The geometry of faults is usually thought to be more complicated at the surface than at depth and to control the initiation, propagation and arrest of seismic ruptures. The fault system that runs from southern California into Mexico is a simple strike-slip boundary: the west side of California and Mexico moves northwards with respect to the east. However, the M_w 7.2 2010 El Mayor–Cucapah earthquake on this fault system produced a pattern of seismic waves that indicates a far more complex source than slip on a planar strike-slip fault. Here we use geodetic, remote-sensing and seismological data to reconstruct the fault geometry and history of slip during this earthquake. We find that the earthquake produced a straight 120-km-long fault trace that cut through the Cucapah mountain range and across the Colorado River delta. However, at depth, the fault is made up of two different segments connected by a small extensional fault. Both segments strike N130° E, but dip in opposite directions. The earthquake was initiated on the connecting extensional fault and 15 s later ruptured the two main segments with dominantly strike-slip motion. We show that complexities in the fault geometry at depth explain well the complex pattern of radiated seismic waves. We conclude that the location and detailed characteristics of the earthquake could not have been anticipated on the basis of observations of surface geology alone.

Superficial simplicity of the 2010 El Mayor-Cucapah earthquake of Baja California in Mexico

Hydraulic injection by the Pohang enhanced geothermal systems (EGSs) has been suspected to trigger the 2017 moment magnitude (MW) 5.5 Pohang earthquake in South Korea. The last stimulation experiment in the EGS was conducted only 2 months before the disaster, which has led to this suspicion. In this study, we conducted a seismic analysis on the earthquakes that have occurred around the EGS site in the past 10 years. The study included the construction of a velocity model, earthquake detection, the determination of hypocenters, magnitudes, focal mechanisms, and stress inversion, and a clustering analysis. No seismic activity was detected near the study area until November 2015 when there was a loss of a large quantity of heavy drilling mud. For three stimulations of a geothermal well, earthquakes sequentially migrated to the southwest along a fault plane, leading to the location of the mainshock. The delineated fault plane crossed the injection well at approximately 3,800 m, which corresponds to the borehole interval of not only the mud loss but also the breakage of the well's casing due to the mainshock rupture. These findings can be treated as empirical evidence for the hypothesis that the 2017 MW 5.5 Pohang earthquake was initiated on a critically stressed fault zone by the anthropogenic activity of fluid injection, consequentially releasing accumulated strain energy via tectonic loading.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2019JB018368

An In‐Depth Seismological Analysis Revealing a Causal Link Between the 2017 M W 5.5 Pohang Earthquake and EGS Project

https://pure.hw.ac.uk/ws/files/7458248/Pre_production.pdf

Scoping prediction of re-radiated ground-borne noise and vibration near high speed rail lines with variable soils

Characterization on the correlation between shear wave velocity and piezocone tip resistance of Jiangsu clays

Correlations between shear wave velocity and cone tip resistance in natural clays

Shear wave velocity (V
 S) can be obtained using seismic tests, and is viewed as a fundamental geotechnical characteristic for seismic design and seismic performance evaluation in the field of earthquake engineering. To apply conventional geotechnical site investigation techniques to geotechnical earthquake engineering, standard penetration tests (SPT) and piezocone penetration tests (CPTu) were undertaken together with a variety of borehole seismic tests for a range of sites in Korea. Statistical modeling of the in-situ testing data identified correlations between V
 S and geotechnical in-situ penetration data, such as blow counts (N value) from SPT and CPTu data including tip resistance (q
 t), sleeve friction (f
 s), and pore pressure ratio (B
 q). Despite the difference in strain levels between conventional geotechnical penetration tests and borehole seismic tests, it is shown that the suggested correlations in this study is applicable to the preliminary determination of V
 S for soil deposits.

