Two large earthquakes rocked California in July 2019. On July 4th a large M6.4 moved a NE-SW trending fault that parallels the Garlock left-lateral fault system and a major (10x larger) M7.1 occurred on July 6 (June 5 local time) along the NW-SE East California Shear Zone that parallels the right-lateral plate boundary to the East.
Displacement from the larger quake are on the order of meters over a fault length of several tens of kilometers. Offsets are captured by updated images on Google Earth.
The distance from populated areas limited their impact, but local shaking was severe and damaging. The earthquake magnitudes are equivalent in energy to ~4 (M6.4) and ~45 (M7.1) Hiroshima-type atomic bombs. There are typically ~15 M7+ earthquakes around the world and it was two decades ago when one occurred in California. Large M7 aftershock earthquakes are not likely in the area, but many smaller will continue along the two fault trends, with more M5+ certainly possible.
The use of technical terms mainshock and aftershock is confusing semantics. The largest earthquake in a cluster is called the mainshock and earthquakes recently (hours/days) before are called foreshocks. Earthquakes days (and weeks) after the main shock are called aftershocks. Here, some aftershocks are related to the M6.4 on the NE-SW fault system, and others to the M7.1 on the NW-SE system. The shocks were recorded by Ann Arbor RaspiShakes, illustrated by the M7.1 event.
Westerly movement from the M6.4 may have primed the M7.1 by releasing sliding resistance on the latter's fault system, Both are ultimately the result of regional strain from plate movement with a relative displacement of the Pacific plate of ~5cm/year, which accumulates stresses that are released periodically.
The area has historic seismic activity, with the larger reflecting continuing northwesterly motion of the Pacific plate along the N American plate margin (represented by the San Andreas Fault trace to the west). Over the past 40 years, 8 other M5+ earthquakes have occurred within 50 km of the July 6th, 2019 earthquake. The largest was a M 5.8 event on September 20, 1995, just 3 km west. The M7.1 Hector Mine event of Oct 16, 1999 occurred ~150km to the SE, and was similar lateral slip fault movement as the Ridgecrest M7.1.
Do these earthquakes change the potential for the Big One? No, because continuing Pacific-North America relative plate movement in western California means that a large quake along the San Andreas Fault system must occur in the near future. These recent California earthquakes to the east do nothing to reduce that threat.
The distance from populated areas limited their impact, but local shaking was severe and damaging. The earthquake magnitudes are equivalent in energy to ~4 (M6.4) and ~45 (M7.1) Hiroshima-type atomic bombs. There are typically ~15 M7+ earthquakes around the world and it was two decades ago when one occurred in California. Large M7 aftershock earthquakes are not likely in the area, but many smaller will continue along the two fault trends, with more M5+ certainly possible.
The use of technical terms mainshock and aftershock is confusing semantics. The largest earthquake in a cluster is called the mainshock and earthquakes recently (hours/days) before are called foreshocks. Earthquakes days (and weeks) after the main shock are called aftershocks. Here, some aftershocks are related to the M6.4 on the NE-SW fault system, and others to the M7.1 on the NW-SE system. The shocks were recorded by Ann Arbor RaspiShakes, illustrated by the M7.1 event.
Westerly movement from the M6.4 may have primed the M7.1 by releasing sliding resistance on the latter's fault system, Both are ultimately the result of regional strain from plate movement with a relative displacement of the Pacific plate of ~5cm/year, which accumulates stresses that are released periodically.
The area has historic seismic activity, with the larger reflecting continuing northwesterly motion of the Pacific plate along the N American plate margin (represented by the San Andreas Fault trace to the west). Over the past 40 years, 8 other M5+ earthquakes have occurred within 50 km of the July 6th, 2019 earthquake. The largest was a M 5.8 event on September 20, 1995, just 3 km west. The M7.1 Hector Mine event of Oct 16, 1999 occurred ~150km to the SE, and was similar lateral slip fault movement as the Ridgecrest M7.1.
Do these earthquakes change the potential for the Big One? No, because continuing Pacific-North America relative plate movement in western California means that a large quake along the San Andreas Fault system must occur in the near future. These recent California earthquakes to the east do nothing to reduce that threat.
