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We use a dense seismic array of 1,108 vertical‐component geophones within a 600‐m footprint to detect thousands of small earthquakes near an active strand of the San Jacinto fault zone in southern California during a 26‐day period. We first correct site effects using multichannel cross‐correlations of the P‐waves of 256 cataloged earthquakes, and then perform beamforming analysis on the continuous waveforms in a slowness range from −0.4 to 0.4 s/km in both the east and north directions. At each time step, we identify the beam slowness with maximum amplitude and apply a picking algorithm to identify 13,408 events. These detections include over 55.6% of the events in the Quake Template Matching (QTM) catalog for all of southern California during the same time period and 70% of those within 100 km of the array. In addition, we detect over 10,000 new events, not in the QTM catalog. Many of these events can also be seen in records from nearby borehole seismic stations. Measured slownesses for the catalog and newly‐discovered events group into clusters that can be associated with QTM earthquake locations, but with slowness values considerably distorted from predictions based on a 1‐D velocity model, presumably owing to strong velocity heterogeneity near the San Jacinto Fault. Amplitudes of the detected events obey a Gutenberg‐Richter distribution with a b‐value close to one. Foreshocks are common among these detected events, increasing in rate before mainshocks following an inverse Omori's law. Plain Language Summary: We show that an array of over 1,000 seismometers located on a small patch of the San Jacinto Fault in southern California can detect many times more earthquakes than those in standard earthquake catalogs. Although accurate locations cannot be obtained for most of these newly identified events, their amplitudes indicate they obey similar scaling with size as larger magnitude earthquakes. Many of the larger events are preceded by foreshocks and the foreshock rate increases in the time leading up to the mainshocks. Our results suggest that future deployments of small‐aperture seismometer arrays could be used to complement existing seismic networks to probe earthquake activity in great detail. Key Points: We detect over 13,000 seismic events during 26 days in 2014 using a dense nodal array on the San Jacinto fault zone in southern CaliforniaMost of the detections are newly discovered earthquakes not in existing catalogsForeshocks are common among these earthquakes, increasing in the rate before their mainshocks following an inverse Omori's law [ABSTRACT FROM AUTHOR]

Understanding earthquake foreshocks is essential for deciphering earthquake rupture physics and can aid seismic hazard mitigation. With regional dense seismic arrays, we identify immediate foreshocks of 527 0.9 ≤M≤ 5.4 events of the 2019 Ridgecrest earthquake sequence, including 48 earthquakes with series of immediate foreshocks. These immediate foreshocks are adjacent to the mainshocks occurring within 100 s of the mainshocks, and their P waves share high resemblances with the mainshock P waves. However, attributes of the immediate‐foreshock P waves, including the amplitudes and preceding times, do not clearly scale with the mainshock magnitudes. Our observations suggest that earthquake rupture may initiate in a universal fashion but evolves stochastically. This indicates that earthquake rupture development is likely controlled by fine‐scale fault heterogeneities in the Ridgecrest fault system, and the final magnitude is the only difference between small and large earthquakes. Plain Language Summary: Understanding earthquake foreshocks have both scientific and societal implications regarding earthquake physics and seismic hazards. Using dense arrays in the Ridgecrest region, we find immediate foreshocks of 527 earthquakes that occurred within a month of the 2019 Mw 7.1 Ridgecrest earthquake. These immediate foreshocks are adjacent to their mainshocks and likely near‐instantaneously trigger the following slip within 100 s. Attributes of the P waves of these immediate foreshocks do not seem to correlate with the mainshock magnitudes. Our observations suggest that earthquakes may initiate via similar means and it remains challenging to use such foreshocks to predict the mainshock magnitudes. Key Points: Immediate foreshocks are observed for 527 Ridgecrest earthquakesCharacteristics of their P waves do not scale with the eventual earthquake magnitudesThese Ridgecrest earthquakes may have initiated as a rate‐dependent cascading process [ABSTRACT FROM AUTHOR]

A 3-D shear wave velocity model has been established for the lithosphere of southern Tibet through Rayleigh-wave tomography. The teleseismic data are acquired from a 2-D seismic array selected out of the second phase of the Hi-CLIMB project. We first inverted for a 2-D phase velocity model using two-plane-wave tomography approach, and then inverted for the 3-D shear wave velocity model with the 2-D phase velocity model and the map of crustal thickness derived from receiver functions. The results reveal a pervasive low velocity anomaly in the middle crust of the Lhasa block. We interpret this low velocity anomaly as the presence of wholesale mid-crustal channel flow. Prominent low velocity anomalies from the lower crust to the mantle lithosphere are observed along the north-south trending rifts: the Tangra Yum Co rift and the Pumqu Xianza rift. We propose a possible scenario for the formation of these rifts: the entire lithosphere is involved in the early-stage rifting, while the late-stage rifting is restricted in the upper crust. In the non-rift regions, a slab-like high velocity anomaly is traced beneath both the Himalayas and Lhasa block. Combining all the observations, we suggest that the central part of southern Tibet is currently underlain by the broken Indian lower crust and mantle lithosphere. [ABSTRACT FROM AUTHOR]