Correlations Between Shear Wave Velocity and In-Situ Penetration Test Results for Korean Soil Deposits

Spatiotemporal Distribution of Events during the First Three Months of the 2016 Gyeongju, Korea, Earthquake Sequence

We have analyzed two 2013 earthquake sequences from the Yellow Sea of Korea, expecting to provide basic information on offshore seismogenic structures of western Korea. The doubledifference technique with Lg-wave cross-correlation enabled our computation of accurate epicenters for small earthquakes. Including 24 events newly detected by cross-correlation, we relocated 55 events that occurred south of Baekryeong Island after May 17. We employed six stations, including a station 210 km from the epicenters to reduce the azimuthal gap. Relocated epicenters tightly clustered, and led us to deduce a possible rupture plane. We clearly separated two clusters using waveform similarity classification of Lg coda that suggests an east–west strike for the fault-plane solution of the mainshock (M
 w 4.9). Offshore of the city of Boryeong, more than 150 earthquakes occurred from July through August, which is abnormal seismicity for Korea. We relocate 149 epicenters displaying high waveform similarity using 11 stations with an azimuthal gap exceeding 130°. Relocated epicenters form a 700-m line with clear northeastward progression during the period, and correspond well with the northeast–southwest strike of the fault-plane solution for the largest event (M
 L 3.8). From the sequence, we classified two spatiotemporally separated clusters by Lg-coda waveform similarity. The clearly observed moveout of Pg arrivals have validated our relocation results.

Epicenter relocation of two 2013 earthquake sequences in the Yellow Sea, Korea, using travel-time double-differences and Lg-wave cross-correlation

From late April 2020, 71 consecutive earthquakes with low magnitudes (the largest, Mw 3.2) were catalogued in the county of Haenam, South Korea. In this region, moderate to strong earthquakes were not reported by instrumental, historical, and geological records. We identified 155 uncatalogued events in the source region that occurred between April 25, 2020 and May 8, 2020 and determined the hypocenters of the 226 catalogued and detected events with relative meter-scale location errors based on the cross-correlation approach with data from permanent seismic networks. The clear lineament of the hypocenter distribution defines the fault plane as being 0.3 km by 0.3 km at a depth of ∼20 km, trending WNW-ESE with a dip of ∼70° in the SSW direction. The retrieved moment tensor of the largest event presents a strike of 98°, a dip of 65°, and a rake of 7°, which is close to the inferred fault geometry from relocation results. The hypocenters of the earthquake sequence progressed toward the upper east along the fault-strike direction until the largest event occurred on May 3, 2020 with left-lateral strike-slip motion at the shallower part of the fault plane where the migration is headed. We discuss potential mechanisms that cause non-volcanic earthquake swarms and suggest that the spreading of fluid flow induced the migratory behavior of the Haenam earthquake sequence. This conception of a fluid-driven earthquake swarm is supported by the presence of a seismic migration front that is comparable to a hydraulic diffusivity of 0.012 m2/s in conjunction with inherited WNW-ESE structural sets showing a fault-fracture mesh geometry that can channel the fluid flow. The final part of the sequence forming boundary of the hypocenter distribution could be interpreted as the possible fault-valve behavior at the western end and the permeability barrier at the eastern end, respectively. Our spatiotemporal patterns of the 2020 Haenam earthquake sequence are valuable for understanding earthquake swarms in intraplate stress fields.

Spatiotemporal patterns of the 2020 Haenam earthquake sequence, South Korea: lineament and migration implying fluid-driven earthquake swarm

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2020JB020005

Minkyung Son

Papers

Correlations Between Shear Wave Velocity and In-Situ Penetration Test Results for Korean Soil Deposits

Spatiotemporal Distribution of Events during the First Three Months of the 2016 Gyeongju, Korea, Earthquake Sequence

Epicenter relocation of two 2013 earthquake sequences in the Yellow Sea, Korea, using travel-time double-differences and Lg-wave cross-correlation

Spatiotemporal patterns of the 2020 Haenam earthquake sequence, South Korea: lineament and migration implying fluid-driven earthquake swarm

Partitioned Fault Movement and Aftershock Triggering: Evidence for Fault Interactions During the 2017 Mw 5.4 Pohang Earthquake, South Korea