Aardbevingen #21

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Magnitude Mw 6.7
Region MAURITIUS - REUNION REGION
Date time 2021-05-12 14:05:16.8 UTC
Location 17.31 S ; 66.45 E
Depth 10 km
Distances 995 km ENE of Port Louis, Mauritius / pop: 155,000 / local time: 18:05:16.8 2021-05-12
415 km NE of Port Mathurin, Mauritius / pop: 6,000 / local time: 18:05:16.8 2021-05-12

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Magnitude Mw 6.6
Region NIAS REGION, INDONESIA
Date time 2021-05-14 06:33:08.0 UTC
Location 0.16 N ; 96.65 E
Depth 10 km
Distances 322 km SW of Padangsidempuan, Indonesia / pop: 100,000 / local time: 13:33:08.0 2021-05-14
268 km SSW of Singkil, Indonesia / pop: 46,800 / local time: 13:33:08.0 2021-05-14

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Here's what we know about the 6.0 Antelope Valley Earthquake

A 6.0 earthquake shook up Northern California on Thursday, with tremors widely felt throughout the Bay Area and down the San Joaquin Valley as far south as Visalia.

The Chronicle spoke with Sarah Minson, a research geophysicist at the U.S. Geological Survey’s Earthquake Science Center, about the high-magnitude temblor, dubbed the Antelope Valley Earthquake after its epicenter south of Lake Tahoe. Here’s what we know so far.

How big was this earthquake?

Thursday’s 6.0-magnitude shaker was among the largest ever recorded in the remote region between Lake Tahoe and Mono Lake on the California-Nevada border. Only two others — one recorded in September 1994, the other in June 1933 — were bigger at 6.1 each, geologists said.

In the past century, 33 temblors that occurred within a 100 mile radius of Antelope Valley had a magnitude above 5.0 on the 10-point Richter scale used to measure earthquakes’ size.

Thursday’s quake was on par with the the 6.0 Napa quake of 2014 and weaker than the 6.9-magnitude Loma Prieta earthquake that devastated the Bay Area in 1989. Other previous 6.0-range quakes include the 6.5-magnitude Coalinga earthquake that struck Fresno County in 1983 and the 6.7-magnitude Northridge earthquake that hit the San Fernando Valley in 1994.

Geologists recorded dozens of aftershocks. Is that normal?

In short, “yes,” Minson said.

Aftershocks follow the largest shock of an earthquake sequence and help release pent-up stress kicked off by intial quake. Their power decays over time, she said, meaning reverberations can continue for a period of weeks, months or even years.

More than 60 aftershocks had been recorded as of Thursday evening, including one 5.2 shaker. But according to Minson, there was “nothing special” about the cascading quakes.

“It’s like when you knock over one domino and it knocks over a bunch of dominoes around it,” Minson said.

Did the earthquake occur on a fault line?

The San Andreas Fault, running the length of the California coastline, is among the largest and most active fault zones in the world. It’s where the Pacific and the North American tectonic plates collide, giving rise to frequent seismic activity and prompting perennial speculation about the Big One.

But the area east of the Sierra Nevada — where the Antelope Valley Earthquake and its ensuing aftershocks occurred — is firmly located on the North American tectonic plate, far from the massive plate boundary that splinters the Bay Area.

The North American plate is moving toward the Pacific Ocean by about 20 milometers a year, stretching the land east of the Sierra Nevada “like taffy,” Minson said. That westward motion gives the terrain its distinctively hilly topography and makes it highly prone to earthquakes and aftershocks.

Where was the earthquake’s epicenter?

The epicenter hit a sparsely populated stretch along the California-Nevada border halfway between Lake Tahoe and Mono Lake, geologists said.

The area is so remote that the U.S. Geological Survey’s automated earthquake detector erroneously pegged the temblor to the closest census-designated place: Smith Valley, Nevada, population 1,627.

The actual epicenter was in California, about 32 miles south of Smith Valley, Minson said.

How common are earthquakes along the California-Nevada border?

The area east of the Sierra Nevada is an active earthquake zone. The quakes typically “fly under the radar” in the sparsely populated region, Minson said, but the area is “quite seismically active.”

Could the 6.0 quake be a prelude to a bigger quake?

“There is no particular reason to think one big earthquake is related to any other big earthquake,” Minson said, adding that high magnitude shakers are exceedingly rare and do not typically occur in tandem.

But double quakes do occur on rare occasions, she said, citing the two Ridgecrest earthquakes, which hit southeast California less than two days apart in July 2019. The first 6.4 magnitude quake rocked its Indian Wells Valley epicenter after a series of small foreshocks. A 7.1-magnitude monster followed 34 hours later.

Despite their close timing and proximity, geologists don’t have clear evidence the two high-magnitude earthquakes were related, Minson said.

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As of now, the seismic coverage on this earthquake is limited. However, data from various sea level buoys indicated a significat tsunami wave. In addition, the wide spread aftershocks in the region indicate a much larger rupture than a moment magnitude of 7.5 would anticipate.

Thus, a larger moment magnitude was used to simulate the tsunami.It was estimated that the total earthquake rupture which caused the tsunami was equivalent to a moment magnitude of about 8.3. The earthquake rupture model used to estimate the tsunami had a total rupture length of 210km and a maximum displacement of 20m.

The resulting tsunami pattern corresponds well the buoy data found here SEA LEVEL STATION MONITORING FACILITY (ioc-sealevelmonitoring.org):

King Edward Island: 0.7m
Syowa Station (Antarctica): 0.6m
Marion Island: 0.4m
St. Helena: 0.4m
Stanley station (South Georgia): 0.3m
Takoradi (Ghana): 0.2m
Reunion Island: 0.2m
Base Prat (Antarctica): 0.1m
Right now, it is unclear how the actual situation looks like. And the current tsunami simulation is an educated guess on what has happened on August 12th, 2021 in the Souther Atlantic Ocean.

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Op woensdag 11 augustus 2021 21:21 schreef Frutsel het volgende:
[ afbeelding ]

7.1 ten zuiden van de Filipijnen

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Op donderdag 12 augustus 2021 22:18 schreef Frutsel het volgende:
7.5 bij south sandwich islands

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Op zaterdag 14 augustus 2021 15:14 schreef Momo het volgende:
[ twitter ]

Dat is niet mooi voor een kwetsbaar land als Haïti...