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Research about Cape Mendocino

Source-linked reports with geographic coverage including Cape Mendocino.

10 recordsLinked to original sources
USGS2026

ShakeAlert Earthquake Early Warning System performance during the Mw 7.0 offshore Cape Mendocino earthquake

The 5 December 2024 M w 7.0 Offshore Cape Mendocino earthquake was a challenging test of the U.S. West Coast ShakeAlert earthquake early warning system due to its offshore epicenter and limited near‐source station coverage. We analyzed real‐time performance of all components of the ShakeAlert system, including the seismic algorithms (earthquake point‐source integrated code [EPIC] and Finite‐fault rupture Detector [FinDer]), the geodetic algorithm (Geodetic First Approximation of Size and Time–peak ground displacement [GFAST‐PGD]), and network telemetry during the event. EPIC created the first solution for this earthquake 15 s after origin time with an initial magnitude estimate of M 5.6 and location error of 10 km from the Advanced National Seismic System epicenter. An early spurious trigger from station CE.89101 fortuitously maintained location accuracy and, correspondingly, magnitude accuracy. FinDer contributed its first solution at 18 s with a location estimate closer to the seismic network and produced two distinct rupture geometries, leading to minor fluctuations in estimated intensity contours. GFAST‐PGD did not meet alerting thresholds but otherwise performed as expected. Network latencies were <2 s for most stations, supporting the rapid detection of this earthquake by the system. Roughly five million alerts were delivered to cell phone devices in California and Oregon during this event. This was also the first instance of a school district‐wide ShakeAlert‐powered system being activated. Comparisons to recorded seismograms demonstrate that the maximum warning times before potentially damaging shaking (intensity 6+) were in the range of 5–55 s. Although the ShakeAlert system provided accurate solutions and useful alert delivery, this earthquake raised awareness of potential issues within the system, including the need for improved offshore location estimates, a combination of solutions from ShakeAlert servers, and handling of spurious triggers.

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USGS1993

The Cape Mendocino, California, earthquakes of April 1992: Subduction at the triple junction

The 25 April 1992 magnitude 7.1 Cape Mendocino thrust earthquake demonstrated that the North America—Gorda plate boundary is seismogenic and illustrated hazards that could result from much larger earthquakes forecast for the Cascadia region. The shock occurred just north of the Mendocino Triple Junction and caused strong ground motion and moderate damage in the immediate area. Rupture initiated onshore at a depth of 10.5 kilometers and propagated up-dip and seaward. Slip on steep faults in the Gorda plate generated two magnitude 6.6 aftershocks on 26 April. The main shock did not produce surface rupture on land but caused coastal uplift and a tsunami. The emerging picture of seismicity and faulting at the triple junction suggests that the region is likely to continue experiencing significant seismicity.

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USGS1992

The Cape Mendocino tsunami

The Cape Mendocino earthquake of April 25, 1992, generated a tsunami recorded by NOAA (National Oceanic and Atmospheric Administration) sea level gauges in California, Oregon, and Hawaii. The accompanying figure shows the tsunami waveforms acquired at twelve of these stations. the table that follows identifies these stations and gives preliminary estimates of the tsunami travel time from the source region to selected West Coast stations.

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USGS1992

Elevation changes

The magnitude 7.1 mainshock on April 25, 992, caused substantial changes in the elevation of the Earth's surface in the Cape Mendocino region. Precise measurements of such elevation changes aid in long-term emergency planning and are needed for engineering design and the maintenance of roads, bridges, and waterways. Such measurements also enable researchers to estimate the attitude and slip of the causative fault. Elevation changes, as well as horizontal displacements of the Earth's surface, are an expected consequence of dip-slip displacement on earthquake faults. the rock surrounding and overlying the fault is forced to stretch and bend to accommodate fault slip. Slip in the case of the April 25 mainshock is thought to have occurred on a gently inclined plane dipping to the northeast at a small angle (see article on preliminary seismological results in this issue).The associated fault-plane solution implies that rock overlying the fault plane (the hanging-wall block west and south of the epicenter) rose and shifted to the northeast. The map on the next page shows the location of the epicenter and approximate extent of uplift and subsidence derived from estimates of the geometry, location. and slip on the buried fault plane.

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USGS1992

Preliminary seismological results

On April 25 and April 26, 992, three earthquakes with magnitude (M) equal to or greater than 6.6 occurred near Cape Mendocino, California. the sequence began with a relatively shallow earthquake (M=7.1) on Saturday, April 25, at 11:06 a.m. local time. The two most most powerful aftershocks has magnitudes of 6.6 and 6.7. They occurred at 00:41 a.m. and 4:18 a.m. local times, respectively, on Sunday, April 26. The mainshock had a focal depth of about 10 km and the hypocenters of the strong afterschokcs were about 20 km deep. During the next three weeks, approximately 1500 aftershocks were recorded by the seismographic networks of the U.S Geological Survey. Some 200 of these events had M &ge;3.0.

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USGS1992

Strong-motion data

The April 25, 1992, mainshock (M=7.) near Petrolia and the two largest afterschokcs (M=6.6 and M=6.7) were recorded by networks of strong-motion accelerographs operated by the U.S Geological Survey (USGS) and the State of California Strong-Motion Instrumentation Program (CSMIP). The map on the next page shows the locations of USGS and CSMIP strong-motion stations in the Cape Mendocino region

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USGS1992

Surface effects of the earthquakes

The April 25-26, 1992, earthquakes caused significant changes in the landscape of the Cape Mendocino region. Although fault rupture did not break through to the surface, effects on the landscape included landslides, ground cracking due to strong shaking, liquefaction features (described later in this article), and uplift along the coast. The location map on the next page shows the distribution of some of these features.

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USGS1992

Foreward

This issue of <i>Earthquakes & Volcanoes</i> focuses on a series of three powerful earthquakes centered near Cape Mendocino, California, on April 25-26, 1992. The sequence began with a magnitude 7.1 mainshock at 11:06 a.m. (local time) on April 25. Strong aftershocks with magnitudes of 6.6 and 6.7 occurred on April 26 at 00:41 a.m. and at 4:18 a.m., respectively.

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