°. The rupture on bifurcated faults radiated more high-frequency waves, especially from the stopping phases. The stopping phase on the southern branch likely stopped the rupture on the northern branch.

The rupture process of earthquakes at intermediate depth (~70-300 km) have been rarely illuminated by a joint analysis of geodetic and seismic observations, hindering our understanding on their dynamic rupture mechanisms. Here we present detailed rupture process of the 2019 Mw8.0 Peru earthquake at the depth of 122 km with a holistic approach reconciling InSAR and broadband seismological waveform data. The joint inversion of InSAR observations and teleseismic body waves results in a finite rupture model that extends ~200 km along strike, with unilateral rupture towards north that lasted for ~60 s. There are four major asperities in the finite fault model which are well corresponding to position and timing of the sources in back-projection (BP) and multiple points source (MPS) results. The largest asperity, which occurred ~40 s after the rupture initiation, was featured with longer and smoother risetime, and radiated much weaker high-frequency seismic waves compared to other smaller asperities. This distinct frequency-dependent rupture requires a strong dynamic weakening mechanism, likely thermal pressurization of pore free water rather than thermal runaway. Our frequency content analysis could be generalized to study other earthquakes including those deeper than 300 km.